Abstract: X-ray investigations on single crystals of a series of terminally dicyanovinyl-substituted quaterthiophenes and co-evaporated blend layers with C(60) give insight into molecular packing behavior and morphology, which are crucial parameters in the field of organic electronics. Structural characteristics on various levels and length scales are correlated with the photovoltaic performance of bulk heterojunction small-molecule organic solar cells.
Abstract: We observe hybrid states of cavity photons and Tamm plasmons in an organic microcavity with an incorporated thin silver layer of increasing thickness up to 40 nm. Via mu-photoluminescence spectroscopy, we investigate their angular dependence. At oblique angles, we observe a TE-TM polarization splitting of more than 40 meV for each mode. An analytical model is developed to describe the coupling of Tamm plasmons and cavity photons and to account for the splitting of the orthogonally polarized resonances. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3681374]
Abstract: Direct laser interference patterning (DLIP) is used to fabricate large area, two-dimensional periodic surface patterns on polyethylene terephthalate (PET) substrates to enhance the performance of ZnPc:C60 solar cells by light concentration in the absorber layer. Comparing the power conversion effi ciencies to the reference cell on fl at PET, a relative increase of 21% is observed for the hexagonal pattern with 0.7 mu m period, depicted in the fi gure.
Abstract: Exciton generation and transfer processes in a multilayer organic light-emitting diode (OLED) are studied in order to realize OLEDs with warm white color coordinates and high color-rendering index (CRI). We investigate a host-guest-system containing four phosphorescent emitters and two matrix materials with different transport properties. We show, by time-resolved spectroscopy, that an energy back-transfer from the blue emitter to the matrix materials occurs, which can be used to transport excitons to the other emitter molecules. Furthermore, we investigate the excitonic and electronic transfer processes by designing suitable emission layer stacks. As a result, we obtain an OLED with Commission Internationale de lEclairage (CIE) coordinates of (0.444;0.409), a CRI of 82, and a spectrum independent of the applied current. The OLED shows an external quantum efficiency of 10% and a luminous efficacy of 17.4 lm/W at 1000 cd/m(2). (C) 2012 American Institute of Physics. [doi:10.1063/1.3679549]
Abstract: In this work, we use different encapsulations to protect vacuum-evaporated small molecule organic solar cells with a simple p-i-i-stack for lifetime studies. Our devices use ZnPc and C60 as active materials. Lifetimes (T50) in a range from 300 h for un-encapsulated devices to 4000 h for glass-encapsulated have been observed. We use a model to distinguish between the water vapor transmission rate (WVTR) of the barrier and an additional WVTR of the aluminum top electrode. For all observed devices a loss of 50% of initial efficiency is observed when 10 mg m(-2) water entered the device. The losses are related to a reduction of short circuit current density only, whereas open circuit voltage and fill factor remains unaffected. We relate this to an interaction of the water molecules with C60. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We have developed semi-transparent n-i-p organic solar cells (OSC) with free-standing multi-wall carbon nanotube (f-CNT) sheets as transparent top electrodes. By a simple and damage-free room temperature orthogonal liquid solution assisted self-laminating process, f-CNT top electrodes are successfully deposited on top of ZnPc:C(60) bulk heterojunction small molecule OSCs. The cells show high fill factors above 58% as well as efficiencies up to 1.5% and greater long-term stability compared to the device having a metal electrode. For the given cell structure with f-CNT semi-transparent electrodes, the influence of an optical spacer on light absorption is studied by a systematic variation of the hole transport layer thickness, supported by optical simulations. The results strongly indicate that OSCs with f-CNT top electrodes and optimized thin film stack are highly promising for semi-transparent OSCs, which can be used in tandem devices, in tinted smart windows, and similar applications by a simple and damage-free process in roll-to-roll configuration that can be scaled to large area manufacturing. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: A novel approach for alternating current (AC)-driven organic light-emitting devices is reported, which uses the concept of molecular doping in organic semiconductors. Doped organic charge-transport layers are used to generate charge carriers within the device, hence eliminating the need for injecting charge carriers from external electrodes. Bright luminance of up to 1000 cd m-2 is observed when the device is driven with an AC bias. The luminance observed is attributed to charge-carrier generation and recombination, leading to the formation of excitons within the device, without injection of charge carriers through external electrodes. A mechanism for internal charge-carrier generation and the device operation is proposed.
Abstract: Efficient single bulk heterojunction organic solar cells based on blends of a fluorinated zinc phthalocyanine as electron donor and fullerene C60 as electron acceptor are reported. In comparison to the commonly used absorber zinc phthalocyanine, the fluorination of the molecule to F4ZnPc leads to an increase in ionisation potential and subsequently to an improvement of about 170 mV in the open circuit voltage of organic solar cells, while the short circuit current density and fill factor remain nearly unchanged. Similar to ZnPc:C60-based devices, the device characteristics of F4ZnPc:C60 solar cells can be further enhanced by improving the blend layer morphology by substrate heating during deposition. F4ZnPc is an efficient donor material that can achieve a 4.6% power conversion efficiency in single heterojunction organic solar cells.
Abstract: Due to their effective short channel length of only a few hundred nanometers, vertical organic triodes (VOTs) have a high potential to overcome problems of low current densities and switching speed in current organic field effect transistors (OFETs). Furthermore, VOTs are easy to build because no sub-structuring of the base contact is necessary. Nevertheless, these devices are poorly investigated. In literature, two different working mechanisms are suggested: hot carrier transport through the metallic base or transport of charge carriers through a permeable base electrode. As a strong asymmetry is expected for function principle based on hot carriers, we are able to distinguish between both mechanisms by examining the bidirectional transmission properties of the VOT consisting of electron transporting materials. We show that high transmission values (> 95%) are possible for both directions, suggesting a base contact with openings forming a grid electrode. (C) 2012 American Institute of Physics. [doi:10.1063/1.3686744]
Abstract: Direct structuring techniques are an indispensable need for future low-cost applications of organic semiconductor materials in e. g. active matrix displays or integrated circuits. We demonstrate direct structuring of a small molecule organic semiconductor by a photolithography lift off process under ambient conditions. To show compatibility of this process, we fabricate organic thin film transistors (OTFT) containing the benchmark electron transporting semiconductor C60 as active material in a top-contact geometry. C60 as electron transporting semiconductor serves as good indicator for contamination and degradation caused by the structuring procedure. To disclose influences of structuring, we discuss the OTFT performance for different channel lengths from 100 mu m down to 2.7 mu m. In particular, we show that lithography processing gives rise to increased contact resistances. Apart from that, mobility of C60 as material parameter is only weakly affected which underlines the compatibility of the suggested structuring procedure. The potential of this structuring procedure for future integration of driving transistors in active matrix displays is demonstrated. (C) 2011 Elsevier B. V. All rights reserved.
Abstract: We examine spontaneous emission in organic electroluminescent devices and investigate the influence of the local photonic mode density on the emissive properties of molecular emitters. Spontaneous emission in organic electroluminescent devices is modeled by means of an approximate closed-form solution for the exciton rate equation, which yields the efficiency of conversion of electrical charges into molecular excited states. The exciton decay rate and the efficiency of conversion of molecular excitation into far-field radiated photons are described using a state-of-the-art classical electromagnetic formalism suitable to model multilayered organic light-emitting diodes (OLEDs). We present an in-depth analysis of the influence of optical microcavities and the corresponding resonant modes on the luminescent properties of organic molecules. Near-field coupling and coupling to metallic reflectors are demonstrated as the main effects responsible for environment-induced modifications of the rate and efficiency of spontaneous emission. The extent to which the excitonic decay rate is modified by the optical microcavity (Purcell effect) is shown to be strictly dependent on the intrinsic luminescence quantum yield of the molecular emitter. The modeling formalism is successfully validated against experimental results obtained on three series of small-molecule p-i-n OLED samples, featuring phosphorescent or fluorescent molecular emitters, with a widely varying thickness of the optical microcavity. We demonstrate that, within its limits of validity, the theoretical treatment in this work provides a rigorous quantitative description of spontaneous emission in organic luminescent devices and allows for the identification of the factors determining the OLED internal and external quantum efficiencies.
Abstract: We present a comprehensive investigation of morphological changes inside the active layer of an organic solar cell induced by substrate heating during layer deposition by thermal evaporation in ultra-high vacuum. To explore the trends observed in solar cell devices, we apply absorption and photoluminescence spectroscopy, atomic force microscopy, X-ray diffraction, and organic field effect transistor measurements. The material combination we use comprises unsubstituted dicyanovinyl end-capped quaterthiophene (DCV4T) as the donor material mixed with C-60 as the acceptor. The solar cell power conversion efficiency decreases with increasing substrate temperature during film deposition due to changes in the crystalline structure of the oligothiophene phase, leading to a decrease in absorption strength. Photoluminescence measurements show that substrate heating increases the amount of phase separation between the donor and acceptor, and topology and structure investigations reveal large aggregates of polycrystalline DCV4T at the surface. However, the fill factor is increased for higher substrate temperatures due to better transport properties. The highest efficiency obtained with this material combination and stack design is 3.0% under AM1.5g illumination. (c) 2012 Elsevier B.V. All rights reserved.
Abstract: For high efficiencies, organic solar cells have to harvest a large part of the solar spectrum. However, in particular for small molecule based cells, efficient infrared absorbers are still rare. We investigate here two aza-bodipy dyes which have promising properties. Upon benzannulation of the pyrrole of difluorobora-1,3,5,7-tetraphenyl-aza-dipyrromethene (Ph-4-bodipy), the thin film absorption maximum is shifted by about 70-773 nm compared to the non-annulated molecule. The thin film properties of both materials are investigated and vacuum-processed solar cells using a mip-architecture (metal intrinsic p-doped) are compared. With the new benzannulated difluoro-bora-bis-(1-phenyl-indoyl)-azamethine (Ph-2-benz-bodipy) as donor material, these planar heterojunction solar cells show an open-circuit voltage of 0.65 V, a fill factor of 65%, and an external quantum efficiency extending up to 860 nm. The mismatch corrected power conversion efficiency reaches 1.1%. (C) 2012 Published by Elsevier B.V.
Abstract: 2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-MeO-DMBI-I) was synthesized and employed as a strong n-type dopant for fullerene C-60, a well-known n-channel semiconductor. The coevaporated thin films showed a maximum conductivity of 5.5 S/cm at a doping concentration of 8.0 wt% (14 mol %), which is the highest value reported to date for molecular n-type conductors, o-MeO-DMBI-I can be stored and handled in air for extended periods without degradation and is thus promising for various organic electronic devices.
Abstract: We report on an alternating current (AC) organic electroluminance (EL) device with high brightness of over 1500 cdm(-2) and very low threshold voltage for the onset of AC luminance (10 V). The device consists of organic layers enclosed in a pair of either silicon dioxide (SiO2) or hafnium dioxide (HfO2) insulating dielectric layers. We observe internal charge carrier generation and luminance due to charge carrier recombination within the device under the AC drive without charge injection. We compare AC-EL demonstrated for the two devices and show that the threshold voltage for the onset of luminance can be significantly lowered using high dielectric constant (kappa) HfO2 insulating layers. Further, we discuss the limitations of internal charge carrier generation and the device performance supported with the results from time resolved measurements. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3692776]
Abstract: In this work we introduce a simple yet very efficient method of heat dissipation by immersing OLED device into hydrofluoroether (HFE) fluids. It is shown that due to highly fluorous nature of this class of fluids, HFE do not damage organic semiconductors which are comprised in the OLED stack and therefore can be used as encapsulation media. HFE also have high thermal conductivity, low viscosity and can efficiently dissipate the heat by means of natural convection with laminar flow. By employing HFE we were able to significantly improve the OLED operating dynamic range. Lifetimes of OLEDs operating in HFE at high currents can be improved by about a factor of 8. Furthermore, HFE fluid significantly improves the light outcoupling by a factor of 70% due to higher than air refractive index (n = 1.3). (C) 2011 Elsevier B. V. All rights reserved.
Abstract: Two optical Tamm plasmons and a discretized microcavity state are observed simultaneously in an organic microcavity by angle-resolved photoluminescence spectroscopy. The Tamm plasmons form as a result of a 40 nm silver layer placed between the bottom distributed Bragg reflector and the lambda/2 cavity layer. This silver layer is perforated by round holes of a few microns size, generating optical mesas from which discretized microcavity states are observed concurrently. The discretization and the intensity of the different states are studied as a function of angle and hole diameter and compared to analytical calculations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3690053]
Abstract: The structural properties and charge carrier mobility of pentacene doped by 2,3,5, 6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and 2,2-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ) are studied by X-ray diffraction, scanning electron microscopy, field effect transistor measurements, and space charge limited currents (SCLC). We observe the presence of polycrystalline and amorphous domains within the doped pentacene film grown under co-deposition conditions. The appearance of the amorphous phase is induced by the molecular dopants F4-TCNQ and F6-TCNNQ. A strong drop of crystallite size is obtained at a doping concentration of around 7 and 4 wt.%, respectively. The loss of the polycrystalline structure is correlated to a strong decrease of the charge carrier mobility in pentacene in horizontal and vertical film structures. We discuss typical scenarios of charge transport for polycrystalline and amorphous thin films in order to explain the observed loss of mobility originated by the doping induced structural phase transition. In this way an optimum doping concentration for highest conductivity with acceptable mobility is determined which can help to improve the performance of organic solar cells and organic high-frequency rectification diodes. (C) 2011 Elsevier B. V. All rights reserved.
Abstract: White top-emitting organic light-emitting diodes (OLEDs) attract much attention, as they are optically independent from the substrate used. While monochrome top-emitting OLEDs can be designed easily to have high-emission efficiency, white light emission faces obstacles. The commonly used thin metal layers as top electrodes turn the device into a microresonator having detrimental narrow and angular dependent emission characteristics. Here we report on a novel concept to improve the color quality and efficiency of white top-emitting OLEDs. We laminate a refractive index-matched microlens film on the top-emitting device. The microlens film acts both as outcoupling-enhancing film and an integrating element, mixing the optical modes to a broadband spectrum.
Abstract: The change of morphology in mixed layers due to different substrate temperature Tsub of organic solar cells containing C(60) and zinc phthalocyanine (ZnPc) is studied. Heating the substrate during deposition of the bulk heterojunction C(60):ZnPc leads to a significant improvement of solar cell performance, mainly due to an increase in photocurrent and fill factor (FF). This is attributed to improved charge carrier percolation pathways within the C(60):ZnPc blend. Using atomic force microscopy, scanning electron microscopy, transmission electron microscopy, organic field effect transistor, X-ray diffraction, and absorption measurements, we observe aggregation, cluster-like, and polycrystalline growth of the heated bulk layer. This provides better transport percolation paths by inducing a phase separation of the molecules. Heated blend layer with thickness of 60 nm shows high performance without loss in FF. When heating the substrate to the optimum temperature of 110 degrees C, a power conversion efficiency of 3.0% is achieved, compared to 1.4% for an identical device prepared on a substrate held at room temperature. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: We present investigations of top emitting organic light emitting devices (OLED) comprising n- and p-doped organic charge transport layers. It has been found previously that in comparison to noninverted p-i-n OLEDs, inverted n-i-p OLEDs show reduced device performances after fabrication. These differences can be eliminated by subsequent thermal annealing of the whole n-i-p OLED. After this process, the n-i-p OLED exhibits a superior low driving voltage of 2.9 V at 1000 cd/m(2) and shows an increase in external quantum efficiency from 11% to almost 15% which we ascribe to a modified charge balance within the intrinsic organic emission layer. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3559847]
Abstract: We show that S-kinks in the current voltage characteristics, which decrease the fill factor significantly, can be caused by a strong imbalance of charge carrier mobilities (hole mobility in donor and electron mobility in acceptor) in planar/flat heterojunction organic solar cells. Electrical simulations according to a drift-diffusion model predict the occurrence of an S-kink for a mobility mismatch factor larger than 100. By combining a low-mobility donor material, (1,2,3,4,9,10,11,12-octaphenyl-diindeno[1,2,3-cd: 1â,2â,3â-lm]perylene), with the acceptors C(60) and N, Nâ-dimethylperylene-3,4: 9,10-dicarboximide, which show different electron mobilities, we experimentally verify the predictions. Our results demonstrate that not only interface effects but also the photoactive material itself can cause S-kinks. (C) 2011 American Institute of Physics. [doi:10.1063/1.3553764]
Abstract: We present an investigation of the intrinsic chemical degradation mechanisms of phosphorescent OLEDs based on the common sky blue emitter bis(2-(4,6-difluorophenyl)-pyridyl-N,C2â)iridium(III)picolinate (FIrpic). The OLEDs are investigated using the laserdesorption/ionization time-of-flight mass spectrometry. The comparison between the collected spectra for electrically aged and unaged OLEDs allows the identification of different reaction products, like fragments which are mainly related to the chemical dissociation of FIrpic molecules during the OLED operation. We present different reaction pathways of the blue emitter FIrpic. One proposed reaction indicates that the short lifetimes of the OLEDs may be related to the irreversible dissociation of the FIrpic molecule by the loss of carbon dioxide (CO(2)) from the picolinate ligand. Additionally, the formation of chemical complexes between different fragments of the FIrpic molecules with their neighbour materials is visible. The cesium adducts of FIrpic formation indicate a possible contribution of the dopant to the OLED degradation process. Finally, we show rearrangement effects in the Cs doped electron transporting layer. This rearrangement is indicated by the presence of m/z signals of the BPhen dimer and adducts of the dimer with Cs. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: A series of novel aza-diisoindolmethine dyes 9 with six different aryl and heteroaryl groups at the indole moiety have been synthesized by the addition of aryl Grignard compounds to phthalodinitrile and subsequent reaction with formamide. A plausible reaction mechanism, through a Leuckart-Wallach-type reduction has been confirmed by means of DFT calculations of the related transition and intermediate states. The corresponding boron difluoride complexes (10) of 9 were prepared in a subsequent reaction step and the spectroscopic and electrochemical properties of 9 and 10 have been investigated both experimentally and theoretically. The aza-diisoindolmethines 9 exhibit an absorption maximum in the range from 615 to 720 nm, whereas the complexes 10 show a bathochromically shifted absorption maximum between 681 and 793 nm. Measurements of 9 and 10 by cyclic voltammetry display fully reversible redox waves for the reduction and oxidation with higher potentials for 10. From the measured redox potentials, the HOMO and LUMO energy levels were calculated for 9 and 10. The frontier orbital energies, the energies of the absorption bands, as well as the orbitals involved in the absorption process were calculated with DFT and compared to the measured results of 9 and 10. The absorption maximum can be related to an intense HOMO-LUMO transition and the more-pronounced stabilization of the LUMO upon complexation is the origin of the bathochromic shift of the absorption. Additionally, single-crystal structures for two species, 10d and 10 f, are reported.
Abstract: We introduce the material hexaazatriphenylene hexacarbonitrile (HATCN) as electron conducting window layer for separating the photoactive region from the cathode in organic p-i-n type solar cells. HATCN has a wide band gap of 3.3 eV and is thus transparent in the visible range of the solar spectrum. Its electrical properties can be tuned by means of molecular n-doping which leads to an increase of electron conductivity by several orders of magnitude up to 2.2 x 10(-4) S/cm. However, an application in photovoltaic devices is restrained by its exceptionally high electron affinity, estimated 4.8 eV, which introduces an electron injection barrier to the photoactive acceptor material C(60). Here, we present a strategy to remove this barrier by means of introducing doped and undoped C(60) intermediate layers, thus demonstrating the importance of energy level matching in a multiple layer structure and the advantages of Fermi level control by doping. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand-alone electrodes for organic solar cells have been optimized using a solvent post-treatment method. The treated PEDOT: PSS films show enhanced conductivities up to 1418 S cm(-1), accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT: PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO-free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc): fullerene C(60) bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre-heating the PEDOT: PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT: PSS with solvent and thermal post-treatment can be a very promising electrode material for highly efficient flexible ITO-free organic solar cells.
Abstract: Efficient synthesis of a series of terminally dicyanovinyl (DCV)-substituted oligothiophenes, DCVnT 1-6, without solubilizing side chains synthesized via a novel convergent approach and their application as electron donors in vacuum-processed m-i-p-type planar and p-i-n-type bulk heterojunction organic solar cells is described. Purification of the products via gradient sublimation yields thermally highly stable organic semiconducting materials in single crystalline quality which allows for X-ray structure analysis. Important insights into the packing features and intermolecular interactions of these promising solar cell materials are provided. Optical absorption spectra and electrochemical properties of the oligomers are investigated and valuable structure-property relationships deduced. Photovoltaic devices incorporating DCVnTs 4-6 showed power conversion efficiencies up to 2.8% for planar and 5.2% for bulk heterojunction organic solar cells under full sun illumination (mismatch corrected simulated AM 1.5G sunlight). The 5.2% efficiency shown here represents one of the highest values ever reported for organic vacuum-deposited single heterojunction solar cells.
Abstract: A homogeneous microcavity is excited in two tightly focused spatially separated spots with a few mu m distance. The radius of the generated cavity modes exceeds the excited regions such that the quasimodes of both spots overlap in space. Coherent coupling of the two spots via stimulated emission is observed, exhibiting a slightly lowered lasing threshold. Furthermore, the emission from both spots is phase locked with a phase difference of either 0 or m. depending on their separation. Correspondingly, the distribution of the electromagnetic field is symmetric or anti-symmetric, and the emission occurs parallel to the cavity normal or symmetrically at non-zero angles, respectively. This behaviour is understood to be determined by the spatial distribution of gain and absorption across the active cavity medium, which we confirm by numerical calculations. (C) 2010 Elsevier Ltd. All rights reserved.
Notes: 10th International Conference on Physics of Light-Matter Coupling in Nanostructures, Cuernavaca, MEXICO, APR 12-16, 2010
Abstract: We present the material 2,3,10,11-tetrabutyl-1,4,9,12-tetraphenyl - diindeno[1,2,3-cd: 1â,2â,3â-lm] perylene (Bu4-Ph4-DIP) as alternative green donor for bulk heterojunction small molecule organic solar cells (SMOSC). It is shown that Bu4-Ph4-DIP exhibits suitable absorption characteristics to be a potential material to fill the absorption gap between the commonly used standard absorbers ZnPc and C(60). Devices with bulk heterojunctions of Bu4-Ph4-DIP:C(60) display very high open circuit voltages of 0.99 v, high fill factors of up to 57%, and experiments yield promising efficiencies of eta > 2%. Such green-blue absorbing SMOSC are characterized by current voltage and external quantum efficiency measurements, and material properties are studied. It is shown that the devices are responsive to substrate heating, and that different donor-acceptor mixing ratios can increase device performance. Possible influences of mixing ratio and heating on device morphology and electrical properties are discussed. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: Using spectroscopic experiments, we show the transition from a single cavity mode into two optical Tamm states upon incorporating a thin metal layer into a planar organic microcavity. The eigenenergies of both modes strongly depend on the thickness of the metal and both adjacent dielectric layers. We show that both resonances cannot have the same wavelength indicating a clear anticrossing behavior. The experimentally observed splitting of the cavity mode into two resonances is confirmed via transfer matrix calculations.
Abstract: The performance of transparent metal top contacts in organic solar cells can strongly be improved by employing surfactant layers. We use scanning electron microscopy to investigate the change in morphology upon insertion of an Al surfactant layer between 4,7-diphenyl-1,10-phenanthroline (BPhen) and a silver top contact. UV photoelectron spectroscopy measurements show the changes in energetic alignments at different steps of the organic/metal interface formation. Furthermore, using X-ray photoelectron spectroscopy depth profiling, we compare the differing intermixing processes happening within the two samples. Thereby, we can show that Al binds to BPhen molecules, acting as surfactant for subsequently deposited Ag layers, while Ag without any Al surfactant layer penetrates into and intermixes with the BPhen layer. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: Using spectroscopic experiments, we show the transition from a single cavity mode into two optical Tamm states upon incorporating a thin metal layer into a planar organic microcavity. The eigenenergies of both modes strongly depend on the thickness of the metal and both adjacent dielectric layers. We show that both resonances cannot have the same wavelength indicating a clear anticrossing behavior. The experimentally observed splitting of the cavity mode into two resonances is confirmed via transfer matrix calculations.
Abstract: We present the material 2,3,10,11-tetrabutyl-1,4,9,12-tetraphenyl - diindeno[1,2,3-cd: 1',2',3'-lm] perylene (Bu4-Ph4-DIP) as alternative green donor for bulk heterojunction small molecule organic solar cells (SMOSC). It is shown that Bu4-Ph4-DIP exhibits suitable absorption characteristics to be a potential material to fill the absorption gap between the commonly used standard absorbers ZnPc and C(60). Devices with bulk heterojunctions of Bu4-Ph4-DIP:C(60) display very high open circuit voltages of 0.99 v, high fill factors of up to 57%, and experiments yield promising efficiencies of eta > 2%. Such green-blue absorbing SMOSC are characterized by current voltage and external quantum efficiency measurements, and material properties are studied. It is shown that the devices are responsive to substrate heating, and that different donor-acceptor mixing ratios can increase device performance. Possible influences of mixing ratio and heating on device morphology and electrical properties are discussed. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: We present an investigation of the intrinsic chemical degradation mechanisms of phosphorescent OLEDs based on the common sky blue emitter bis(2-(4,6-difluorophenyl)-pyridyl-N,C2')iridium(III)picolinate (FIrpic). The OLEDs are investigated using the laserdesorption/ionization time-of-flight mass spectrometry. The comparison between the collected spectra for electrically aged and unaged OLEDs allows the identification of different reaction products, like fragments which are mainly related to the chemical dissociation of FIrpic molecules during the OLED operation. We present different reaction pathways of the blue emitter FIrpic. One proposed reaction indicates that the short lifetimes of the OLEDs may be related to the irreversible dissociation of the FIrpic molecule by the loss of carbon dioxide (CO2) from the picolinate ligand. Additionally, the formation of chemical complexes between different fragments of the FIrpic molecules with their neighbour materials is visible. The cesium adducts of FIrpic formation indicate a possible contribution of the dopant to the OLED degradation process. Finally, we show rearrangement effects in the Cs doped electron transporting layer. This rearrangement is indicated by the presence of m/z signals of the BPhen dimer and adducts of the dimer with Cs. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: The electronic and geometrical structure of single difluoro-bora-1,3,5,7-tetraphenyl-azadipyrromethene (aza-BODIPY) molecules adsorbed on the Au(111) surface is investigated by low temperature scanning tunneling microscopy and spectroscopy in conjunction with ab initio density functional theory simulations of the density of states and of the interaction with the substrate. Our DFT calculations indicate that the aza-BODIPY molecule forms a chemical bond with the Au(111) substrate, with distortion of the molecular geometry and significant charge transfer between the molecule and the substrate. Nevertheless, most likely due to the low corrugation of the Au(111) surface, diffusion of the molecule is observed for applied bias in excess of 1 V.
Abstract: The synthesis of five homoleptic transition metal complexes of bis-(phenyl)-diisoindol-aza-methene is described together with the optical, electrochemical and thermal properties of these compounds. Additionally, crystal structures for the Co and the Zn complex are reported.
Abstract: The performance of transparent metal top contacts in organic solar cells can strongly be improved by employing surfactant layers. We use scanning electron microscopy to investigate the change in morphology upon insertion of an Al surfactant layer between 4,7-diphenyl-1,10-phenanthroline (BPhen) and a silver top contact. UV photoelectron spectroscopy measurements show the changes in energetic alignments at different steps of the organic/metal interface formation. Furthermore, using X-ray photoelectron spectroscopy depth profiling, we compare the differing intermixing processes happening within the two samples. Thereby, we can show that Al binds to BPhen molecules, acting as surfactant for subsequently deposited Ag layers, while Ag without any Al surfactant layer penetrates into and intermixes with the BPhen layer. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: The electronic and geometrical structure of single difluoro-bora-1,3,5,7-tetraphenyl-azadipyrromethene (aza-BODIPY) molecules adsorbed on the Au(111) surface is investigated by low temperature scanning tunneling microscopy and spectroscopy in conjunction with ab initio density functional theory simulations of the density of states and of the interaction with the substrate. Our DFT calculations indicate that the aza-BODIPY molecule forms a chemical bond with the Au(111) substrate, with distortion of the molecular geometry and significant charge transfer between the molecule and the substrate. Nevertheless, most likely due to the low corrugation of the Au(111) surface, diffusion of the molecule is observed for applied bias in excess of 1 V.
Abstract: The synthesis of five homoleptic transition metal complexes of bis-(phenyl)-diisoindol-aza-methene is described together with the optical, electrochemical and thermal properties of these compounds. Additionally, crystal structures for the Co and the Zn complex are reported.
Abstract: Zinc phthalocyanine (ZnPc), C(32)F(11)N(8)Zn, is a planar organic molecule having numerous optical and electrical applications in organic electronics. This work investigates the influence of various deposition parameters on the morphology of vapour thermal evaporated ZnPc films. For this purpose, ZnPc is deposited at different substrate temperatures up to 90 degrees C and film thickness up to 50 nm onto various substrates. The morphology of this ZnPc layers is characterised by X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM) methods. XRD measurements show that all ZnPc films are crystalline in a triclinic (alpha-ZnPc) or monoclinic (gamma-ZnPc) phase, independent from the kind of substrate, layer thickness, or substrate temperature. The ZnPc powder, the starting product for the thermally evaporated ZnPc films, is present in the stable monoclinic beta-phase. Thus, the stacking of the ZnPc molecules changes completely during deposition. The crystallite size perpendicular to the substrate determined by XRD microstructure analysis is in the range of the layer thickness while the lateral size, obtained by AFM, is increasing with substrate temperature and film thickness. AFM and XRR show an increase of the layer roughness for thicker ZnPc layers and higher substrate temperatures during film deposition. The strain in the ZnPc films decreases for higher substrate temperatures due to enhanced thermal relaxation and for thicker ZnPc films due to lower surface tension. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We present small molecule organic solar cells (SMOSC) based on flat heterojunctions (FHJ) of the alternative green donor 2,3,10,11-tetrapropyl-1,4,9,12-tetraphenyl-diindeno[ 1,2,3-cd: 1â, 2â, 3â-lm]perylene (P4-Ph4-DIP) and the fullerene C(60). P4-Ph4-DIP absorbs in the green spectral range and thus fills the spectral gap that standard absorber materials (zinc or copper phthalocyanine for red and C(60) for blue absorption) leave, thus allowing broad coverage of the sun spectrum, which is of major interest for tandem devices. The materials properties of P4-Ph4-DIP are studied, and SMOSC are characterized by current voltage, external quantum efficiency, and aging measurements. The solar cells display very high fill factors FF > 76% and open circuit voltages V(OC) of close to 1 V. Mismatch-corrected efficiencies of up to 1.9% are obtained. Aging measurements show that C(60) in conjunction with P4-Ph4-DIP yields extremely stable devices. We observe approximate to 88% of the initial efficiency after 2500 h illumination at 999 mW/cm(2) illumination intensity, with no observable change in short-circuit current density. Furthermore, we also show that a systematic variation of donor thickness in FHJ can be combined with transfer matrix formalism-based optical simulations and the continuity equation for excitons to reliably determine the exciton diffusion length L(D). A value of 9 +/- 1 nm is found for P4-Ph4-DIP.
Abstract: In this paper, two vacuum processed single heterojunction organic solar cells with complementary absorption are described and the construction and optimization of tandem solar cells based on the combination of these heterojunctions demonstrated. The red-absorbing heterojunction consists of C(60) and a fluorinated zinc phthalocyanine derivative (F4-ZnPc) that leads to a 0.1-0.15 V higher open circuit voltage V(proportional to) than the commonly used ZnPc. The second heterojunction incorporates C(60) and a dicyanovinyl-capped sexithiophene derivative (DCV6T) that mainly absorbs in the green. The combination of both heterojunctions into one tandem solar cell leads to an absorption over the whole visible range of the sun spectrum. Thickness variations of the transparent p-doped optical spacer between both subcells in the tandem solar cell is shown to lead to a significant change in short circuit current density j(sc) due to optical interference effects, whereas V(proportional to) and fill factor are hardly affected. The maximum efficiency eta of about 5.6% is found for a spacer thickness of 150-165 nm. Based on the optimized 165nm thick spacer, effects of intensity and angle of illumination, and temperature on a tandem device are investigated. Variations in illumination intensity lead to a linear change in jsc over three orders of magnitude and a nearly constant eta in the range of 30 to 310 mW cm(-2). Despite the stacked heterojunctions, the performance of the tandem device is robust against different illumination angles: j(sc) and eta closely follow a cosine behavior between 0 degrees and 70 degrees. Investigations of the temperature behavior of the tandem device show an increase in eta of 0.016 percentage points per Kelvin between -20 degrees C and 25 degrees C followed by a plateau up to 50 C. Finally, further optimization of the tandem stack results in a certified. of (6.07 +/- 0.24)% on (1.9893 +/- 0.0060) cm(2) (Fraunhofer ISE), i.e., areas large enough to be of relevance for modules.
Abstract: We report on highly enhanced light outcoupling of bi-directional red phosphorescent organic light-emitting diodes (BiOLEDs) by strong micro-cavity effects. Inserting additional metal layers between the indium tin oxide (ITO) anode and the hole transporting layer (HTL) and controlling the thickness of the counter metal electrode result in dramatic changes in electroluminescence spectra and angular distribution. We find that the external quantum efficiency (EQE) for both bottom and top emitting side can be significantly improved: the overall EQE reaches up to 24.24%. (C) 2011 American Institute of Physics. [doi:10.1063/1.3628292]
Abstract: We determine the mobility-lifetime product (mu tau) of free charge carriers in pristine zinc phthalocyanine (ZnPc) films using photocurrent measurements. The photocurrent is proportional to the free charge carrier generation efficiency (eta) and the mu tau product, and the carrier collection length is directly proportional to the latter. The mu tau product is thus an important parameter for the electronic quality of a material. We further determine the dominant photocarrier generation mechanisms in ZnPc. The free carrier generation efficiency is estimated from total carrier collection and electric field-induced photoluminescence quenching measurements. Using eta and the electric field dependence of the photocurrent, we estimate the mu tau product of holes in ZnPc to be about 3x10(-11) cm(2)/V.
Abstract: We characterize a series of dicyanovinyl-terthiophenes with different alkyl side chains. Variations of side chain substitution patterns and length mainly affect the morphology of the evaporated thin films, which in turn sensitively influences properties like absorption, energy levels, and thin film roughness. To investigate changes in transfer processes between electron donor (D) and acceptor (A) molecules due to side chain variations, we use photoinduced absorption spectroscopy (PIA). PIA probes the long-living photoexcited species at the D-A interface: triplet excitons, cations, and anions. For a blend layer of dicyanovinyl-terthiophene and the electron acceptor fullerene C(60), an energy transfer via the singlet and triplet manifold of C(60) occurs. The recombination dynamics of the triplet excitons reveal two components that differ in their lifetime and generation rate by 1 order of magnitude. By comparing the dynamics of triplet excitons in neat and blend layers, we estimate the energy transfer efficiency in dependence of the type of side chain. The compound with methyl side chains shows remarkable properties regarding thin film absorption, surface roughness, and energy transfer efficiency, which we attribute to the specific nanomorphology of the thin film.
Abstract: Thin conductive, hydrophobic films of poly(3,4-ethylenedioxythiophene) or PEDOT were synthesized on-substrate in the presence of the organic electron acceptor and dehydrogenating agent 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) using a versatile processing procedure. Significant polymerization in the processing solution was delayed by using common aprotic, ethereal solvents with low dielectric constants to prevent solvating the EDOT:DDQ charge transfer complex into radicals. Polymerization was initiated by an increase in concentration upon solvent evaporation during spin coating. A hydrophobic polymer matrix additive of polyvinyl acetate was used to aid in film formation, and a post-treatment rinse with acetonitrile was necessary to obtain a conductive film. Conductivities ranged from 17 to 59 S/cm, where the higher values were achieved at the cost of transparency. A work function of 4.62 eV was determined by UV photoelectron spectroscopy for one film recipe. When comparing conductive AFM results of PEDOT: DDQ to highly conductive, ethylene glycol-treated PEDOT: poly(styrenesulfonate) or PSS, surface currents were orders of magnitude higher for PEDOT: DDQ than for PEDOT: PSS. If optimized further, less acidic and hydrophobic PEDOT: DDQ films have the potential to replace PEDOT: PSS for use as a transparent electrode or charge transport layer in organic solar cells, organic light emitting diodes, touch screens, and other optoelectronic devices. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We report on efficient and stable ITO-free small molecule organic solar cells with conductive poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) electrodes using a post-treatment process, causing selective removal of PSS. The solar cells with post-treated PEDOT:PSS electrodes show significantly improved short circuit current densities and efficiencies compared to untreated devices. Moreover, the removal of PSS by the post-treatment significantly improves the lifetime of devices, which are more resistant to loss of fill factor compared to untreated devices. (C) 2011 American Institute of Physics. [doi:10.1063/1.3634015]
Abstract: The electrical and optical properties of molecular thin films are widely used, for instance in organic electronics, and depend strongly on the molecular arrangement of the organic layers. It is shown here how atomic structural information can be obtained from molecular films without further knowledge of the single-crystal structure. C60 fullerene was chosen as a representative test material. A 250 nm C60 film was investigated by grazing-incidence X-ray diffraction and the data compared with a Bragg-Brentano X-ray diffraction measurement of the corresponding C60 powder. The diffraction patterns of both powder and film were used to calculate the pair distribution function (PDF), which allowed an investigation of the short-range order of the structures. With the help of the PDF, a structure model for the C60 molecular arrangement was determined for both C60 powder and thin film. The results agree very well with a classical whole-pattern fitting approach for the C60 diffraction patterns.
Abstract: We investigate organic light-emitting diodes (OLEDs) comprising the singlet emitter system 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped into aluminium tris(8-hydroxyquinoline) (Alq(3)) at high excitation densities. With the OLED active area reduced to 100 x 100 mu m(2), current densities up to 800 A/cm(2) are achieved in pulsed operation. These devices exhibit an intense electroluminescence (EL) turn-on peak on the nanosecond time scale. With the help of streak camera measurements, we prove that the steady state EL of the fluorescent OLEDs is reduced due to singlet-triplet quenching. We demonstrate that short electrical pulses with a rise time of 10 ns make the separation of singlet emission and singlet-triplet quenching in time domain possible. By modeling the singlet and triplet population dynamics in the emission layer, we find that the triplet-triplet annihilation-rate coefficient in doped fluorescent materials is triplet-density dependent at high excitation density. The increased triplet lifetime usually observed in host: guest systems due to triplet trapping on guest molecules vanishes at high current densities. An increase in current density leads to an increased triplet-triplet annihilation rate, while the triplet density in the emission layer stays constant.
Abstract: A series of novel thiophene-substituted aza-BODIPY dyes were synthesized by means of a standard procedure and complemented by a Stille-coupling of a brominated species with 2-tributylstannylthiophene. The optical as well as the electrochemical properties of the compounds were investigated and compared to result of density functional theory (DFT) calculations. The influence of the thiophene substituents is discussed in dependence of the position at the aza-BODIPY core regarding the HOMO and LUMO frontier orbitals. The different distributions of the HOMO and LUMO coefficients over the BODIPY core lead to a variable influence of the thiophene substituents on the HOMO and LUMO energies, being the origin of the tunable optical and electrochemical properties. (C) 2011 Elsevier Ltd. All rights reserved.
Abstract: For improving the lifetime of organic light emitting diodes (OLEDs), it is essential to understand the chemical reaction pathways involved in the degradation process of these devices. Using the laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) technique, an OLED based on the phosphorescent emitter tris(phenylpyridine)iridium (Ir(ppy)(3)) was investigated. We show that the chemical dissociation reaction of the Ir(ppy)(3) molecule and an additional complexation of its fragments with the used hole blocking material 4,7-diphenyl-1, 10-phenanthroline is the main intrinsic degradation mechanism. This finding underlines the exciton induced dissociation mechanism of phosphorescent emitters, even at emitters without any metal-oxygen- bonds in the ligand system. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3617459]
Abstract: We report on improved and controlled light outcoupling of transparent organic light-emitting diodes (TOLEDs) by inserting thin silver layers between the indium tin oxide anode and the hole transporting layer. The introduction of Ag layers influences both the bottom and top emission of the TOLEDs, and it results in dramatic changes in the electroluminescence spectra and angular distribution. We find that the overall external quantum efficiency can be increased up to 18.8%, and the ratio of bottom and top emission can be almost identical. (C) 2011 Optical Society of America
Abstract: Small molecule organic solar cells were studied with respect to water and oxygen induced degradation by mapping the spatial distribution of reaction products in order to elucidate the degradation patterns and failure mechanisms. The active layers consist of a 30 nm bulk heterojunction formed by the donor material zinc-phthalocyanine (ZnPc) and the acceptor material Buckminsterfullerene (C(60)) followed by 30 nm C60 for additional absorption. The active layers are sandwiched between 6 nm 4,7-diphenyl-1, 10-phenanthroline (Bphen) and 30 nm N,Nâ-diphenyl-N,Nâ-bis(3-methylphenyl)-[1,1â-biPhenyl]-4,4â-diamine p-doped with C(60)F(36) (MeO-TPD:C(60)F(36)), which acted as hole transporting layer. Indium-tinoxide (ITO) and aluminum served as hole and electron collecting electrode, respectively. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) in conjunction with isotopic labeling using H(2) (18)O and (18)O(2) provided information on where and to what extent the atmosphere had reacted with the device. A comparison was made between the use of a humid (oxygen free) atmosphere, a dry oxygen atmosphere, and a dry (oxygen free) nitrogen atmosphere during testing of devices that were kept in the dark and devices that were subjected to illumination under simulated sunlight. It was found that water significantly causes the device to degrade. The two most significant degradation mechanisms are diffusion of water through the aluminum electrode resulting in massive formation of aluminum oxide at the BPhen/Al interface, and diffusion of water into the ZnPc:C(60) layer where ZnPc becomes oxidized. Finally, diffusion from the electrodes was found to have no or a negligible effect on the device lifetime. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We show that the number of extracted charge carriers is a suitable measure to compare lifetime measurements on organic solar cells at different intensities. In detail, we used pin-structures with active layers containing a bulk heterojunction of Zincphthalocyanine (ZnPc) and C(60). Extended lifetime measurements under constant monochromatic or white illumination at defined temperatures of 50 degrees C or 90 degrees C are done. On the one hand, we show that the number of extracted charge carriers is important to determine the degree of degradation. On the other hand, our results show that the energy of irradiated photons is significant for accelerated measurements. This is an major advantage for the realisation of accelerated lifetime measurements. Additionally, we find that not single charge carriers, but excitons cause the degradation of the observed solar cells. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: A large number of flexible polymer solar modules comprising 16 serially connected individual cells was prepared at the experimental workshop at Riso DTU. The photoactive layer was prepared from several varieties of P3HT (Merck, Plextronics, BASF and Riso DTU) and two varieties of ZnO (nanoparticulate, thin film) were employed as electron transport layers. The devices were all tested at Riso DTU and the functional devices were subjected to an inter-laboratory study involving the performance and the stability of modules over time in the dark, under light soaking and outdoor conditions. 24 laboratories from 10 countries and across four different continents were involved in the studies. The reported results allowed for analysis of the variability between different groups in performing lifetime studies as well as performing a comparison of different testing procedures. These studies constitute the first steps toward establishing standard procedures for an OPV lifetime characterization. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We investigate a set of transparent organic LEDs (TOLEDs) with different organic capping layer (OC) thicknesses to understand the capping layer effect. We find that thickness variation of the OC strongly influences the emission properties of TOLEDs and exhibits different trends for top or bottom emission. The external quantum efficiency for the top side can be enhanced by a factor of 63%, but that of the bottom side only by 4% compared to a reference device without an OC. Additionally, we demonstrate that the introduction of the OC is an effective method to control the bottom-to-top emission ratio within a measured range from 2.87 to 6.05. (C) 2011 Optical Society of America
Abstract: We demonstrate highly efficient small molecule organic light emitting diodes and organic solar cells based on the p-i-n-type structure using the fluorinated fullerene molecule C(60)F(36) as p-dopant in the hole transport layer. We present synthesis, chemical analysis, and energy level investigation of the dopant as well as the conductivity of organic layers consisting of a matrix of N,N,Nâ,Nâ-tetrakis 4-methoxyphenyl-benzidine(MeO-TPD) or N,Nâ-[(Diphenyl-N,Nâ-bis)9, 9,-dimethyl-fluoren-2-yl]-benzidine(BF-DPB) doped by the fullerene compound. State of the art organic p-i-n devices containing C(60)F(36) show efficiencies comparable to devices with the commonly used p-dopant2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)-TCNQ). The advantages of the fullerene based dopant are the low volatility and high thermal stability, which is beneficial for device operation under elevated temperature. These properties make C(60)F(36) highly attractive for the usage as p-dopant in a broad spectrum of organic p-i-n devices like organic light emitting diodes, solar cells, memories, or transistors. (C) 2011 American Institute of Physics. [doi:10.1063/1.3590142]
Abstract: We demonstrate that the laser output power of an organic microcavity saturates with the pump energy shortly above the lasing threshold. The extent of the saturation correlates with the volume of the optical mode and is strongly enhanced in a small-mode volume lambda/2 microcavity. The laser emission characteristics under different pumping conditions unambiguously show that the effect is due to the finite number of absorbing molecules per mode that are efficiently saturated at higher pump powers. A modified set of rate equations is applied to obtain a theoretical description of the lasing dynamics and to account for the flattening of the integrated output power above the lasing threshold. These equations provide a basis to describe the occurrence of saturation at high excitation levels in dependence on the pump pulse duration and the number of absorbing molecules in the mode volume. (C) 2011 American Institute of Physics. [doi:10.106311.3593188]
Abstract: We investigate the optical characteristics of transparent organic light-emitting diodes (TOLEDs) with thickness variation of the top metal electrode. We find that the thickness variation of the top cathode shows a different influence on the bottom and top side emissions, respectively. The overall efficiency which is the sum of the bottom and top emissions can be enhanced up to 17.7% of external quantum efficiency (EQE) and 25.1 lm/W of power efficiency (PE) at a driving current density of 15 mA/cm(2). Furthermore, we carefully investigate the angular distribution of the bottom and top side emissions separately with a spectrogoniometer. We find that both considerably differ from a Lambertian assumption and exhibit opposite trends, depending on the top metal cathode thickness. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We present highly efficient, semitransparent small molecule organic solar cells. The devices employ an indium tin oxide-free top contact, consisting of thin metal films. An additional organic layer is used to enhance light outcoupling. The solar cell incorporates two stacked subcells, each containing a donor: acceptor bulk heterojunction. The two subcells have complementary absorbers, with separate blue (C(60)), red (fluorinated zinc phthalocyanine), and green (dicyanovinyl oligothiophene derivative) absorbing molecules. A power conversion efficiency of 4.9 +/- 0.2% is obtained for the device having an average transmission of 24% in the visible range. (C) 2011 American Institute of Physics. [doi:10.1063/1.3610551]
Abstract: The effect of injection and extraction barriers on flat heterojunction (FHJ) and bulk heterojunction (BHJ) organic solar cells is analyzed. The barriers are realized by a combination of p-type materials with HOMOs varying between -5.0 and -5.6 eV as hole-transport layer (HTL) and as donor in vacuum-evaporated multilayer p-i-metal small-molecule solar cells. The HTL/donor interface can be seen as a model for the influence of contacts in organic solar cells in general. Using drift-diffusion simulations we are well able to reproduce and explain the experimental I-V curves qualitatively. In FHJ solar cells the open-circuit voltage (V(oc)) is determined by the donor and is independent of the HTL. In BHJ solar cells, however, V(oc) decreases if injection barriers are present. This different behavior is caused by a blocking of the charge carriers at a spatially localized donor/acceptor heterojunction, which is only present in the FHJ solar cells. The forward current is dominated by the choice of HTL. An energy mismatch in the HOMOs leads to kinks in the I-V curves in the cases for which V(oc) is independent of the HTL.
Abstract: We show that S-kinks in the current voltage characteristics, which decrease the fill factor significantly, can be caused by a strong imbalance of charge carrier mobilities (hole mobility in donor and electron mobility in acceptor) in planar/flat heterojunction organic solar cells. Electrical simulations according to a drift-diffusion model predict the occurrence of an S-kink for a mobility mismatch factor larger than 100. By combining a low-mobility donor material, (1,2,3,4,9,10,11,12-octaphenyl-diindeno[1,2,3-cd: 1',2',3'-lm]perylene), with the acceptors C(60) and N, N'-dimethylperylene-3,4: 9,10-dicarboximide, which show different electron mobilities, we experimentally verify the predictions. Our results demonstrate that not only interface effects but also the photoactive material itself can cause S-kinks. (C) 2011 American Institute of Physics. [doi:10.1063/1.3553764]
Abstract: We present investigations of top emitting organic light emitting devices (OLED) comprising n- and p-doped organic charge transport layers. It has been found previously that in comparison to noninverted p-i-n OLEDs, inverted n-i-p OLEDs show reduced device performances after fabrication. These differences can be eliminated by subsequent thermal annealing of the whole n-i-p OLED. After this process, the n-i-p OLED exhibits a superior low driving voltage of 2.9 V at 1000 cd/m(2) and shows an increase in external quantum efficiency from 11% to almost 15% which we ascribe to a modified charge balance within the intrinsic organic emission layer. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3559847]
Abstract: We discuss the electrical calcium test--a method to measure very small rates of water vapor permeation through barrier films with high throughput. The sensitivity range for our design is found to be 10(-5) to 15 g/(m(2) d). Moreover, a closer look at the importance of electrodes series resistance is taken: We show that permeation rates are underestimated if it is neglected. Taking this series resistance and Fickian diffusion into account not only the steady, but also the transient state of the permeation curve can be fitted. Using this approach, permeation barriers with different permeabilities are evaluated leading to water vapor transmission rates well comparable to coulometric measurements. The calcium layer morphology is investigated by atomic force microscopy measurements indicating microscopical inhomogeneities during degradation. Variations of electrode material and calcium layer thickness are carried out to examine their influence on the measured permeation. Additionally, optical and electrical calcium tests are compared. Small differences in the time dependence are observed and discussed.
Abstract: We discuss the electrical calcium test - a method to measure very small rates of water vapor permeation through barrier films with high throughput. The sensitivity range for our design is found to be 10(-5) to 15 g/(m(2) d). Moreover, a closer look at the importance of electrodes series resistance is taken: We show that permeation rates are underestimated if it is neglected. Taking this series resistance and Fickian diffusion into account not only the steady, but also the transient state of the permeation curve can be fitted. Using this approach, permeation barriers with different permeabilities are evaluated leading to water vapor transmission rates well comparable to coulometric measurements. The calcium layer morphology is investigated by atomic force microscopy measurements indicating microscopical inhomogeneities during degradation. Variations of electrode material and calcium layer thickness are carried out to examine their influence on the measured permeation. Additionally, optical and electrical calcium tests are compared. Small differences in the time dependence are observed and discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3633956]
Abstract: Thin conductive, hydrophobic films of poly(3,4-ethylenedioxythiophene) or PEDOT were synthesized on-substrate in the presence of the organic electron acceptor and dehydrogenating agent 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) using a versatile processing procedure. Significant polymerization in the processing solution was delayed by using common aprotic, ethereal solvents with low dielectric constants to prevent solvating the EDOT:DDQ charge transfer complex into radicals. Polymerization was initiated by an increase in concentration upon solvent evaporation during spin coating. A hydrophobic polymer matrix additive of polyvinyl acetate was used to aid in film formation, and a post-treatment rinse with acetonitrile was necessary to obtain a conductive film. Conductivities ranged from 17 to 59 S/cm, where the higher values were achieved at the cost of transparency. A work function of 4.62 eV was determined by UV photoelectron spectroscopy for one film recipe. When comparing conductive AFM results of PEDOT: DDQ to highly conductive, ethylene glycol-treated PEDOT: poly(styrenesulfonate) or PSS, surface currents were orders of magnitude higher for PEDOT: DDQ than for PEDOT: PSS. If optimized further, less acidic and hydrophobic PEDOT: DDQ films have the potential to replace PEDOT: PSS for use as a transparent electrode or charge transport layer in organic solar cells, organic light emitting diodes, touch screens, and other optoelectronic devices. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: In this paper, two vacuum processed single heterojunction organic solar cells with complementary absorption are described and the construction and optimization of tandem solar cells based on the combination of these heterojunctions demonstrated. The red-absorbing heterojunction consists of C(60) and a fluorinated zinc phthalocyanine derivative (F4-ZnPc) that leads to a 0.1-0.15 V higher open circuit voltage V(proportional to) than the commonly used ZnPc. The second heterojunction incorporates C(60) and a dicyanovinyl-capped sexithiophene derivative (DCV6T) that mainly absorbs in the green. The combination of both heterojunctions into one tandem solar cell leads to an absorption over the whole visible range of the sun spectrum. Thickness variations of the transparent p-doped optical spacer between both subcells in the tandem solar cell is shown to lead to a significant change in short circuit current density j(sc) due to optical interference effects, whereas V(proportional to) and fill factor are hardly affected. The maximum efficiency eta of about 5.6% is found for a spacer thickness of 150-165 nm. Based on the optimized 165nm thick spacer, effects of intensity and angle of illumination, and temperature on a tandem device are investigated. Variations in illumination intensity lead to a linear change in jsc over three orders of magnitude and a nearly constant eta in the range of 30 to 310 mW cm(-2). Despite the stacked heterojunctions, the performance of the tandem device is robust against different illumination angles: j(sc) and eta closely follow a cosine behavior between 0 degrees and 70 degrees. Investigations of the temperature behavior of the tandem device show an increase in eta of 0.016 percentage points per Kelvin between -20 degrees C and 25 degrees C followed by a plateau up to 50 C. Finally, further optimization of the tandem stack results in a certified. of (6.07 +/- 0.24)% on (1.9893 +/- 0.0060) cm(2) (Fraunhofer ISE), i.e., areas large enough to be of relevance for modules.
Abstract: We report on efficient and stable ITO-free small molecule organic solar cells with conductive poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) electrodes using a post-treatment process, causing selective removal of PSS. The solar cells with post-treated PEDOT:PSS electrodes show significantly improved short circuit current densities and efficiencies compared to untreated devices. Moreover, the removal of PSS by the post-treatment significantly improves the lifetime of devices, which are more resistant to loss of fill factor compared to untreated devices. (C) 2011 American Institute of Physics. [doi:10.1063/1.3634015]
Abstract: A series of novel thiophene-substituted aza-BODIPY dyes were synthesized by means of a standard procedure and complemented by a Stille-coupling of a brominated species with 2-tributylstannylthiophene. The optical as well as the electrochemical properties of the compounds were investigated and compared to result of density functional theory (DFT) calculations. The influence of the thiophene substituents is discussed in dependence of the position at the aza-BODIPY core regarding the HOMO and LUMO frontier orbitals. The different distributions of the HOMO and LUMO coefficients over the BODIPY core lead to a variable influence of the thiophene substituents on the HOMO and LUMO energies, being the origin of the tunable optical and electrochemical properties. (C) 2011 Elsevier Ltd. All rights reserved.
Abstract: We report on the resistive switching effect in metal/organic semiconductor/metal structures and present a general explanation of the switching mechanism in these devices. The J-V characteristics of metal/tris(8-hydroxyquinolinato) aluminum (Alq(3))/metal devices will be discussed and it will be further shown that these sustain only a limited number of switching cycles. Besides Alq(3), we also investigate other organic semiconductor materials and obtain a bistable behavior, which is independent of the organic material but dependent on the current injection conditions of the interface between the organic material and the metal top electrode. Further, we investigate the material independent switching effect using impedance spectroscopy and can disclose a transition from capacitive to resistive behavior at switching voltages. We propose that the switching in metal/organic semiconductor/metal structures is caused by the growth and rupture of resistive filaments in the organic semiconductor. (C) 2011 American Institute of Physics. [doi:10.1063/1.3656432]
Abstract: The electrical and optical properties of molecular thin films are widely used, for instance in organic electronics, and depend strongly on the molecular arrangement of the organic layers. It is shown here how atomic structural information can be obtained from molecular films without further knowledge of the single-crystal structure. C60 fullerene was chosen as a representative test material. A 250 nm C60 film was investigated by grazing-incidence X-ray diffraction and the data compared with a Bragg-Brentano X-ray diffraction measurement of the corresponding C60 powder. The diffraction patterns of both powder and film were used to calculate the pair distribution function (PDF), which allowed an investigation of the short-range order of the structures. With the help of the PDF, a structure model for the C60 molecular arrangement was determined for both C60 powder and thin film. The results agree very well with a classical whole-pattern fitting approach for the C60 diffraction patterns.
Abstract: We investigate organic light-emitting diodes (OLEDs) comprising the singlet emitter system 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped into aluminium tris(8-hydroxyquinoline) (Alq(3)) at high excitation densities. With the OLED active area reduced to 100 x 100 mu m(2), current densities up to 800 A/cm(2) are achieved in pulsed operation. These devices exhibit an intense electroluminescence (EL) turn-on peak on the nanosecond time scale. With the help of streak camera measurements, we prove that the steady state EL of the fluorescent OLEDs is reduced due to singlet-triplet quenching. We demonstrate that short electrical pulses with a rise time of 10 ns make the separation of singlet emission and singlet-triplet quenching in time domain possible. By modeling the singlet and triplet population dynamics in the emission layer, we find that the triplet-triplet annihilation-rate coefficient in doped fluorescent materials is triplet-density dependent at high excitation density. The increased triplet lifetime usually observed in host: guest systems due to triplet trapping on guest molecules vanishes at high current densities. An increase in current density leads to an increased triplet-triplet annihilation rate, while the triplet density in the emission layer stays constant.
Abstract: We report on highly enhanced light outcoupling of bi-directional red phosphorescent organic light-emitting diodes (BiOLEDs) by strong micro-cavity effects. Inserting additional metal layers between the indium tin oxide (ITO) anode and the hole transporting layer (HTL) and controlling the thickness of the counter metal electrode result in dramatic changes in electroluminescence spectra and angular distribution. We find that the external quantum efficiency (EQE) for both bottom and top emitting side can be significantly improved: the overall EQE reaches up to 24.24%. (C) 2011 American Institute of Physics. [doi:10.1063/1.3628292]
Abstract: We determine the mobility-lifetime product (mu tau) of free charge carriers in pristine zinc phthalocyanine (ZnPc) films using photocurrent measurements. The photocurrent is proportional to the free charge carrier generation efficiency (eta) and the mu tau product, and the carrier collection length is directly proportional to the latter. The mu tau product is thus an important parameter for the electronic quality of a material. We further determine the dominant photocarrier generation mechanisms in ZnPc. The free carrier generation efficiency is estimated from total carrier collection and electric field-induced photoluminescence quenching measurements. Using eta and the electric field dependence of the photocurrent, we estimate the mu tau product of holes in ZnPc to be about 3x10(-11) cm(2)/V.
Abstract: The effect of injection and extraction barriers on flat heterojunction (FHJ) and bulk heterojunction (BHJ) organic solar cells is analyzed. The barriers are realized by a combination of p-type materials with HOMOs varying between -5.0 and -5.6 eV as hole-transport layer (HTL) and as donor in vacuum-evaporated multilayer p-i-metal small-molecule solar cells. The HTL/donor interface can be seen as a model for the influence of contacts in organic solar cells in general. Using drift-diffusion simulations we are well able to reproduce and explain the experimental I-V curves qualitatively. In FHJ solar cells the open-circuit voltage (V(oc)) is determined by the donor and is independent of the HTL. In BHJ solar cells, however, V(oc) decreases if injection barriers are present. This different behavior is caused by a blocking of the charge carriers at a spatially localized donor/acceptor heterojunction, which is only present in the FHJ solar cells. The forward current is dominated by the choice of HTL. An energy mismatch in the HOMOs leads to kinks in the I-V curves in the cases for which V(oc) is independent of the HTL.
Abstract: We demonstrate that the laser output power of an organic microcavity saturates with the pump energy shortly above the lasing threshold. The extent of the saturation correlates with the volume of the optical mode and is strongly enhanced in a small-mode volume lambda/2 microcavity. The laser emission characteristics under different pumping conditions unambiguously show that the effect is due to the finite number of absorbing molecules per mode that are efficiently saturated at higher pump powers. A modified set of rate equations is applied to obtain a theoretical description of the lasing dynamics and to account for the flattening of the integrated output power above the lasing threshold. These equations provide a basis to describe the occurrence of saturation at high excitation levels in dependence on the pump pulse duration and the number of absorbing molecules in the mode volume. (C) 2011 American Institute of Physics. [doi:10.106311.3593188]
Abstract: We demonstrate highly efficient small molecule organic light emitting diodes and organic solar cells based on the p-i-n-type structure using the fluorinated fullerene molecule C(60)F(36) as p-dopant in the hole transport layer. We present synthesis, chemical analysis, and energy level investigation of the dopant as well as the conductivity of organic layers consisting of a matrix of N,N,N',N'-tetrakis 4-methoxyphenyl-benzidine(MeO-TPD) or N,N'-[(Diphenyl-N,N'-bis)9, 9,-dimethyl-fluoren-2-yl]-benzidine(BF-DPB) doped by the fullerene compound. State of the art organic p-i-n devices containing C(60)F(36) show efficiencies comparable to devices with the commonly used p-dopant2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)-TCNQ). The advantages of the fullerene based dopant are the low volatility and high thermal stability, which is beneficial for device operation under elevated temperature. These properties make C(60)F(36) highly attractive for the usage as p-dopant in a broad spectrum of organic p-i-n devices like organic light emitting diodes, solar cells, memories, or transistors. (C) 2011 American Institute of Physics. [doi:10.1063/1.3590142]
Abstract: Small molecule organic solar cells were studied with respect to water and oxygen induced degradation by mapping the spatial distribution of reaction products in order to elucidate the degradation patterns and failure mechanisms. The active layers consist of a 30 nm bulk heterojunction formed by the donor material zinc-phthalocyanine (ZnPc) and the acceptor material Buckminsterfullerene (C(60)) followed by 30 nm C60 for additional absorption. The active layers are sandwiched between 6 nm 4,7-diphenyl-1, 10-phenanthroline (Bphen) and 30 nm N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biPhenyl]-4,4'-diamine p-doped with C(60)F(36) (MeO-TPD:C(60)F(36)), which acted as hole transporting layer. Indium-tinoxide (ITO) and aluminum served as hole and electron collecting electrode, respectively. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) in conjunction with isotopic labeling using H(2) (18)O and (18)O(2) provided information on where and to what extent the atmosphere had reacted with the device. A comparison was made between the use of a humid (oxygen free) atmosphere, a dry oxygen atmosphere, and a dry (oxygen free) nitrogen atmosphere during testing of devices that were kept in the dark and devices that were subjected to illumination under simulated sunlight. It was found that water significantly causes the device to degrade. The two most significant degradation mechanisms are diffusion of water through the aluminum electrode resulting in massive formation of aluminum oxide at the BPhen/Al interface, and diffusion of water into the ZnPc:C(60) layer where ZnPc becomes oxidized. Finally, diffusion from the electrodes was found to have no or a negligible effect on the device lifetime. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We present highly efficient, semitransparent small molecule organic solar cells. The devices employ an indium tin oxide-free top contact, consisting of thin metal films. An additional organic layer is used to enhance light outcoupling. The solar cell incorporates two stacked subcells, each containing a donor: acceptor bulk heterojunction. The two subcells have complementary absorbers, with separate blue (C(60)), red (fluorinated zinc phthalocyanine), and green (dicyanovinyl oligothiophene derivative) absorbing molecules. A power conversion efficiency of 4.9 +/- 0.2% is obtained for the device having an average transmission of 24% in the visible range. (C) 2011 American Institute of Physics. [doi:10.1063/1.3610551]
Abstract: We investigate the optical characteristics of transparent organic light-emitting diodes (TOLEDs) with thickness variation of the top metal electrode. We find that the thickness variation of the top cathode shows a different influence on the bottom and top side emissions, respectively. The overall efficiency which is the sum of the bottom and top emissions can be enhanced up to 17.7% of external quantum efficiency (EQE) and 25.1 lm/W of power efficiency (PE) at a driving current density of 15 mA/cm(2). Furthermore, we carefully investigate the angular distribution of the bottom and top side emissions separately with a spectrogoniometer. We find that both considerably differ from a Lambertian assumption and exhibit opposite trends, depending on the top metal cathode thickness. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We characterize a series of dicyanovinyl-terthiophenes with different alkyl side chains. Variations of side chain substitution patterns and length mainly affect the morphology of the evaporated thin films, which in turn sensitively influences properties like absorption, energy levels, and thin film roughness. To investigate changes in transfer processes between electron donor (D) and acceptor (A) molecules due to side chain variations, we use photoinduced absorption spectroscopy (PIA). PIA probes the long-living photoexcited species at the D-A interface: triplet excitons, cations, and anions. For a blend layer of dicyanovinyl-terthiophene and the electron acceptor fullerene C(60), an energy transfer via the singlet and triplet manifold of C(60) occurs. The recombination dynamics of the triplet excitons reveal two components that differ in their lifetime and generation rate by 1 order of magnitude. By comparing the dynamics of triplet excitons in neat and blend layers, we estimate the energy transfer efficiency in dependence of the type of side chain. The compound with methyl side chains shows remarkable properties regarding thin film absorption, surface roughness, and energy transfer efficiency, which we attribute to the specific nanomorphology of the thin film.
Abstract: For improving the lifetime of organic light emitting diodes (OLEDs), it is essential to understand the chemical reaction pathways involved in the degradation process of these devices. Using the laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) technique, an OLED based on the phosphorescent emitter tris(phenylpyridine)iridium (Ir(ppy)(3)) was investigated. We show that the chemical dissociation reaction of the Ir(ppy)(3) molecule and an additional complexation of its fragments with the used hole blocking material 4,7-diphenyl-1, 10-phenanthroline is the main intrinsic degradation mechanism. This finding underlines the exciton induced dissociation mechanism of phosphorescent emitters, even at emitters without any metal-oxygen- bonds in the ligand system. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3617459]
Abstract: We report on improved and controlled light outcoupling of transparent organic light-emitting diodes (TOLEDs) by inserting thin silver layers between the indium tin oxide anode and the hole transporting layer. The introduction of Ag layers influences both the bottom and top emission of the TOLEDs, and it results in dramatic changes in the electroluminescence spectra and angular distribution. We find that the overall external quantum efficiency can be increased up to 18.8%, and the ratio of bottom and top emission can be almost identical. (C) 2011 Optical Society of America
Abstract: We present a simulation model for the analysis of charge-carrier transport in organic thin-film devices, and apply it to a three-color white hybrid organic light-emitting diode (OLED) with fluorescent blue and phosphorescent red and green emission. We simulate a series of single-carrier devices, which reconstruct the OLED layer sequence step by step. Thereby, we determine the energy profiles for hole and electron transport, show how to discern bulk from interface limitation, and identify trap states.
Abstract: A concentration gradient in a mixed absorber layer with increasing content of donor (acceptor) towards the hole (electron) collecting contact could improve the charge carrier collection in bulk heterojunction organic solar cells. We study p-i-metal type solar cells where the gradient in a 45 nm thick ZnPc:C(60) absorber layer is introduced by varying the deposition rate during co-evaporation. It is shown that the observed increase in the performance is mainly caused by a better energy level alignment and reduced recombination at the p-side. A significant influence on charge carrier transport is not observed. However, regions with a concentration of less than 20% of one component do not fully contribute to the photocurrent. Voltage dependent external quantum efficiency data are used to identify the photoactive regions. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand-alone electrodes for organic solar cells have been optimized using a solvent post-treatment method. The treated PEDOT: PSS films show enhanced conductivities up to 1418 S cm(-1), accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT: PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO-free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc): fullerene C(60) bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre-heating the PEDOT: PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT: PSS with solvent and thermal post-treatment can be a very promising electrode material for highly efficient flexible ITO-free organic solar cells.
Abstract: Zinc phthalocyanine (ZnPc), C(32)F(11)N(8)Zn, is a planar organic molecule having numerous optical and electrical applications in organic electronics. This work investigates the influence of various deposition parameters on the morphology of vapour thermal evaporated ZnPc films. For this purpose, ZnPc is deposited at different substrate temperatures up to 90 degrees C and film thickness up to 50 nm onto various substrates. The morphology of this ZnPc layers is characterised by X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM) methods. XRD measurements show that all ZnPc films are crystalline in a triclinic (alpha-ZnPc) or monoclinic (gamma-ZnPc) phase, independent from the kind of substrate, layer thickness, or substrate temperature. The ZnPc powder, the starting product for the thermally evaporated ZnPc films, is present in the stable monoclinic beta-phase. Thus, the stacking of the ZnPc molecules changes completely during deposition. The crystallite size perpendicular to the substrate determined by XRD microstructure analysis is in the range of the layer thickness while the lateral size, obtained by AFM, is increasing with substrate temperature and film thickness. AFM and XRR show an increase of the layer roughness for thicker ZnPc layers and higher substrate temperatures during film deposition. The strain in the ZnPc films decreases for higher substrate temperatures due to enhanced thermal relaxation and for thicker ZnPc films due to lower surface tension. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We present small molecule organic solar cells (SMOSC) based on flat heterojunctions (FHJ) of the alternative green donor 2,3,10,11-tetrapropyl-1,4,9,12-tetraphenyl-diindeno[ 1,2,3-cd: 1`, 2`, 3`-lm]perylene (P4-Ph4-DIP) and the fullerene C(60). P4-Ph4-DIP absorbs in the green spectral range and thus fills the spectral gap that standard absorber materials (zinc or copper phthalocyanine for red and C(60) for blue absorption) leave, thus allowing broad coverage of the sun spectrum, which is of major interest for tandem devices. The materials properties of P4-Ph4-DIP are studied, and SMOSC are characterized by current voltage, external quantum efficiency, and aging measurements. The solar cells display very high fill factors FF > 76% and open circuit voltages V(OC) of close to 1 V. Mismatch-corrected efficiencies of up to 1.9% are obtained. Aging measurements show that C(60) in conjunction with P4-Ph4-DIP yields extremely stable devices. We observe approximate to 88% of the initial efficiency after 2500 h illumination at 999 mW/cm(2) illumination intensity, with no observable change in short-circuit current density. Furthermore, we also show that a systematic variation of donor thickness in FHJ can be combined with transfer matrix formalism-based optical simulations and the continuity equation for excitons to reliably determine the exciton diffusion length L(D). A value of 9 +/- 1 nm is found for P4-Ph4-DIP.
Abstract: A concentration gradient in a mixed absorber layer with increasing content of donor (acceptor) towards the hole (electron) collecting contact could improve the charge carrier collection in bulk heterojunction organic solar cells. We study p-i-metal type solar cells where the gradient in a 45 nm thick ZnPc:C(60) absorber layer is introduced by varying the deposition rate during co-evaporation. It is shown that the observed increase in the performance is mainly caused by a better energy level alignment and reduced recombination at the p-side. A significant influence on charge carrier transport is not observed. However, regions with a concentration of less than 20% of one component do not fully contribute to the photocurrent. Voltage dependent external quantum efficiency data are used to identify the photoactive regions. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: Efficient synthesis of a series of terminally dicyanovinyl (DCV)-substituted oligothiophenes, DCVnT 1-6, without solubilizing side chains synthesized via a novel convergent approach and their application as electron donors in vacuum-processed m-i-p-type planar and p-i-n-type bulk heterojunction organic solar cells is described. Purification of the products via gradient sublimation yields thermally highly stable organic semiconducting materials in single crystalline quality which allows for X-ray structure analysis. Important insights into the packing features and intermolecular interactions of these promising solar cell materials are provided. Optical absorption spectra and electrochemical properties of the oligomers are investigated and valuable structure-property relationships deduced. Photovoltaic devices incorporating DCVnTs 4-6 showed power conversion efficiencies up to 2.8% for planar and 5.2% for bulk heterojunction organic solar cells under full sun illumination (mismatch corrected simulated AM 1.5G sunlight). The 5.2% efficiency shown here represents one of the highest values ever reported for organic vacuum-deposited single heterojunction solar cells.
Abstract: A homogeneous microcavity is excited in two tightly focused spatially separated spots with a few mu m distance. The radius of the generated cavity modes exceeds the excited regions such that the quasimodes of both spots overlap in space. Coherent coupling of the two spots via stimulated emission is observed, exhibiting a slightly lowered lasing threshold. Furthermore, the emission from both spots is phase locked with a phase difference of either 0 or m. depending on their separation. Correspondingly, the distribution of the electromagnetic field is symmetric or anti-symmetric, and the emission occurs parallel to the cavity normal or symmetrically at non-zero angles, respectively. This behaviour is understood to be determined by the spatial distribution of gain and absorption across the active cavity medium, which we confirm by numerical calculations. (C) 2010 Elsevier Ltd. All rights reserved.
Abstract: The change of morphology in mixed layers due to different substrate temperature Tsub of organic solar cells containing C(60) and zinc phthalocyanine (ZnPc) is studied. Heating the substrate during deposition of the bulk heterojunction C(60):ZnPc leads to a significant improvement of solar cell performance, mainly due to an increase in photocurrent and fill factor (FF). This is attributed to improved charge carrier percolation pathways within the C(60):ZnPc blend. Using atomic force microscopy, scanning electron microscopy, transmission electron microscopy, organic field effect transistor, X-ray diffraction, and absorption measurements, we observe aggregation, cluster-like, and polycrystalline growth of the heated bulk layer. This provides better transport percolation paths by inducing a phase separation of the molecules. Heated blend layer with thickness of 60 nm shows high performance without loss in FF. When heating the substrate to the optimum temperature of 110 degrees C, a power conversion efficiency of 3.0% is achieved, compared to 1.4% for an identical device prepared on a substrate held at room temperature. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: A series of novel aza-diisoindolmethine dyes 9 with six different aryl and heteroaryl groups at the indole moiety have been synthesized by the addition of aryl Grignard compounds to phthalodinitrile and subsequent reaction with formamide. A plausible reaction mechanism, through a Leuckart-Wallach-type reduction has been confirmed by means of DFT calculations of the related transition and intermediate states. The corresponding boron difluoride complexes (10) of 9 were prepared in a subsequent reaction step and the spectroscopic and electrochemical properties of 9 and 10 have been investigated both experimentally and theoretically. The aza-diisoindolmethines 9 exhibit an absorption maximum in the range from 615 to 720 nm, whereas the complexes 10 show a bathochromically shifted absorption maximum between 681 and 793 nm. Measurements of 9 and 10 by cyclic voltammetry display fully reversible redox waves for the reduction and oxidation with higher potentials for 10. From the measured redox potentials, the HOMO and LUMO energy levels were calculated for 9 and 10. The frontier orbital energies, the energies of the absorption bands, as well as the orbitals involved in the absorption process were calculated with DFT and compared to the measured results of 9 and 10. The absorption maximum can be related to an intense HOMO-LUMO transition and the more-pronounced stabilization of the LUMO upon complexation is the origin of the bathochromic shift of the absorption. Additionally, single-crystal structures for two species, 10d and 10 f, are reported.
Abstract: We introduce the material hexaazatriphenylene hexacarbonitrile (HATCN) as electron conducting window layer for separating the photoactive region from the cathode in organic p-i-n type solar cells. HATCN has a wide band gap of 3.3 eV and is thus transparent in the visible range of the solar spectrum. Its electrical properties can be tuned by means of molecular n-doping which leads to an increase of electron conductivity by several orders of magnitude up to 2.2 x 10(-4) S/cm. However, an application in photovoltaic devices is restrained by its exceptionally high electron affinity, estimated 4.8 eV, which introduces an electron injection barrier to the photoactive acceptor material C(60). Here, we present a strategy to remove this barrier by means of introducing doped and undoped C(60) intermediate layers, thus demonstrating the importance of energy level matching in a multiple layer structure and the advantages of Fermi level control by doping. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: We investigate a set of transparent organic LEDs (TOLEDs) with different organic capping layer (OC) thicknesses to understand the capping layer effect. We find that thickness variation of the OC strongly influences the emission properties of TOLEDs and exhibits different trends for top or bottom emission. The external quantum efficiency for the top side can be enhanced by a factor of 63%, but that of the bottom side only by 4% compared to a reference device without an OC. Additionally, we demonstrate that the introduction of the OC is an effective method to control the bottom-to-top emission ratio within a measured range from 2.87 to 6.05. (C) 2011 Optical Society of America
Abstract: A large number of flexible polymer solar modules comprising 16 serially connected individual cells was prepared at the experimental workshop at Riso DTU. The photoactive layer was prepared from several varieties of P3HT (Merck, Plextronics, BASF and Riso DTU) and two varieties of ZnO (nanoparticulate, thin film) were employed as electron transport layers. The devices were all tested at Riso DTU and the functional devices were subjected to an inter-laboratory study involving the performance and the stability of modules over time in the dark, under light soaking and outdoor conditions. 24 laboratories from 10 countries and across four different continents were involved in the studies. The reported results allowed for analysis of the variability between different groups in performing lifetime studies as well as performing a comparison of different testing procedures. These studies constitute the first steps toward establishing standard procedures for an OPV lifetime characterization. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: The dissociation of excitons and the creation of charges in the active bulk layer in small molecule organic solar cells is significantly influenced by the morphology of the active layer. Here, we influence the active layer morphology of dicyanovinyl-quinquethiophene (donor): fullerene C(60) (acceptor) blend layers by deposition on heated substrates. The signatures of the donor cations and triplet excitons are investigated by photoinduced absorption spectroscopy (PIA) at different measurement temperatures. With increasing measurement temperatures, we observe a decrease in triplet exciton generation rate, accompanied by an increase in cation generation rate. At room temperature we compare the dynamics of donor cations in blend layers deposited at room temperature (T(sub) = 30 degrees C) and blend layers deposited on a heated substrate (T(sub) = 80 degrees C) by PIA. The cation lifetime (approximate to 10 - 100 mu s) is significantly increased in the heated layer (T(sub) = 80 degrees C), whereas the cation generation rate is decreased in the heated layer compared to the unheated layer (T(sub) = 30 degrees C). Impedance spectroscopy of heated (T(sub) = 80 degrees C) and unheated (T(sub) = 30 degrees C) solar cells exhibits a similar increase in carrier lifetime for the heated layer. Furthermore, we determine the lifetimes of free (1-5 mu s) and trapped charges (1 s) by impedance spectroscopy and hence assign the optically detected cation signatures to shallow trap states. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We present efficient, semitransparent small molecule organic solar cells. The devices employ an indium tin oxide-free top contact, consisting of thin metal films and an additional organic capping layer for enhanced light in/outcoupling. The solar cell encorporates a bulk heterojunction with the donor material Ph(2)-benz-bodipy, an infrared absorber. Combination of Ph2-benz-bodipy with C(60) as acceptor leads to devices with high open circuit voltages of up to 0.81V and short circuit current densities of 5-6 mA/cm(2), resulting in efficiences of 2.2%-2.5%. At the same time, the devices are highly transparent, with an average transmittance in the visible range (400-750 nm) of up to 47.9%, with peaks at 538nm of up to 64.2% and an average transmittance in the yellow-green range of up to 61.8%. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3660708]
Abstract: We review top-emitting organic light-emitting diodes (OLEDs), which are beneficial for lighting and display applications, where non-transparent substrates are used. The optical effects of the microcavity structure as well as the loss mechanisms are discussed. Outcoupling techniques and the work on white top-emitting OLEDs are summarized. We discuss the power dissipation spectra for a monochrome and a white top-emitting OLED and give quantitative reports on the loss channels. Furthermore, the development of inverted top-emitting OLEDs is described. (C) 2011 Optical Society of America
Abstract: We study the morphology and optical properties of vacuum deposited films of the alpha-omega-bis-(dicyanovinylen)-sexithiophene, comprising four butyl side chains DCV6T-Bu(1,2,5,6) (DCV6T-Bu). An absorption band showing vibronic substructure indicates ordered molecular arrangement in the solid state. The room temperature (RT) self-organization is confirmed by X-ray diffraction (XRD). For films grown on heated substrates, XRD analysis and atomic force microscopy display increased crystallinity with larger domain size. In correlation to the XRD data, with increasing substrate temperature the absorption of the heated films becomes more structured and continuously shifts to longer wavelengths. Further, the hole mobility in DCV6T-Bu/C60 planar heterojunction (PHJ) devices, utilizing DCV6T-Bu films grown at RT and elevated substrate temperature is investigated using the charge extraction by linearly increasing voltage method. The derived values of the activation energy are consistent with the corresponding DCV6T-Bu film morphology. However, the charge carrier mobility does not increase with improving molecular order, as is evident by the obtained mobility values of 1.0 x 10 (6) cm(2)/Vs for the RT and 3.1 x 10 (7) cm(2)/Vs for the heated device, respectively. Finally, DCV6T-Bu/C60 PHJ solar cells consisting of absorber layers deposited on heated and unheated substrates are compared. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We investigate white top-emitting organic light-emitting diodes (OLEDs) based on a heterostructure of down-conversion (DC) layers. The white DCOLED comprises consecutive organic conversion layers of 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped in a matrix of tris(8-hydroxy-quinolinato)aluminum (Alq(3)), and N4,N41'-bis-(4-tert-butyl-phenyl)-N4, N4'-di-fluoranthen-3-yl-diphenylether-4,4'-diamine (OYSE). The DC layers also function as capping layers to enhance the light outcoupling and optical modification of the underlying blue OLED. White light emission with CIE color coordinates of (0.27, 0.26) and a color rendering index of 60 is achieved. Furthermore, the spectral angular dependence of the white device is examined. (C) 2011 Elsevier B. V. All rights reserved.
Abstract: We introduce 2,3,8,9,14,15-hexachloro-5,6,11,12,17,18-hexaazatrinaphthylene (HATNA-Cl(6)) as n-dopable electron transport material (ETM) for small molecule organic solar cells. Because of its large optical energy gap of 2.7 eV and its well suited energy level positions, the material can be implemented as a semitransparent spacer layer between the reflecting metal contact and the photoactive C(60) acceptor layer in p-i-n type solar cells. By varying the ETM thickness, it is possible to shift the position of the photoactive area with respect to the interference maximum of the optical field distribution. Applying n-HATNA-Cl(6) instead of the parasitically absorbing reference ETM n-C(60) results in a considerably improved photocurrent density and accordingly in a higher efficiency. At d(ETM) = 100 nm the power conversion efficiency is more than doubled as it increases from (100 nm n-C(60))=0.5% to (100 nm n-HATNA-Cl(6))=1.1%. (C) 2011 American Institute of Physics. [doi:10.1063/1.3664828]
Abstract: A series of dicyanovinyl-oligothiophenes are investigated concerning their thermal stability, absorption in thin film, and hole mobility. Due to very high extinction coefficients, these materials are interesting for application as donor in solar cells. The quinquethiophene DCV(2)-5T, which shows a hole mobility of 2.2 x 10(-5) cm(2)/Vs, is used as donor material in a flat heterojunction organic small molecule solar cells. Despite a very thin donor layer of only 6 nm, these devices exhibit in a planar heterojunction with 15 nm C(60) an efficiency of up to 2.8% with a fill factor of up to 58%, a short circuit current density of 5.2 mA/cm(2), an open circuit voltage of 1.03 V. and an external quantum efficiency of 30% in the green spectral range. (C) 2011 Elsevier B.V. All rights reserved.
Abstract: We show that the number of extracted charge carriers is a suitable measure to compare lifetime measurements on organic solar cells at different intensities. In detail, we used pin-structures with active layers containing a bulk heterojunction of Zincphthalocyanine (ZnPc) and C(60). Extended lifetime measurements under constant monochromatic or white illumination at defined temperatures of 50 degrees C or 90 degrees C are done. On the one hand, we show that the number of extracted charge carriers is important to determine the degree of degradation. On the other hand, our results show that the energy of irradiated photons is significant for accelerated measurements. This is an major advantage for the realisation of accelerated lifetime measurements. Additionally, we find that not single charge carriers, but excitons cause the degradation of the observed solar cells. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: We report recent results on top-emitting organic light-emitting diodes (OLEDs) using color conversion layers (CCLs) embedded into the electron transport layer of the OLED structure. The method of color conversion provides the possibility to generate a color stable emission with operating lifetime. Due to a constant ratio between absorbed blue emission and converter emission, the spectral shape remains for all time. This guarantees constant color coordinates of the OLED, which is essential for lighting applications. It is shown that OLEDs using conversion layers reach external quantum efficiencies (EQE) which can be higher than the corresponding blue top-emitting OLED. The used conversion layer thickness is below 100 nm, reaching Commission Internationale de l'Eclairage (CIE) coordinates of (0.23; 0.27) close to the Planckian locus at a maximum EQE of 3.16% using a blue fluorescent emitter system. Furthermore, we show that the excitation mechanism of the conversion layer is caused by absorption and no parasitic electrical excitation is taking place. Investigations on the emission color over the lifetime show color-stability over a period of up to 2200 h. (C) 2011 American Institute of Physics. [doi:10.1063/1.3656452]
Abstract: A key feature of stacked organic solar cells is an efficient recombination contact at the interface between the solar cells in the stack. Here, an electron current has to be converted into a hole current without loss of energy. Furthermore, the recombination contact has to be highly transparent. We present a new approach for small molecule organic solar cells using highly doped organic layers. Our approach adapts the use of tunnel diodes known from inorganic tandem solar cells. We compare a metal cluster based recombination contact reported in literature to the new approach using different organic tandem solar cell structures. For this purpose, current-voltage characteristics of adequate solar cells are measured. The experiments show that highly doped layers as recombination contacts in tandem organic solar cells are superior to the metal cluster based approach. The proposed concept allows an addition of the open circuit voltages of the subcells of a tandem solar cell, without absorption or reflection at the recombination contact. The results further show that our concept does not depend on the specific choice of materials as it is seen for metal cluster based recombination contacts. It therefore represents a general approach which is compatible to mass manufacturing. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3467786]
Abstract: Weaker triplet-triplet annihilation (TTA) is observed in organic solid mixed systems comprising the phosphorescent emitter Ir(ppy)(2)(acac), which has a smaller dipole moment than the reference compound Ir(ppy)(3). Spectral and time-resolved analysis show that the weaker TTA can be explained by lower emitter aggregation, which reduces diffusion-based TTA.
Abstract: We present small molecule solar cells with alpha,omega-bis-(dicyanovinylene)-sexithiophene:C(60) mixed heterojunctions, reaching power conversion efficiencies of 4.9 +/- 0.2%. We use substrate heating during deposition of the mixed layer to achieve an optimized morphology and show that this significantly improves the internal quantum efficiencies (IQEs) to values approaching 70%. By optical modeling, we evaluate the amount of loss due to absorption in inactive layers and show that IQE of the active layer itself is about 80%. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3475766]
Abstract: We report on red top-emitting organic light-emitting diode structures with higher order cavities. The emission zone is placed in the first, second, and third antinodes of the electric field in the cavity by increasing the hole transport layer thickness. Furthermore, the thicknesses of the cathode and the capping layer are varied to achieve high efficiencies. Using doped charge transport layers and a phosphorescent emitter, we reach up to 29%, 17%, and 12% external quantum efficiencies for first, second, and third order devices, respectively. An optical model is further used to analyze the angular dependent emission. (C) 2010 American Institute of Physics. [doi:10.1063/1.3530447]
Abstract: The external quantum efficiency of organic light-emitting diodes (OLEDs) is limited by several loss mechanisms. By applying a numerical model for the efficiency analysis of OLED devices, we analyze the distribution of the different energy loss mechanisms in bottom and top emission organic light-emitting diodes. We validate the findings by the comparison with experimental data measured on red state-of-the-art p-i-n devices containing the red phosphorescent emitting dye iridium(III)bis[2-methyldibenzo-(f, h)quinoxaline](acetylacetonate) [Ir(MDQ)(2)(acac)]. The model is used to design extremely efficient bottom and top emission diodes with 21% and 27% external quantum efficiencies, respectively. (C) 2010 American Institute of Physics. [doi:10.1063/1.3527936]
Abstract: We report a technique to prepare hemispherical resonators on a distributed Bragg reflector (DBR). This so-called hydrophobic spreading allows the creation of hemispherical structures with diameters ranging from 5 to 50 mu m. By embedding semiconductor nanocrystal quantum rods (NQRs) into these structures, we achieve a coupling of their emission into whispering-gallery modes. Although the NQR-emission is confined in three dimensions, the DBR is transparent for the excitation, allowing selective excitation of different regions of the hemisphere. Employing a two-dimensional model to approximately describe relevant modes, we are able to estimate the refractive index and diameters of the hemispheres from spectral data. (C) 2010 American Institute of Physics. [doi:10.1063/1.3517566]
Abstract: Organic Zener diodes with a precisely adjustable reverse breakdown from -3 to -15 V without any influence on the forward current-voltrage curve are realized This is accomplished by controlling the width of the charge depletion zone in a pin diode with an accuracy of one nanometer independently of the doping concentration and the thickness of the intrinsic layer The breakdown effect with its exponential current voltage behavior and a weak temperature dependence is explained by a tunneling mechanism across the highest occupied molecular orbital lowest unoccupied molecular orbital gap of neighboring molecules The experimental data are confirmed by a minimal Hamiltonian model approach including coherent tunneling and incoherent hopping processes as possible charge transport pathways through the effective device region
Abstract: Beside inorganic LEDs and fluorescent lamps, organic light-emitting diodes (OLEDs) are evolving into a serious alternative to incandescent lamps. Up to now, it was assumed that all-phosphorescent OLEDs are required for reaching sufficiently high efficiencies. However, the stability of phosphorescent blue emitters is a major challenge. We present a novel approach to achieve highly efficient (up to 90 lm/W at 1000 cd/m(2) using a macroextractor) white light emission from OLEDs. The here presented combination of a fluorescent blue and a phosphorescent red emitter simultaneously allows for a strong blue emission and efficient triplet transfer to the phosphor. The spectrum is extended in the green and yellow region by a full phosphorescent unit stacked on top of the triplet harvesting device. This superposition of four different emitters results in color coordinates close to illuminant A and a color rendering index of 80. Furthermore, color stability is given with respect to varying driving conditions and estimations of the electrical and optical efficiencies are provided. (C) 2010 American Institute of Physics. [doi:10.1063/1.3516481]
Abstract: We detect and measure the trap charges in a small molecular bulk heterojunction solar cell under operating conditions. The trap-charge density is estimated from capacitance measurements with light incident on the sample. At high intensities (similar to 1 sun, 100 mW/cm(2)), the trapped charge concentration leads to a spatial distortion of the electric field in the device. The lower limit of the trap-charge density is estimated to be 6 x 10(16) cm(-3). The frequency dependence of the capacitance suggests that the charges are trapped in a manifold of deep states present in the energy gap of the semiconductors. The distortion of the electric field by this trap charge affects the charge-carrier collection efficiency.
Abstract: We demonstrate top-emitting organic light-emitting diodes (OLEDs) with white emission spectra, employing a three color hybrid cavity structure with two highly efficient phosphorescent orange-red and green emitters (iridium(III)bis(2-methyldibenzo-[f,h]chinoxalin)(acetylacetonate) (Ir(MDQ)(2)(acac)) and tris(2-phenylpyridin) iridium(III) (Ir(ppy)(3))) and the stable blue fluorescent emitter 2,5,8,11-tetra-tert-butylperylene (TBPe). In contrast to Lambertian emission characteristics, our devices show enhanced emission in the forward direction up to angles of 50 degrees. In order to optically adjust the balance of blue and red light in the emission spectra, the influence of two different anode mirror materials are studied. We further test varied electron blocking layer materials to improve the device performance. An optimized layer structure with a hybrid anode mirror yields luminous efficacies of 13.3 lm/W (4.9% external quantum efficiency (EQE)) for the blocker material 2,2â,7,7â-tetrakis-(N,N-diphenylamino)-9,9â-spirobifluoren (S-TAD) and 12.4 lm/W (5.3% EQE) for N,Nâ-di(naphthalen-1-yl)-N,Nâ-diphenyl-benzidine (NPB), respectively, at approximate luminances of 1000 cd/m(2). Using an aluminum anode, the emission shows Commission Internationale dâEclairage chromaticity coordinates (CIE 1931) of (0.420, 0.407) at approximately 1000 cd/m(2) and color rendering indices of up to 77. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: Organic Zener diodes with a precisely adjustable reverse breakdown from -3 to -15 V without any influence on the forward current-voltrage curve are realized This is accomplished by controlling the width of the charge depletion zone in a pin diode with an accuracy of one nanometer independently of the doping concentration and the thickness of the intrinsic layer The breakdown effect with its exponential current voltage behavior and a weak temperature dependence is explained by a tunneling mechanism across the highest occupied molecular orbital lowest unoccupied molecular orbital gap of neighboring molecules The experimental data are confirmed by a minimal Hamiltonian model approach including coherent tunneling and incoherent hopping processes as possible charge transport pathways through the effective device region
Abstract: Beside inorganic LEDs and fluorescent lamps, organic light-emitting diodes (OLEDs) are evolving into a serious alternative to incandescent lamps. Up to now, it was assumed that all-phosphorescent OLEDs are required for reaching sufficiently high efficiencies. However, the stability of phosphorescent blue emitters is a major challenge. We present a novel approach to achieve highly efficient (up to 90 lm/W at 1000 cd/m(2) using a macroextractor) white light emission from OLEDs. The here presented combination of a fluorescent blue and a phosphorescent red emitter simultaneously allows for a strong blue emission and efficient triplet transfer to the phosphor. The spectrum is extended in the green and yellow region by a full phosphorescent unit stacked on top of the triplet harvesting device. This superposition of four different emitters results in color coordinates close to illuminant A and a color rendering index of 80. Furthermore, color stability is given with respect to varying driving conditions and estimations of the electrical and optical efficiencies are provided. (C) 2010 American Institute of Physics. [doi:10.1063/1.3516481]
Abstract: The external quantum efficiency of organic light-emitting diodes (OLEDs) is limited by several loss mechanisms. By applying a numerical model for the efficiency analysis of OLED devices, we analyze the distribution of the different energy loss mechanisms in bottom and top emission organic light-emitting diodes. We validate the findings by the comparison with experimental data measured on red state-of-the-art p-i-n devices containing the red phosphorescent emitting dye iridium(III)bis[2-methyldibenzo-(f, h)quinoxaline](acetylacetonate) [Ir(MDQ)(2)(acac)]. The model is used to design extremely efficient bottom and top emission diodes with 21% and 27% external quantum efficiencies, respectively. (C) 2010 American Institute of Physics. [doi:10.1063/1.3527936]
Abstract: We report a technique to prepare hemispherical resonators on a distributed Bragg reflector (DBR). This so-called hydrophobic spreading allows the creation of hemispherical structures with diameters ranging from 5 to 50 mu m. By embedding semiconductor nanocrystal quantum rods (NQRs) into these structures, we achieve a coupling of their emission into whispering-gallery modes. Although the NQR-emission is confined in three dimensions, the DBR is transparent for the excitation, allowing selective excitation of different regions of the hemisphere. Employing a two-dimensional model to approximately describe relevant modes, we are able to estimate the refractive index and diameters of the hemispheres from spectral data. (C) 2010 American Institute of Physics. [doi:10.1063/1.3517566]
Abstract: A series of oligothiophenes that are end-capped with dicyanovinyl (DCV) and 1,3,2-(2H)-dioxaborine (DOB) moieties has been prepared using standard procedures. Their optoelectronic properties have been investigated by cyclic voltammetry and optical absorption. The optical absorption has been measured both in solution and thin film state. (C) 2010 Elsevier Ltd. All rights reserved.
Abstract: We have investigated conductivity and ageing of ZnPc/C60 solar cells as they are influenced by the charge carrier mobility and variation of the potential barrier height. The test structures with a reasonable energy conversion efficiency of similar to 1.5% were investigated. The samples were aged for 1300 h upon illumination by the blue LED with peak emission at 475 nm. Upon ageing the devices showed a strong and fast degradation of the efficiency, short circuit current and of the fill factor within several hours followed by a much slower decrease of them. The reference samples kept in the dark at the room temperature did show only very small changes in their I-V curves. Carrier mobility dependencies on electric field strength at different temperatures were measured by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. It was demonstrated that mobility values decrease during degradation as compared to the reference samples. Nevertheless only mobility changes cannot explain the observed drop of device current. The increase of the effective barrier height by about 0.1 eV from similar to 0.55 eV up to similar to 0.65 eV was observed in the aged samples. Meanwhile thermal activation energy values of the electrical conductivity grew from about 0.28 eV prior to degradation up to about 0.34 eV after ageing. (c) 2010 Elsevier B.V. All rights reserved.
Notes: 2nd International Conference on Physics of Optical Materials and Devices, Herceg Novi, MONTENEGRO, AUG 27-30, 2009
Abstract: We present 2,3,10,11-tetrapropyl-1,4,9,12-tetraphenyl-diindeno[1,2,3-cd:1â,2â,3â- lm]perylene (P4-Ph4-DIP) as new green donor for small molecule organic solar cells (SMOSC). P4-Ph4-DIP absorbs in the green spectral range of 490-580 nm and thus is a decisive building block towards broad coverage of the sun spectrum. SMOSC with P4-Ph4-DIP as donor and C(60) as blue-absorbing acceptor display very high fill factors FF > 76% and open circuit voltages of 0.99 V, reaching eta = 1.92% for 12 nm P4-Ph4-DIP. Current voltage and external quantum efficiency spectra suggest that an exciton diffusion length E(D) < 20 nm is the main limiting factor. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: A series of oligothiophenes that are end-capped with dicyanovinyl (DCV) and 1,3,2-(2H)-dioxaborine (DOB) moieties has been prepared using standard procedures. Their optoelectronic properties have been investigated by cyclic voltammetry and optical absorption. The optical absorption has been measured both in solution and thin film state. (C) 2010 Elsevier Ltd. All rights reserved.
Abstract: We present 2,3,10,11-tetrapropyl-1,4,9,12-tetraphenyl-diindeno[1,2,3-cd:1',2',3'-lm ]perylene (P4-Ph4-DIP) as new green donor for small molecule organic solar cells (SMOSC). P4-Ph4-DIP absorbs in the green spectral range of 490-580 nm and thus is a decisive building block towards broad coverage of the sun spectrum. SMOSC with P4-Ph4-DIP as donor and C(60) as blue-absorbing acceptor display very high fill factors FF > 76% and open circuit voltages of 0.99 V, reaching eta = 1.92% for 12 nm P4-Ph4-DIP. Current voltage and external quantum efficiency spectra suggest that an exciton diffusion length E(D) < 20 nm is the main limiting factor. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: We show that capping layers of tris-(8-hydroxy-quinolinato)-aluminum Alq(3) enable increased absorption and photocurrent in organic solar cells (OSCs) when using transparent metal films as top electrodes. Furthermore, by varying the capping layer thickness, the optical field in the OSC is tuned for selective wavelengths, opening a possibility of influencing the external quantum efficiency for specific absorber materials. It is described how a second maximum of the optical field intensity can be utilized, which is a concept significant for tandem solar cells. Indium tin oxide (ITO)-free OSCs are presented which show the influence of capping layer on efficiency, saturation, fill factor, and open-circuit voltage, with numerical calculations supporting the experimental evidence of layer-selective enhancement. (C) 2010 American Institute of Physics. [doi:10.1063/1.3311559]
Abstract: Organic blue light-emitting devices are essential for the development of future light sources and display technology. Here, we present a highly efficient host-guest system suitable for blue light emission, consisting of the wide gap host material 3,6-di(9-carbazolyl)-9-(2-ethylhexyl)carbazole (TCz1) and the phosphorescent blue emitter iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C(2')]picolinate (FIrpic). We investigate charge carrier balance as a function of hole blocking layer thickness. For optimized structures, devices with a quantum efficiency as high as 14.3% and a luminous efficacy of 21 lm/W at a luminance of 1000 cd/m(2) are realized.
Abstract: We present first results of a new developed large area manufacturing system for organic light-emitting diodes (OLED) dedicated to lighting and signage applications The system combines high throughput with flexibility in production at highly-precise deposition conditions After introducing the system with its modules, first results for organic and metal layer deposition properties are shown Next orange/red p-i-n type OLED samples are prepared on large ITO substrates and the characterization by luminance and current measurements are presented The devices achieve very high efficiencies up to 31 cd/A on large area substrates which are comparable to devices on smaller substrates (C) 2009 Elsevier B V. All rights reserved
Abstract: Current matching of the subcells is crucial to optimize the performance of tandem solar cells. Due to the thin film optics of organic solar cells, the position of the two subcells relative to the reflecting electrode becomes a very important issue. This is demonstrated for an indium tin oxide (ITO)/pin/pii/Al structure with thin intrinsic absorbing layers consisting of zinc-phthalocyanine and fullerene C(60) and a metal-free lossless recombination contact between the subcells. By keeping the thickness of the absorbing layers constant and changing only the thickness of the inner p-doped transparent layer in 16 steps from 0 to 186 nm, the distance of the ITO-sided subcell from the reflecting electrode (Al) is systematically varied. Thus, the p-doped layer works as an optical spacer between both subcells. The influence of its thickness on the thin film optics is shown in optical simulations and confirmed with current-voltage measurements. If both subcells are separated only by the recombination contact, they are positioned in the first interference maximum of the incident light and the currents of the individual subcells nearly matches. By increasing the spacer layer thickness, the ITO-sided subcell is moved to the first interference minimum, limiting the measured short circuit current density j(sc) of the tandem solar cell to about 1/2 of its initial value without spacer. At a spacer thickness of about 140 nm, j(sc) recovers in the second interference maximum to nearly its original value. Within this series, an almost constant high fill factor of about 59% as well as a constant open circuit voltage of 1.06 V is observed, showing that the Ohmic losses in the spacer are negligible. The power conversion efficiency of these devices reaches nearly 4% in the first and approximately 3.6% in the second interference maximum, respectively, in an outdoor test at 1 sun. Furthermore, it is shown that for thicker absorber layers, an optimized current density cannot be reached in the first, but only in the second optical interference maximum, making the presented optical spacer an essential component for efficient organic tandem devices.
Abstract: In this study, we examine organic light emitting diodes (OLEDs) having Al top electrodes deposited on organic layers by direct-current magnetron sputtering. The OLEDs consisted of electronically doped transport layers and phosphorescent emission layer were characterized by typical current-voltage-luminance measurement. They showed higher leakage currents, decreased forward currents, and corresponding increases of driving voltage after the sputter deposition on the organic layers. The OLEDs exhibited randomly distributed bright spots on the active area, and the bright spots were investigated by scanning electron microscopy/energy-dispersive X-ray spectroscopy. In order to prove the origins of sputter damage, simple organic/Al layer samples were made and investigated by ellipsometry and laser-induced desorption/ionization time-of-flight mass spectrometry. The results are compared with previous works addressing the fundamental phenomena of magnetron sputtering. We conclude that the high leakage current originated from a penetration of sputtered metal atoms into the underlying organic layers, and the decrease of forward current resulted from an interface degradation caused by the radiation of plasma, which reduces charge carrier injection preferentially at the Al/organic layer interface. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: We investigate the end-capped oligothiophene derivative alpha,omega-bis-(dicyanovinylene)-sexithiophene with ethyl side chains (DCV6T) as donor material in heterojunctions with C60. The effect of the substrate temperature on the morphology and related photophysical properties of single DCV6T and mixed DCV6T:C60 layers is investigated. Single layers of DCV6T show crystalline features in UV-visible absorption and x-ray diffraction when grown on a substrate heated to 90 degrees C. Investigations of DCV6T:C60 mixed layers by atomic force microscopy, UV-visible absorption, and photoluminescence measurements reveal that the elevated substrate temperature induces an increased phase separation between the two materials with larger domain size and higher surface roughness. Based on these observations, we present mixed heterojunction solar cells where the power conversion efficiency (eta(PCE)) is increased from 1.6% to 3.8% by increasing the substrate temperature from 30 to 90 degrees C, respectively.
Abstract: Cu and Zn Phtalocyanines (CuPc and ZnPc), and C60 are materials frequently used for organic Solar cell engineering. Their energy levels form a donor-acceptor junction, and they have high absorption coefficients and a complementary absorption for the Sun spectrum. We have investigated ageing properties of ZnPc/C60 Solar cells as they are influenced by the charge carrier mobility and variation of the potential barrier height of the ZnPc/C60 interface. The structures ITO/ZnPc/C60/C60: AOB/Al with a reasonable energy conversion efficiency of similar to 1.5% were investigated. The samples were aged for 1300 hours upon illumination with blue LED, with peak emission at 475 nm, and incident light power density of 10 mW/cm(2). The aged devices showed a strong and fast degradation of the short circuit current and of the fill factor after several hours followed by an almost constant behaviour of these values. The reference samples kept in the dark at the room temperature did show only very small changes in their I-V curves. Carrier mobility dependencies on electric field strength at different temperatures were measured by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. It was demonstrated that mobility values decrease during degradation as compared to the reference samples. Nevertheless only mobility changes cannot explain the observed drop of device current. The increase of the effective barrier height at the interface of ZnPc and C60 by about 0.1 eV from similar to 0.55 eV up to similar to 0.65 eV was observed in the aged samples. Meanwhile thermal activation energy values of the electrical conductivity grew from about 0.28 eV prior to degradation up to about 0.34 eV after ageing.
Abstract: This work addresses the long-term aging of organic solar cells based on a. at zinc phthalocyanine (ZnPc)/C(60) heterojunction. We investigate both the typical degradation behavior of the short circuit current and of the saturated photocurrent, defined as I(-1V). The latter remains constant after a relatively small initial decay, which is directly related to a substantial reduction of the contribution of excitons generated in C(60) to the external quantum efficiency. Mass spectroscopy analysis of the organic material after aging revealed oxidized C(60) and sub-products thereof. The reduction of I(-1V) is thus attributed to the reaction of C(60) molecules with oxygen impurities. The results strongly suggest that ZnPc/C(60) photovoltaic cells are intrinsically very stable on a time scale of thousand of hours if such reactions are prevented. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: A detailed investigation of the diffusion of triplet excitons in a layer of N,N'-di-1-naphthalenyl-N,N'-diphenyl-[1,1' : 4', 1 '' : 4 '',1'''-quaterphenyl]-4,4'''-diamine (4P-NPD) incorporated in organic light-emitting diodes is presented. An appropriate method to measure the triplet diffusion length in fluorescent host materials is the spatial separation of the site of exciton generation from the site of radiative triplet decay by inserting a host spacer layer of varying thickness. However, cavity effects, the quenching and blocking of excitons at the boundaries of the spacer layer, and direct charge-carrier recombination in the sensing layer need to be taken into account. We use a specially designed layer stack, which excludes the influence of cavity effects on the measurements and a strongly quenching sensing layer, which ensures well-defined boundary conditions. The quenching of excitons by the sensing layer, the generation zone, and direct charge-carrier recombination are investigated experimentally and their influence on the extracted diffusion length are discussed. The significance of triplet-triplet annihilation in this analysis is estimated by a current-dependent evaluation. An analytic model for the dependence of the sensing layer emission on the spacer thickness is presented, which includes the important effects. By this means, we find a triplet diffusion length of 11 +/- 3 nm in 4P-NPD.
Abstract: We detect and measure the trap charges in a small molecular bulk heterojunction solar cell under operating conditions. The trap-charge density is estimated from capacitance measurements with light incident on the sample. At high intensities (similar to 1 sun, 100 mW/cm(2)), the trapped charge concentration leads to a spatial distortion of the electric field in the device. The lower limit of the trap-charge density is estimated to be 6 x 10(16) cm(-3). The frequency dependence of the capacitance suggests that the charges are trapped in a manifold of deep states present in the energy gap of the semiconductors. The distortion of the electric field by this trap charge affects the charge-carrier collection efficiency.
Abstract: We demonstrate top-emitting organic light-emitting diodes (OLEDs) with white emission spectra, employing a three color hybrid cavity structure with two highly efficient phosphorescent orange-red and green emitters (iridium(III)bis(2-methyldibenzo-[f,h]chinoxalin)(acetylacetonate) (Ir(MDQ)(2)(acac)) and tris(2-phenylpyridin) iridium(III) (Ir(ppy)(3))) and the stable blue fluorescent emitter 2,5,8,11-tetra-tert-butylperylene (TBPe). In contrast to Lambertian emission characteristics, our devices show enhanced emission in the forward direction up to angles of 50 degrees. In order to optically adjust the balance of blue and red light in the emission spectra, the influence of two different anode mirror materials are studied. We further test varied electron blocking layer materials to improve the device performance. An optimized layer structure with a hybrid anode mirror yields luminous efficacies of 13.3 lm/W (4.9% external quantum efficiency (EQE)) for the blocker material 2,2',7,7'-tetrakis-(N,N-diphenylamino)-9,9'-spirobifluoren (S-TAD) and 12.4 lm/W (5.3% EQE) for N,N'-di(naphthalen-1-yl)-N,N'-diphenyl-benzidine (NPB), respectively, at approximate luminances of 1000 cd/m(2). Using an aluminum anode, the emission shows Commission Internationale d'Eclairage chromaticity coordinates (CIE 1931) of (0.420, 0.407) at approximately 1000 cd/m(2) and color rendering indices of up to 77. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: We have investigated conductivity and ageing of ZnPc/C60 solar cells as they are influenced by the charge carrier mobility and variation of the potential barrier height. The test structures with a reasonable energy conversion efficiency of similar to 1.5% were investigated. The samples were aged for 1300 h upon illumination by the blue LED with peak emission at 475 nm. Upon ageing the devices showed a strong and fast degradation of the efficiency, short circuit current and of the fill factor within several hours followed by a much slower decrease of them. The reference samples kept in the dark at the room temperature did show only very small changes in their I-V curves. Carrier mobility dependencies on electric field strength at different temperatures were measured by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. It was demonstrated that mobility values decrease during degradation as compared to the reference samples. Nevertheless only mobility changes cannot explain the observed drop of device current. The increase of the effective barrier height by about 0.1 eV from similar to 0.55 eV up to similar to 0.65 eV was observed in the aged samples. Meanwhile thermal activation energy values of the electrical conductivity grew from about 0.28 eV prior to degradation up to about 0.34 eV after ageing. (c) 2010 Elsevier B.V. All rights reserved.
Abstract: We present small molecule solar cells with alpha,omega-bis-(dicyanovinylene)-sexithiophene:C(60) mixed heterojunctions, reaching power conversion efficiencies of 4.9 +/- 0.2%. We use substrate heating during deposition of the mixed layer to achieve an optimized morphology and show that this significantly improves the internal quantum efficiencies (IQEs) to values approaching 70%. By optical modeling, we evaluate the amount of loss due to absorption in inactive layers and show that IQE of the active layer itself is about 80%. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3475766]
Abstract: Weaker triplet-triplet annihilation (TTA) is observed in organic solid mixed systems comprising the phosphorescent emitter Ir(ppy)(2)(acac), which has a smaller dipole moment than the reference compound Ir(ppy)(3). Spectral and time-resolved analysis show that the weaker TTA can be explained by lower emitter aggregation, which reduces diffusion-based TTA.
Abstract: We compare the degradation of organic light emitting diodes (OLEDs) by UV light and by electrical driving. We prove that the exponential dependence of the half-lifetime on the current density known from electrical aging is also valid for UV-degradation. The influence of excitons on the degradation of OLEDs is determined and we experimentally distinguish between the influence of singlet and triplet excitons. We conclude that singlet excitons are the main cause of degradation for Spiro-DPVBi(2,2('),7,7(')-tetrakis(2,2-diphenylvinyl)spiro-9,9(')-bifluo rene)-based OLEDs by a comparison of the degradation of electrically driven and UV-excited OLEDs. (c) 2010 American Institute of Physics. [doi: 10.1063/1.3460285]
Abstract: A key feature of stacked organic solar cells is an efficient recombination contact at the interface between the solar cells in the stack. Here, an electron current has to be converted into a hole current without loss of energy. Furthermore, the recombination contact has to be highly transparent. We present a new approach for small molecule organic solar cells using highly doped organic layers. Our approach adapts the use of tunnel diodes known from inorganic tandem solar cells. We compare a metal cluster based recombination contact reported in literature to the new approach using different organic tandem solar cell structures. For this purpose, current-voltage characteristics of adequate solar cells are measured. The experiments show that highly doped layers as recombination contacts in tandem organic solar cells are superior to the metal cluster based approach. The proposed concept allows an addition of the open circuit voltages of the subcells of a tandem solar cell, without absorption or reflection at the recombination contact. The results further show that our concept does not depend on the specific choice of materials as it is seen for metal cluster based recombination contacts. It therefore represents a general approach which is compatible to mass manufacturing. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3467786]
Abstract: We report on red top-emitting organic light-emitting diode structures with higher order cavities. The emission zone is placed in the first, second, and third antinodes of the electric field in the cavity by increasing the hole transport layer thickness. Furthermore, the thicknesses of the cathode and the capping layer are varied to achieve high efficiencies. Using doped charge transport layers and a phosphorescent emitter, we reach up to 29%, 17%, and 12% external quantum efficiencies for first, second, and third order devices, respectively. An optical model is further used to analyze the angular dependent emission. (C) 2010 American Institute of Physics. [doi:10.1063/1.3530447]
Abstract: We introduce single-carrier devices with electrical doped layers as a concept for the characterization of charge-carrier transport in organic semiconductors. In this approach, individual organic layers from a multilayer device are investigated in single-carrier test devices, where they are enclosed by symmetrically arranged electrical doped layers of equal thickness and composition. Single carrier devices without electrical doped layers are usually difficult to interpret due to an uncertainty about interface dipole effects between the metal contacts and pristine organic layers. In comparison, our devices show Ohmic contacts at the electrodes as well as zero built-in voltage and thus allow a more direct insight into charge-carrier transport. State-of-the-art simulation models are applied to analyze current-voltage characteristics and evaluate crucial parameters such as energy barriers between adjacent organic layers and mobilities. (c) 2010 American Institute of Physics. [doi: 10.1063/1.3460528]
Abstract: We study the influence of different substrate temperatures during the deposition of the ZnPc:C-60 blend layer in bulk heterojunction organic solar cells. It is shown that substrate heating during evaporation leads to a significant improvement in the solar cell performance mainly due to an increase in photocurrent and fill factor (FF). This is attributed to improved morphology resulting in better charge carrier percolation pathways within the ZnPc:C-60 blend, leading to reduced transport losses. Using this method, blend layer thicknesses of 150 nm are possible without loss in FF, which requires a three-dimensional interpenetrating network without isolated clusters. When heating the substrate up to 110 degrees C, an efficiency of 2.56% is achieved compared to 1.59% for an identical device prepared at room temperature.
Abstract: We show that the fullerene C-70 is suitable to replace fullerene C-60, which is commonly used as electron transporter and acceptor in small-molecule organic solar cells. It is shown that the higher absorption of C-70 leads to high external quantum efficiencies of over 50% in the spectral range of 500-700 nm. By optimizing the energy level alignment to hole transport layers, the absorption, and the ratio of C-70:zinc phthalocyanine (ZnPc) in a bulk heterojunction solar cell, an efficiency of eta=2.87% is achieved. This is a substantial improvement over an identical solar cell employing C-60 having eta=2.27%. The efficiency increase is due to a higher photocurrent, while fill factor and open-circuit voltage for C-70 and C-60-containing organic solar cells remain comparable.
Abstract: White organic light-emitting diodes (OLEDs) are highly efficient large-area light sources that may play an important role in solving the global energy crisis, while also opening novel design possibilities in general lighting applications. Usually, highly efficient white OLEDs are designed by combining three phosphorescent emitters for the colors blue, green, and red. However, this procedure is not ideal as it is difficult to find sufficiently stable blue phosphorescent emitters. Here, a novel approach to meet the demanding power efficiency and device stability requirements is discussed: a triplet harvesting concept for hybrid white OLED, which combines a blue fluorophor with red and green phosphors and is capable of reaching an internal quantum efficiency of 100% if a suitable blue emitter with high-lying triplet transition is used is introduced. Additionally, this concept paves the way towards an extremely simple white OLED design, using only a single emitter layer.
Abstract: We present semitransparent small-molecule organic solar cells (OSC) deposited by thermal evaporation onto indium tin oxide (ITO)-coated glass substrates. The devices employ ITO-free ultrathin metal layers as top electrodes, containing 1 nm metal surfactant interlayer for improved morphology. Using a bulk heterojunction of zinc phthalocyanine and C(60), sandwiched in between doped dedicated transport layers for efficient charge carrier extraction, power conversion efficiencies comparable to conventional OSC with an intransparent thick back electrode and similar device layout are achieved: the semitransparent OSC yield power conversion efficiencies well above 2% with external quantum efficiencies above 30%-40%. Organic light incoupling layers improve the transmission to up to 50% in the visible part of the optical spectrum.
Abstract: We analyze the polarization dependence of the discrete photonic states in an organic microcavity system that is laterally confined on the micron scale. Via microscopic photoluminescence measurements on square shaped photonic dots, we demonstrate a polarization splitting that increases up to 10 meV for higher order modes. These experimental results are evaluated using a vectorial waveguide approach and the splitting is attributed to phase shifts, which occur its a result Of reflection at the side walls of the mesa structures. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3259719]
Abstract: We present an in-depth analysis of the angle and mode tuning dependence of the polarization splitting in detuned organic microcavities. The system consists of highly reflecting dielectric mirrors comprising a continuous wedge-shaped cavity layer of 4,4'-bis(N-carbazole)-biphenyl, doped with 2 wt % of 4,4'-bis[(N-carbazole)styryl]-biphenyl as the active material. By varying the measurement position across the sample, we shift the resonant mode with respect to the center of the cavity stop band. Starting from small detunings, the splitting value increases non-linearly up to 58 meV, when the resonant modes reach the stop band edge. We compare this behavior with results from transfer matrix calculations and theoretical predictions. (c) 2009 American Institute of Physics
Abstract: We investigate the doping behavior of the strongly electron accepting molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane coevaporated with the host molecule N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine by photoemission spectroscopy and conductivity measurements. Using interface resolved measurements, we compare the alignment on different substrates and investigate the effects of varying doping concentrations on the Fermi level position. We find that at high doping concentrations the Fermi level gets pinned at the exponentially decaying tail of the highest Occupied molecular orbital and compare these results with different dopants and host molecules. The measurement of the doping dependent space charge layer thickness yields information on the amount of free charge carriers and thereby the efficiency of the doping. (C) 2009 American Institute of Physics. [doi:10.1063/1.3259436]
Abstract: The development of white organic light-emitting diodes(1) (OLEDs) holds great promise for the production of highly efficient large-area light sources. High internal quantum efficiencies for the conversion of electrical energy to light have been realized(2-4). Nevertheless, the overall device power efficiencies are still considerably below the 60-70 lumens per watt of fluorescent tubes, which is the current benchmark for novel light sources. Although some reports about highly power-efficient white OLEDs exist(5,6), details about structure and the measurement conditions of these structures have not been fully disclosed: the highest power efficiency reported in the scientific literature is 44 lm W-1 (ref. 7). Here we report an improved OLED structure which reaches fluorescent tube efficiency. By combining a carefully chosen emitter layer with high-refractive-index substrates(8,9), and using a periodic outcoupling structure, we achieve a device power efficiency of 90 lm W-1 at 1,000 candelas per square metre. This efficiency has the potential to be raised to 124 lm W-1 if the light outcoupling can be further improved. Besides approaching internal quantum efficiency values of one, we have also focused on reducing energetic and ohmic losses that occur during electron-photon conversion. We anticipate that our results will be a starting point for further research, leading to white OLEDs having efficiencies beyond 100 lm W-1. This could make white-light OLEDs, with their soft area light and high colour-rendering qualities, the light sources of choice for the future.
Abstract: The efficiency roll-off at high brightness levels is a key factor limiting the application of organic light emitting diodes. We investigate triplet-triplet annihilation in an archetype phosphorescent host-guest system. We show that the currently used host-guest systems are not at the physical limit set by intrinsic annihilation, but have an increased roll-off due to aggregate formation. The existence of these aggregates is directly proven by transmission electron microscopy.
Abstract: Room-temperature multimode laser emission is observed in a microcavity consisting of dielectric mirrors and small-molecular-weight organic photonic dots as a cavity layer. The structure shows simultaneous lasing of a wide variety of transverse modes. A comparison of the laser operating characteristics with those of unpatterned structures shows an enhancement in the spontaneous emission coupling factor by more than two orders of magnitude due to the lateral confinement. The spectral features are in quantitative agreement with calculations of quantized photonic states in three-dimensional optical cavities.
Abstract: For improving the lifetime of organic light emitting devices (OLEDs), the analysis of the chemical degradation requires a deep understanding of the involved reaction pathways. We show that the dissociation reactions of phosphorescent emitters and the additional complexations with the used surrounding blocking layers are the dominant intrinsic degradation mechanisms in long living p-i-n type OLEDs. We use the laser desorption/ionization (LDI) time-of-flight mass spectrometry to correlate the laser-induced ion formation with the observed lifetime of the organic devices. The superlinear correlation between the LDI forced reactions and the lifetimes allows the prediction of the lifetime of an OLED with new materials.
Abstract: We present zinc phthalocyanine (ZnPc):C-60 bulk-heterojunction top-illuminated organic solar cells using ultrathin metal layers as transparent top contacts. We show that solar cell performance sensitively depends on the interface and morphology of the cathode, which can be influenced by varying the composition and layer structure of the metal contact. We investigate various metal combinations, such as 3 nm Al/8 nm Ag and 7 nm Al/14 nm Ag, to illustrate the necessity to find a suitable combination of morphology and electrical and optical properties. Solar cells using standard materials and a 1 nm Al/14 nm Ag cathode exhibit promising efficiencies of over 2.2%.
Abstract: The concept of an additional capping layer, which is deposited onto the semitransparent top contact, is applied to minimize microcavity effects for white light emission from top-emitting organic light emitting devices (OLEDs). The influence on the optical properties of such devices with silver as top electrode material is discussed using an analytical method and numerical simulations. The results of the theoretical findings are experimentally verified for inverted top emitting devices on opaque substrates, showing broad spectral bandwidth and angle-independent color coordinates.
Abstract: Holy(4-vinylpyridine)-block-poly(4-iodo-styrene), P4VP-b-PS(l), block copolymers obtained by iodination of readily available P4VP-b-PS block copolymers strongly adhere to variety of polar substrates including Si wafers, glasses, or metal oxide surfaces by a polar P4VP block, forming polymer brushes of moderately stretched PS(l) chains. Kumada catalyst-transfer polycondensation (KCTP) from the P4VP-b-PS(l) brushes results into planar brushes of the graft copolymer in which relatively short (similar to 10 nm) poly(3-hexylthiophene), P3HT, grafts emanate from the surface-tethered PS(l) chains. Grafting of the P3HT leads to significant stretching of the PS(l) backbone as a result of increased excluded volume interactions. Specific adsorption of the P4VP block to polar surfaces was utilized in this work to pattern the P4VP(25)-b-PS(l)(350) brush. The microscopically structured P4VP(25)-b-PS(l)(350) brush was converted into the respectively patterned P4VP-PS(l)-g-P3HT one using KCTP. We also demonstrated that KCTP from functional block copolymers is an attractive option for nanostructuring with polymer brushes. P4VP(75)-b-PS(l)(313) micelles obtained in selective solvent for the PS(l) block form a quasi-ordered hexagonal array on Si wafer. The P4VP(75)-b-PS(l)(313) monolayer preserves the characteristic quasi-regular arrangement of the micelles even after extensive rinsing with various solvents. Although the grafting of P3HT from the nanopatterned P4VP(75)-b-PS(l)(313) brush destroys the initial order; the particulate morphology in the resulting film is preserved. We believe that the developed method to structured brushes of conductive polymers can be further exploited in novel stimuli-responsive materials, optoectronic devices, and sensors.
Abstract: We present metal multilayers consisting of aluminum and silver in different combinations serving as semitransparent top contacts for organic solar cells. Scanning electron microscopy, atomic force microscopy, and optical spectroscopy are used to illustrate how ultrathin Al interlayers influence the morphology of Ag layers evaporated on top of organic materials and how closed layers with good conductivity can be achieved. Multilayer metal contacts are used to fabricate top-illuminated small-molecule organic solar cells (SM-OSCs) which reach efficiencies comparable to conventional SM-OSCs that employ tin-doped indium oxide as electrode. It is shown that combinations of Al and Au lead to similar results, suggesting a similar mechanism for the influence on morphological development of both Ag and Au.
Abstract: Triplet-triplet annihilation (TTA) between excited states of host and guest species in organic mixed films, comprising a phosphorescent heavy-metal complex, is usually only accessed indirectly, as one is only able to monitor the radiative guest phosphorescence. We compare two different host materials 4,4â,4 ââ-tris(N-carbazolyl)-triphenylamine (TCTA) and 4,4â-N,Nâ-dicarbazole-biphenyl (CBP) in combination with the green emitter fac-tris(2-phenylpyridine) iridium which differ in their triplet state energy. At excitation levels in the saturation range of guest emission, we observe increasing non-linearities inphotoluminescence transients with increasing excitation power for the CBP host material, which can directly be attributed to TTA of host material, which can directly be attributed to TTA of host and guest excited states. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: Metal organic fluorescent and phosphorescent emitters are widely used in organic light emitting devices (OLEDs). Iridium-based triplet emitters are known to undergo chemical reactions with other materials during OLED aging. The material tris(8-hydroxy-quinolinato)aluminum (Alq(3)), which is widely used as electron transporting material and green fluorescent emitter, degrades mainly during hole transport. We investigate the chemical changes in Alq(3) during device aging: using laser desorption ionization time-of-flight mass spectrometry, we study the reaction products found in degraded OLEDs. Similar to the reactions known from the phosphorescent iridium-based emitters, Alq(3) undergoes a dissociation and further reaction with the hole blocking material 4,7-diphenyl-1,10-phenanthroline.
Abstract: For organic solar cells, effective absorption over a wide wavelength range is important. A simple donor-acceptor pair is usually not sufficient to reach this goal. Thus, it would be desirable to utilize multiple photoactive materials in a single cell. In this work, two hole conducting materials, pentacene and zinc phthalocyanine (ZnPc), and electron conducting C(60) are chosen to construct three-component heterojunctions aiming at improved effective photon harvesting in organic solar cells. It is found that in pentacene/ZnPc/C(60) multiple heterojunctions, part of the excitons in pentacene reach the ZnPc/C(60) interface, where efficient exciton separation occurs and contributes to the photocurrent (PC). Triplet excitons are confirmed to be the major origin of PC by transient PC response measurements, suggesting that triplet-to-triplet energy transfer from pentacene to ZnPc is responsible for the improved PC of pentacene/ZnPc/C(60) multiheterojunctions. Furthermore, exothermic energy transfer from ZnPc to the lower lying triplet levels of pentacene is employed for extending the absorption range and enlarging the absorption intensity. To realize such a structure, an ultrathin ZnPc layer is embedded in the pentacene film in pentacene/C(60) single heterojunctions, leading to an enhanced quantum efficiency in the long wavelength range compared to the reference cell. These findings pave a way to efficient photovoltaic cells with a wide photoresponse ranging from near UV through the visible to the near infrared. (C) 2009 American Institute of Physics. [doi:10.1063/1.3187904]
Abstract: Room-temperature multimode laser emission is observed in a microcavity consisting of dielectric mirrors and small-molecular-weight organic photonic dots as a cavity layer. The structure shows simultaneous lasing of a wide variety of transverse modes. A comparison of the laser operating characteristics with those of unpatterned structures shows an enhancement in the spontaneous emission coupling factor by more than two orders of magnitude due to the lateral confinement. The spectral features are in quantitative agreement with calculations of quantized photonic states in three-dimensional optical cavities.
Abstract: The concept of an additional capping layer, which is deposited onto the semitransparent top contact, is applied to minimize microcavity effects for white light emission from top-emitting organic light emitting devices (OLEDs). The influence on the optical properties of such devices with silver as top electrode material is discussed using an analytical method and numerical simulations. The results of the theoretical findings are experimentally verified for inverted top emitting devices on opaque substrates, showing broad spectral bandwidth and angle-independent color coordinates.
Abstract: Triplet-triplet annihilation (TTA) between excited states of host and guest species in organic mixed films, comprising a phosphorescent heavy-metal complex, is usually only accessed indirectly, as one is only able to monitor the radiative guest phosphorescence. We compare two different host materials 4,4',4 ''-tris(N-carbazolyl)-triphenylamine (TCTA) and 4,4'-N,N'-dicarbazole-biphenyl (CBP) in combination with the green emitter fac-tris(2-phenylpyridine) iridium which differ in their triplet state energy. At excitation levels in the saturation range of guest emission, we observe increasing non-linearities inphotoluminescence transients with increasing excitation power for the CBP host material, which can directly be attributed to TTA of host material, which can directly be attributed to TTA of host and guest excited states. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: For improving the lifetime of organic light emitting devices (OLEDs), the analysis of the chemical degradation requires a deep understanding of the involved reaction pathways. We show that the dissociation reactions of phosphorescent emitters and the additional complexations with the used surrounding blocking layers are the dominant intrinsic degradation mechanisms in long living p-i-n type OLEDs. We use the laser desorption/ionization (LDI) time-of-flight mass spectrometry to correlate the laser-induced ion formation with the observed lifetime of the organic devices. The superlinear correlation between the LDI forced reactions and the lifetimes allows the prediction of the lifetime of an OLED with new materials.
Abstract: Holy(4-vinylpyridine)-block-poly(4-iodo-styrene), P4VP-b-PS(l), block copolymers obtained by iodination of readily available P4VP-b-PS block copolymers strongly adhere to variety of polar substrates including Si wafers, glasses, or metal oxide surfaces by a polar P4VP block, forming polymer brushes of moderately stretched PS(l) chains. Kumada catalyst-transfer polycondensation (KCTP) from the P4VP-b-PS(l) brushes results into planar brushes of the graft copolymer in which relatively short (similar to 10 nm) poly(3-hexylthiophene), P3HT, grafts emanate from the surface-tethered PS(l) chains. Grafting of the P3HT leads to significant stretching of the PS(l) backbone as a result of increased excluded volume interactions. Specific adsorption of the P4VP block to polar surfaces was utilized in this work to pattern the P4VP(25)-b-PS(l)(350) brush. The microscopically structured P4VP(25)-b-PS(l)(350) brush was converted into the respectively patterned P4VP-PS(l)-g-P3HT one using KCTP. We also demonstrated that KCTP from functional block copolymers is an attractive option for nanostructuring with polymer brushes. P4VP(75)-b-PS(l)(313) micelles obtained in selective solvent for the PS(l) block form a quasi-ordered hexagonal array on Si wafer. The P4VP(75)-b-PS(l)(313) monolayer preserves the characteristic quasi-regular arrangement of the micelles even after extensive rinsing with various solvents. Although the grafting of P3HT from the nanopatterned P4VP(75)-b-PS(l)(313) brush destroys the initial order; the particulate morphology in the resulting film is preserved. We believe that the developed method to structured brushes of conductive polymers can be further exploited in novel stimuli-responsive materials, optoectronic devices, and sensors.
Abstract: We fabricated OLEDs having the evaporated Yb cathode and the sputtered Al cathode. Magnetron sputtering is a versatile deposition method, but the energetic particles during sputtering process can damage the underlying organic layers. We applied various process parameters to explain the role of Ar neutrals quoting the former researches. OLEDs showed almost comparable degradation under the discharge voltage of 300 V, but serious degradations were found at the discharge voltage of over 300V and the low process pressure. The degradations influenced on the lifetime of OLED, and the measured luminance decay was much larger than the OLED having the evaporated Yb cathode due to the high leakage currents.
Abstract: We present zinc phthalocyanine (ZnPc):C(60) bulk-heterojunction top-illuminated organic solar cells using ultrathin metal layers as transparent top contacts. We show that solar cell performance sensitively depends on the interface and morphology of the cathode, which can be influenced by varying the composition and layer structure of the metal contact. We investigate various metal combinations, such as 3 nm Al/8 nm Ag and 7 nm Al/14 nm Ag, to illustrate the necessity to find a suitable combination of morphology and electrical and optical properties. Solar cells using standard materials and a 1 nm Al/14 nm Ag cathode exhibit promising efficiencies of over 2.2%.
Abstract: We present metal multilayers consisting of aluminum and silver in different combinations serving as semitransparent top contacts for organic solar cells. Scanning electron microscopy, atomic force microscopy, and optical spectroscopy are used to illustrate how ultrathin Al interlayers influence the morphology of Ag layers evaporated on top of organic materials and how closed layers with good conductivity can be achieved. Multilayer metal contacts are used to fabricate top-illuminated small-molecule organic solar cells (SM-OSCs) which reach efficiencies comparable to conventional SM-OSCs that employ tin-doped indium oxide as electrode. It is shown that combinations of Al and Au lead to similar results, suggesting a similar mechanism for the influence on morphological development of both Ag and Au.
Abstract: The efficiency roll-off at high brightness levels is a key factor limiting the application of organic light emitting diodes. We investigate triplet-triplet annihilation in an archetype phosphorescent host-guest system. We show that the currently used host-guest systems are not at the physical limit set by intrinsic annihilation, but have an increased roll-off due to aggregate formation. The existence of these aggregates is directly proven by transmission electron microscopy.
Abstract: We study the influence of different substrate temperatures during the deposition of the ZnPc:C(60) blend layer in bulk heterojunction organic solar cells. It is shown that substrate heating during evaporation leads to a significant improvement in the solar cell performance mainly due to an increase in photocurrent and fill factor (FF). This is attributed to improved morphology resulting in better charge carrier percolation pathways within the ZnPc:C(60) blend, leading to reduced transport losses. Using this method, blend layer thicknesses of 150 nm are possible without loss in FF, which requires a three-dimensional interpenetrating network without isolated clusters. When heating the substrate up to 110 degrees C, an efficiency of 2.56% is achieved compared to 1.59% for an identical device prepared at room temperature.
Abstract: We experimentally investigate variable laser mode excitation in an organic photonic dot microcavity by shifting the excitation beam position. The sample comprises two highly reflective dielectric mirrors (R > 99.9%) and a square-shaped organic dye mesa of a DCM doped (2 wt%) Alq(3)-matrix. Its wavelength-size (approximate to 5 x 5 mu m(2)) transforms the cavity mode dispersion to a set of discrete states, each with a different intensity distribution of the electromagnetic field in space. Numerical simulations, including absorption and gain, confirm the experimentally observed relation between mode distribution and progression on the excitation condition. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3222981]
Abstract: Recent advances in the field of p-i-n type organic light emitting diodes (OLEDs) are reviewed. OLEDs are energy efficient light sources, which will be used in the near future in commercial display and lighting applications. In particular, p-i-n type OLEDs consisting of doped charge transport layers and intrinsic emission layers have been very successful in reducing the operational voltages and increasing the power efficiency. After shortly introducing the p-i-n concept, the advantages of this concept in terms of low operating voltages, high power efficiency and long lifetime are described. In particular, the latest reports on monochrome, white, and top-emission p-i-n OLEDs are summarized.
Abstract: We show that the fullerene C(70) is suitable to replace fullerene C(60), which is commonly used as electron transporter and acceptor in small-molecule organic solar cells. It is shown that the higher absorption of C(70) leads to high external quantum efficiencies of over 50% in the spectral range of 500-700 nm. By optimizing the energy level alignment to hole transport layers, the absorption, and the ratio of C(70):zinc phthalocyanine (ZnPc) in a bulk heterojunction solar cell, an efficiency of eta=2.87% is achieved. This is a substantial improvement over an identical solar cell employing C(60) having eta=2.27%. The efficiency increase is due to a higher photocurrent, while fill factor and open-circuit voltage for C(70) and C(60)-containing organic solar cells remain comparable.
Abstract: We use photoelectron spectroscopy to study the electronic structure and energy level alignment throughout an organic light-emitting diode. The structure under investigation is a state-of-the-art long-living red phosphorescent device composed of doped charge-injection layers, charge-blocking layers, and an emission layer. By consecutively building up the whole device, the key parameters of every interface are measured. Our results show that the doped layers have a significant influence on the device energetics, especially in controlling the built-in potential, and that there are mostly only small dipoles present at the interfaces of the intrinsic organic layers.
Abstract: White organic light-emitting diodes (OLEDs) are highly efficient large-area light sources that may play an important role in solving the global energy crisis, while also opening novel design possibilities in general lighting applications. Usually, highly efficient white OLEDs are designed by combining three phosphorescent emitters for the colors blue, green, and red. However, this procedure is not ideal as it is difficult to find sufficiently stable blue phosphorescent emitters. Here, a novel approach to meet the demanding power efficiency and device stability requirements is discussed: a triplet harvesting concept for hybrid white OLED, which combines a blue fluorophor with red and green phosphors and is capable of reaching an internal quantum efficiency of 100% if a suitable blue emitter with high-lying triplet transition is used is introduced. Additionally, this concept paves the way towards an extremely simple white OLED design, using only a single emitter layer.
Abstract: The development of white organic light-emitting diodes(1) (OLEDs) holds great promise for the production of highly efficient large-area light sources. High internal quantum efficiencies for the conversion of electrical energy to light have been realized(2-4). Nevertheless, the overall device power efficiencies are still considerably below the 60-70 lumens per watt of fluorescent tubes, which is the current benchmark for novel light sources. Although some reports about highly power-efficient white OLEDs exist(5,6), details about structure and the measurement conditions of these structures have not been fully disclosed: the highest power efficiency reported in the scientific literature is 44 lm W-1 (ref. 7). Here we report an improved OLED structure which reaches fluorescent tube efficiency. By combining a carefully chosen emitter layer with high-refractive-index substrates(8,9), and using a periodic outcoupling structure, we achieve a device power efficiency of 90 lm W-1 at 1,000 candelas per square metre. This efficiency has the potential to be raised to 124 lm W-1 if the light outcoupling can be further improved. Besides approaching internal quantum efficiency values of one, we have also focused on reducing energetic and ohmic losses that occur during electron-photon conversion. We anticipate that our results will be a starting point for further research, leading to white OLEDs having efficiencies beyond 100 lm W-1. This could make white-light OLEDs, with their soft area light and high colour-rendering qualities, the light sources of choice for the future.
Abstract: We use photoelectron spectroscopy to study the electronic structure and energy level alignment throughout an organic light-emitting diode. The structure under investigation is a state-of-the-art long-living red phosphorescent device composed of doped charge-injection layers, charge-blocking layers, and an emission layer. By consecutively building up the whole device, the key parameters of every interface are measured. Our results show that the doped layers have a significant influence on the device energetics, especially in controlling the built-in potential, and that there are mostly only small dipoles present at the interfaces of the intrinsic organic layers.
Abstract: We present the material alpha-omega-bis-(dicyanovinylen)-sexithiophen (DCV6T) as donor material in organic solar cells. A systematic study on the potential of DCV6T is given for different active layer concepts. DCV6T is a member of a class of acceptor-substituted oligothiophenes, which showed efficiencies of up to 3.4% and open circuit voltages (V(oc)) of 1.0 V, which were recently reported [K. Schulze Adv. Mater. (Weinheim, Ger.) 18, 2875 (2006)]. To verify the potential of the material (DCV6T), organic solar cells with planar heterojunctions, bulk heterojunctions, and a hybrid-planar-mixed heterojunction are investigated. The planar heterojunction solar cells of DCV6T and C60 show the highest V(oc) of 0.90 V. The mixed heterojunction solar cells have improved currents but a lower V(oc) of 0.82 V. The solar cell using the hybrid-planar-mixed heterojunction achieves the best combination of parameters. It has a V(oc) of 0.88 V, a short circuit current (j(sc)) of 5.7 +/- 0.4 mA cm(-2), a fill factor of 41.6%, and a power conversion efficiency of 2.1 +/- 0.2%.
Abstract: We show the impact of lateral Structuring on the optical properties of organic thin films in microcavities. The active material tris-(8-hydroxy quinoline) aluminium (Alq3) was deposited using shadow mask evaporation, resulting in various micron sized shapes. A simplified box model is employed to describe the steady state electric field distribution in these mesa cavities. The complex mode structure measured in transmission and photoluminescence is confirmed by a transfer matrix calculation and a Fourier transform of the internal electric field distribution.
Notes: 7th International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD â07), Newark, DE, SEP 24-27, 2007
Abstract: In this work we study the input-output characteristics of an organic vertical cavity surface emitting laser (OVCSEL) with an active layer of Alq3:DCM over 3 orders of magnitude in pump intensity. We find that lasing from this standard inhomogeneously broadened gain medium cannot be described using a single lasing threshold value. Instead, an inhomogeneously broadened distribution of spontaneous emission factors, beta, must be employed in order to correctly model the measured input-output characteristics.
Notes: 7th International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD â07), Newark, DE, SEP 24-27, 2007
Abstract: In this paper we focus on the current status of organic solar cells based on small molecules. Since their discovery, much progress has been made, and the main steps are highlighted that led to the current state-of-the-art devices. However, organic solar cells still need to be improved further, and the main strategies for improving the power conversion efficiency, namely raising the open circuit voltage V-oc and increasing the short circuit current density J(sc), are discussed. In theory, power conversion efficiencies of around 15% should be possible with a single heterojunction; for higher efficiencies, stacked solar cell concepts have to be employed.
Abstract: In this contribution, it is shown that the method of laser-desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) is a powerful technique for analyzing complete organic devices, such as organic light-emitting diodes (OLEDs) or organic solar cells. LDI-TOF-MS has the potential to analyze fully processed organic devices without special pretreatment such as dissolving the device, peeling off, the metal, cathode, or using additional matrix materials. Thus, devices may be analysed as they are with a minimum of measurement artefacts. It is demonstrated that the method allows an analysis of complex organic multilayer devices, their composition, and incorporated impurities. It even allows possible electrochemical reaction products caused by device degradation to be analyzed. Thus, LDI-TOF-MS has major advantages compared to measurements of dissolved samples. As an example, the identification of all of the materials used in a complete OLED is shown Furthermore, a detailed chemical analysis of long-term driven OLEDs, including the detection of degradation products, is presented. From these data, several degradation mechanisms can be distinguished.
Abstract: The aim of this article is to investigate the origin of the open circuit voltage (V-oc) in organic heterojunction solar cells. The studied devices consist of buckminsterfullerene C-60 as acceptor material and an oligophenyl-derivative 4,4â-bis-(N,N-diphenylamino)quaterphenyl (4P-TPD) as donor material. These photoactive materials are sandwiched between indium tin oxide and p-doped hole transport layers. Using two different p-doped hole transport layers, the built-in voltage of the solar cells is independently changed from the metal contacts. The influence of the built-in voltage on the V-oc is investigated in bulk and planar heterojunctions. In bulk heterojunctions, in which doped transport layers border directly on the photoactive blend layer, V-oc cannot exceed the built-in voltage significantly. Though, in planar heterojunctions, V-oc, is identical with the splitting of quasi-Fermi levels at the donor-acceptor interface and is thus primarily determined by the difference of the lowest unoccupied molecular orbital of C-60 and the highest occupied molecular orbital of 4P-TPD. In planar heterojunctions, the open circuit voltage can exceed the built-in voltage. Furthermore, the investigations show that the efficiency of organic solar cells can be improved by using p-doped charge transport layers with optimized energy level alignment to the active materials. The optimized planar heterojunction shows a fill factor of up to 65.5% and a V-oc of 0.95 V. For solar cells with insufficient energy level alignment between the photoactive layer system and the hole transport layer, a reduced V-oc in bulk heterojunction cells and a characteristic S shape of the I-V characteristics in planar heterojunction cells are observed. (C) 2008 American Institute of Physics.
Abstract: We show that ultrathin metal layers (Ag or Al/Ag) are feasible as transparent top contacts for zinc phthalocyanine: C-60 bulk-heterojunction inverted organic solar cells thermally evaporated on glass substrates. Furthermore, it is demonstrated that the introduction of an organic capping layer drastically increases light incoupling and photon harvesting, in accordance with optical simulations. Proof of principle tin-doped indium oxide (ITO)-free solar cells employing a transparent metal contact and a capping layer reach efficiencies of 1.06%, compared to 0.69% without addition of the capping layer. (c) 2008 American Institute of Physics.
Abstract: We discuss exciton transport and energy transfer in organic solar cells based on dicyanovinyl-terthiophene (DCV3T). Time-resolved surface luminescence quenching experiments on double layers of DCV3T and zinc-phthalocyanine as luminescence quencher are analyzed in the framework of a three-level luminescence model with an initially absorbing state of short lifetime that relaxes to a longer living emitting state. Luminescence quenching of the emitting state is assigned to Forster-type energy transfer with an apparent Forster radius of 2.1 nm, which is in accordance with the Forster radius obtained from the spectral overlap integral. A diffusion based analysis for the emitting state yields a diffusion length of L-e=6.9 nm. The short living initial state is quenched by diffusion of the excitons to the interface with a diffusion length of L-a=5.3 nm. External quantum efficiency measurements of the photocurrent in a corresponding organic solar cell structure evaluated with a two-level diffusion model give a diffusion length of L-EQE=6.0 nm, whereas a two-level model for stationary luminescence quenching yields a diffusion length of L-cw=12 nm. This suggests that only one type of the excitons contributes to the photocurrent. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2976317]
Abstract: One key problem in optimizing organic solar cells is to maximize the absorption of incident light and to keep the charge carrier transport paths as short as possible in order to minimize recombination losses during the charge carrier extraction. The large versatility of organic semiconductors and compositions requires specific optimization of each system. Due to the small thickness of the functional layers in the order of several ten nanometres, coherent optics has to be considered and therefore interference effects play a dominant role. Here we present and discuss concepts for light trapping in organic solar cells. These are wide gap layers in planar solar cells, folded solar cell architectures benefiting from the illumination under inclined incident angles and multiple reflections and absorptions as well as diffraction gratings embossed into the photoactive layer. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: Recently, several memory devices based on organic semiconductors were reported. In most cases, single organic layers were employed, and the switching mechanisms were not fully understood. Here, we report on a novel device structure based on electrically doped organic heterostructures showing highly stable memory behavior. The organic layers are embedded between a bottom indium tin oxide and a top metal contact and form a quantum well-like structure. The device shows a hysteresis in the current-voltage characteristics and well-defined switching behavior. We achieved reproducible bistable electrical switching and stable memory phenomena in these organic multilayer heterostructure devices. The two states were retained for up to several days without noticeable decrease of their on/off ratio. Further, it was found that the ratio of the on/off current depends on the writing voltage, allowing the use as multistate memory.
Abstract: New 2,2â-difluoro-1,3,2-dioxaborines with pronounced electron accepting properties have been prepared by reaction of quinizarines/halo-substituted quinizarines with BF3 center dot Et2O. From the results of electrochemical analysis, the electron affinities (EAs) of 1,3,2-dioxaborines can be increased when the hydrogen atoms are replaced by halogen ones. Furthermore, conductance of zinc phthalocyanine (ZnPc), a most versatile organic (photo)conductors used in organic light-emitting diodes (OLEDs), can be enhanced greatly when 4,9,10,11,12-pentachloro-2,2,7,7-tetrafluoro-2,7-dihydro-1,3,6,8-tetraox a-2,7-dibora-benzo[e]pyrene (4c) doped as charge-transport materials which suggesting halogen substitution is a versatile tool for creating efficient new p-dopants. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: We compare between two derivatives of dicyanovinyl-quinquethiophenes with different alkyl side chain lengths. Both materials show comparable open circuit voltages V-oc in organic solar cells with fullerene C-60 as acceptor, as expected since they have the same highest occupied molecular orbital energy. However, differences in the current-voltage-characteristics, particularly in the fill factor, are observed. We analyze both derivatives in hole-only devices and find a difference in the hole injection between the doped hole transport layer and the oligothiophenes. Additionally, we determine the hole mobility of the two materials and explain the different behaviors of the two materials in solar cells. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2990071]
Abstract: We report on the use of a thin layer of poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as anode for bottom emission organic light emitting diodes (OLEDs). The combination of polymer anodes with OLEDs having either electrically doped or undoped hole transport layers in direct contact with the polymer is shown. We discuss the impact of the annealing conditions of the polymer on the OLED lifetime in comparison to indium tin oxide anodes. Supported by a differential thermal analysis of PEDOT: PSS, a strong influence of residual water in the polymer on the device lifetime is found. Additional heating of the polymer anode in a dry ambient prior to OLED deposition is necessary to achieve high device lifetimes. At a luminance of 260 cd/m(2), pin-OLEDs on a PEDOT:PSS anode show no measurable device degradation during 5200 h of operation. (c) 2008 American Institute of Physics.
Abstract: The implementation of proper charge carrier transport materials in p-i-n type organic solar cells strongly influences the device performance. Our investigation focuses on the substitution of the standard layer sequence used at the side of electron transport, usually consisting of either C-60/exciton blocking layer/metal or n-C-60/metal by a layer sequence including a window layer. Here, we evaluate the transparent electron transport material 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), which guarantees a loss free charge extraction from the active materials due to a good energy level alignment as well as exciton blocking due to its wide bandgap. It is demonstrated that upon the exchange of the electron transport layer n-C-60 for n-NTCDA, the solar power conversion efficiency of a p-i-n device can be increased by 10%. (C) 2008 American Institute of Physics.
Abstract: We present an organic microcavity laser with wide tunability in the range of 595-650 nm, having a threshold as low as 2 nJ/pulse. The active medium consists of the organic composite tris(8-hydroxy quinoline) aluminum (Alq(3)) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) embedded in between two distributed Bragg reflectors. The inhomogeneously broadened emission from DCM is exploited for lasing by means of a tunable Fabry-Perot-type interferometer. Continuous tuning is achieved by varying the thickness of the active layer. The device shows a high photostability under direct excitation in the absorption band of the guest molecule, thus, allowing to monitor the optical gain spectrum of the active medium. (c) 2008 American Institute of Physics.
Abstract: Light-emitting diodes based on organic semiconductors show promising features for display and lighting applications. A vertical in-line deposition technique for organic light-emitting diode (OLED) manufacturing was developed. OLED devices with electrically doped transport layers show low operating voltage, high efficiency and long lifetime. The preparation of p-i-n type devices was performed with the in-line fabrication tool resulting in highly efficient OLED with low operating voltage. The lowest operating voltage was achieved for green diodes with 2.9 V for 100 cd/m(2). This demonstrates that the p-i-n device concept can be applied under manufacturing conditions. In-line manufactured highly efficient red, green and blue OLED are presented. One important aspect for fabrication cost is the used ground contact, which is commonly made by indium tin oxide (ITO). For low cost fabrication an alternative for ITO has to be used. In this work, ITO was replaced by aluminium doped zinc oxide (ZAO). The results are comparable to OLEDs using ITO as transparent conductive oxide. (C) 2007 Elsevier B.V. All rights reserved.
Notes: 6th International Conference on Coatings on Glass and Plasics, Dresden, GERMANY, JUN 18-22, 2006
Abstract: We demonstrate near-infrared (NIR) organic light-emitting diodes containing the phthalocyanines of copper (CuPc), palladium (PdPc), and platinum (PtPc) as emitting material. The devices show NIR emission from the triplet excitonic states of those phthalocyanines at 1095, 1025, and 966 nm, respectively. A yellow singlet emitter serves as host for the emitter materials, reducing triplet exciton quenching and improving energy transfer to the emitter. Using the emitter PtPc as guest and the yellow singlet emitter as host, an external quantum efficiency of 0.3% is achieved for infrared light emission at 966 nm. Due to the use of electrically doped charge transport layers, operation at voltages significantly below 3 V is possible. Light output reaches 80 mu W/cm(2) at a current density of 140 mA/cm(2). (c) 2008 American Institute of Physics.
Abstract: A power conversion efficiency of 3.4% with an open-circuit voltage of 1 V was recently demonstrated in a thin film solar cell utilizing fullerene C-60 as acceptor and a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor [K. Schulze , Adv. Mater. (Weinheim, Ger.) 18, 2872 (2006)]. This prompted us to systematically study the energy- and electron transfer processes at the oligothiophene:fullerene heterojunction for a homologous series of these oligothiophenes. Cyclic voltammetry and ultraviolet photoelectron spectroscopy data show that the heterojunction is modified due to tuning of the highest occupied molecular orbital energy for different oligothiophene chain lengths, while the lowest unoccupied molecular orbital energy remains essentially fixed due to the presence of electron-withdrawing end groups (dicyanovinyl) attached to the oligothiophene. Use of photoinduced absorption (PA) allows the study of the electron transfer process at the heterojunction to C-60. Quantum-chemical calculations performed at the density functional theory and/or time-dependent density functional theory level and cation absorption spectra of diluted DCVnT provide an unambiguous identification of the transitions observed in the PA spectra. Upon increasing the effective energy gap of the donor-acceptor pair by increasing the ionization energy of the donor, photoinduced electron transfer is eventually replaced with energy transfer, which alters the photovoltaic operation conditions. The optimum open-circuit voltage of a solar cell is thus a trade-off between efficient charge separation at the interface and maximized effective gap. It appears that the open-circuit voltages of 1.0-1.1 V in our solar cell devices have reached an optimum since higher voltages result in a loss in charge separation efficiency.
Abstract: The authors present an alternative method for the determination of the charge carrier mobility of organic thin films. In contrast to known methods like space charge limited current, field effect transistor and time of flight approaches, we determine the charge carrier mobility of a mixed film, serving as emission layer, within the conventional multilayer device architecture. We make use of a strong delayed generation feature in the electroluminescent decay, following a short voltage pump pulse in a time-resolved set-up. Taking into account the preferentially electron transporting properties of the film, the mobility of a N,N-di(naphthalen-2-yl)-N,Nâ-diphenyl-benzidine (NPB):tris(1 phenylisoquinoline) iridium [Ir(piq)(3)] mixed film is found to be on the order of 10(-5) cm(2) (Vs)(-1). (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Notes: Physics School Meeting of the German-Physical-Society, Bad Honnef, GERMANY, SEP 23-28, 2007
Abstract: We present a detailed study of the thickness dependence of the photoluminescence of ultrathin organic films on Au(111). For the evaluation of the observed intensities a new differential analysis method is introduced. The obtained quantum yield indicates a complete quenching in the thickness range below 4 monolayers (ML) for the two crystal polymorphs and saturates above 12 ML. Several models of energy transfer were applied to our data. We conclude that only consideration of both exciton diffusion and dissociation at the organic-metal interface in combination with classical energy transfer results in a complete and satisfactory description.
Abstract: We demonstrate enhanced light outcoupling from bottom emitting monochrome high- efficiency red, green, and blue organic light emitting diodes by adding silver layers on the indium tin oxide (ITO) anode. The devices contain the phosphorescent emitting dyes tris(2- phenylpyridine )iridium and iridium (III) bis[2- methyldibenzo-(f, h)quinoxaline] acetylacetonate , and the blue singlet emitter 2,2â, 7,7â- tetrakis 2,2- (diphenylvinyl) spiro- 9,9â-bifluorene. We follow the p- i- n doping concept to increase the power efficiencies. We reach 81 lm/ W for red, 101 lm/ W for green, and 4.0 lm/ W for blue color. These efficiencies are improved by up to a factor of 2.3 compared to standard ITO devices, which we attribute to microcavity amplification between the cathode and the Ag layer. (c) 2008 American Institute of Physics.
Abstract: Thin films of the perylene derivative N,Nâ-dimethylperylene-3,4,9,10-bis-dicarboximide (MePTCDI) incorporated in SiO2 matrix at various concentrations are obtained by condensation of host and dye in high vacuum. Photoluminescence spectroscopy is applied to study the spectral properties of the layers. Significant alterations in luminescence spectra in dependence on dye quantity are explained as a consequence of dye aggregation and resonant energy transfer. We demonstrate that the deposition geometry and preparation conditions offer an effective way to reduce the possibilities for non-radiative transitions, thus increasing the photoluminescence quantum efficiency. (c) 2008 Elsevier B.V. All rights reserved.
Abstract: We discuss the importance of appropriate charge carrier confinement and exciton management for the realization of highly efficient and stable organic light-emitting diodes (OLEDs). As an example, we choose red p-i-n-type OLEDs based on the iridium-based electrophosphorescent emitter Ir(MDQ)(2)(acac) doped in alpha-NPD as host material. We show how an appropriate choice of the hole blocking layer material allows external quantum efficiencies as high as 20% for this emitter. At the same time, the display-relevant brightness of 100 cd/m(2) is reached at an operation voltage of only 2.4 V, which is close to the thermodynamic limit. As a result, a high total power efficiency of 37.5 lm/W at 100 cd/m(2) is reached. In a further step, we study the influence of the blocker materials on device lifetime. We investigate the chemical reactions causing the degradation process by use of matrix assisted laser desorption time-of-flight mass spectrometry. It can be shown that discovered degradation reactions can be suppressed by an appropriate choice of the hole blocking material. (c) 2008 American Institute of Physics.
Abstract: We report on organic p-i-n homojunctions composed of differently doped vacuum-deposited pentacene layers. We observe a remarkably high built-in voltage of 1.65 V. An analysis of the current-voltage characteristics under dark and illuminated conditions reveals that the open-circuit voltage is directly related to the built-in voltage and that the recombination process is influenced by the distinct charge transport properties of electrons and holes in the pentacene film. By a comparison with p-i-p and n-i-n single-carrier homojunctions, deep trap states located around 0.63 eV below the electron transport level are shown to influence the properties.
Abstract: The performance of organic electronic devices, such as organic light emitting diodes, transistors, or organic solar cells, depends critically on the chemical composition of the metal/organic and organic/metal interfaces which inject or extract charges into or from the device. By combining a number of techniques, such as x-ray photoemission spectroscopy (XPS) sputter depth profiling, XPS itself, secondary ion mass spectrometry, and laser desorption/ionization time-of-flight mass spectrometry, we investigate the reasons for differences in charge injection from metallic bottom and top contacts into either preferentially hole or preferentially electron transporting materials. We find that the deposition of metal onto organic semiconductors creates an organic-inorganic mixed interlayer in between the organic bulk material and the metal. In the case of electron injection, this interlayer acts as highly doped injection layer, while for hole injection, no significant improvement is visible. In addition to the self-doping, some cathode materials form partially oxidized metal-on-organic interfaces caused by oxygen in the residual gas. Depending on the evaporation conditions, the oxygen content varies. The effect of the oxygen incorporation, the origin, and the binding behavior in between the metal-on-organic interlayer is investigated and discussed. In contrast, organic materials evaporated on top of metals create an abrupt interface, where no self-doping effect is observed.
Abstract: Recently, several memory devices based on organic semiconductors were reported. In most cases, single organic layers were employed, and the switching mechanisms were not fully understood. Here, we report on a novel device structure based on electrically doped organic heterostructures showing highly stable memory behavior. The organic layers are embedded between a bottom indium tin oxide and a top metal contact and form a quantum well-like structure. The device shows a hysteresis in the current-voltage characteristics and well-defined switching behavior. We achieved reproducible bistable electrical switching and stable memory phenomena in these organic multilayer heterostructure devices. The two states were retained for up to several days without noticeable decrease of their on/off ratio. Further, it was found that the ratio of the on/off current depends on the writing voltage, allowing the use as multistate memory.
Abstract: The authors present an alternative method for the determination of the charge carrier mobility of organic thin films. In contrast to known methods like space charge limited current, field effect transistor and time of flight approaches, we determine the charge carrier mobility of a mixed film, serving as emission layer, within the conventional multilayer device architecture. We make use of a strong delayed generation feature in the electroluminescent decay, following a short voltage pump pulse in a time-resolved set-up. Taking into account the preferentially electron transporting properties of the film, the mobility of a N,N-di(naphthalen-2-yl)-N,N'-diphenyl-benzidine (NPB):tris(1 phenylisoquinoline) iridium [Ir(piq)(3)] mixed film is found to be on the order of 10(-5) cm(2) (Vs)(-1). (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: We present an organic microcavity laser with wide tunability in the range of 595-650 nm, having a threshold as low as 2 nJ/pulse. The active medium consists of the organic composite tris(8-hydroxy quinoline) aluminum (Alq(3)) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) embedded in between two distributed Bragg reflectors. The inhomogeneously broadened emission from DCM is exploited for lasing by means of a tunable Fabry-Perot-type interferometer. Continuous tuning is achieved by varying the thickness of the active layer. The device shows a high photostability under direct excitation in the absorption band of the guest molecule, thus, allowing to monitor the optical gain spectrum of the active medium. (c) 2008 American Institute of Physics.
Abstract: We report on organic p-i-n homojunctions composed of differently doped vacuum-deposited pentacene layers. We observe a remarkably high built-in voltage of 1.65 V. An analysis of the current-voltage characteristics under dark and illuminated conditions reveals that the open-circuit voltage is directly related to the built-in voltage and that the recombination process is influenced by the distinct charge transport properties of electrons and holes in the pentacene film. By a comparison with p-i-p and n-i-n single-carrier homojunctions, deep trap states located around 0.63 eV below the electron transport level are shown to influence the properties.
Abstract: Light-emitting diodes based on organic semiconductors show promising features for display and lighting applications. A vertical in-line deposition technique for organic light-emitting diode (OLED) manufacturing was developed. OLED devices with electrically doped transport layers show low operating voltage, high efficiency and long lifetime. The preparation of p-i-n type devices was performed with the in-line fabrication tool resulting in highly efficient OLED with low operating voltage. The lowest operating voltage was achieved for green diodes with 2.9 V for 100 cd/m(2). This demonstrates that the p-i-n device concept can be applied under manufacturing conditions. In-line manufactured highly efficient red, green and blue OLED are presented. One important aspect for fabrication cost is the used ground contact, which is commonly made by indium tin oxide (ITO). For low cost fabrication an alternative for ITO has to be used. In this work, ITO was replaced by aluminium doped zinc oxide (ZAO). The results are comparable to OLEDs using ITO as transparent conductive oxide. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: We present a detailed study of the thickness dependence of the photoluminescence of ultrathin organic films on Au(111). For the evaluation of the observed intensities a new differential analysis method is introduced. The obtained quantum yield indicates a complete quenching in the thickness range below 4 monolayers (ML) for the two crystal polymorphs and saturates above 12 ML. Several models of energy transfer were applied to our data. We conclude that only consideration of both exciton diffusion and dissociation at the organic-metal interface in combination with classical energy transfer results in a complete and satisfactory description.
Abstract: In this work we study the input-output characteristics of an organic vertical cavity surface emitting laser (OVCSEL) with an active layer of Alq3:DCM over 3 orders of magnitude in pump intensity. We find that lasing from this standard inhomogeneously broadened gain medium cannot be described using a single lasing threshold value. Instead, an inhomogeneously broadened distribution of spontaneous emission factors, beta, must be employed in order to correctly model the measured input-output characteristics.
Abstract: We show the impact of lateral Structuring on the optical properties of organic thin films in microcavities. The active material tris-(8-hydroxy quinoline) aluminium (Alq3) was deposited using shadow mask evaporation, resulting in various micron sized shapes. A simplified box model is employed to describe the steady state electric field distribution in these mesa cavities. The complex mode structure measured in transmission and photoluminescence is confirmed by a transfer matrix calculation and a Fourier transform of the internal electric field distribution.
Abstract: A power conversion efficiency of 3.4% with an open-circuit voltage of 1 V was recently demonstrated in a thin film solar cell utilizing fullerene C(60) as acceptor and a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor [K. Schulze , Adv. Mater. (Weinheim, Ger.) 18, 2872 (2006)]. This prompted us to systematically study the energy- and electron transfer processes at the oligothiophene:fullerene heterojunction for a homologous series of these oligothiophenes. Cyclic voltammetry and ultraviolet photoelectron spectroscopy data show that the heterojunction is modified due to tuning of the highest occupied molecular orbital energy for different oligothiophene chain lengths, while the lowest unoccupied molecular orbital energy remains essentially fixed due to the presence of electron-withdrawing end groups (dicyanovinyl) attached to the oligothiophene. Use of photoinduced absorption (PA) allows the study of the electron transfer process at the heterojunction to C(60). Quantum-chemical calculations performed at the density functional theory and/or time-dependent density functional theory level and cation absorption spectra of diluted DCVnT provide an unambiguous identification of the transitions observed in the PA spectra. Upon increasing the effective energy gap of the donor-acceptor pair by increasing the ionization energy of the donor, photoinduced electron transfer is eventually replaced with energy transfer, which alters the photovoltaic operation conditions. The optimum open-circuit voltage of a solar cell is thus a trade-off between efficient charge separation at the interface and maximized effective gap. It appears that the open-circuit voltages of 1.0-1.1 V in our solar cell devices have reached an optimum since higher voltages result in a loss in charge separation efficiency.
Abstract: White organic light emitting diodes harvesting triplet excitons from the fluorescent blue emitter N,N(')-di-1-naphthalenyl-N,N(')-diphenyl-[1,1':4',1 '':4 '',1'"-quaterphenyl]-4,4'-diamine (4P-NPD) are presented. Direct doping of the phosphorescent orange iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) into 4P-NPD results in a strongly reduced efficiency roll-off as compared to separate emission layers, and yields 49.3 lm W(-1) total external power efficiency (24.1% quantum efficiency) at a luminance of 1000 cd m(-2) [CIE 1931 chromaticity coordinates (0.49,0.41)], measured in an integrating sphere. Introduction of an exciton balancing interlayer improves the chromaticity (0.43,0.43) toward the CIE illuminant A warm white point and keeps a high efficiency of 40.7 lm W(-1), 20.3%. (c) 2008 American Institute of Physics.
Abstract: We demonstrate near-infrared (NIR) organic light-emitting diodes containing the phthalocyanines of copper (CuPc), palladium (PdPc), and platinum (PtPc) as emitting material. The devices show NIR emission from the triplet excitonic states of those phthalocyanines at 1095, 1025, and 966 nm, respectively. A yellow singlet emitter serves as host for the emitter materials, reducing triplet exciton quenching and improving energy transfer to the emitter. Using the emitter PtPc as guest and the yellow singlet emitter as host, an external quantum efficiency of 0.3% is achieved for infrared light emission at 966 nm. Due to the use of electrically doped charge transport layers, operation at voltages significantly below 3 V is possible. Light output reaches 80 mu W/cm(2) at a current density of 140 mA/cm(2). (c) 2008 American Institute of Physics.
Abstract: We discuss the importance of appropriate charge carrier confinement and exciton management for the realization of highly efficient and stable organic light-emitting diodes (OLEDs). As an example, we choose red p-i-n-type OLEDs based on the iridium-based electrophosphorescent emitter Ir(MDQ)(2)(acac) doped in alpha-NPD as host material. We show how an appropriate choice of the hole blocking layer material allows external quantum efficiencies as high as 20% for this emitter. At the same time, the display-relevant brightness of 100 cd/m(2) is reached at an operation voltage of only 2.4 V, which is close to the thermodynamic limit. As a result, a high total power efficiency of 37.5 lm/W at 100 cd/m(2) is reached. In a further step, we study the influence of the blocker materials on device lifetime. We investigate the chemical reactions causing the degradation process by use of matrix assisted laser desorption time-of-flight mass spectrometry. It can be shown that discovered degradation reactions can be suppressed by an appropriate choice of the hole blocking material. (c) 2008 American Institute of Physics.
Abstract: We demonstrate enhanced light outcoupling from bottom emitting monochrome high- efficiency red, green, and blue organic light emitting diodes by adding silver layers on the indium tin oxide (ITO) anode. The devices contain the phosphorescent emitting dyes tris(2- phenylpyridine )iridium and iridium (III) bis[2- methyldibenzo-(f, h)quinoxaline] acetylacetonate , and the blue singlet emitter 2,2', 7,7'- tetrakis 2,2- (diphenylvinyl) spiro- 9,9'-bifluorene. We follow the p- i- n doping concept to increase the power efficiencies. We reach 81 lm/ W for red, 101 lm/ W for green, and 4.0 lm/ W for blue color. These efficiencies are improved by up to a factor of 2.3 compared to standard ITO devices, which we attribute to microcavity amplification between the cathode and the Ag layer. (c) 2008 American Institute of Physics.
Abstract: We compare between two derivatives of dicyanovinyl-quinquethiophenes with different alkyl side chain lengths. Both materials show comparable open circuit voltages V(oc) in organic solar cells with fullerene C(60) as acceptor, as expected since they have the same highest occupied molecular orbital energy. However, differences in the current-voltage-characteristics, particularly in the fill factor, are observed. We analyze both derivatives in hole-only devices and find a difference in the hole injection between the doped hole transport layer and the oligothiophenes. Additionally, we determine the hole mobility of the two materials and explain the different behaviors of the two materials in solar cells. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2990071]
Abstract: We discuss exciton transport and energy transfer in organic solar cells based on dicyanovinyl-terthiophene (DCV3T). Time-resolved surface luminescence quenching experiments on double layers of DCV3T and zinc-phthalocyanine as luminescence quencher are analyzed in the framework of a three-level luminescence model with an initially absorbing state of short lifetime that relaxes to a longer living emitting state. Luminescence quenching of the emitting state is assigned to Forster-type energy transfer with an apparent Forster radius of 2.1 nm, which is in accordance with the Forster radius obtained from the spectral overlap integral. A diffusion based analysis for the emitting state yields a diffusion length of L(e)=6.9 nm. The short living initial state is quenched by diffusion of the excitons to the interface with a diffusion length of L(a)=5.3 nm. External quantum efficiency measurements of the photocurrent in a corresponding organic solar cell structure evaluated with a two-level diffusion model give a diffusion length of L(EQE)=6.0 nm, whereas a two-level model for stationary luminescence quenching yields a diffusion length of L(cw)=12 nm. This suggests that only one type of the excitons contributes to the photocurrent. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2976317]
Abstract: In this paper we focus on the current status of organic solar cells based on small molecules. Since their discovery, much progress has been made, and the main steps are highlighted that led to the current state-of-the-art devices. However, organic solar cells still need to be improved further, and the main strategies for improving the power conversion efficiency, namely raising the open circuit voltage V(oc) and increasing the short circuit current density J(sc), are discussed. In theory, power conversion efficiencies of around 15% should be possible with a single heterojunction; for higher efficiencies, stacked solar cell concepts have to be employed.
Abstract: The performance of organic electronic devices, such as organic light emitting diodes, transistors, or organic solar cells, depends critically on the chemical composition of the metal/organic and organic/metal interfaces which inject or extract charges into or from the device. By combining a number of techniques, such as x-ray photoemission spectroscopy (XPS) sputter depth profiling, XPS itself, secondary ion mass spectrometry, and laser desorption/ionization time-of-flight mass spectrometry, we investigate the reasons for differences in charge injection from metallic bottom and top contacts into either preferentially hole or preferentially electron transporting materials. We find that the deposition of metal onto organic semiconductors creates an organic-inorganic mixed interlayer in between the organic bulk material and the metal. In the case of electron injection, this interlayer acts as highly doped injection layer, while for hole injection, no significant improvement is visible. In addition to the self-doping, some cathode materials form partially oxidized metal-on-organic interfaces caused by oxygen in the residual gas. Depending on the evaporation conditions, the oxygen content varies. The effect of the oxygen incorporation, the origin, and the binding behavior in between the metal-on-organic interlayer is investigated and discussed. In contrast, organic materials evaporated on top of metals create an abrupt interface, where no self-doping effect is observed.
Abstract: One key problem in optimizing organic solar cells is to maximize the absorption of incident light and to keep the charge carrier transport paths as short as possible in order to minimize recombination losses during the charge carrier extraction. The large versatility of organic semiconductors and compositions requires specific optimization of each system. Due to the small thickness of the functional layers in the order of several ten nanometres, coherent optics has to be considered and therefore interference effects play a dominant role. Here we present and discuss concepts for light trapping in organic solar cells. These are wide gap layers in planar solar cells, folded solar cell architectures benefiting from the illumination under inclined incident angles and multiple reflections and absorptions as well as diffraction gratings embossed into the photoactive layer. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: In this contribution, it is shown that the method of laser-desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) is a powerful technique for analyzing complete organic devices, such as organic light-emitting diodes (OLEDs) or organic solar cells. LDI-TOF-MS has the potential to analyze fully processed organic devices without special pretreatment such as dissolving the device, peeling off, the metal, cathode, or using additional matrix materials. Thus, devices may be analysed as they are with a minimum of measurement artefacts. It is demonstrated that the method allows an analysis of complex organic multilayer devices, their composition, and incorporated impurities. It even allows possible electrochemical reaction products caused by device degradation to be analyzed. Thus, LDI-TOF-MS has major advantages compared to measurements of dissolved samples. As an example, the identification of all of the materials used in a complete OLED is shown Furthermore, a detailed chemical analysis of long-term driven OLEDs, including the detection of degradation products, is presented. From these data, several degradation mechanisms can be distinguished.
Abstract: We show that ultrathin metal layers (Ag or Al/Ag) are feasible as transparent top contacts for zinc phthalocyanine: C(60) bulk-heterojunction inverted organic solar cells thermally evaporated on glass substrates. Furthermore, it is demonstrated that the introduction of an organic capping layer drastically increases light incoupling and photon harvesting, in accordance with optical simulations. Proof of principle tin-doped indium oxide (ITO)-free solar cells employing a transparent metal contact and a capping layer reach efficiencies of 1.06%, compared to 0.69% without addition of the capping layer. (c) 2008 American Institute of Physics.
Abstract: The implementation of proper charge carrier transport materials in p-i-n type organic solar cells strongly influences the device performance. Our investigation focuses on the substitution of the standard layer sequence used at the side of electron transport, usually consisting of either C(60)/exciton blocking layer/metal or n-C(60)/metal by a layer sequence including a window layer. Here, we evaluate the transparent electron transport material 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), which guarantees a loss free charge extraction from the active materials due to a good energy level alignment as well as exciton blocking due to its wide bandgap. It is demonstrated that upon the exchange of the electron transport layer n-C(60) for n-NTCDA, the solar power conversion efficiency of a p-i-n device can be increased by 10%. (C) 2008 American Institute of Physics.
Abstract: The aim of this article is to investigate the origin of the open circuit voltage (V(oc)) in organic heterojunction solar cells. The studied devices consist of buckminsterfullerene C(60) as acceptor material and an oligophenyl-derivative 4,4'-bis-(N,N-diphenylamino)quaterphenyl (4P-TPD) as donor material. These photoactive materials are sandwiched between indium tin oxide and p-doped hole transport layers. Using two different p-doped hole transport layers, the built-in voltage of the solar cells is independently changed from the metal contacts. The influence of the built-in voltage on the V(oc) is investigated in bulk and planar heterojunctions. In bulk heterojunctions, in which doped transport layers border directly on the photoactive blend layer, V(oc) cannot exceed the built-in voltage significantly. Though, in planar heterojunctions, V(oc), is identical with the splitting of quasi-Fermi levels at the donor-acceptor interface and is thus primarily determined by the difference of the lowest unoccupied molecular orbital of C(60) and the highest occupied molecular orbital of 4P-TPD. In planar heterojunctions, the open circuit voltage can exceed the built-in voltage. Furthermore, the investigations show that the efficiency of organic solar cells can be improved by using p-doped charge transport layers with optimized energy level alignment to the active materials. The optimized planar heterojunction shows a fill factor of up to 65.5% and a V(oc) of 0.95 V. For solar cells with insufficient energy level alignment between the photoactive layer system and the hole transport layer, a reduced V(oc) in bulk heterojunction cells and a characteristic S shape of the I-V characteristics in planar heterojunction cells are observed. (C) 2008 American Institute of Physics.
Abstract: We report on the use of a thin layer of poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as anode for bottom emission organic light emitting diodes (OLEDs). The combination of polymer anodes with OLEDs having either electrically doped or undoped hole transport layers in direct contact with the polymer is shown. We discuss the impact of the annealing conditions of the polymer on the OLED lifetime in comparison to indium tin oxide anodes. Supported by a differential thermal analysis of PEDOT: PSS, a strong influence of residual water in the polymer on the device lifetime is found. Additional heating of the polymer anode in a dry ambient prior to OLED deposition is necessary to achieve high device lifetimes. At a luminance of 260 cd/m(2), pin-OLEDs on a PEDOT:PSS anode show no measurable device degradation during 5200 h of operation. (c) 2008 American Institute of Physics.
Abstract: Thin films of the perylene derivative N,N'-dimethylperylene-3,4,9,10-bis-dicarboximide (MePTCDI) incorporated in SiO2 matrix at various concentrations are obtained by condensation of host and dye in high vacuum. Photoluminescence spectroscopy is applied to study the spectral properties of the layers. Significant alterations in luminescence spectra in dependence on dye quantity are explained as a consequence of dye aggregation and resonant energy transfer. We demonstrate that the deposition geometry and preparation conditions offer an effective way to reduce the possibilities for non-radiative transitions, thus increasing the photoluminescence quantum efficiency. (c) 2008 Elsevier B.V. All rights reserved.
Abstract: White organic light emitting diodes harvesting triplet excitons from the fluorescent blue emitter N,N-â-di-1-naphthalenyl-N,N-â-diphenyl-[1,1â:4â,1 ââ:4 ââ,1ââ-quaterphenyl]-4,4â-diamine (4P-NPD) are presented. Direct doping of the phosphorescent orange iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) into 4P-NPD results in a strongly reduced efficiency roll-off as compared to separate emission layers, and yields 49.3 lm W-1 total external power efficiency (24.1% quantum efficiency) at a luminance of 1000 cd m(-2) [CIE 1931 chromaticity coordinates (0.49,0.41)], measured in an integrating sphere. Introduction of an exciton balancing interlayer improves the chromaticity (0.43,0.43) toward the CIE illuminant A warm white point and keeps a high efficiency of 40.7 lm W-1, 20.3%. (c) 2008 American Institute of Physics.
Abstract: New 2,2'-difluoro-1,3,2-dioxaborines with pronounced electron accepting properties have been prepared by reaction of quinizarines/halo-substituted quinizarines with BF3 center dot Et2O. From the results of electrochemical analysis, the electron affinities (EAs) of 1,3,2-dioxaborines can be increased when the hydrogen atoms are replaced by halogen ones. Furthermore, conductance of zinc phthalocyanine (ZnPc), a most versatile organic (photo)conductors used in organic light-emitting diodes (OLEDs), can be enhanced greatly when 4,9,10,11,12-pentachloro-2,2,7,7-tetrafluoro-2,7-dihydro-1,3,6,8-tetraox a-2,7-dibora-benzo[e]pyrene (4c) doped as charge-transport materials which suggesting halogen substitution is a versatile tool for creating efficient new p-dopants. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: We investigate the thickness-dependent dielectric function of ultrathin titanyl phthalocyanine (TiOPc) layers on Au(111) by in situ spectroscopy during film growth. The absorption features reveal a complex structural development of the film, described by the observation of different crystal phases. The experiment allows addressing the thickness dependence of the oscillator strength in ultrathin films for the first time. Surprisingly, the oscillator strength increases with thickness by more than a factor of 3 and does not saturate until a thickness of 20 monolayers is reached.
Abstract: The authors investigate phosphorescent organic light emitting diodes comprising mixed films of fac tris(2-phenylpyridine) iridium dispersed in 4,4(â),4(â)-tris(N-carbazolyl)-triphenylamine (TCTA) as emission layer (EML). Based on the results of photoluminescence experiments, they intermit the EML with thin neat layers of TCTA acting as an exciton blocking layer inside the EML which suppresses triplet-triplet annihilation. They show that this EML structure leads to an improved roll-off behavior: Starting at the initial external quantum efficiencies (IEQE) of 15.8% and 14.4% at low brightness for the reference and interlayer device, respectively, those structures yield critical current densities j(c) of 140 mA/cm(2) and 270 mA/cm(2) defining the current density of half-value IEQE.(c) 2007 American Institute of Physics.
Abstract: Carrier transport in organic films is usually dominated by hopping process, leading to different temperature dependence from that of inorganic crystals. The aurhors demonstrate that n-doped C-60 films show temperature dependence analogous to inorganic semiconductors. At low temperatures, the conductivity increases with temperature, around room temperature, a maximum is reached and then the conductivity decreases. These observations are confirmed by the dependence of mobility on doping level. In contrast to previous reports for organic thin films, the C-60 films show a decrease of mobility with increasing doping levels, i.e., they follow the well-known Matthiessen rule which is generally observed in inorganic semiconductors.
Abstract: The authors compare organic solar cells using two different transparent conductive oxides as anode: indium tin oxide (ITO) and three kinds of aluminum doped zinc oxide (ZAO). These anodes with different work functions are used for small molecule photovoltaic devices based on an oligothiophene derivative as donor and fullerene C-60 as acceptor molecule. It turns out that cells on ITO and ZAO have virtually identical properties. In particular, the authors demonstrate that the work function of the anode does not influence the V-oc of the photovoltaic device due to the use of doped transport layers. (C) 2007 American Institute of Physics.
Abstract: Using ultraviolet photoemission spectroscopy, we investigated the energy level alignment at the interfaces of typical anodes used in organic electronics, indium tin oxide (ITO) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), with the oligomeric hole transport material N,N,N-â,N-â-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD), and studied the influence of electrical interface doping by the strong electron acceptor tetrafluoro tetracyanoquinodimethane (F-4-TCNQ). The fundamentally different anode materials with work functions of 4.40 eV (ITO) and 4.85 eV (PEDOT:PSS) show different hole injection barriers, which also depend on the thickness of the F-4-TCNQ interface dopant layer. PEDOT:PSS anodes exhibit a consistently lower hole injection barrier to MeO-TPD compared to ITO by 0.1 eV. We attribute this low hole injection barrier to additional charge transfer reactions at the PEDOT:PSS/MeO-TPD interface. In contrast, the deposition of the electron acceptor at the interface helps significantly to lower the hole injection barrier for ITO anodes. (C) 2007 American Institute of Physics.
Abstract: The best polymeric solar cells reported so far are based on a so-called bulk heterojunction of a polythiophene as donor and a soluble fullerene derivative as acceptor. However, these cells still suffer from an unsatisfying photovoltage, typically below 0.7 V. Here, we show that we can achieve higher photovoltages using a new terthiophene end-capped with electron withdrawing dicyanovinyl groups (DCV3T) that increase both the ionization energy and even more strongly the electron affinity of the compound. The new material is tested in cells using a photoactive heterojunction to separate the excitones generated in the oligomer and a p-doped wide-gap transport layer. The solar cells show an open circuit voltage of up to 1.04 V and a broad spectral sensitivity band ranging from 420 nm to 620 nm. Solar cells based on such oligothiophenes are promising candidates for stacked organic solar cells tailored to the sun-spectrum. Moreover, we present first examples of a new concept for organic solar cells: By blending DCV3T with fullerene C-60, an enhanced generation of triplet excitons on the oligomer can be achieved via a back and forth transfer of excitons (ping-pong-efffect).
Abstract: We investigate the dissociation and the photochemical behavior of different organic semiconductor materials typically employed in organic light emitting diodes (OLED) and other organic electronic devices, using laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS). The study is subdivided into two parts: The first part is an investigation of several hole conductive materials from the class of triarylamines commonly used in OLEDs and organic solar cells. Upon LDI measurements, this class of materials does not show any photochemical reactions. The second part of the work deals with two typical electron conductive materials: tris(8-hydroxyquinolinato)-aluminum (Alq(3)) and 4,7-diphenyl-1, 10-phenanthroline (BPhen). These materials show photochemical reactions in terms of oligomerization. The individual reaction pathways will be discussed in detail. We compare these reactions by further studying two similar molecules (1,1â-biphenyl-4â-oxy)bis(8-hydroxy-2-methylquinolinato)-aluminum (BAlq) and bathocuproine (BCP). Finally, we discuss a possible relation of the results to degradation mechanisms taking place in organic devices. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: A novel concept for white organic light emitting diodes (OLEDs) enabling the utilization of all electrically generated excitons for light generation is introduced. The key feature is a fluorescent blue emitter with high triplet energy, rendering it possible to harvest its triplet excitons by letting them diffuse to an orange phosphorescent iridium complex.
Abstract: We report on the optical mode structure of laterally confined organic microcavities. For preparation, an organic semiconductor is evaporated through a mask with square sized holes, resulting in photonic dots with approximately 5 mu m diameter. Using a microscope setup, we observe a complex mode structure in transmission and photoluminescence. From the mode mapping, we conclude a strong three-dimensional optical confinement. The near and far field spectra are modeled by transfer matrix calculations and a Fourier transform of the internal electric field distribution, respectively. (C) 2007 American Institute of Physics.
Abstract: Photoinduced and transient absorption spectroscopy is used to study triplet exciton dynamics in thin films of a new thiophene-based oligomer (DCV3T) and blends of DCV3T and fullerene C-60. We find enhanced DCV3T triplet exciton generation in the blend layer, which is explained as an excitonic ping-pong effect: singlet energy transfer from DCV3T to C-60, followed by immediate intersystem crossing to C-60 and triplet exciton back-transfer. Estimations of the rate constants involved show that the ping-pong effect has on overall efficiency close to unity. The singlet-singlet energy transfer from DCV3T to C-60 is demonstrated by efficient quenching of DCV3T luminescence in the blend, leading to sensitized emission of C-60. We discuss a promising new concept of 1 solar cells with an enlarged active-layer thickness based on potentially long-ranged triplet exciton diffusion in combination with efficient intersystem crossing.
Abstract: The authors report the observation of an intense conical emission from an organic microcavity. The emission is characterized experimentally and modeled as a higher order Fabry-Perot mode predicted 45 years ago by Kastler [Appl. Opt. 1, 17 (1962)]. A distinct cone is only visible above the normal mode laser threshold which implies a stimulated emission nature of the off-axial light. The input/output characteristics of normal and cone emissions hint that both originate from one and the same lasing mode. (c) 2007 American Institute of Physics.
Abstract: We investigate quenching processes which contribute to the roll-off in quantum efficiency of phosphorescent organic light-emitting diodes (OLEDâs) at high brightness: triplet-triplet annihilation, energy transfer to charged molecules (polarons), and dissociation of excitons into free charge carriers. The investigated OLEDâs comprise a host-guest system as emission layer within a state-of-the-art OLED structure-i.e., a five-layer device including doped transport and thin charge carrier and exciton blocking layers. In a red phosphorescent device, N,N-â-di(naphthalen-2-yl)- N,N-â-diphenyl-benzidine is used as matrix and tris(1-phenylisoquinoline) iridium [Ir(piq)(3)] as emitter molecule. This structure is compared to a green phosphorescent OLED with a host-guest system comprising the matrix 4,4(â),4(â)-tris (N-carbazolyl)-triphenylamine and the well-known triplet emitter fac-tris(2-phenylpyridine) iridium [Ir(ppy)(3)]. The triplet-triplet annihilation is characterized by the rate constant k(TT) which is determined by time-resolved photoluminescence experiments. To investigate triplet-polaron quenching, unipolar devices were prepared. A certain exciton density, created by continuous-wave illumination, is analyzed as a function of current density flowing through the device. This delivers the corresponding rate constant k(P). Field-induced quenching is not observed under typical OLED operation conditions. The experimental data are implemented in an analytical model taking in account both triplet-triplet annihilation and triplet-polaron quenching. It shows that both processes strongly influence the OLED performance. Compared to the red Ir(piq)(3) OLED, the green Ir(ppy)(3) device shows a stronger efficiency roll-off which is mainly due to a longer phosphorescent lifetime tau and a thinner exciton formation zone w.
Abstract: Interference effects in top-emitting organic light-emitting diodes (OLEDs) containing an air gap of 15 mu m are studied over a range of angles in order to obtain information about the spatial coherence properties. The devices display effects similar to a Fabry-Perot Etalon illuminated by a source with a broad spectral range, but the interference effects diminish rapidly with angle, which is inexplicable with simple thin film optics. The results suggest that this effect is due to the spatial coherence of the OLED. From an analysis of the interference pattern a number of device parameters can also be determined. (c) 2007 American Institute of Physics.
Abstract: We report on the dynamics of laser emission from an anisotropic organic microcavity filled with the guest-host composite of tris-(8-hydroxy quinoline) aluminium (Alq(3)) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). In a single microcavity, a linesplitting of 0.18 THz between two perpendicularly polarized laser modes is observed. We ascribe this effect to an optical anisotropy in the distributed Bragg reflectors surrounding the organic layer. The temporal behavior of the electromagnetic field is studied by an up-conversion technique and shows an optical beating of 0.18 THz. Two modeling approaches are used to gain insight in the temporal evolution and phase behavior of the two modes. Both point towards the presence of a phase-coupling mechanism in this system.
Notes: Spring Meeting of the German-Physical-Society, Frankfurt, GERMANY, MAR 17, 2006
Abstract: We investigate triplet-triplet annihilation in molecular host-guest systems where triplets are localized on spatially separated guest molecules. Our results indicate that the dominant mechanism of annihilation is single-step long-range (Forster-type) energy transfer between two excited guests. This mechanism leads to a fundamental limit for the efficiency of phosphorescent organic light emitting diodes at high luminance. Our model is confirmed by photoluminescence decay experiments on 2,3,7,8,12,13,17,18-octaethylporphine platinum as guest in a host matrix of 4,4(â)-N,N-â-dicarbazole-biphenyl.
Abstract: Aluminum doped zinc oxide (ZAO) is presented in this letter as an alternative transparent electrode: optimized ZAO films offer excellent parameters for organic light emitting diodes (OLEDs). The ZAO films are applied to various p-i-n-type OLEDs. By using green phosphorescent molecules in a double emitter structure, very high efficiencies were obtained, namely, 54.6 cd/A and 61.5 lm/W for 100 cd/m(2) at 2.78 V. Additionally, white OLEDs on ZAO demonstrated pure white emission independent of the luminance and high efficiencies of 12.6 cd/A and 14.5 lm/W for 100 cd/m(2) at 2.6 V, which is comparable to indium-tin-oxide based white OLEDs. (c) 2007 American Institute of Physics.
Abstract: Up to now, most organic light emitting diodes (OLEDs) have utilized inorganic materials as transport anodes. In this study, we show that conductive polymers are suitable for this purpose as well. Polyaniline anodes, with a conductivity of 200 S/cm, are used to inject holes into the adjacent organic layers. Due to electrical doping of the electron and hole transport layer with an intrinsic emission layer sandwiched in between (pin-OLED), the devices reach high luminance at low voltage. The phosphorescent emitters Ir(MDQ)(2)(acac) and Ir(ppy)(3), as well as the fluorescent emitter Spiro-DPVBi, are implemented within pin-OLEDs using a polyaniline anode. By the use of different host materials, a green double-emitting OLED is demonstrated and compared to the corresponding single emission layer device. Furthermore, a white OLED combining fluorescent and phosphorescent emitting layers is presented, reaching 8.9 lm/W at 1000 cd/m(2). The results demonstrate an efficient charge carrier injection from the polymer into the p-type doped hole transport layer, leading to good power efficiencies of the OLEDs. (c) 2007 American Institute of Physics.
Abstract: The photovoltaic performance of zinc phthalocyanine (ZnPc)/C-60 heterojunctions is improved by alternately depositing ultrathin active layers between the two bulk layers. Using a proper multilayer structure, a 60% improvement in conversion efficiency under AM1.5 illumination is obtained, in comparison with that of flat heterojunctions. Transmission electron microscope investigations reveal an effective phase separation of ZnPc and C-60 and a high degree of crystalline ordering of C-60 in this system. The authors propose that an interpenetrating interface is formed, which facilitates both exciton separation and charge transport, and enhances the photocurrent from ZnPc due to an extended photoactive region. (C) 2007 American Institute of Physics.
Abstract: We investigate the dissociation and the photochemical behavior of different organic semiconductor materials typically employed in organic light emitting diodes (OLED) and other organic electronic devices, using laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS). The study is subdivided into two parts: The first part is an investigation of several hole conductive materials from the class of triarylamines commonly used in OLEDs and organic solar cells. Upon LDI measurements, this class of materials does not show any photochemical reactions. The second part of the work deals with two typical electron conductive materials: tris(8-hydroxyquinolinato)-aluminum (Alq(3)) and 4,7-diphenyl-1, 10-phenanthroline (BPhen). These materials show photochemical reactions in terms of oligomerization. The individual reaction pathways will be discussed in detail. We compare these reactions by further studying two similar molecules (1,1'-biphenyl-4'-oxy)bis(8-hydroxy-2-methylquinolinato)-aluminum (BAlq) and bathocuproine (BCP). Finally, we discuss a possible relation of the results to degradation mechanisms taking place in organic devices. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: A novel concept for white organic light emitting diodes (OLEDs) enabling the utilization of all electrically generated excitons for light generation is introduced. The key feature is a fluorescent blue emitter with high triplet energy, rendering it possible to harvest its triplet excitons by letting them diffuse to an orange phosphorescent iridium complex.
Abstract: We investigate triplet-triplet annihilation in molecular host-guest systems where triplets are localized on spatially separated guest molecules. Our results indicate that the dominant mechanism of annihilation is single-step long-range (Forster-type) energy transfer between two excited guests. This mechanism leads to a fundamental limit for the efficiency of phosphorescent organic light emitting diodes at high luminance. Our model is confirmed by photoluminescence decay experiments on 2,3,7,8,12,13,17,18-octaethylporphine platinum as guest in a host matrix of 4,4'-N,N'-dicarbazole-biphenyl.
Abstract: We investigate triplet-triplet annihilation in molecular host-guest systems where triplets are localized on spatially separated guest molecules. Our results indicate that the dominant mechanism of annihilation is single-step long-range (Forster-type) energy transfer between two excited guests. This mechanism leads to a fundamental limit for the efficiency of phosphorescent organic light emitting diodes at high luminance. Our model is confirmed by photoluminescence decay experiments on 2,3,7,8,12,13,17,18-octaethylporphine platinum as guest in a host matrix of 4,4(')-N,N-'-dicarbazole-biphenyl.
Abstract: The photovoltaic performance of zinc phthalocyanine (ZnPc)/C-60 heterojunctions is improved by alternately depositing ultrathin active layers between the two bulk layers. Using a proper multilayer structure, a 60% improvement in conversion efficiency under AM1.5 illumination is obtained, in comparison with that of flat heterojunctions. Transmission electron microscope investigations reveal an effective phase separation of ZnPc and C-60 and a high degree of crystalline ordering of C-60 in this system. The authors propose that an interpenetrating interface is formed, which facilitates both exciton separation and charge transport, and enhances the photocurrent from ZnPc due to an extended photoactive region. (C) 2007 American Institute of Physics.
Abstract: Interference effects in top-emitting organic light-emitting diodes (OLEDs) containing an air gap of 15 mu m are studied over a range of angles in order to obtain information about the spatial coherence properties. The devices display effects similar to a Fabry-Perot Etalon illuminated by a source with a broad spectral range, but the interference effects diminish rapidly with angle, which is inexplicable with simple thin film optics. The results suggest that this effect is due to the spatial coherence of the OLED. From an analysis of the interference pattern a number of device parameters can also be determined. (c) 2007 American Institute of Physics.
Abstract: We report on the dynamics of laser emission from an anisotropic organic microcavity filled with the guest-host composite of tris-(8-hydroxy quinoline) aluminium (Alq(3)) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). In a single microcavity, a linesplitting of 0.18 THz between two perpendicularly polarized laser modes is observed. We ascribe this effect to an optical anisotropy in the distributed Bragg reflectors surrounding the organic layer. The temporal behavior of the electromagnetic field is studied by an up-conversion technique and shows an optical beating of 0.18 THz. Two modeling approaches are used to gain insight in the temporal evolution and phase behavior of the two modes. Both point towards the presence of a phase-coupling mechanism in this system.
Abstract: Highly conductive polymer anodes are compared to ITO contacts for organic LEDs (OLEDs). The polymer PEDOT.PSS exhibits a conductivity of 500 S cm(-1) and enables improved device performance and lower operating voltages compared to ITO-based devices (see figure). Red, green, and blue OLEDs with polymer anodes achieve device efficiencies of 15.9, 63.5, and 4.0 lm W-1, respectively.
Abstract: We investigate quenching processes which contribute to the roll-off in quantum efficiency of phosphorescent organic light-emitting diodes (OLED's) at high brightness: triplet-triplet annihilation, energy transfer to charged molecules (polarons), and dissociation of excitons into free charge carriers. The investigated OLED's comprise a host-guest system as emission layer within a state-of-the-art OLED structure-i.e., a five-layer device including doped transport and thin charge carrier and exciton blocking layers. In a red phosphorescent device, N,N-'-di(naphthalen-2-yl)- N,N-'-diphenyl-benzidine is used as matrix and tris(1-phenylisoquinoline) iridium [Ir(piq)(3)] as emitter molecule. This structure is compared to a green phosphorescent OLED with a host-guest system comprising the matrix 4,4('),4(')-tris (N-carbazolyl)-triphenylamine and the well-known triplet emitter fac-tris(2-phenylpyridine) iridium [Ir(ppy)(3)]. The triplet-triplet annihilation is characterized by the rate constant k(TT) which is determined by time-resolved photoluminescence experiments. To investigate triplet-polaron quenching, unipolar devices were prepared. A certain exciton density, created by continuous-wave illumination, is analyzed as a function of current density flowing through the device. This delivers the corresponding rate constant k(P). Field-induced quenching is not observed under typical OLED operation conditions. The experimental data are implemented in an analytical model taking in account both triplet-triplet annihilation and triplet-polaron quenching. It shows that both processes strongly influence the OLED performance. Compared to the red Ir(piq)(3) OLED, the green Ir(ppy)(3) device shows a stronger efficiency roll-off which is mainly due to a longer phosphorescent lifetime tau and a thinner exciton formation zone w.
Abstract: Up to now, most organic light emitting diodes (OLEDs) have utilized inorganic materials as transport anodes. In this study, we show that conductive polymers are suitable for this purpose as well. Polyaniline anodes, with a conductivity of 200 S/cm, are used to inject holes into the adjacent organic layers. Due to electrical doping of the electron and hole transport layer with an intrinsic emission layer sandwiched in between (pin-OLED), the devices reach high luminance at low voltage. The phosphorescent emitters Ir(MDQ)(2)(acac) and Ir(ppy)(3), as well as the fluorescent emitter Spiro-DPVBi, are implemented within pin-OLEDs using a polyaniline anode. By the use of different host materials, a green double-emitting OLED is demonstrated and compared to the corresponding single emission layer device. Furthermore, a white OLED combining fluorescent and phosphorescent emitting layers is presented, reaching 8.9 lm/W at 1000 cd/m(2). The results demonstrate an efficient charge carrier injection from the polymer into the p-type doped hole transport layer, leading to good power efficiencies of the OLEDs. (c) 2007 American Institute of Physics.
Abstract: Photoinduced and transient absorption spectroscopy is used to study triplet exciton dynamics in thin films of a new thiophene-based oligomer (DCV3T) and blends of DCV3T and fullerene C-60. We find enhanced DCV3T triplet exciton generation in the blend layer, which is explained as an excitonic ping-pong effect: singlet energy transfer from DCV3T to C-60, followed by immediate intersystem crossing to C-60 and triplet exciton back-transfer. Estimations of the rate constants involved show that the ping-pong effect has on overall efficiency close to unity. The singlet-singlet energy transfer from DCV3T to C-60 is demonstrated by efficient quenching of DCV3T luminescence in the blend, leading to sensitized emission of C-60. We discuss a promising new concept of 1 solar cells with an enlarged active-layer thickness based on potentially long-ranged triplet exciton diffusion in combination with efficient intersystem crossing.
Abstract: Using ultraviolet photoemission spectroscopy, we investigated the energy level alignment at the interfaces of typical anodes used in organic electronics, indium tin oxide (ITO) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), with the oligomeric hole transport material N,N,N-',N-'-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD), and studied the influence of electrical interface doping by the strong electron acceptor tetrafluoro tetracyanoquinodimethane (F-4-TCNQ). The fundamentally different anode materials with work functions of 4.40 eV (ITO) and 4.85 eV (PEDOT:PSS) show different hole injection barriers, which also depend on the thickness of the F-4-TCNQ interface dopant layer. PEDOT:PSS anodes exhibit a consistently lower hole injection barrier to MeO-TPD compared to ITO by 0.1 eV. We attribute this low hole injection barrier to additional charge transfer reactions at the PEDOT:PSS/MeO-TPD interface. In contrast, the deposition of the electron acceptor at the interface helps significantly to lower the hole injection barrier for ITO anodes. (C) 2007 American Institute of Physics.
Abstract: We investigate the thickness-dependent dielectric function of ultrathin titanyl phthalocyanine (TiOPc) layers on Au(111) by in situ spectroscopy during film growth. The absorption features reveal a complex structural development of the film, described by the observation of different crystal phases. The experiment allows addressing the thickness dependence of the oscillator strength in ultrathin films for the first time. Surprisingly, the oscillator strength increases with thickness by more than a factor of 3 and does not saturate until a thickness of 20 monolayers is reached.
Abstract: The best polymeric solar cells reported so far are based on a so-called bulk heterojunction of a polythiophene as donor and a soluble fullerene derivative as acceptor. However, these cells still suffer from an unsatisfying photovoltage, typically below 0.7 V. Here, we show that we can achieve higher photovoltages using a new terthiophene end-capped with electron withdrawing dicyanovinyl groups (DCV3T) that increase both the ionization energy and even more strongly the electron affinity of the compound. The new material is tested in cells using a photoactive heterojunction to separate the excitones generated in the oligomer and a p-doped wide-gap transport layer. The solar cells show an open circuit voltage of up to 1.04 V and a broad spectral sensitivity band ranging from 420 nm to 620 nm. Solar cells based on such oligothiophenes are promising candidates for stacked organic solar cells tailored to the sun-spectrum. Moreover, we present first examples of a new concept for organic solar cells: By blending DCV3T with fullerene C-60, an enhanced generation of triplet excitons on the oligomer can be achieved via a back and forth transfer of excitons (ping-pong-efffect).
Abstract: The authors investigate phosphorescent organic light emitting diodes comprising mixed films of fac tris(2-phenylpyridine) iridium dispersed in 4,4('),4(')-tris(N-carbazolyl)-triphenylamine (TCTA) as emission layer (EML). Based on the results of photoluminescence experiments, they intermit the EML with thin neat layers of TCTA acting as an exciton blocking layer inside the EML which suppresses triplet-triplet annihilation. They show that this EML structure leads to an improved roll-off behavior: Starting at the initial external quantum efficiencies (IEQE) of 15.8% and 14.4% at low brightness for the reference and interlayer device, respectively, those structures yield critical current densities j(c) of 140 mA/cm(2) and 270 mA/cm(2) defining the current density of half-value IEQE.(c) 2007 American Institute of Physics.
Abstract: Carrier transport in organic films is usually dominated by hopping process, leading to different temperature dependence from that of inorganic crystals. The aurhors demonstrate that n-doped C-60 films show temperature dependence analogous to inorganic semiconductors. At low temperatures, the conductivity increases with temperature, around room temperature, a maximum is reached and then the conductivity decreases. These observations are confirmed by the dependence of mobility on doping level. In contrast to previous reports for organic thin films, the C-60 films show a decrease of mobility with increasing doping levels, i.e., they follow the well-known Matthiessen rule which is generally observed in inorganic semiconductors.
Abstract: The authors report the observation of an intense conical emission from an organic microcavity. The emission is characterized experimentally and modeled as a higher order Fabry-Perot mode predicted 45 years ago by Kastler [Appl. Opt. 1, 17 (1962)]. A distinct cone is only visible above the normal mode laser threshold which implies a stimulated emission nature of the off-axial light. The input/output characteristics of normal and cone emissions hint that both originate from one and the same lasing mode. (c) 2007 American Institute of Physics.
Abstract: Aluminum doped zinc oxide (ZAO) is presented in this letter as an alternative transparent electrode: optimized ZAO films offer excellent parameters for organic light emitting diodes (OLEDs). The ZAO films are applied to various p-i-n-type OLEDs. By using green phosphorescent molecules in a double emitter structure, very high efficiencies were obtained, namely, 54.6 cd/A and 61.5 lm/W for 100 cd/m(2) at 2.78 V. Additionally, white OLEDs on ZAO demonstrated pure white emission independent of the luminance and high efficiencies of 12.6 cd/A and 14.5 lm/W for 100 cd/m(2) at 2.6 V, which is comparable to indium-tin-oxide based white OLEDs. (c) 2007 American Institute of Physics.
Abstract: The authors compare organic solar cells using two different transparent conductive oxides as anode: indium tin oxide (ITO) and three kinds of aluminum doped zinc oxide (ZAO). These anodes with different work functions are used for small molecule photovoltaic devices based on an oligothiophene derivative as donor and fullerene C(60) as acceptor molecule. It turns out that cells on ITO and ZAO have virtually identical properties. In particular, the authors demonstrate that the work function of the anode does not influence the V(oc) of the photovoltaic device due to the use of doped transport layers. (C) 2007 American Institute of Physics.
Abstract: Highly conductive polymer anodes are compared to ITO contacts for organic LEDs (OLEDs). The polymer PEDOT.PSS exhibits a conductivity of 500 S cm(-1) and enables improved device performance and lower operating voltages compared to ITO-based devices (see figure). Red, green, and blue OLEDs with polymer anodes achieve device efficiencies of 15.9, 63.5, and 4.0 lm W-1, respectively.
Abstract: We report on the optical mode structure of laterally confined organic microcavities. For preparation, an organic semiconductor is evaporated through a mask with square sized holes, resulting in photonic dots with approximately 5 mu m diameter. Using a microscope setup, we observe a complex mode structure in transmission and photoluminescence. From the mode mapping, we conclude a strong three-dimensional optical confinement. The near and far field spectra are modeled by transfer matrix calculations and a Fourier transform of the internal electric field distribution, respectively. (C) 2007 American Institute of Physics.
Abstract: The authors present time-resolved studies of laser emission from an organic microcavity filled with a guest-host composite of tris(8-hydroxy quinoline) aluminium and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran. A line splitting of 0.18 THz between two perpendicularly polarized lines is observed, resulting from an optical anisotropy in the distributed Bragg reflectors. This particular behavior is attributed to oblique columnar structures shaped by off-axial preparatory conditions. By means of an upconversion setup an oscillation at 5.5 ps period time and phase coupling of the modes are observed. A rate-equation-based approach is utilized for modeling the temporal behavior. (c) 2006 American Institute of Physics.
Abstract: White organic light emitting diodes combining the phosphorescent green and orange-red emitting systems fac tris(2-phenylpyridine) iridium doped 4,4(â),4(â)-tris(N-carbazolyl)-triphenylamine (TCTA) and iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) doped N,N-â-di(naphthalen-1-yl)-N,N-â-diphenyl-benzidine with the blue fluorescent dye 2,2(â),7,7(â)-tetrakis(2,2-diphenylvinyl)spiro-9,9(â)-bifluorene (Spiro-DPVBi) are presented. By introducing a thin layer of coevaporated TCTA and 2,2(â),2(â) (1,3,5-benzenetriyl) tris-[1-phenyl-1H-benzimidazole] between the phosphorescent and the fluorescent region, both singlet and triplet excitons are confined efficiently, whereas charge carriers still pass easily this interlayer. Furthermore, the interlayer suppresses Dexter transfer of the phosphorescent excitons to the nonradiative triplet state of Spiro-DPVBi. Best devices reach a current efficiency of 16.3 cd/A at 100 cd/m(2) and a color rendering index of 85 at warm white CIE chromaticity coordinates of (0.47, 0.42). Due to the use of electrically doped charge transport layers, 100 cd/m(2) are obtained at 2.95 V with a power efficiency of 17.4 lm/W. (c) 2006 American Institute of Physics.
Abstract: The optical outcoupling of top-emitting organic light-emitting diodes (OLEDs) can be improved by a thin dielectric capping layer on top of the transparent cathode. We investigate the emission properties of a set of top-emitting OLEDs with the same device structure, but different organic capping layer thicknesses to understand the capping layer effect. We demonstrate that the distribution pattern of the emitted light from top-emitting OLEDs depends strongly on the capping layer thickness, showing not only a maximum current efficiency enhancement by a factor of 1.38 (up to 78 cd/A in forward direction at 1000 cd/m(2)), but also a quantum efficiency improvement by 35%. This leads to a device efficiency of up to 17.8% and 69 lm/W at a brightness of 1000 cd/m(2). We show that this efficiency enhancement is not due to the redistribution of emitted light, but mainly due to the improvement of outcoupling efficiency by changing the optical structure of the devices with the organic capping layer. The maximum outcoupling efficiency is achieved at a capping layer thickness where the top contact stack (cathode+capping layer) shows low absorption and high reflection. (c) 2006 American Institute of Physics.
Abstract: Organic solar cells based on a new oligothiophene derivative (see figure) and fullerence C-60 exhibit power efficiencies of up to 3.4%. alpha,alphaâ-Bis(2,2-dicyanovinyl)-quinquethiophene (DCV5T) features a reduced optical gap with high absorption coefficients between 450 and 650 nm. Dicyanovinyl substituents on the oligothiophene rings lower the bandgap and increase the ionization energy of these oligomers, enabling the formation of photoactive heterojunctions with C-60 and yielding open-circuit photovoltages as high as 1 V.
Abstract: In organic light emitting diodes (OLEDs) with transparent electrodes, the luminance usually becomes inhomogeneous if the size of the pixel increases above 10 mm. A theoretical model for inhomogeneous voltage and luminance in OLEDs is provided together with an approximate analytical solution for the problem in case of cylindrical symmetry. Experimental observations of inhomogeneous luminance are compared with numerical simulations based on the theoretical model, proving the applicability of the approximations made in the theoretical model. (c) 2006 American Institute of Physics.
Abstract: We demonstrate extremely stable and highly efficient red p-i-n-type organic light emitting diodes (OLEDs) based on an iridium-based electrophosphorescent dye in suitable host materials. The OLEDs reach lifetimes well above 1x10(7) h at 100 cd/m(2) initial luminance and reach at the same time a performance of 12.4% external quantum efficiency. This high lifetime is attributed to a combination of the low current density needed to reach a certain luminance and to the high stability of the materials against both charge carriers and excitons. (c) 2006 American Institute of Physics.
Abstract: We present a study on n doping of C-60 thin films by acridine orange base [3,6-bis(dimethylamino)acridine(AOB)] combining conductivity, field effect, and Seebeck measurements. An increase of more than six orders of magnitude in conductivity is observed for a doping ratio of 6 mol %, accompanied by a decrease in the activation energy from 0.64 to 0.15 eV compared to the undoped C-60. We observe a clear doping effect immediately after sample preparation, but also a further activation by annealing or illumination. The field effect and Seebeck measurements confirm n-type conduction of C-60 thin films and show that deep donor states are formed in AOB-doped C-60 thin films. A field effect mobility of 0.2 cm(2)/V s is achieved for a doping level of 1.8 mol %. Near Infrared (NIR) and Fourier transform infrared (FTIR) spectra demonstrate electron transfer from the dopant to the matrix: For C-60 doped with AOB, C-60(-) is present in NIR absorption and FTIR spectra. On the other hand, a peak corresponding to acridine orange [3,6-bis(dimethylamino)acridinium chloride (AOBH(+))] is also observed in the FTIR spectrum of C-60:AOB, where AOBH(+) corresponds to AOB with one additional proton attached. Electrochemical data of AOB and AOBH(+) in acetontrile suggest that the AOB radical cation is not stable, but is rapidly transformed into a compound with similar properties to AOBH(+). Conductivities of C-60 thin films doped with bis(ethylenedithio)-tetrathiafulvalene were also investigated to confirm that the doping effect of AOB in C-60 does not result from a simple electron transfer from AOB to C-60.(c) 2006 American Institute of Physics.
Abstract: Thin vacuum deposited films of aluminum tris(8-hydroxyquinolinate) (Alq(3)) embedded in SiO2 matrix at concentrations corresponding to a single-molecule distribution are investigated. The spectral properties of the films are studied by luminescence spectroscopy. It is established that the dilution of Alq(3) in the matrix leads to blueshift of the luminescence maximum up to 0.28 eV compared to pure Alq(3) layers. In contrast to the recently reported facial isomer of Alq(3) in the crystalline delta phase, we conclude from our spectral data the observation of facial isomer molecules. (c) 2006 American Institute of Physics.
Abstract: We demonstrate high-efficiency top emitting organic light-emitting diodes employing silver (Ag) for both anode and cathode. Following the p-i-n doping and double emission layer concepts, the devices show a very high efficiency of 50 cd/A at 1000 cd/m2 with a driving voltage of only 2.85 V. The efficiency can be further improved to 78 cd/A by tuning the optical structure with an organic capping layer. A simple explanation based on the transmittance of the top contact cannot explain this efficiency enhancement. Instead, we theoretically show that this capping effect is dependent on the overall optical structure of the device. (c) 2006 American Institute of Physics.
Abstract: Recently, supersonic molecular-beam epitaxy (SuMBE) was invented as an alternative method for the deposition of organic material, using higher kinetic energies for deposition than conventional organic molecular-beam epitaxy (OMBE). Using titanyl phthalocyanine (TiOPc) as a model substance, we show that the SuMBE deposition results in increased crystal quality of the deposited material. This is induced by the high kinetic energy of the molecular-beam in SuMBE, which leads to increased molecular mobility on the surface, resulting in larger crystal sizes and higher crystal quality. Alternatively, similar films as made by OMBE can be deposited by SuMBE at lower substrate temperatures. This temperature reduction may be of interest for the deposition of stacked organic devices on underlying heat sensitive layers, as they are quite common in organic electronic devices. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Femtosecond pump-probe spectroscopy is applied to thin films of the quasi-one-dimensional organic semiconductor 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). We present transient absorption spectra over a broad spectral range. Ultrafast intraband relaxation in the S, manifold towards the border of the Brillouin zone is shown to depend on temperature and excitation density. The intraband relaxation time is of the order of 100 fs. At high excitation densities (> 10(19) cm(-3)), the major de-excitation mechanism for the relaxed excitons is exciton-exciton annihilation. The experimental decay dynamics can be explained very well by two alternative annihilation models: one-dimensional diffusion limited bimolecular recombination or single-step long range Forster-type annihilation. In contrast, a three-dimensional diffusion limited annihilation model is significantly inferior. For all three models, we extract annihilation rates, diffusion constants, diffusion lengths, and Forster radii for room and liquid Helium temperature. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: We present a comprehensive experimental and theoretical study of the optical properties of matrix-isolated molecules of the two perylene derivatives N,N-â-dimethylperylene-3,4,9,10-dicarboximide (MePTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). A solid solution of the dyes in an SiO2 matrix exhibits monomer-like behavior. Transient absorption pump-probe spectroscopy in the range 1.2-2.6 eV has been performed on an ultrafast time scale. The differential transmittance reveals contributions from ground-state bleaching, stimulated emission, and excited-state absorption. Both systems exhibit broad excited-state absorption features below 2.0 eV with a clear peak around 1.8 eV. The spectra can be consistently explained by the results of quantum-chemical calculations. We have applied both the coupled cluster singles and doubles (CCSD) model and the multireference-determinant single and double configuration-interaction (MRD-CI) technique on the basis of the intermediate neglect of differential overlap (INDO) Hamiltonian. The results are insensitive to whether the geometry is optimized for the electronic ground state or first excited state. The experimental polarization anisotropies for the two major transitions are in agreement with the calculated polarizations.
Abstract: The authors compare red, green, and blue top- and conventional bottom-emitting organic light-emitting diodes (OLEDs) based on iridium complexes. Surprisingly, red top-emitting OLEDs are much more efficient than their bottom-emitting counterpart, while for green and blue devices, top and bottom emitters are comparable. The authors explain these results by an improved internal quantum efficiency only in the red top-emitting OLED: Due to strong microcavity effect, spontaneous emission of emitters in all top-emitting devices is enhanced. However, only the internal quantum yield of the red emitters is then improved due to its comparable radiative and nonradiative decay rates. They also show that the efficiency roll-off at higher current densities is reduced in top-emitting OLEDs. (c) 2006 American Institute of Physics.
Abstract: The authors compare red, green, and blue top- and conventional bottom-emitting organic light-emitting diodes (OLEDs) based on iridium complexes. Surprisingly, red top-emitting OLEDs are much more efficient than their bottom-emitting counterpart, while for green and blue devices, top and bottom emitters are comparable. The authors explain these results by an improved internal quantum efficiency only in the red top-emitting OLED: Due to strong microcavity effect, spontaneous emission of emitters in all top-emitting devices is enhanced. However, only the internal quantum yield of the red emitters is then improved due to its comparable radiative and nonradiative decay rates. They also show that the efficiency roll-off at higher current densities is reduced in top-emitting OLEDs. (c) 2006 American Institute of Physics.
Abstract: Thin vacuum deposited films of aluminum tris(8-hydroxyquinolinate) (Alq(3)) embedded in SiO2 matrix at concentrations corresponding to a single-molecule distribution are investigated. The spectral properties of the films are studied by luminescence spectroscopy. It is established that the dilution of Alq(3) in the matrix leads to blueshift of the luminescence maximum up to 0.28 eV compared to pure Alq(3) layers. In contrast to the recently reported facial isomer of Alq(3) in the crystalline delta phase, we conclude from our spectral data the observation of facial isomer molecules. (c) 2006 American Institute of Physics.
Abstract: We present a comprehensive experimental and theoretical study of the optical properties of matrix-isolated molecules of the two perylene derivatives N,N-'-dimethylperylene-3,4,9,10-dicarboximide (MePTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). A solid solution of the dyes in an SiO2 matrix exhibits monomer-like behavior. Transient absorption pump-probe spectroscopy in the range 1.2-2.6 eV has been performed on an ultrafast time scale. The differential transmittance reveals contributions from ground-state bleaching, stimulated emission, and excited-state absorption. Both systems exhibit broad excited-state absorption features below 2.0 eV with a clear peak around 1.8 eV. The spectra can be consistently explained by the results of quantum-chemical calculations. We have applied both the coupled cluster singles and doubles (CCSD) model and the multireference-determinant single and double configuration-interaction (MRD-CI) technique on the basis of the intermediate neglect of differential overlap (INDO) Hamiltonian. The results are insensitive to whether the geometry is optimized for the electronic ground state or first excited state. The experimental polarization anisotropies for the two major transitions are in agreement with the calculated polarizations.
Abstract: Recently, supersonic molecular-beam epitaxy (SuMBE) was invented as an alternative method for the deposition of organic material, using higher kinetic energies for deposition than conventional organic molecular-beam epitaxy (OMBE). Using titanyl phthalocyanine (TiOPc) as a model substance, we show that the SuMBE deposition results in increased crystal quality of the deposited material. This is induced by the high kinetic energy of the molecular-beam in SuMBE, which leads to increased molecular mobility on the surface, resulting in larger crystal sizes and higher crystal quality. Alternatively, similar films as made by OMBE can be deposited by SuMBE at lower substrate temperatures. This temperature reduction may be of interest for the deposition of stacked organic devices on underlying heat sensitive layers, as they are quite common in organic electronic devices. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: We demonstrate high-efficiency top emitting organic light-emitting diodes employing silver (Ag) for both anode and cathode. Following the p-i-n doping and double emission layer concepts, the devices show a very high efficiency of 50 cd/A at 1000 cd/m2 with a driving voltage of only 2.85 V. The efficiency can be further improved to 78 cd/A by tuning the optical structure with an organic capping layer. A simple explanation based on the transmittance of the top contact cannot explain this efficiency enhancement. Instead, we theoretically show that this capping effect is dependent on the overall optical structure of the device. (c) 2006 American Institute of Physics.
Abstract: We report the experimental observation of polarization splitting at normal incidence and terahertz oscillations of transmitted light from a single planar microcavity. Optical anisotropy in the SiO2/TiO2-dielectric mirrors leads to two perpendicularly polarized transmission modes. We ascribe the anisotropy to oblique columnar structures in the dielectrics resulting from off-axial growth of the microcavity structure. We apply an up-conversion setup for temporally and spectrally resolved measurements and obtain a corresponding beating of 1.25 THz. Time resolved measurements yield a cavity photon lifetime of 0.65 ps, corresponding to a Q value of 1600. To explain our observations we introduce a Fourier-transform based analytical model. (c) 2006 American Institute of Physics.
Abstract: Femtosecond pump-probe spectroscopy is applied to thin films of the quasi-one-dimensional organic semiconductor 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). We present transient absorption spectra over a broad spectral range. Ultrafast intraband relaxation in the S, manifold towards the border of the Brillouin zone is shown to depend on temperature and excitation density. The intraband relaxation time is of the order of 100 fs. At high excitation densities (> 10(19) cm(-3)), the major de-excitation mechanism for the relaxed excitons is exciton-exciton annihilation. The experimental decay dynamics can be explained very well by two alternative annihilation models: one-dimensional diffusion limited bimolecular recombination or single-step long range Forster-type annihilation. In contrast, a three-dimensional diffusion limited annihilation model is significantly inferior. For all three models, we extract annihilation rates, diffusion constants, diffusion lengths, and Forster radii for room and liquid Helium temperature. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: We present a study on n doping of C-60 thin films by acridine orange base [3,6-bis(dimethylamino)acridine(AOB)] combining conductivity, field effect, and Seebeck measurements. An increase of more than six orders of magnitude in conductivity is observed for a doping ratio of 6 mol %, accompanied by a decrease in the activation energy from 0.64 to 0.15 eV compared to the undoped C-60. We observe a clear doping effect immediately after sample preparation, but also a further activation by annealing or illumination. The field effect and Seebeck measurements confirm n-type conduction of C-60 thin films and show that deep donor states are formed in AOB-doped C-60 thin films. A field effect mobility of 0.2 cm(2)/V s is achieved for a doping level of 1.8 mol %. Near Infrared (NIR) and Fourier transform infrared (FTIR) spectra demonstrate electron transfer from the dopant to the matrix: For C-60 doped with AOB, C-60(-) is present in NIR absorption and FTIR spectra. On the other hand, a peak corresponding to acridine orange [3,6-bis(dimethylamino)acridinium chloride (AOBH(+))] is also observed in the FTIR spectrum of C-60:AOB, where AOBH(+) corresponds to AOB with one additional proton attached. Electrochemical data of AOB and AOBH(+) in acetontrile suggest that the AOB radical cation is not stable, but is rapidly transformed into a compound with similar properties to AOBH(+). Conductivities of C-60 thin films doped with bis(ethylenedithio)-tetrathiafulvalene were also investigated to confirm that the doping effect of AOB in C-60 does not result from a simple electron transfer from AOB to C-60.(c) 2006 American Institute of Physics.
Abstract: The authors present time-resolved studies of laser emission from an organic microcavity filled with a guest-host composite of tris(8-hydroxy quinoline) aluminium and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran. A line splitting of 0.18 THz between two perpendicularly polarized lines is observed, resulting from an optical anisotropy in the distributed Bragg reflectors. This particular behavior is attributed to oblique columnar structures shaped by off-axial preparatory conditions. By means of an upconversion setup an oscillation at 5.5 ps period time and phase coupling of the modes are observed. A rate-equation-based approach is utilized for modeling the temporal behavior. (c) 2006 American Institute of Physics.
Abstract: In organic light emitting diodes (OLEDs) with transparent electrodes, the luminance usually becomes inhomogeneous if the size of the pixel increases above 10 mm. A theoretical model for inhomogeneous voltage and luminance in OLEDs is provided together with an approximate analytical solution for the problem in case of cylindrical symmetry. Experimental observations of inhomogeneous luminance are compared with numerical simulations based on the theoretical model, proving the applicability of the approximations made in the theoretical model. (c) 2006 American Institute of Physics.
Abstract: The optical outcoupling of top-emitting organic light-emitting diodes (OLEDs) can be improved by a thin dielectric capping layer on top of the transparent cathode. We investigate the emission properties of a set of top-emitting OLEDs with the same device structure, but different organic capping layer thicknesses to understand the capping layer effect. We demonstrate that the distribution pattern of the emitted light from top-emitting OLEDs depends strongly on the capping layer thickness, showing not only a maximum current efficiency enhancement by a factor of 1.38 (up to 78 cd/A in forward direction at 1000 cd/m(2)), but also a quantum efficiency improvement by 35%. This leads to a device efficiency of up to 17.8% and 69 lm/W at a brightness of 1000 cd/m(2). We show that this efficiency enhancement is not due to the redistribution of emitted light, but mainly due to the improvement of outcoupling efficiency by changing the optical structure of the devices with the organic capping layer. The maximum outcoupling efficiency is achieved at a capping layer thickness where the top contact stack (cathode+capping layer) shows low absorption and high reflection. (c) 2006 American Institute of Physics.
Abstract: White organic light emitting diodes combining the phosphorescent green and orange-red emitting systems fac tris(2-phenylpyridine) iridium doped 4,4('),4(')-tris(N-carbazolyl)-triphenylamine (TCTA) and iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) doped N,N-'-di(naphthalen-1-yl)-N,N-'-diphenyl-benzidine with the blue fluorescent dye 2,2('),7,7(')-tetrakis(2,2-diphenylvinyl)spiro-9,9(')-bifluorene (Spiro-DPVBi) are presented. By introducing a thin layer of coevaporated TCTA and 2,2('),2(') (1,3,5-benzenetriyl) tris-[1-phenyl-1H-benzimidazole] between the phosphorescent and the fluorescent region, both singlet and triplet excitons are confined efficiently, whereas charge carriers still pass easily this interlayer. Furthermore, the interlayer suppresses Dexter transfer of the phosphorescent excitons to the nonradiative triplet state of Spiro-DPVBi. Best devices reach a current efficiency of 16.3 cd/A at 100 cd/m(2) and a color rendering index of 85 at warm white CIE chromaticity coordinates of (0.47, 0.42). Due to the use of electrically doped charge transport layers, 100 cd/m(2) are obtained at 2.95 V with a power efficiency of 17.4 lm/W. (c) 2006 American Institute of Physics.
Abstract: We demonstrate extremely stable and highly efficient red p-i-n-type organic light emitting diodes (OLEDs) based on an iridium-based electrophosphorescent dye in suitable host materials. The OLEDs reach lifetimes well above 1x10(7) h at 100 cd/m(2) initial luminance and reach at the same time a performance of 12.4% external quantum efficiency. This high lifetime is attributed to a combination of the low current density needed to reach a certain luminance and to the high stability of the materials against both charge carriers and excitons. (c) 2006 American Institute of Physics.
Abstract: We report the experimental observation of polarization splitting at normal incidence and terahertz oscillations of transmitted light from a single planar microcavity. Optical anisotropy in the SiO2/TiO2-dielectric mirrors leads to two perpendicularly polarized transmission modes. We ascribe the anisotropy to oblique columnar structures in the dielectrics resulting from off-axial growth of the microcavity structure. We apply an up-conversion setup for temporally and spectrally resolved measurements and obtain a corresponding beating of 1.25 THz. Time resolved measurements yield a cavity photon lifetime of 0.65 ps, corresponding to a Q value of 1600. To explain our observations we introduce a Fourier-transform based analytical model. (c) 2006 American Institute of Physics.
Abstract: Organic solar cells based on a new oligothiophene derivative (see figure) and fullerence C-60 exhibit power efficiencies of up to 3.4%. alpha,alpha'-Bis(2,2-dicyanovinyl)-quinquethiophene (DCV5T) features a reduced optical gap with high absorption coefficients between 450 and 650 nm. Dicyanovinyl substituents on the oligothiophene rings lower the bandgap and increase the ionization energy of these oligomers, enabling the formation of photoactive heterojunctions with C-60 and yielding open-circuit photovoltages as high as 1 V.
Abstract: The use of doped wide-gap charge transport layers with high conductivity and low absorption in the visible range enables one to achieve high internal quantum efficiencies and to optimize the devices with respect to optical interference effects. Here, it is shown that this architecture is particularly useful for stacking several cells on top of each other. The doping eases the recombination of the majority carriers at the interface between the cells, whereas the recombination centers are hidden for excitons and minority carriers. By stacking two p-i-n cells both with a phthalocyanine-fullerene blend as photoactive layer, a power efficiency of up to 3.8% at simulated AM1.5 illumination as compared to 2.1% for the respective single p-i-n cell has been achieved. Numerical simulations of the optical field distribution based on the transfer-matrix formalism are applied for optimization. The concept paves the way to even higher efficiencies by stacking several p-i-n cells with different photoactive materials that together cover the full visible spectrum. (c) 2005 American Institute of Physics.
Abstract: High-performance organic light-emitting diodes (OLEDs) are promoting future applications of solid-state lighting and flat-panel displays. We demonstrate here that the performance demands for OLEDs are met by the PIN (p-doped hole-transport layer/intrinsically conductive emission layer/n-doped electron-transport layer) approach. This approach enables high current efficiency, low driving voltage, as well as long OLED lifetimes. Data on very-high-efficiency diodes (power efficiencies exceeding 70 lm/W) incorporating a double-emission layer, comprised of two bipolar layers doped with tris(phenylpyridine)iridium [Ir(ppy)(3)], into the PIN architecture are shown. Lifetimes of more than 220,000 hours at a brightness of 150 cd/m(2) are reported for a red PIN diode. The PIN approach further allows the integration of highly efficient top-emitting diodes on a wide range of substrates. This is an important factor, especially for display applications where the compatibility of PIN OLEDs with various kinds of substrates is a key advantage. The PIN concept is very compatible with different backplanes, including passive-matrix substrates as well as active-matrix substrates on low-temperature polysilicon (LTPS) or, in particular, amorphous silicon (a- Si).
Abstract: nWe present a comprehensive study of ultrafast relaxation properties of optical excitations in thin films of quasi-1D stacked organic materials PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N-'-dimethylperylene-3,4,9,10-dicarboximide) over five decades of time. Pump-probe experiments reveal excitonic intraband relaxation time constants of 65 fs for MePTCDI and 100 fs for PTCDA. The initial time-resolved luminescence anisotropy is consistent with the exciton model of Davydov-split states. The subsequent decay of the anisotropy can be explained with a thermally activated exciton hopping process. A full understanding of the pump-probe experiments calls for an explanation beyond the models presently available.
Abstract: We investigate the dynamics of the organic laser guest-host composite of tris-(8-hydroxy quinoline) aluminium and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran embedded in a high-Q (Q approximate to 4500) double distributed Bragg reflector microcavity using subpicosecond up-conversion techniques. Lasing is observed at a threshold of 0.4 nJ/20 mu J cm(-2) with a linewidth of 0.05 nm (resolution limit). We observe a strongly nonlinear intensity-dependent delay of the emitted radiation burst. All experimental results are successfully modeled by a set of nonlinear rate equations, emphasizing the importance of a feedback mechanism for lasing. (C) 2005 American Institute of Physics.
Abstract: We demonstrate highly efficient, vapor-deposited blue organic light-emitting diodes (OLEDs) operating at low voltage. For reaching deep-blue color, we used two new fluorophores, 9,10-bis(9,9â-spirobi[9H-fluorene]-2-yl)anthracene (Spiro-Anthracene) from Covion, and 4,4â-bis-(N,N-diphenylamino)-tetraphenyl (4P-TPD) from Syntec-Sensient, sandwiched in between p- and n-type doped wide band-gap transport layers and appropriate blocking layers. These p-i-n OLED devices show high luminance and efficiency at low operating voltages. Both dyes emit deep-blue light at color coordinates of x = 0.15 and y = 0.09 (4P-TPD) and x=0.15 and y=0.18 (Spiro-Anthracene). Optimized devices containing Spiro-Anthracene reach a luminance of 100 and 1000 cd/m(2) already at a voltage of 2.9 and 3.4 V, respectively. At the same time, a deep-blue color with CIE color coordinates of x = 0.14 and y = 0.14 as well as good current efficiencies (3.9 cd/A at 100 cd/m(2)) and quantum efficiencies (3.7% at 100 cd/m(2)) are reached, which shows that the concept of doped transport layers and appropriate fluorescent emitters can be applied successfully to the preparation of blue OLEDs. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: We excite excitonic wave packets in biased semiconductor superlattices with spectrally shaped ultrashort optical pulses. We tailor the shape and phase of the pulse spectrum in order to control the coherent dynamics of the excitonic wave packets formed from a superposition of three excitonic states. Via careful shaping, we are able to excite either wave packets that exhibit standard Bloch oscillations (BO's) or breathing-mode (BM) motion. These two types of motion are characterized by the presence (BO) or absence (BM) of an internal intraband polarization caused by the electron-hole separation within the excitonic wave packet. The wave packet evolution is monitored using spectrally resolved four-wave mixing. This ability to control the BO's provides a way to control the emitted THz radiation.
Abstract: We report an investigation of the excitonic properties of thin crystalline films of the archetypal organic semiconductor PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) grown on poly- and single crystalline surfaces. A sensitive setup capable of measuring the optical properties of ultrathin organic molecular crystals via differential reflectance spectroscopy (DRS) is presented. This tool allows to carry out measurements in situ, i.e., during the actual film growth, and over a wide spectral range, even on single crystalline surfaces with high symmetry or metallic surfaces, where widely used techniques like reflection anisotropy spectroscopy (RAS) or fluorescence excitation spectroscopy fail. The spectra obtained by DRS resemble mainly the absorption of the films if transparent substrates are used, which simplifies the analysis. In the case of mono- to multilayer films of PTCDA on single crystalline muscovite mica(0001) and Au(111) substrates, the formation of the solid state absorption from monomer to dimer and further to crystal-like absorption spectra can be monitored.
Abstract: We realize p- and n-type doping of the organic semiconductor zinc-phthalocyanine using a novel strong organic donor. This allows us to demonstrate the first stable and reproducible organic p-n homojunctions. The diodes show very high built-in potentials, attractive, e.g., for organic solar cells. However, the diode characteristics cannot be described by the standard Shockley theory of the p-n junction since the ideality factor strongly increases with decreasing temperature. We show that this behavior can be explained by deviations from the Einstein relation for disordered materials.
Abstract: We demonstrate highly efficient, vapor-deposited blue organic light-emitting diodes (OLEDs) operating at low voltage. For reaching deep-blue color, we used two new fluorophores, 9,10-bis(9,9'-spirobi[9H-fluorene]-2-yl)anthracene (Spiro-Anthracene) from Covion, and 4,4'-bis-(N,N-diphenylamino)-tetraphenyl (4P-TPD) from Syntec-Sensient, sandwiched in between p- and n-type doped wide band-gap transport layers and appropriate blocking layers. These p-i-n OLED devices show high luminance and efficiency at low operating voltages. Both dyes emit deep-blue light at color coordinates of x = 0.15 and y = 0.09 (4P-TPD) and x=0.15 and y=0.18 (Spiro-Anthracene). Optimized devices containing Spiro-Anthracene reach a luminance of 100 and 1000 cd/m(2) already at a voltage of 2.9 and 3.4 V, respectively. At the same time, a deep-blue color with CIE color coordinates of x = 0.14 and y = 0.14 as well as good current efficiencies (3.9 cd/A at 100 cd/m(2)) and quantum efficiencies (3.7% at 100 cd/m(2)) are reached, which shows that the concept of doped transport layers and appropriate fluorescent emitters can be applied successfully to the preparation of blue OLEDs. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: We propose a new type of epitaxy, line-on-line coincidence (LOL), which explains the ordering in the organic-organic heterolayer system PTCDA on HBC on graphite. LOL epitaxy is similar to point-on-line coincidence (POL) in the sense that all overlayer molecules lie on parallel, equally spaced lines. The key difference to POL is that these lines are not restricted to primitive lattice lines of the substrate lattice. Potential energy calculations demonstrate that this new type of epitaxy is indeed characterized by a minimum in the overlayer-substrate interaction potential.
Abstract: We report on investigations of optical excitations in polycrystalline organic molecular crystals with quasi-1D-stacked crystal structure and negative exciton dispersion. As model system, we choose thin films of the perylene derivative 3,4,9, 10-perylenetetracarboxylic dianhydride (PTCDA). Using pump-probe spectroscopy, we show how the relaxation from the absorbing state towards the border of the Brillouin zone occurs on a 120 fs timescale. Time-resolved luminescence anisotropy gives evidence that as a result of the coherent coupling between adjacent stacks, populations of the Davydovsplit states that are prepared during photo-excitation relax into the emitting states in less than 5 ps. The behavior of the luminescence anisotropy can be explained by the orientation of the two PTCDA molecules in the unit cell. However, a full understanding of the ultrafast pump-probe anisotropy requires novel explanations beyond current models. (c) 2004 Elsevier B.V. All rights reserved.
Abstract: We realize p- and n-type doping of the organic semiconductor zinc-phthalocyanine using a novel strong organic donor. This allows us to demonstrate the first stable and reproducible organic p-n homojunctions. The diodes show very high built-in potentials, attractive, e.g., for organic solar cells. However, the diode characteristics cannot be described by the standard Shockley theory of the p-n junction since the ideality factor strongly increases with decreasing temperature. We show that this behavior can be explained by deviations from the Einstein relation for disordered materials.
Abstract: The use of doped wide-gap charge transport layers with high conductivity and low absorption in the visible range enables one to achieve high internal quantum efficiencies and to optimize the devices with respect to optical interference effects. Here, it is shown that this architecture is particularly useful for stacking several cells on top of each other. The doping eases the recombination of the majority carriers at the interface between the cells, whereas the recombination centers are hidden for excitons and minority carriers. By stacking two p-i-n cells both with a phthalocyanine-fullerene blend as photoactive layer, a power efficiency of up to 3.8% at simulated AM1.5 illumination as compared to 2.1% for the respective single p-i-n cell has been achieved. Numerical simulations of the optical field distribution based on the transfer-matrix formalism are applied for optimization. The concept paves the way to even higher efficiencies by stacking several p-i-n cells with different photoactive materials that together cover the full visible spectrum. (c) 2005 American Institute of Physics.
Abstract: High-performance organic light-emitting diodes (OLEDs) are promoting future applications of solid-state lighting and flat-panel displays. We demonstrate here that the performance demands for OLEDs are met by the PIN (p-doped hole-transport layer/intrinsically conductive emission layer/n-doped electron-transport layer) approach. This approach enables high current efficiency, low driving voltage, as well as long OLED lifetimes. Data on very-high-efficiency diodes (power efficiencies exceeding 70 lm/W) incorporating a double-emission layer, comprised of two bipolar layers doped with tris(phenylpyridine)iridium [Ir(ppy)(3)], into the PIN architecture are shown. Lifetimes of more than 220,000 hours at a brightness of 150 cd/m(2) are reported for a red PIN diode. The PIN approach further allows the integration of highly efficient top-emitting diodes on a wide range of substrates. This is an important factor, especially for display applications where the compatibility of PIN OLEDs with various kinds of substrates is a key advantage. The PIN concept is very compatible with different backplanes, including passive-matrix substrates as well as active-matrix substrates on low-temperature polysilicon (LTPS) or, in particular, amorphous silicon (a- Si).
Abstract: We report on investigations of optical excitations in polycrystalline organic molecular crystals with quasi-1D-stacked crystal structure and negative exciton dispersion. As model system, we choose thin films of the perylene derivative 3,4,9, 10-perylenetetracarboxylic dianhydride (PTCDA). Using pump-probe spectroscopy, we show how the relaxation from the absorbing state towards the border of the Brillouin zone occurs on a 120 fs timescale. Time-resolved luminescence anisotropy gives evidence that as a result of the coherent coupling between adjacent stacks, populations of the Davydovsplit states that are prepared during photo-excitation relax into the emitting states in less than 5 ps. The behavior of the luminescence anisotropy can be explained by the orientation of the two PTCDA molecules in the unit cell. However, a full understanding of the ultrafast pump-probe anisotropy requires novel explanations beyond current models. (c) 2004 Elsevier B.V. All rights reserved.
Notes: 6th International Conference on Excitonic Processes in Condensed Matter (EXCON 04), Cracow, POLAND, JUL 06-09, 2004
Abstract: We report an investigation of the excitonic properties of thin crystalline films of the archetypal organic semiconductor PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) grown on poly- and single crystalline surfaces. A sensitive setup capable of measuring the optical properties of ultrathin organic molecular crystals via differential reflectance spectroscopy (DRS) is presented. This tool allows to carry out measurements in situ, i.e., during the actual film growth, and over a wide spectral range, even on single crystalline surfaces with high symmetry or metallic surfaces, where widely used techniques like reflection anisotropy spectroscopy (RAS) or fluorescence excitation spectroscopy fail. The spectra obtained by DRS resemble mainly the absorption of the films if transparent substrates are used, which simplifies the analysis. In the case of mono- to multilayer films of PTCDA on single crystalline muscovite mica(0001) and Au(111) substrates, the formation of the solid state absorption from monomer to dimer and further to crystal-like absorption spectra can be monitored.
Abstract: We excite excitonic wave packets in biased semiconductor superlattices with spectrally shaped ultrashort optical pulses. We tailor the shape and phase of the pulse spectrum in order to control the coherent dynamics of the excitonic wave packets formed from a superposition of three excitonic states. Via careful shaping, we are able to excite either wave packets that exhibit standard Bloch oscillations (BOâs) or breathing-mode (BM) motion. These two types of motion are characterized by the presence (BO) or absence (BM) of an internal intraband polarization caused by the electron-hole separation within the excitonic wave packet. The wave packet evolution is monitored using spectrally resolved four-wave mixing. This ability to control the BOâs provides a way to control the emitted THz radiation.
Abstract: nWe present a comprehensive study of ultrafast relaxation properties of optical excitations in thin films of quasi-1D stacked organic materials PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N-â-dimethylperylene-3,4,9,10-dicarboximide) over five decades of time. Pump-probe experiments reveal excitonic intraband relaxation time constants of 65 fs for MePTCDI and 100 fs for PTCDA. The initial time-resolved luminescence anisotropy is consistent with the exciton model of Davydov-split states. The subsequent decay of the anisotropy can be explained with a thermally activated exciton hopping process. A full understanding of the pump-probe experiments calls for an explanation beyond the models presently available.
Abstract: We propose a new type of epitaxy, line-on-line coincidence (LOL), which explains the ordering in the organic-organic heterolayer system PTCDA on HBC on graphite. LOL epitaxy is similar to point-on-line coincidence (POL) in the sense that all overlayer molecules lie on parallel, equally spaced lines. The key difference to POL is that these lines are not restricted to primitive lattice lines of the substrate lattice. Potential energy calculations demonstrate that this new type of epitaxy is indeed characterized by a minimum in the overlayer-substrate interaction potential.
Abstract: We investigate the dynamics of the organic laser guest-host composite of tris-(8-hydroxy quinoline) aluminium and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran embedded in a high-Q (Q approximate to 4500) double distributed Bragg reflector microcavity using subpicosecond up-conversion techniques. Lasing is observed at a threshold of 0.4 nJ/20 mu J cm(-2) with a linewidth of 0.05 nm (resolution limit). We observe a strongly nonlinear intensity-dependent delay of the emitted radiation burst. All experimental results are successfully modeled by a set of nonlinear rate equations, emphasizing the importance of a feedback mechanism for lasing. (C) 2005 American Institute of Physics.
Abstract: We reconstruct a âtestâ wave function in a strongly coupled, graded well-width superlattice by resolving the spatial extension of the interband polarisation and deducing the wave function employing non-linear optical spectroscopy. The graded gap superlattice allows us to precisely control the distance between âtestâ and âprobeâ wave functions. By spatially tuning one wave function with respect to the other and recording the amplitude and the sign of the modulation of the spectrally resolved four-wave-mixing (FWM) signal with respect to delay, we are able to reconstruct the âtestâ wave function. Our numerical simulation of the third-order response of an inhomogeneously broadened system reproduces the experimental data in great detail. The wave function used for the modelling is computed by a one-dimensional transfer matrix model including electron-hole Coulomb interaction. Our experimental scheme inherently allows us to quantitatively distinguish between non-linear mechanisms leading to the FWM signal, namely phase-space filling and excitation-induced dephasing.
Abstract: We introduce a p-i-n-type heterojunction architecture for organic solar cells where the active region is sandwiched between two doped wide-gap layers. The term p-i-n means here a layer sequence in the form p-doped layer, intrinsic layer and n-doped layer. The doping is realized by controlled co-evaporation using organic dopants and leads to conductivities of 10(-4) to 10(-5) S/cm in the p- and n-doped wide-gap layers, respectively. The photoactive layer is formed by a mixture of phthalocyanine zinc (ZnPc) and the fullerene C-60 and shows mainly amorphous morphology. As a first step towards p-i-n structures, we show the advantage of using wide-gap layers in M-i-p-type diodes (metal layer-intrinsic layer-p-doped layer). The solar cells exhibit a maximum external quantum efficiency of 40% between 630-nm and 700-nm wavelength. With the help of an optical multilayer model, we optimize the optical properties of the solar cells by placing the active region at the maximum of the optical field distribution. The results of the model are largely confirmed by the experimental findings. For an optically optimized device, we find an internal quantum efficiency of around 82% under short-circuit conditions. Adding a layer of 10-nm thickness of the red material N,N-â-dimethylperylene-3,4:9,10-dicarboximide (Me-PTCDI) to the active region, a power-conversion efficiency of 1.9% for a single cell is obtained. Such optically thin cells with high internal quantum efficiency are an important step towards high-efficiency tandem cells. First tandem cells which are not yet optimized already show 2.4% power-conversion efficiency under simulated AM 1.5 illumination of 125 mW/cm(2) .
Abstract: We studied the growth and properties of titanyl phthalocyanine (TiOPc) thin films made by supersonic molecular beam epitaxy (SuMBE). Interesting differences in the growth properties on amorphous (quartz) and crystalline (mica) substrates were found, indicating that SuMBE gives rise to an epitaxy of disc-like organic molecules on crystalline substrates. The combined control of the kinetic energy of the molecules in the supersonic beam specific to SuMBE and of the substrate temperature during deposition are the key parameters used to determine the final properties of the films. We show that SuMBE is a well-suited epitaxy method for the deposition of relatively large organic molecules, leading to layers of thin organic (single-)crystals with lateral dimensions in the micrometer range. By SuMBE we can control the growth of different polymorphs of TiOPc. We found and studied two ways to produce films of red and infrared absorbing phase II TiOPc, which is of interest for applications in organic solar cells. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: We investigated photoluminescence of the phosphorescent molecule platinum(II)-2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin. In neat films prepared by organic molecular beam deposition or precipitation from solution, we observe a near infrared emission feature around 1.65 eV, caused by aggregation effects. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: The optical properties of microcavities (MCs) are strongly dependent on both polarization of incident and emitted light and its angle of observation. Here we report the measurements of cw- and time-resolved photoluminescence (PL) observed at negative detuning and at resonance for s- and p-polarization in the strong coupling regime of a planar MC containing J-aggregates of a cyanine dye. Following non-resonant excitation, the emission spectra consist of three types of features: direct J-aggregate exciton emission, polariton emission, and uncoupled monomer emission through the transmission maxima of the distributed Bragg reflector beyond the stop-band. We compare our experimental results with a transfer-matrix calculation of the transmission for s- and p-polarization and explain the different positions of the polariton branches, the stop-band width, and the high- and low energy transmission maxima of the MC. Time-resolved PL experiments show an increase in the decay lifetime of the exciton-like mode when it is positioned far from the cavity mode. Close to resonance, the lower polariton branch decays with the natural lifetime of the J-aggregates. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: We present an approach to stable n-type doping of organic matrices using organic dopants. In order to circumvent stability limitations inherent to strong organic donors, we produce the donor from a stable precursor compound in situ. As an example, the cationic dye pyronin B chloride is studied as a dopant in a 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) matrix. Conductivities of up to 1.9 x 10(-4) S cm(-1) are obtained for doped NTCDA, two orders of magnitude higher than the conductivity of NTCDA doped with bis(ethylenedithio)-tetrathiafulvalene as investigated previously, and four orders of magnitude higher than nominally undoped NTCDA films. Field-effect measurements are used to prove n-type conduction and to study the doping effect further. The findings are interpreted using a model of transport in disordered solids using a recently published model. Combined FTIR, UV-vis, and mass spectroscopy investigations suggest the formation of leuco pyronin B during sublimation of pyronin B chloride.
Abstract: We show that the performance of organic solar cells can be strongly improved using PIN structures with doped wide-gap transport layers and a phthalocyanine-fullerene blend as a photoactive layer. Such cells only absorb light in the photoactive region and thus avoid recombination losses at the contacts and make optimum use of the light reflected at the top contact. They reach approximately 1.9% power efficiency at simulated AM 1.5 illumination, nearly 100% internal quantum efficiency and a fill factor of approximately 50%. We show that it is possible to stack two or more of these optically thin PIN cells into one sandwich structure without significant losses. This principle should soon enable us to prepare organic solar cells with outstandingly high efficiency. In the solar cells, a new technique of n-doping by cationic dyes is used. Using fullerene or perylene derivatives as a matrix, it yields thermally very stable layers with high conductivity. (C) 2003 Elsevier B.V. All rights reserved.
Notes: Symposium on Thin Film and Nono-Structured Materials for Photovoltaics, Strasbourg, FRANCE, JUN, 2003
Abstract: We describe a simple organic solar cell structure that allows to study in detail loss mechanisms due to non-ohmic contacts, structural drawbacks and material selection: the MIP-type (metal-intrinsic-p-doped) structure discussed here represents the p-side and the active layer within a PIN type solar cell architecture. We here adapt the bulk heterojunction concept using blends of zinc-phthalocyanine (ZnPc) and buckminsterfullerene C-60. Furthermore, we use doped wide-gap materials as hole transport layers that enable a more sophisticated solar cell development. The samples are characterized by combination of current voltage characteristics, impedance spectroscopy and capacitance-voltage measurements. We present an evolution of MIP-type structures which improves all solar cell parameters; in particular, excessive series resistance and contact problems reducing the fill factor could be almost completely eliminated. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: We report theoretical and experimental results for the intraband dynamics of biased semiconductor superlattices excited by ultrashort optical pulses. The theoretical model used employs an excitonic basis that includes 1s and all higher-energy in-plane excitonic states. These excitonic states are used to calculate the intraband polarization and terahertz emission of the superlattice system in response to excitation via an ultrashort optical pulse. Our results show that the higher in-plane excitonic states often modify considerably the terahertz emission relative to the results obtained using a 1s exciton basis, but that under some excitation conditions a 1s exciton basis gives accurate results. Good agreement between experimental and theoretical results is obtained.
Abstract: Green phosphorescent organic light-emitting devices (OLEDs) employing tris(2-phenylpyridine) iridium doped into a wide energy gap hole transport host have been studied. N,N,N-â,N-â-tetrakis(4-methoxyphenyl)-benzidine doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane is used as a hole injection and transport layer, 4,7-diphenyl-1,10-phenanthroline and cesium are coevaporated as a n-doped electron transport layer, and an intrinsic emission layer is sandwiched between these two doped layer. Such a p-i-n device features efficient carrier injection from both contacts into the doped transport layers and low ohmic losses in these highly conductive layers. Thus, low operating voltages are obtained compared to conventional undoped OLEDs. By modifying the device structure, we optimized the carrier balance in the emission layer and at its interfaces. For the optimized device, the maximum power efficiency is 53 lm/W, and a luminance of 1000 cd/m(2) is reached at 3.1 V with a power efficiency of 45 lm/W. (C) 2004 American Institute of Physics.
Abstract: We present results on p-i-n-type photovoltaic devices based on donor-acceptor organic small molecule blends and doped wide-gap charge transport layers. The doped transport layers are formed using a high vacuum co-evaporation deposition technique (i.e. co-sublimation of matrix and dopant). Solar cell devices have been fabricated based on a blend of zinc-phthalocyanine as donor (D) and fullerene (C-60) as electron acceptor (A) with doped charge transport layers. Optionally, a layer of neat layer of a perylene dye, perlenetetracarboxylic-bis-benzimidazole, is added to extend the absorption spectrum of the active layer system. The energy conversion efficiency of these cells under 1 sun, standard AM1.5 illumination (100 mW/cm(2)) is up to 1.9%. This represents an almost twofold improvement as compared to previously reported cells with similar active layer system in a conventional architecture. In this report, I-V characteristics, power conversion efficiencies, the dependence of short circuit current on incident white light intensity and absorption spectra of the active layer system are discussed. (C) 2003 Elsevier B.V. All rights reserved.
Notes: Symposium on Thin Film and Nono-Structured Materials for Photovoltaics, Strasbourg, FRANCE, JUN, 2003
Abstract: We report the generation of terahertz (THz) radiation in a biased semiconductor superlattice by difference-frequency mixing of interband transitions. The Wannier-Stark spectrum of the superlattice was selectively excited by two spectrally narrow laser lines. The emitted THz radiation was measured as a function of the energetic splitting and spectral position of the bichromatic excitation. The generation of tunable THz radiation is verified. The results clearly show enhanced THz emission when either continuum or 1s exciton wave packets are excited. Good agreement is obtained between the experiment and the results of an exciton model of the nonlinear coherent dynamics. (C) 2004 American Institute of Physics.
Abstract: We present results on p-i-n-type photovoltaic devices based on donor-acceptor organic small molecule blends and doped wide-gap charge transport layers. The doped transport layers are formed using a high vacuum co-evaporation deposition technique (i.e. co-sublimation of matrix and dopant). Solar cell devices have been fabricated based on a blend of zinc-phthalocyanine as donor (D) and fullerene (C-60) as electron acceptor (A) with doped charge transport layers. Optionally, a layer of neat layer of a perylene dye, perlenetetracarboxylic-bis-benzimidazole, is added to extend the absorption spectrum of the active layer system. The energy conversion efficiency of these cells under 1 sun, standard AM1.5 illumination (100 mW/cm(2)) is up to 1.9%. This represents an almost twofold improvement as compared to previously reported cells with similar active layer system in a conventional architecture. In this report, I-V characteristics, power conversion efficiencies, the dependence of short circuit current on incident white light intensity and absorption spectra of the active layer system are discussed. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: We report theoretical and experimental results for the intraband dynamics of biased semiconductor superlattices excited by ultrashort optical pulses. The theoretical model used employs an excitonic basis that includes 1s and all higher-energy in-plane excitonic states. These excitonic states are used to calculate the intraband polarization and terahertz emission of the superlattice system in response to excitation via an ultrashort optical pulse. Our results show that the higher in-plane excitonic states often modify considerably the terahertz emission relative to the results obtained using a 1s exciton basis, but that under some excitation conditions a 1s exciton basis gives accurate results. Good agreement between experimental and theoretical results is obtained.
Abstract: We show that the performance of organic solar cells can be strongly improved using PIN structures with doped wide-gap transport layers and a phthalocyanine-fullerene blend as a photoactive layer. Such cells only absorb light in the photoactive region and thus avoid recombination losses at the contacts and make optimum use of the light reflected at the top contact. They reach approximately 1.9% power efficiency at simulated AM 1.5 illumination, nearly 100% internal quantum efficiency and a fill factor of approximately 50%. We show that it is possible to stack two or more of these optically thin PIN cells into one sandwich structure without significant losses. This principle should soon enable us to prepare organic solar cells with outstandingly high efficiency. In the solar cells, a new technique of n-doping by cationic dyes is used. Using fullerene or perylene derivatives as a matrix, it yields thermally very stable layers with high conductivity. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: We reconstruct a 'test' wave function in a strongly coupled, graded well-width superlattice by resolving the spatial extension of the interband polarisation and deducing the wave function employing non-linear optical spectroscopy. The graded gap superlattice allows us to precisely control the distance between 'test' and 'probe' wave functions. By spatially tuning one wave function with respect to the other and recording the amplitude and the sign of the modulation of the spectrally resolved four-wave-mixing (FWM) signal with respect to delay, we are able to reconstruct the 'test' wave function. Our numerical simulation of the third-order response of an inhomogeneously broadened system reproduces the experimental data in great detail. The wave function used for the modelling is computed by a one-dimensional transfer matrix model including electron-hole Coulomb interaction. Our experimental scheme inherently allows us to quantitatively distinguish between non-linear mechanisms leading to the FWM signal, namely phase-space filling and excitation-induced dephasing.
Abstract: We present an approach to stable n-type doping of organic matrices using organic dopants. In order to circumvent stability limitations inherent to strong organic donors, we produce the donor from a stable precursor compound in situ. As an example, the cationic dye pyronin B chloride is studied as a dopant in a 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) matrix. Conductivities of up to 1.9 x 10(-4) S cm(-1) are obtained for doped NTCDA, two orders of magnitude higher than the conductivity of NTCDA doped with bis(ethylenedithio)-tetrathiafulvalene as investigated previously, and four orders of magnitude higher than nominally undoped NTCDA films. Field-effect measurements are used to prove n-type conduction and to study the doping effect further. The findings are interpreted using a model of transport in disordered solids using a recently published model. Combined FTIR, UV-vis, and mass spectroscopy investigations suggest the formation of leuco pyronin B during sublimation of pyronin B chloride.
Abstract: Green phosphorescent organic light-emitting devices (OLEDs) employing tris(2-phenylpyridine) iridium doped into a wide energy gap hole transport host have been studied. N,N,N-',N-'-tetrakis(4-methoxyphenyl)-benzidine doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane is used as a hole injection and transport layer, 4,7-diphenyl-1,10-phenanthroline and cesium are coevaporated as a n-doped electron transport layer, and an intrinsic emission layer is sandwiched between these two doped layer. Such a p-i-n device features efficient carrier injection from both contacts into the doped transport layers and low ohmic losses in these highly conductive layers. Thus, low operating voltages are obtained compared to conventional undoped OLEDs. By modifying the device structure, we optimized the carrier balance in the emission layer and at its interfaces. For the optimized device, the maximum power efficiency is 53 lm/W, and a luminance of 1000 cd/m(2) is reached at 3.1 V with a power efficiency of 45 lm/W. (C) 2004 American Institute of Physics.
Abstract: This report demonstrates external power conversion efficiencies of 2% under 100 mW/cm(2) simulated AM1.5 illumination for organic thin-film photovoltaic cells using a phthalocyanine-fullerene (ZnPc/C-60) bulk heterojunction as an active layer, embedded into a p-i-n type architecture with doped wide-gap charge transport layers. For an optically optimized device, we found internal quantum efficiency (IQE) of above 80% under short circuit conditions. Such optically thin cells with high internal quantum efficiency are an important step towards high efficiency tandem cells. The p-i-n architecture allows for the design of solar cells with high internal quantum efficiency where only the photoactive region absorbs visible light and recombination losses at contacts are avoided. The IN characteristics, power conversion efficiencies, the dependence of short circuit current on incident white light intensity, incident photon to collected electron efficiency (IPCE) and absorption spectra of the active layer system are discussed.
Abstract: The triphenylmethane dye crystal violet (CV) and its leuco base, leucocrystal violet (LCV), are investigated as dopants for n-type doping of fullerene C-60. Conductivities up to 8 x 10(-3) S/cm at 30degreesC are achieved when C-60 is doped with CV. Mass spectroscopy and optical spectroscopy (UV/VIS/NIR absorption and Fourier transform infrared transmission) confirm that the leuco base LCV is formed during sublimation of the cationic CV dye. When the commercially available LCV was directly used as a dopant in C-60, a maximum conductivity of 1.3 x 10(-2) S/cm was obtained at 30degreesC. We found that in both cases, the leuco base became reoxidized to the cationic form by electron transfer to electron-accepting matrixes, leading to the doping effect. The donor properties of LCV in a charge-transfer complex with 7,7,8,8-tetracyanoquinodimethane (TCNQ) were confirmed by UV/VIS/NIR absorption and Fourier transform infrared (FTIR) spectroscopy. C-60 anions were observed in the FTIR or NIR absorption spectra of the mixed films of C-60 and LCV. Photoinduced charge transfer between LCV and C-60 provides free electrons, which increase the n-type conductivity. The electron transfer becomes irreversible by hydride abstraction.
Abstract: We demonstrate high-efficiency organic light-emitting diodes by incorporating a double-emission layer (D-EML) into p-i-n-type cell architecture. The D-EML is comprised of two layers with ambipolar transport characteristics, both doped with the green phosphorescent dye tris(phenylpyridine)iridium. The D-EML system of two bipolar layers leads to an expansion of the exciton generation region. Due to its self-balancing character, accumulation of charge carriers at the outer interfaces is avoided. Thus, a power efficiency of approximately 77 lm/W and an external quantum efficiency of 19.3% are achieved at 100 cd/m(2) with an operating voltage of only 2.65 V. More importantly, the efficiency decays only weakly with increasing brightness, and a power efficiency of 50 lm/W is still obtained even at 4000 cd/m(2). (C) 2004 American Institute of Physics.
Abstract: We introduce a p-i-n-type heterojunction architecture for organic solar cells where the active region is sandwiched between two doped wide-gap layers. The term p-i-n means here a layer sequence in the form p-doped layer, intrinsic layer and n-doped layer. The doping is realized by controlled co-evaporation using organic dopants and leads to conductivities of 10(-4) to 10(-5) S/cm in the p- and n-doped wide-gap layers, respectively. The photoactive layer is formed by a mixture of phthalocyanine zinc (ZnPc) and the fullerene C-60 and shows mainly amorphous morphology. As a first step towards p-i-n structures, we show the advantage of using wide-gap layers in M-i-p-type diodes (metal layer-intrinsic layer-p-doped layer). The solar cells exhibit a maximum external quantum efficiency of 40% between 630-nm and 700-nm wavelength. With the help of an optical multilayer model, we optimize the optical properties of the solar cells by placing the active region at the maximum of the optical field distribution. The results of the model are largely confirmed by the experimental findings. For an optically optimized device, we find an internal quantum efficiency of around 82% under short-circuit conditions. Adding a layer of 10-nm thickness of the red material N,N-'-dimethylperylene-3,4:9,10-dicarboximide (Me-PTCDI) to the active region, a power-conversion efficiency of 1.9% for a single cell is obtained. Such optically thin cells with high internal quantum efficiency are an important step towards high-efficiency tandem cells. First tandem cells which are not yet optimized already show 2.4% power-conversion efficiency under simulated AM 1.5 illumination of 125 mW/cm(2) .
Abstract: We describe a simple organic solar cell structure that allows to study in detail loss mechanisms due to non-ohmic contacts, structural drawbacks and material selection: the MIP-type (metal-intrinsic-p-doped) structure discussed here represents the p-side and the active layer within a PIN type solar cell architecture. We here adapt the bulk heterojunction concept using blends of zinc-phthalocyanine (ZnPc) and buckminsterfullerene C-60. Furthermore, we use doped wide-gap materials as hole transport layers that enable a more sophisticated solar cell development. The samples are characterized by combination of current voltage characteristics, impedance spectroscopy and capacitance-voltage measurements. We present an evolution of MIP-type structures which improves all solar cell parameters; in particular, excessive series resistance and contact problems reducing the fill factor could be almost completely eliminated. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: This report demonstrates external power conversion efficiencies of 2% under 100 mW/cm(2) simulated AM1.5 illumination for organic thin-film photovoltaic cells using a phthalocyanine-fullerene (ZnPc/C-60) bulk heterojunction as an active layer, embedded into a p-i-n type architecture with doped wide-gap charge transport layers. For an optically optimized device, we found internal quantum efficiency (IQE) of above 80% under short circuit conditions. Such optically thin cells with high internal quantum efficiency are an important step towards high efficiency tandem cells. The p-i-n architecture allows for the design of solar cells with high internal quantum efficiency where only the photoactive region absorbs visible light and recombination losses at contacts are avoided. The IN characteristics, power conversion efficiencies, the dependence of short circuit current on incident white light intensity, incident photon to collected electron efficiency (IPCE) and absorption spectra of the active layer system are discussed.
Abstract: We present luminescence quenching experiments and determine the exciton diffusion length in polycrystalline thin films of PTCDA (perylene-3,4,9,10-tetracarboxylic-dianhydride). From an analysis of time-resolved experiments, we can distinguish between exciton transport during an ultra-fast initial relaxation phase and transport in the long-living emitting states. The temperature dependence of the exciton diffusion constant in the emitting states indicates thermally activated hopping. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: We report the generation of terahertz (THz) radiation in a biased semiconductor superlattice by difference-frequency mixing of interband transitions. The Wannier-Stark spectrum of the superlattice was selectively excited by two spectrally narrow laser lines. The emitted THz radiation was measured as a function of the energetic splitting and spectral position of the bichromatic excitation. The generation of tunable THz radiation is verified. The results clearly show enhanced THz emission when either continuum or 1s exciton wave packets are excited. Good agreement is obtained between the experiment and the results of an exciton model of the nonlinear coherent dynamics. (C) 2004 American Institute of Physics.
Abstract: The triphenylmethane dye crystal violet (CV) and its leuco base, leucocrystal violet (LCV), are investigated as dopants for n-type doping of fullerene C-60. Conductivities up to 8 x 10(-3) S/cm at 30degreesC are achieved when C-60 is doped with CV. Mass spectroscopy and optical spectroscopy (UV/VIS/NIR absorption and Fourier transform infrared transmission) confirm that the leuco base LCV is formed during sublimation of the cationic CV dye. When the commercially available LCV was directly used as a dopant in C-60, a maximum conductivity of 1.3 x 10(-2) S/cm was obtained at 30degreesC. We found that in both cases, the leuco base became reoxidized to the cationic form by electron transfer to electron-accepting matrixes, leading to the doping effect. The donor properties of LCV in a charge-transfer complex with 7,7,8,8-tetracyanoquinodimethane (TCNQ) were confirmed by UV/VIS/NIR absorption and Fourier transform infrared (FTIR) spectroscopy. C-60 anions were observed in the FTIR or NIR absorption spectra of the mixed films of C-60 and LCV. Photoinduced charge transfer between LCV and C-60 provides free electrons, which increase the n-type conductivity. The electron transfer becomes irreversible by hydride abstraction.
Abstract: We demonstrate high-efficiency organic light-emitting diodes by incorporating a double-emission layer (D-EML) into p-i-n-type cell architecture. The D-EML is comprised of two layers with ambipolar transport characteristics, both doped with the green phosphorescent dye tris(phenylpyridine)iridium. The D-EML system of two bipolar layers leads to an expansion of the exciton generation region. Due to its self-balancing character, accumulation of charge carriers at the outer interfaces is avoided. Thus, a power efficiency of approximately 77 lm/W and an external quantum efficiency of 19.3% are achieved at 100 cd/m(2) with an operating voltage of only 2.65 V. More importantly, the efficiency decays only weakly with increasing brightness, and a power efficiency of 50 lm/W is still obtained even at 4000 cd/m(2). (C) 2004 American Institute of Physics.
Abstract: We present luminescence quenching experiments and determine the exciton diffusion length in polycrystalline thin films of PTCDA (perylene-3,4,9,10-tetracarboxylic-dianhydride). From an analysis of time-resolved experiments, we can distinguish between exciton transport during an ultra-fast initial relaxation phase and transport in the long-living emitting states. The temperature dependence of the exciton diffusion constant in the emitting states indicates thermally activated hopping. (C) 2004 Elsevier B.V. All rights reserved.
Notes: 325th Wilhelm and Else Heraeus Seminar on Organic Molecular Solids, Bad Honnef, GERMANY, MAY 17-19, 2004
Abstract: The optical properties of microcavities (MCs) are strongly dependent on both polarization of incident and emitted light and its angle of observation. Here we report the measurements of cw- and time-resolved photoluminescence (PL) observed at negative detuning and at resonance for s- and p-polarization in the strong coupling regime of a planar MC containing J-aggregates of a cyanine dye. Following non-resonant excitation, the emission spectra consist of three types of features: direct J-aggregate exciton emission, polariton emission, and uncoupled monomer emission through the transmission maxima of the distributed Bragg reflector beyond the stop-band. We compare our experimental results with a transfer-matrix calculation of the transmission for s- and p-polarization and explain the different positions of the polariton branches, the stop-band width, and the high- and low energy transmission maxima of the MC. Time-resolved PL experiments show an increase in the decay lifetime of the exciton-like mode when it is positioned far from the cavity mode. Close to resonance, the lower polariton branch decays with the natural lifetime of the J-aggregates. (C) 2004 Elsevier B.V. All rights reserved.
Notes: 325th Wilhelm and Else Heraeus Seminar on Organic Molecular Solids, Bad Honnef, GERMANY, MAY 17-19, 2004
Abstract: We studied the growth and properties of titanyl phthalocyanine (TiOPc) thin films made by supersonic molecular beam epitaxy (SuMBE). Interesting differences in the growth properties on amorphous (quartz) and crystalline (mica) substrates were found, indicating that SuMBE gives rise to an epitaxy of disc-like organic molecules on crystalline substrates. The combined control of the kinetic energy of the molecules in the supersonic beam specific to SuMBE and of the substrate temperature during deposition are the key parameters used to determine the final properties of the films. We show that SuMBE is a well-suited epitaxy method for the deposition of relatively large organic molecules, leading to layers of thin organic (single-)crystals with lateral dimensions in the micrometer range. By SuMBE we can control the growth of different polymorphs of TiOPc. We found and studied two ways to produce films of red and infrared absorbing phase II TiOPc, which is of interest for applications in organic solar cells. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: We investigated photoluminescence of the phosphorescent molecule platinum(II)-2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin. In neat films prepared by organic molecular beam deposition or precipitation from solution, we observe a near infrared emission feature around 1.65 eV, caused by aggregation effects. (C) 2004 Elsevier B.V. All rights reserved.
Notes: 325th Wilhelm and Else Heraeus Seminar on Organic Molecular Solids, Bad Honnef, GERMANY, MAY 17-19, 2004
Abstract: In this paper, we discuss recent experiments which prove that evaporated organic films can be efficiently doped by co-evaporation with organic dopant molecules. Key advantages for devices are the high conductivity and the formation of ohmic contacts despite large energetic barriers. For p-type doping, efficient doping is possible for a variety of polycrystalline and amorphous materials. Despite the differences in the microscopic behavior, all basic effects known from doped inorganic semiconductors are found in organics as well. However, efficient n-type doping with stable molecular dopants is still a challenge. Organic light emitting diodes (OLED) with conductivity doped transport layers show significantly improved properties: For instance, we have achieved a brightness of 100 cd/m(2) already at a voltage of 2.55 V, well below previous results for undoped devices. The advantages of doping are even more pronounced for top-emitting, inverted OLED structures: Due to the ohmic contacts nearly independent of the contact properties, it is possible to realize inverted top-emitting devices with parameters comparable to standard devices. Our doping technology is thus a significant advantage for active-matrix OLED displays and other displays on opaque substrate. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: For high electric fields, the lifetime of Wannier-Stark ladder states in a periodic potential is reduced by the fundamental process of Zener tunneling. We report on the analysis of the coherence lifetime of such states in semiconductor superlattices by interband spectroscopy. The reduction of lifetime by strong coupling between bands can only in the first approximation be described by the well-known Zener theory. A recently developed theoretical model is applied to calculate directly the tunneling probability of Wannier-Stark states as a function of the electric field. The theoretical results compare well with experiment, reproducing the complex interplay of both nonresonant and resonant Zener tunneling to higher bands. By comparing experiment and theory for a superlattice with a symmetric and one with a nonsymmetric potential, we can draw conclusions on a very general basis about the sensitive dependence of Zener tunneling on the specific dispersion relation of the carriers.
Abstract: We present a systematic study on photovoltaic devices that combine an organic small molecule photoactive donor-acceptor bulk heterojunction system with controlled doping of the charge transport layers. The doped transport layers are formed using high vacuum co-evaporation deposition technique (i.e. co-sublimation of matrix and dopant). Solar cell devices have been fabricated based on zinc-phthalocyanine (ZnPc) as donor (D) and fullerene (C-60) as electron acceptor (A) with doped charge transport layers. The cells show a short circuit current, I-sc = 1.5 mA/cm(2), an open circuit voltage, V-oc = 450 mV, a fill factor, FF = 0.5, and a power conversion efficiency, eta(e) = 3.37% under 1/10 sun (10 mW/cm(2)) white light illumination. In addition, these bulk-heterojunction photovoltaic devices were characterized under 1 sun (100 mW/cm(2)) white light illumination showing I-sc = 6.3 mA/cm(2), V-oc = 500 mV, and eta(e) = 1.04%. We have observed that the performance of such 'bulk-heterojunction' photovoltaic devices is critically dependent on the transport properties of the interpenetrating network D/A system and doped charge transport layers. (C) 2002 Elsevier B.V. All rights reserved.
Abstract: We present an approach to stable n-type doping of organic matrices using organic dopands. To circumvent stability limitations inherent in strong organic donors, we produce the donor from a stable precursor compound in situ. As an example, pyronin B chloride is studied as a dopant in a 1,4,5,8-naphthalene tetracarboxylic dianhydride matrix. Conductivities up to 2x10(-4) S/cm are obtained, which is two orders of magnitude higher than obtained previously using bis(ethylenedithio)-tetrathiafulvalene as a dopant [A. Nollau, M. Pfeiffer, T. Fritz, and K. Leo, J. Appl. Phys. 87, 4340 (2000)]. Field-effect measurements are used to prove n-type conduction. Other matrices which can be doped are N,N'-dimethyl-perylene-3,4,9,10-tetracarboxylic diimide and fullerene C-60, frequently used in organic solar cells. Visible light and Fourier-transform infrared spectroscopy confirm the donor properties of pyronin B. (C) 2003 American Institute of Physics.
Abstract: We demonstrated high-efficiency low-voltage stable inverted transparent electrophosphorescent organic light-emitting diodes employing an indium-tin-oxide coated glass substrate directly as cathode and a semitransparent top Au thin film as anode. The structure contains an iridium-complex doped emissive layer sandwiched in between n- and p-doped charge transport layer with appropriate blocking layers to form a nip structure. The devices are about 50% transparent and emit green light from both sides with peak external quantum efficiency (EQE) of 4.08% (14.3 cd/A). At 100 cd/m(2), the EQE is 3.8% (13 cd/A) at an operating voltage of 4.3 V. The devices exhibit a lifetime of above 50 hours under continuous constant-current driving for the initial luminance of about 9000 cd/m(2) in vacuum, which project a lifetime of 5000 hours for 100 cd/m(2).
Notes: International Conference on Science and Technology of Synthetic Metals (ICSM 2002), SHANGHAI, PEOPLES R CHINA, JUN 29-JUL 05, 2002
Abstract: We have studied the behavior of various intrinsic emission zones on the characteristics of organic light-emitting diodes with a p-doped hole-transport layer and an n-doped electron-transport layer based on our previous work [J. S. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, K. Leo, and S. Liu, Appl. Phys. Lett. 80, 139 (2002)]. This configuration is referred to as a PiN structure. Because the p- and n-doped regions occupy nearly 80% of the total thickness in our PiN device, the intrinsic region becomes a narrow layer between two doped regions. This intrinsic region includes the region where the radiative recombination occurs. Thus, the nature of this layer plays an important role in determining the actual device performance. Employing 8-tris-hydroxyquinoline aluminum as an emitter, we investigated the influence of the thickness of the emitter layer on the performance of the device. The optimum thickness of the emitter layer is found to be 20 nm. Combining the fluorescence dye doping method, we have optimized the PiN structure device. Two emitter systems have been used: Alq(3) doped with two highly fluorescent laser dyes, Quinacridone or Coumarin 6, respectively. We have demonstrated the influence of the thickness and the doping of the emission zone on the characteristics of a doped emitter device with PiN structure, and obtained higher-efficiency PiN structure devices. The different properties of PiN devices corresponding to two different emitter dopants with different trapping effect are also discussed. (C) 2003 American Institute of Physics.
Abstract: We demonstrate a transparent, inverted, electrophosphorescent n-i-p organic light emitting diode (OLED) exhibiting a luminance of 500 cd/m(2) at 3.1 V, and with a luminous power efficiency of 23 1m/W when light emitted from both top and bottom surfaces is summed. We find that 10% more light is emitted from the top surface; hence a power efficiency of 121m/W is obtained for a device viewed through the top, transparent contact. This device, with applications to head-up and displays employing n-type Si driver circuitry, has significantly higher power efficiency and lower drive voltage than undoped fluorescent inverted OLEDs. Efficient injection of both electrons and holes is made possible by controlled n- and p-doping of the transport layers with high doping levels. The light emitting region is protected from ITO sputtering damage by a 210 nm thick p-doped hole transport layer. The transparency of the device at the peak OLED emission wavelength of 510 nm is (80 +/- 5)%. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: We present an approach to stable n-type doping of organic matrices using organic dopands. To circumvent stability limitations inherent in strong organic donors, we produce the donor from a stable precursor compound in situ. As an example, pyronin B chloride is studied as a dopant in a 1,4,5,8-naphthalene tetracarboxylic dianhydride matrix. Conductivities up to 2x10(-4) S/cm are obtained, which is two orders of magnitude higher than obtained previously using bis(ethylenedithio)-tetrathiafulvalene as a dopant [A. Nollau, M. Pfeiffer, T. Fritz, and K. Leo, J. Appl. Phys. 87, 4340 (2000)]. Field-effect measurements are used to prove n-type conduction. Other matrices which can be doped are N,Nâ-dimethyl-perylene-3,4,9,10-tetracarboxylic diimide and fullerene C-60, frequently used in organic solar cells. Visible light and Fourier-transform infrared spectroscopy confirm the donor properties of pyronin B. (C) 2003 American Institute of Physics.
Abstract: The optical properties of an electrically biased semiconductor superlattice are strongly influenced by coupling to other bands. As these coupling mechanisms depend on the strength of the applied electric field, we find a distinct variation in transition line broadening, dephasing time and spatial extension of the wavefunction. The fundamental Bloch-oscillating transport in a periodic structure is drastically modified and exhibits a strong damping with a surprising revival of polarization coherence which reappears on a picosecond scale.
Abstract: Inverted transparent multi-layered vacuum deposited organic light-emitting diodes (OLEDs) employing an indium-tin-oxide (ITO) coated glass substrate directly as cathode and a semitransparent top Au thin film as anode comprise an emissive layer sandwiched in between n- and p-doped charge transport layer with appropriate blocking layers to form a nil) structure. By doping the 8-tris-hydroxyquinoline-aluminum (Alq(3)) layer with highly fluorescent molecules, coumarin 6, the external current efficiency has been improved by about a factor of 2, up to 6 cd/A, in comparison with intrinsic Alq(3) devices. The devices are about 50% transparent and emit green light from both sides (4.4 V for 100 cd/m(2)). (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: We demonstrated high-efficiency low-voltage stable inverted transparent electrophosphorescent organic light-emitting diodes employing an indium-tin-oxide coated glass substrate directly as cathode and a semitransparent top Au thin film as anode. The structure contains an iridium-complex doped emissive layer sandwiched in between n- and p-doped charge transport layer with appropriate blocking layers to form a nip structure. The devices are about 50% transparent and emit green light from both sides with peak external quantum efficiency (EQE) of 4.08% (14.3 cd/A). At 100 cd/m(2), the EQE is 3.8% (13 cd/A) at an operating voltage of 4.3 V. The devices exhibit a lifetime of above 50 hours under continuous constant-current driving for the initial luminance of about 9000 cd/m(2) in vacuum, which project a lifetime of 5000 hours for 100 cd/m(2).
Abstract: We have studied the behavior of various intrinsic emission zones on the characteristics of organic light-emitting diodes with a p-doped hole-transport layer and an n-doped electron-transport layer based on our previous work [J. S. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, K. Leo, and S. Liu, Appl. Phys. Lett. 80, 139 (2002)]. This configuration is referred to as a PiN structure. Because the p- and n-doped regions occupy nearly 80% of the total thickness in our PiN device, the intrinsic region becomes a narrow layer between two doped regions. This intrinsic region includes the region where the radiative recombination occurs. Thus, the nature of this layer plays an important role in determining the actual device performance. Employing 8-tris-hydroxyquinoline aluminum as an emitter, we investigated the influence of the thickness of the emitter layer on the performance of the device. The optimum thickness of the emitter layer is found to be 20 nm. Combining the fluorescence dye doping method, we have optimized the PiN structure device. Two emitter systems have been used: Alq(3) doped with two highly fluorescent laser dyes, Quinacridone or Coumarin 6, respectively. We have demonstrated the influence of the thickness and the doping of the emission zone on the characteristics of a doped emitter device with PiN structure, and obtained higher-efficiency PiN structure devices. The different properties of PiN devices corresponding to two different emitter dopants with different trapping effect are also discussed. (C) 2003 American Institute of Physics.
Abstract: The Coulomb coupling between Wannier-Stark excitons and the continuum of lower-lying states in an electrically biased superlattice leads not only to pronounced Fano resonances, but has also a heavy impact on the nonlinear polarization dynamics. The strong variation of line broadening in linear spectra with the electric field, however, is not reflected in the dephasing rate. Moreover, increasing the excitation density leads to a linear growth of the time-resolved four-wave-mixing signal instead of a cubic dependence. We present a theory based upon the Born-Markov equations of a superlattice. The unusual polarization dynamics can be traced back to the interference resulting from the simultaneous occupation of different Wannier-Stark subbands and the relaxation of these nonequilibrium distributions.
Abstract: We demonstrate a transparent, inverted, electrophosphorescent n-i-p organic light emitting diode (OLED) exhibiting a luminance of 500 cd/m(2) at 3.1 V, and with a luminous power efficiency of 23 1m/W when light emitted from both top and bottom surfaces is summed. We find that 10% more light is emitted from the top surface; hence a power efficiency of 121m/W is obtained for a device viewed through the top, transparent contact. This device, with applications to head-up and displays employing n-type Si driver circuitry, has significantly higher power efficiency and lower drive voltage than undoped fluorescent inverted OLEDs. Efficient injection of both electrons and holes is made possible by controlled n- and p-doping of the transport layers with high doping levels. The light emitting region is protected from ITO sputtering damage by a 210 nm thick p-doped hole transport layer. The transparency of the device at the peak OLED emission wavelength of 510 nm is (80 +/- 5)%. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: We present a systematic study on photovoltaic devices that combine an organic small molecule photoactive donor-acceptor bulk heterojunction system with controlled doping of the charge transport layers. The doped transport layers are formed using high vacuum co-evaporation deposition technique (i.e. co-sublimation of matrix and dopant). Solar cell devices have been fabricated based on zinc-phthalocyanine (ZnPc) as donor (D) and fullerene (C-60) as electron acceptor (A) with doped charge transport layers. The cells show a short circuit current, I-sc = 1.5 mA/cm(2), an open circuit voltage, V-oc = 450 mV, a fill factor, FF = 0.5, and a power conversion efficiency, eta(e) = 3.37% under 1/10 sun (10 mW/cm(2)) white light illumination. In addition, these bulk-heterojunction photovoltaic devices were characterized under 1 sun (100 mW/cm(2)) white light illumination showing I-sc = 6.3 mA/cm(2), V-oc = 500 mV, and eta(e) = 1.04%. We have observed that the performance of such âbulk-heterojunctionâ photovoltaic devices is critically dependent on the transport properties of the interpenetrating network D/A system and doped charge transport layers. (C) 2002 Elsevier B.V. All rights reserved.
Abstract: The Coulomb coupling between Wannier-Stark excitons and the continuum of lower-lying states in an electrically biased superlattice leads not only to pronounced Fano resonances, but has also a heavy impact on the nonlinear polarization dynamics. The strong variation of line broadening in linear spectra with the electric field, however, is not reflected in the dephasing rate. Moreover, increasing the excitation density leads to a linear growth of the time-resolved four-wave-mixing signal instead of a cubic dependence. We present a theory based upon the Born-Markov equations of a superlattice. The unusual polarization dynamics can be traced back to the interference resulting from the simultaneous occupation of different Wannier-Stark subbands and the relaxation of these nonequilibrium distributions.
Abstract: Inverted transparent multi-layered vacuum deposited organic light-emitting diodes (OLEDs) employing an indium-tin-oxide (ITO) coated glass substrate directly as cathode and a semitransparent top Au thin film as anode comprise an emissive layer sandwiched in between n- and p-doped charge transport layer with appropriate blocking layers to form a nil) structure. By doping the 8-tris-hydroxyquinoline-aluminum (Alq(3)) layer with highly fluorescent molecules, coumarin 6, the external current efficiency has been improved by about a factor of 2, up to 6 cd/A, in comparison with intrinsic Alq(3) devices. The devices are about 50% transparent and emit green light from both sides (4.4 V for 100 cd/m(2)). (C) 2003 Elsevier Science B.V. All rights reserved.
Notes: Spring Meeting of the European-Materials-Research-Society (E-MRS), STRASBOURG, FRANCE, JUN 18-21, 2002
Abstract: In this paper, we discuss recent experiments which prove that evaporated organic films can be efficiently doped by co-evaporation with organic dopant molecules. Key advantages for devices are the high conductivity and the formation of ohmic contacts despite large energetic barriers. For p-type doping, efficient doping is possible for a variety of polycrystalline and amorphous materials. Despite the differences in the microscopic behavior, all basic effects known from doped inorganic semiconductors are found in organics as well. However, efficient n-type doping with stable molecular dopants is still a challenge. Organic light emitting diodes (OLED) with conductivity doped transport layers show significantly improved properties: For instance, we have achieved a brightness of 100 cd/m(2) already at a voltage of 2.55 V, well below previous results for undoped devices. The advantages of doping are even more pronounced for top-emitting, inverted OLED structures: Due to the ohmic contacts nearly independent of the contact properties, it is possible to realize inverted top-emitting devices with parameters comparable to standard devices. Our doping technology is thus a significant advantage for active-matrix OLED displays and other displays on opaque substrate. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: For high electric fields, the lifetime of Wannier-Stark ladder states in a periodic potential is reduced by the fundamental process of Zener tunneling. We report on the analysis of the coherence lifetime of such states in semiconductor superlattices by interband spectroscopy. The reduction of lifetime by strong coupling between bands can only in the first approximation be described by the well-known Zener theory. A recently developed theoretical model is applied to calculate directly the tunneling probability of Wannier-Stark states as a function of the electric field. The theoretical results compare well with experiment, reproducing the complex interplay of both nonresonant and resonant Zener tunneling to higher bands. By comparing experiment and theory for a superlattice with a symmetric and one with a nonsymmetric potential, we can draw conclusions on a very general basis about the sensitive dependence of Zener tunneling on the specific dispersion relation of the carriers.
Abstract: We demonstrate high-efficiency electrophosphorescent organic light-emitting diodes (PHOLEDs) with double light-emitting layers (D-EMLs) by doping both hole and electron transport hosts with fac tris(2-phenylpyridine)iridium [Ir(ppy)(3)] simultaneously. The D-EMLs PHOLEDs show significantly improved efficiency (peak external quantum efficiency of about 12.6%, corresponding to a current efficiency of 44.3 cd/A) compared to the conventional PHOLEDs with a single EML and either hole or electron transport host doped with Ir(ppy)(3). We attribute this improvement mainly to reduced losses of triplet excitons into regions that are not doped by phosphorescent emitter molecules. (C) 2002 American Institute of Physics.
Abstract: Phonon modes which are strongly coupled to the lowest optical transition of the quasi-one-dimensional organic crystal of MePTCDI (N-N'-dimethylperylene-3,4,9,10-dicarboximide) are directly time-resolved as they appear as coherent wavepackets in femtosecond pump-probe experiments. We observe both coherent oscillations of intramolecular vibrational wavepackets (internal phonons) and, for the first time in a quasi-one-dimensional organic crystal, lower-energetic modulations which are related to coherent lattice phonons (external phonons). The phononic wavepacket motion is attributed to the electronic ground-state potential surface. The coherence decay of all observed vibrations is in the range of some picoseconds. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: We present a study of the Zener effect in the optical absorption of strongly coupled superlattices with both a magnetic and an electric field in growth direction. The in-plane continuum of electron states is discretized due to Landau quantization, which allows to directly observe the transition from discrete to continuum states due to Zener tunneling in a true 1D system.
Abstract: We present an electron energy-loss study of the nature and dispersion of excitons in quasi-one-dimensional 3,4:9,10-perylenetetracarboxylic dianhydride molecular crystals. The observed anisotropy of the excitation energies, the spectral shape and the dispersion indicates the presence of two types of excitons near the excitation onset. These are associated with mixed intramolecular Frenkel and intermolecular charge-transfer excitons. In particular, the excitons exhibit a negative dispersion along the molecular stacks which has important consequences for their recombination.
Abstract: Phonon modes which are strongly coupled to the lowest optical transition of the quasi-one-dimensional organic crystal of MePTCDI (N-Nâ-dimethylperylene-3,4,9,10-dicarboximide) are directly time-resolved as they appear as coherent wavepackets in femtosecond pump-probe experiments. We observe both coherent oscillations of intramolecular vibrational wavepackets (internal phonons) and, for the first time in a quasi-one-dimensional organic crystal, lower-energetic modulations which are related to coherent lattice phonons (external phonons). The phononic wavepacket motion is attributed to the electronic ground-state potential surface. The coherence decay of all observed vibrations is in the range of some picoseconds. (C) 2002 Elsevier Science B.V. All rights reserved.
Notes: 10th International Conference on Phonon Scattering in Condensed Matter, HANOVER, NEW HAMPSHIRE, AUG 12-17, 2001
Abstract: The structure of [4-(phenylazo)phenoxy]hexane-1-thioI (AzoC(6)) self-assembled monolayers (SAMs) on Au(111) has been investigated with scanning tunneling microscopy (STM) and Fourier transform infrared-reflection absorption spectroscopy (FTIR-RAS). The FTIR-RAS results yield a tilt angle of the molecules close to 0degrees, which significantly differs from the 30degrees tilt angle of linear n-alkanethiols. In STM images two types of domains are observed that have equal unit cell dimensions and two molecules per unit cell but show different tunneling contrasts, which is attributed to a different arrangement of the molecules within the unit cell. The relationship of the molecular lattice to the substrate lattice is found to be commensurate.
Abstract: Organic thin film materials are widely investigated for applications in electronic and optoelectronic devices. For OLEDs with low operation voltage, controlled doping of the charge transport layers is particularly important. We have recently shown that starburst like amorphous materials (m-MTDATA) can be doped by strong molecular acceptors like F-4-TCNQ (tetrafluoro-tetracyano-quinodimethane). These amorphous materials have the advantage of being stable up to 100 degreesC and forming smooth surfaces. Very well blocking Mip-type Schottky structures (rectification ratio 10(4)-10(5)) allow to determine doping profiles using CV (capacitance-voltage) spectroscopy. The interface between the doped and the undoped layer can be clearly seen as an abrupt change of the capacitance. The impedance spectra can be understood in terms of an equivalent circuit model. The deduced parameters agree well with the nominal thicknesses of the i- and the p-layer and the independently measured bulk conductivity. (C) 2002 Elsevier Science B.V. All rights reserved.
Notes: E-MRS 2001 Spring Meeting, STRASBOURG, FRANCE, JUN 05-08, 2001
Abstract: We investigate the dynamics of a Bloch-oscillating wave packet in the presence of strong coupling to delocalized above barrier states (Zener tunneling), using time-resolved intraband polarization-sensitive measurements. At a threshold electric field, the resonance of localized and delocalized states causes a quantum beating which is observed as a revival in the intraband polarization. Our numerical simulation visualizes the spatial wave packet decomposition and reformation. The wave packet moves on a ps time scale over a distance of more than 100 nm and sequentially undergoes Bloch oscillations in the below- and above-barrier bands.
Abstract: We have realized a small-molecule organic light-emitting diode where the intrinsic emitter layer is sandwiched by n- and p-doped transport layers with appropriate blocking layers. The diodes based on this pin concept have exponential forward characteristics up to comparatively high current densities. The diodes reach high brightness at very low operating voltage: for instance, 1000 cd/m(2) at a voltage of 2.9 V. Despite the highly doped transport layers, the devices reach very high efficiency for the given emitter system up to high brightness. (C) 2002 American Institute of Physics.
Abstract: We have previously shown that p-type doping of the hole injection and transport layer of an organic light emitting diode (OLED) by coevaporation of two molecules leads to lower operating voltages of the device. In such OLEDs, the use of a proper buffer layer between the doped layer and the light emission layer is essential to yield a high efficiency. Here, we apply the doping concept to OLEDs with a light emission layer doped with a fluorescent dye in order to prove that doping of the transport layer is able to improve the optoelectronic properties of highly efficient OLEDs. The emitter layer consists of quinacridone (QAD) doped aluminum-tris-(8-hydroxy-quinolate) (Alq(3)). The hole injection and transport layer is a Starburst layer p-type-doped with tetrafluoro-tetracyano-quinodimethane (F-4-TCNQ). As buffer layer, a diamine layer (TPD) is used. Holes are injected from untreated ITO electrons via a lithium fluoride (LiF)/aluminum cathode combination. For this OLED layer sequence, we achieved a luminance of 100 cd/m(2) in forward direction at the lowest operating voltage ever reported for nonpolymeric OLEDs (3.2-3.4 V) with a current efficiency of around 10 cd/A. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: We present a study of the Zener effect in the optical absorption of strongly coupled superlattices with both a magnetic and an electric field in growth direction. The in-plane continuum of electron states is discretized due to Landau quantization, which allows to directly observe the transition from discrete to continuum states due to Zener tunneling in a true 1D system.
Abstract: We demonstrate low-voltage inverted transparent vacuum deposited organic light-emitting diodes employing an indium-tin-oxide coated glass substrate directly as cathode and a semitransparent top Au thin film as anode. The devices comprise an intrinsic 8-tris-hydroxyquinoline aluminum (Alq(3)) emitting layer sandwiched in between n- and p-doped charge transport layer with appropriate blocking layers. They exhibit low driving voltages (similar to4 V for a luminance of similar to100 cd/m2). The devices are about 50% transparent in the Alq(3) emission region and emit green light from both sides with a total external current efficiency of about 2.5 cd/A. (C) 2002 American Institute of Physics.
Abstract: Organic thin film materials are widely investigated for applications in electronic and optoelectronic devices. For OLEDs with low operation voltage, controlled doping of the charge transport layers is particularly important. We have recently shown that starburst like amorphous materials (m-MTDATA) can be doped by strong molecular acceptors like F-4-TCNQ (tetrafluoro-tetracyano-quinodimethane). These amorphous materials have the advantage of being stable up to 100 degreesC and forming smooth surfaces. Very well blocking Mip-type Schottky structures (rectification ratio 10(4)-10(5)) allow to determine doping profiles using CV (capacitance-voltage) spectroscopy. The interface between the doped and the undoped layer can be clearly seen as an abrupt change of the capacitance. The impedance spectra can be understood in terms of an equivalent circuit model. The deduced parameters agree well with the nominal thicknesses of the i- and the p-layer and the independently measured bulk conductivity. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: We demonstrate high-efficiency electrophosphorescent organic light-emitting diodes (PHOLEDs) with double light-emitting layers (D-EMLs) by doping both hole and electron transport hosts with fac tris(2-phenylpyridine)iridium [Ir(ppy)(3)] simultaneously. The D-EMLs PHOLEDs show significantly improved efficiency (peak external quantum efficiency of about 12.6%, corresponding to a current efficiency of 44.3 cd/A) compared to the conventional PHOLEDs with a single EML and either hole or electron transport host doped with Ir(ppy)(3). We attribute this improvement mainly to reduced losses of triplet excitons into regions that are not doped by phosphorescent emitter molecules. (C) 2002 American Institute of Physics.
Abstract: We investigate the dynamics of a Bloch-oscillating wave packet in the presence of strong coupling to delocalized above barrier states (Zener tunneling), using time-resolved intraband polarization-sensitive measurements. At a threshold electric field, the resonance of localized and delocalized states causes a quantum beating which is observed as a revival in the intraband polarization. Our numerical simulation visualizes the spatial wave packet decomposition and reformation. The wave packet moves on a ps time scale over a distance of more than 100 nm and sequentially undergoes Bloch oscillations in the below- and above-barrier bands.
Abstract: We have previously shown that p-type doping of the hole injection and transport layer of an organic light emitting diode (OLED) by coevaporation of two molecules leads to lower operating voltages of the device. In such OLEDs, the use of a proper buffer layer between the doped layer and the light emission layer is essential to yield a high efficiency. Here, we apply the doping concept to OLEDs with a light emission layer doped with a fluorescent dye in order to prove that doping of the transport layer is able to improve the optoelectronic properties of highly efficient OLEDs. The emitter layer consists of quinacridone (QAD) doped aluminum-tris-(8-hydroxy-quinolate) (Alq(3)). The hole injection and transport layer is a Starburst layer p-type-doped with tetrafluoro-tetracyano-quinodimethane (F-4-TCNQ). As buffer layer, a diamine layer (TPD) is used. Holes are injected from untreated ITO electrons via a lithium fluoride (LiF)/aluminum cathode combination. For this OLED layer sequence, we achieved a luminance of 100 cd/m(2) in forward direction at the lowest operating voltage ever reported for nonpolymeric OLEDs (3.2-3.4 V) with a current efficiency of around 10 cd/A. (C) 2002 Elsevier Science B.V. All rights reserved.
Notes: E-MRS 2001 Spring Meeting, STRASBOURG, FRANCE, JUN 05-08, 2001
Abstract: We demonstrate low-voltage inverted transparent vacuum deposited organic light-emitting diodes employing an indium-tin-oxide coated glass substrate directly as cathode and a semitransparent top Au thin film as anode. The devices comprise an intrinsic 8-tris-hydroxyquinoline aluminum (Alq(3)) emitting layer sandwiched in between n- and p-doped charge transport layer with appropriate blocking layers. They exhibit low driving voltages (similar to4 V for a luminance of similar to100 cd/m2). The devices are about 50% transparent in the Alq(3) emission region and emit green light from both sides with a total external current efficiency of about 2.5 cd/A. (C) 2002 American Institute of Physics.
Abstract: The structure of [4-(phenylazo)phenoxy]hexane-1-thioI (AzoC(6)) self-assembled monolayers (SAMs) on Au(111) has been investigated with scanning tunneling microscopy (STM) and Fourier transform infrared-reflection absorption spectroscopy (FTIR-RAS). The FTIR-RAS results yield a tilt angle of the molecules close to 0degrees, which significantly differs from the 30degrees tilt angle of linear n-alkanethiols. In STM images two types of domains are observed that have equal unit cell dimensions and two molecules per unit cell but show different tunneling contrasts, which is attributed to a different arrangement of the molecules within the unit cell. The relationship of the molecular lattice to the substrate lattice is found to be commensurate.
Abstract: We have realized a small-molecule organic light-emitting diode where the intrinsic emitter layer is sandwiched by n- and p-doped transport layers with appropriate blocking layers. The diodes based on this pin concept have exponential forward characteristics up to comparatively high current densities. The diodes reach high brightness at very low operating voltage: for instance, 1000 cd/m(2) at a voltage of 2.9 V. Despite the highly doped transport layers, the devices reach very high efficiency for the given emitter system up to high brightness. (C) 2002 American Institute of Physics.
Abstract: We present an electron energy-loss study of the nature and dispersion of excitons in quasi-one-dimensional 3,4:9,10-perylenetetracarboxylic dianhydride molecular crystals. The observed anisotropy of the excitation energies, the spectral shape and the dispersion indicates the presence of two types of excitons near the excitation onset. These are associated with mixed intramolecular Frenkel and intermolecular charge-transfer excitons. In particular, the excitons exhibit a negative dispersion along the molecular stacks which has important consequences for their recombination.
Abstract: We demonstrate the use of a p-doped amorphous starburst amine, 4, 4â, 4â-tris(N, N-diphenyl- amino)triphenylamine (TDATA), doped with a very strong acceptor, tetrafluoro- tetracyano-quinodimethane by controlled coevaporation as an excellent hole injection material for organic light-emitting diodes (OLEDs). Multilayered OLEDs consisting of double hole transport layers of p-doped TDATA and triphenyl-diamine, and an emitting layer of pure 8-tris-hydroxyquinoline aluminum exhibit a very low operating voltage (3.4 V) for obtaining 100 cd/m(2) even for a comparatively large (110 nm) total hole transport layer thickness. (C) 2001 American Institute of Physics.
Abstract: Hexa-peri-hexabenzocoronene (C42H18, HBC) films adsorbed on the Au(111) surface were investigated by means of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). The results show that both methods give comparable results for the electronic structure of the occupied states, if the STS is performed at appropriate parameters. Additionally, STS gives an immediate insight into the unoccupied states. The highest occupied state of multilayer films is found 1.3-1.4 eV below the Fermi levels and the lowest unoccupied state 1.8 eV above the Fermi level. The resulting transport gap is compared to optical absorption measurements. Work-function changes of -0.8 eV indicate an interface dipole, i.e., vacuum level alignment does not occur at the HBC-Au interface. Compared to UPS, the voltage range of the STS measurements is limited to prevent sample damage.
Abstract: The Zener effect in the optical absorption of shallow superlattices is studied theoretically and experimentally. The comparison between the Kane approximation, which is the widely accepted picture so far, and a numerical calculation including all subbands, provides clear evidence for the Zener effect. The experimental spectra are in good agreement with the theory. The measured line broadening as a function of the electric field behaves as predicted by the Zener theory, and is of the same order of magnitude as calculated from the sample parameters. We also suggest absorption measurements in a magnetic field in growth direction, which would provide an even clearer demonstration for the transition between discrete transitions and the continuum due to Zener tunneling.
Abstract: We investigate the energy spectrum and the electron dynamics of a band in a semiconductor superlattice as a function of the electric field. Linear optical spectroscopy shows that, for high fields, the well-known localization of the Bloch states is followed by a field-induced delocalization, associated with Zener breakdown. Using time-resolved measurements, we observe Bloch oscillations in a regime where they are damped by Zener breakdown.
Abstract: PURPOSE. TO investigate the segregation pattern of the mitochondrial DNA mutation at nucleotide position 3460 responsible for Leberâs hereditary optic neuropathy (LHON) and to determine the prevalence of heteroplasmy for the three primary LHON mutations at positions 11778, 3460, and 14484. METHODS. Segregation analysis was performed in a cross-sectional study by determining the level of heteroplasmy in blood leukocytes of 23 LHON patients and unaffected carriers from four unrelated families. One family comprising two affected and three unaffected carriers was followed over 5.5 years for a longitudinal segregation analysis of heteroplasmy. The percentage of mutant mtDNA was determined using a novel procedure of fluorescence-based primer extension and restriction fragment length polymorphism analysis. The prevalence of heteroplasmy was assessed by determining the number of genealogically unrelated LHON pedigrees with heteroplasmic maternal family members from the LHON patient records of the Department of Ophthalmology, University of Tubingen, Germany. RESULTS. The authors observed a marked variability in the degree of heteroplasmy levels within each pedigree and a tendency toward a higher mutant allele frequency in offspring generations. Disease expression was correlated with higher levels of mutant mtDNA molecules. Longitudinal analysis revealed no statistically significant decrease in the heteroplasmy level in the family studied but a reduction of 11% and 12D/o in one affected and one unaffected individual, respectively. In 167 genealogically unrelated LHON families the prevalence of heteroplasmy was 5.6%, 40%, and 36.4% for the 11778, 3460, and 14484 LHON mutations, respectively. CONCLUSIONS. Cross-sectional studies of heteroplasmy for the 3460 LHON mutation suggest that the genotype shifts toward a higher mutational load in offspring generations. Long-term decrease in the blood mutant load in single cases indicates negative selection of the mutant allele in the hematopoietic cell system. The prevalence of heteroplasmy varies significantly between the different primary LHON mutations, suggesting genotypical differences in disease expression.
Abstract: We have directly time resolved coherent phonon oscillations in quasi-one-dimensional organic crystals of MePTCDI (N-N â -dimethvlperylene-3,4,9,10-dicarboximide), using femtosecond pump-probe experiments. We observe both higher-energy oscillations caused by intramolecular vibrations (internal phonons) and, fur the first time in a quasi-one-dimensional organic system, lower-energy modulations which are related to coherent lattice phonons (external phonons). For internal A(g) vibrations, the coherence decay rime of about 2 ps is almost independent of the mode. In contrast, the damping time of the external phonons increases strongly with decreasing energy.
Abstract: For the first time, heterostructures of 3,4,9,10-perylenetetracarboxylic-3,4,9, 10-dianhydride (PTCDA) on hexa-peri-hexabenzocoronene (HBC) on reconstructed Au(1 1 1) have been prepared with molecularly perfect interfaces by organic molecular beam epitaxy. First, we investigate the growth of HBC on reconstructed Au(l 1 I) surfaces. Large domains which are defect free on the molecular scale are observed by scanning tunneling microscopy (STM). While a surface reconstruction of the substrate can still be seen through the molecular film, the periodicity has changed as compared to the bare substrate. The low energy electron diffraction (LEED) images show a hexagonal pattern for the HBC films. From our simulations, it is concluded that HBC grows on such modified reconstructed surfaces in a point-on-line coincident superstructure with a hexagonal unit cell. Second, a monolayer of PTCDA is deposited on HBC and structurally characterized. Despite the inhomogeneous nature of the sample we are still able to obtain molecularly reserved STM images for the PTCDA. The LEED and STR;I measurements reveal the well-known herringbone alignment of the PTCDA molecules with two molecules in a rectangular unit cell, similar to that of the (1 0 2) bulk plane of PTCDA. The LEED measurements and the pronounced two-dimensional moire pattern in the STM images are used to determine precisely the distinctive relative orientation of the PTCDA unit cell with respect to the underlying HBC lattice. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: Coherent phonon oscillations in quasi-1D organic crystals of MePTCDI (N-Nâ-dimethylperylene-3,4,9,10-dicarboximide) are directly time-resolved by femtosecond pump-probe experiments. We observe both oscillations caused by intra-molecular vibrations (internal phonons) and, for the first time in a quasi-1D organic system. modulations which are related to coherent lattice phonons (external phonons). From the observed phonon coherence of a few picoseconds, we assign the observed phonon oscillations to the electronic ground state potential. (C) 2001 Elsevier Science B.V. All rights reserved.
Notes: 30th International Conference on Dynamical Proces Excited States of Solids, VILLEURBANNE, FRANCE, JUL 01-04, 2001
Abstract: The trap properties of the commonly used organic light-emitting diode emitter material tris-8-(hydroxyquinoline) aluminum (Alq(3)) have been investigated using thermally stimulated currents. Based on a model of the field dependence of the thermally stimulated currents, a trap density of 1.3x10(17) cm(-3) for depths ranging from 0.05 to 0.7 eV is obtained, indicating considerable influence on charge carrier statistics. A field-induced lowering of trap depth was observed and explained in the framework of the Poole-Frenkel effect. (C) 2001 American Institute of Physics.
Abstract: Hexa-peri-hexabenzocoronene (C42H18, HBC) films adsorbed on the Au(111) surface were investigated by means of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). The results show that both methods give comparable results for the electronic structure of the occupied states, if the STS is performed at appropriate parameters. Additionally, STS gives an immediate insight into the unoccupied states. The highest occupied state of multilayer films is found 1.3-1.4 eV below the Fermi levels and the lowest unoccupied state 1.8 eV above the Fermi level. The resulting transport gap is compared to optical absorption measurements. Work-function changes of -0.8 eV indicate an interface dipole, i.e., vacuum level alignment does not occur at the HBC-Au interface. Compared to UPS, the voltage range of the STS measurements is limited to prevent sample damage.
Abstract: We demonstrate enhanced hole injection and lowered driving voltage in vacuum-deposited organic light-emitting diodes (OLEDs) with a hole-transport layer using the starburst amine 4,4',4"-tris(N,N-diphenyl-amino)triphenylamine (TDATA) p-doped with a very strong acceptor, tetrafluoro-tetracyano-quinodimethane (F-4-TCNQ) by controlled coevaporation. The doping leads to high conductivity of doped TDATA layers and a high density of equilibrium charge carriers, which facilitates hole injection and transport. Moreover, multilayer OLEDs consisting of double hole-transport layers of thick p-doped TDATA and a thin triphenyl-diamine (TPD) interlayer exhibit very low operating voltages.
Abstract: The organic molecule 3,4,9,10-perylenetetracarboxylic-3,4,9,10-dianhydride (PTCDA) deposited on Au(1 0 0) has been investigated by scanning tunneling spectroscopy (STS). For this purpose, highly ordered ultrathin PTCDA films have been prepared by organic molecular beam epitaxy. Single point current-voltage spectroscopy at fixed tip-sample separations has been performed in UHV on closed PTCDA monolayers as well as on PTCDA islands. The normalized differential conductivity curves calculated from these I-V data reveal a pronounced peak at +0.8 to 0.9 V sample bias. No peak in the normalized differential conductivity was observed for uncovered gold regions. The STS results indicate tunneling via the lowest unoccupied molecular orbital at adequate bias voltages. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: Semiconductor superlattices have been intensively used as a model system for the investigation of high field transport in solids. Here, we discuss ultrafast optical experiments which allow us to monitor the electron wave packet motion in detail. First, we discuss a novel technique which directly traces the center of mass spatial motion. We use this method for the detection of a linear motion of the wave packets, which is superimposed on the harmonic Bloch oscillations. A comparison with theory shows that this coherent analog to the Shapiro effect in superconductors is associated with gain at THz frequencies. In a second part, we discuss recent results which address the Zener breakdown due to tunneling to higher bands by linear spectroscopy, which directly shows the wave function delocalization. We also trace the dynamics of the Zener tunneling by time-resolved spectroscopy and show the damping of Bloch oscillations due to Zener tunneling. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: For the first time, heterostructures of 3,4,9,10-perylenetetracarboxylic-3,4,9, 10-dianhydride (PTCDA) on hexa-peri-hexabenzocoronene (HBC) on reconstructed Au(1 1 1) have been prepared with molecularly perfect interfaces by organic molecular beam epitaxy. First, we investigate the growth of HBC on reconstructed Au(l 1 I) surfaces. Large domains which are defect free on the molecular scale are observed by scanning tunneling microscopy (STM). While a surface reconstruction of the substrate can still be seen through the molecular film, the periodicity has changed as compared to the bare substrate. The low energy electron diffraction (LEED) images show a hexagonal pattern for the HBC films. From our simulations, it is concluded that HBC grows on such modified reconstructed surfaces in a point-on-line coincident superstructure with a hexagonal unit cell. Second, a monolayer of PTCDA is deposited on HBC and structurally characterized. Despite the inhomogeneous nature of the sample we are still able to obtain molecularly reserved STM images for the PTCDA. The LEED and STR;I measurements reveal the well-known herringbone alignment of the PTCDA molecules with two molecules in a rectangular unit cell, similar to that of the (1 0 2) bulk plane of PTCDA. The LEED measurements and the pronounced two-dimensional moire pattern in the STM images are used to determine precisely the distinctive relative orientation of the PTCDA unit cell with respect to the underlying HBC lattice. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: We have directly time resolved coherent phonon oscillations in quasi-one-dimensional organic crystals of MePTCDI (N-N ' -dimethvlperylene-3,4,9,10-dicarboximide), using femtosecond pump-probe experiments. We observe both higher-energy oscillations caused by intramolecular vibrations (internal phonons) and, fur the first time in a quasi-one-dimensional organic system, lower-energy modulations which are related to coherent lattice phonons (external phonons). For internal A(g) vibrations, the coherence decay rime of about 2 ps is almost independent of the mode. In contrast, the damping time of the external phonons increases strongly with decreasing energy.
Abstract: We investigate the energy spectrum and the electron dynamics of a band in a semiconductor superlattice as a function of the electric field. Linear optical spectroscopy shows that, for high fields, the well-known localization of the Bloch states is followed by a field-induced delocalization, associated with Zener breakdown. Using time-resolved measurements, we observe Bloch oscillations in a regime where they are damped by Zener breakdown.
Abstract: We demonstrate the use of a p-doped amorphous starburst amine, 4, 4', 4"-tris(N, N-diphenyl- amino)triphenylamine (TDATA), doped with a very strong acceptor, tetrafluoro- tetracyano-quinodimethane by controlled coevaporation as an excellent hole injection material for organic light-emitting diodes (OLEDs). Multilayered OLEDs consisting of double hole transport layers of p-doped TDATA and triphenyl-diamine, and an emitting layer of pure 8-tris-hydroxyquinoline aluminum exhibit a very low operating voltage (3.4 V) for obtaining 100 cd/m(2) even for a comparatively large (110 nm) total hole transport layer thickness. (C) 2001 American Institute of Physics.
Abstract: The exciton structure of crystalline MePTCDI (N-Nâ-dimethylperylene-3,4,9,10-dicarboximide) is modeled by a one-dimensional Hamiltonian, which includes the interactions between Renkel excitons with several vibronic levels and charge-transfer excitons. Using appropriate fitting parameters, which are verified by quantum chemical calculations, this model can explain the main features of the low temperature absorption spectrum. Polarized absorption spectra show different polarization ratios for the various peaks. This polarization behavior is explained by the varying contribution of the charge-transfer transition dipole, which has a direction different from the Frenkel transition dipole. Our model for the exciton band structure is supported by transient emission measurements.
Notes: 4th International Conference on Excitonic Processes in Condensed Matter (EXCON2000), OSAKA, JAPAN, AUG 22-25, 2000
Abstract: The Zener effect in the optical absorption of shallow superlattices is studied theoretically and experimentally. The comparison between the Kane approximation, which is the widely accepted picture so far, and a numerical calculation including all subbands, provides clear evidence for the Zener effect. The experimental spectra are in good agreement with the theory. The measured line broadening as a function of the electric field behaves as predicted by the Zener theory, and is of the same order of magnitude as calculated from the sample parameters. We also suggest absorption measurements in a magnetic field in growth direction, which would provide an even clearer demonstration for the transition between discrete transitions and the continuum due to Zener tunneling.
Abstract: In molecular crystals a charge carrier and a molecular (Frenkel) exciton are attracted to each other. This attraction arises from the increase of the molecular static polarizability upon electronic excitation, and may be responsible for the formation of a two-particle bound state of the exciton and the free charge. Such charged Frenkel excitons are analogous to trions (bound states of a Wannier-Mott exciton and a charge carrier) in inorganic semiconductors. In this paper we develop a theory of charged excitons in molecular crystals, and apply this theory to study the spectra of charged Frenkel excitons in two-dimensional structures, which serve as a rather good approximation for layered crystals like anthracene, tetracene. We show that the binding energy of charged Frenkel excitons can be of the order of several hundred cm(-1). Thus, they may be stable at room temperatures, in contrast to trions in inorganic semiconductors. Particularly interesting is the possibility of forming charged triplet excitons in the crystal of tetracene where the energy of the lowest triplet exciton is much smaller than the energy of the lowest excitation in ions. This circumstance prevents the transformation of charged triplet excitons into an ion excitation with short lifetime and promotes the formation of rather stable charged triplet excitation. We briefly discuss the optical properties of charged Frenkel excitons and the effects of a static electric field. (C) 2001 Published by Elsevier Science B.V.
Abstract: We use semiconductor superlattices as a model system for the investigation of Fano resonances. In absorption the excitonic transitions of the Wannier-Stark ladder show the typical asymmetric line shape due to coupling to the continuum of lower-lying transitions. The unique feature of these Fano resonances is that they allow to continuously tune the key parameter - the coupling strength Gamma between the discrete state and the degenerate continuum - by varying the bias voltage. Using this feature, we directly show that the Fano coupling leads to a fast polarization decay. We also investigate the dependence of the Fano parameters on the structure of the superlattice and compare with an extensive theoretical model of the resonances. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: In molecular crystals a charge carrier and a molecular (Frenkel) exciton are attracted to each other. This attraction arises from the increase of the molecular static polarizability upon electronic excitation, and may be responsible for the formation of a two-particle bound state of the exciton and the free charge. Such charged Frenkel excitons are analogous to trions (bound states of a Wannier-Mott exciton and a charge carrier) in inorganic semiconductors. In this paper we develop a theory of charged excitons in molecular crystals, and apply this theory to study the spectra of charged Frenkel excitons in two-dimensional structures, which serve as a rather good approximation for layered crystals like anthracene, tetracene. We show that the binding energy of charged Frenkel excitons can be of the order of several hundred cm(-1). Thus, they may be stable at room temperatures, in contrast to trions in inorganic semiconductors. Particularly interesting is the possibility of forming charged triplet excitons in the crystal of tetracene where the energy of the lowest triplet exciton is much smaller than the energy of the lowest excitation in ions. This circumstance prevents the transformation of charged triplet excitons into an ion excitation with short lifetime and promotes the formation of rather stable charged triplet excitation. We briefly discuss the optical properties of charged Frenkel excitons and the effects of a static electric field. (C) 2001 Published by Elsevier Science B.V.
Abstract: The organic molecule 3,4,9,10-perylenetetracarboxylic-3,4,9,10-dianhydride (PTCDA) deposited on Au(1 0 0) has been investigated by scanning tunneling spectroscopy (STS). For this purpose, highly ordered ultrathin PTCDA films have been prepared by organic molecular beam epitaxy. Single point current-voltage spectroscopy at fixed tip-sample separations has been performed in UHV on closed PTCDA monolayers as well as on PTCDA islands. The normalized differential conductivity curves calculated from these I-V data reveal a pronounced peak at +0.8 to 0.9 V sample bias. No peak in the normalized differential conductivity was observed for uncovered gold regions. The STS results indicate tunneling via the lowest unoccupied molecular orbital at adequate bias voltages. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: Semiconductor superlattices have been intensively used as a model system for the investigation of high field transport in solids. Here, we discuss ultrafast optical experiments which allow us to monitor the electron wave packet motion in detail. First, we discuss a novel technique which directly traces the center of mass spatial motion. We use this method for the detection of a linear motion of the wave packets, which is superimposed on the harmonic Bloch oscillations. A comparison with theory shows that this coherent analog to the Shapiro effect in superconductors is associated with gain at THz frequencies. In a second part, we discuss recent results which address the Zener breakdown due to tunneling to higher bands by linear spectroscopy, which directly shows the wave function delocalization. We also trace the dynamics of the Zener tunneling by time-resolved spectroscopy and show the damping of Bloch oscillations due to Zener tunneling. (C) 2001 Elsevier Science B.V. All rights reserved.
Notes: Rutherford Memorial Workshop on Semiconductor Nanostructures, QUEENSTOWN, NEW ZEALAND, FEB 05-09, 2001
Abstract: We demonstrate efficient organic electroluminescent devices with multiple well structure and a p-doped hole injection and transport layer (HTL). The multiple well structure improves the efficiency and the controlled p-doped HTL leads to a lower operating voltage. An amorphous starburst [4,4',4 " -tris(N,N-diphenylamino)triphenylamine] doped with a strong organic acceptor, tetrafluoro-tetracyano-quinodimethane serves as the HTL material, a spiro-linked compound, 2,2',7',7'-tetra-kis-(diphenylamine)-9,9'-spirobifluorene as an interlayer to provide a favorable interface and as a barrier within the Multiple well structure and 8-tris-hydroxyquinoline as an emitter and well. The double-well device exhibits low operating voltage, less than 4 V, for obtaining 100 cd/m(2) and the highest current efficiency exceeding 5 cd/A. Changes in the spectra due to the different well structures are also discussed.
Abstract: We demonstrate efficient organic electroluminescent devices with multiple well structure and a p-doped hole injection and transport layer (HTL). The multiple well structure improves the efficiency and the controlled p-doped HTL leads to a lower operating voltage. An amorphous starburst [4,4â,4 â -tris(N,N-diphenylamino)triphenylamine] doped with a strong organic acceptor, tetrafluoro-tetracyano-quinodimethane serves as the HTL material, a spiro-linked compound, 2,2â,7â,7â-tetra-kis-(diphenylamine)-9,9â-spirobifluorene as an interlayer to provide a favorable interface and as a barrier within the Multiple well structure and 8-tris-hydroxyquinoline as an emitter and well. The double-well device exhibits low operating voltage, less than 4 V, for obtaining 100 cd/m(2) and the highest current efficiency exceeding 5 cd/A. Changes in the spectra due to the different well structures are also discussed.
Abstract: We demonstrate enhanced hole injection and lowered driving voltage in vacuum-deposited organic light-emitting diodes (OLEDs) with a hole-transport layer using the starburst amine 4,4â,4â-tris(N,N-diphenyl-amino)triphenylamine (TDATA) p-doped with a very strong acceptor, tetrafluoro-tetracyano-quinodimethane (F-4-TCNQ) by controlled coevaporation. The doping leads to high conductivity of doped TDATA layers and a high density of equilibrium charge carriers, which facilitates hole injection and transport. Moreover, multilayer OLEDs consisting of double hole-transport layers of thick p-doped TDATA and a thin triphenyl-diamine (TPD) interlayer exhibit very low operating voltages.
Abstract: We present a systematic study on p-type doping of zinc-phthalocyanine by tetrafluoro-tetracyanoquinodimethane as an example of controlled doping of thin organic films by cosublimation of matrix and dopant. The zinc-phthalocyanine layers are prepared both in polycrystalline and amorphous phase by variation of the sublimation conditions. The films are electrically characterized in situ by temperature dependent conductivity and Seebeck and field-effect measurements. In addition to previous work, we show that also amorphous phthalocyanine layers can be doped, i.e., their conductivity increases and their Seebeck coefficient decreases indicating a shift of the Fermi level towards the hole transport level. The field-effect mobility of the polycrystalline samples is in the range of 10(-4)-10(-3) cm(-2)/Vs and increases with increasing dopant concentration. Adapting a percolation model presented by Vissenberg and Matters [Phys. Rev. B, 57, 12 964 (1998)], which assumes hopping transport within a distribution of localized states, we can quantitatively describe the conductivity (in different organic layers) and the field-effect mobility.
Abstract: We present a systematic study on p-type doping of zinc-phthalocyanine by tetrafluoro-tetracyanoquinodimethane as an example of controlled doping of thin organic films by cosublimation of matrix and dopant. The zinc-phthalocyanine layers are prepared both in polycrystalline and amorphous phase by variation of the sublimation conditions. The films are electrically characterized in situ by temperature dependent conductivity and Seebeck and field-effect measurements. In addition to previous work, we show that also amorphous phthalocyanine layers can be doped, i.e., their conductivity increases and their Seebeck coefficient decreases indicating a shift of the Fermi level towards the hole transport level. The field-effect mobility of the polycrystalline samples is in the range of 10(-4)-10(-3) cm(-2)/Vs and increases with increasing dopant concentration. Adapting a percolation model presented by Vissenberg and Matters [Phys. Rev. B, 57, 12 964 (1998)], which assumes hopping transport within a distribution of localized states, we can quantitatively describe the conductivity (in different organic layers) and the field-effect mobility.
Abstract: The trap properties of the commonly used organic light-emitting diode emitter material tris-8-(hydroxyquinoline) aluminum (Alq(3)) have been investigated using thermally stimulated currents. Based on a model of the field dependence of the thermally stimulated currents, a trap density of 1.3x10(17) cm(-3) for depths ranging from 0.05 to 0.7 eV is obtained, indicating considerable influence on charge carrier statistics. A field-induced lowering of trap depth was observed and explained in the framework of the Poole-Frenkel effect. (C) 2001 American Institute of Physics.
Abstract: The exciton structure of crystalline MePTCDI (N-N'-dimethylperylene-3,4,9,10-dicarboximide) is modeled by a one-dimensional Hamiltonian, which includes the interactions between Renkel excitons with several vibronic levels and charge-transfer excitons. Using appropriate fitting parameters, which are verified by quantum chemical calculations, this model can explain the main features of the low temperature absorption spectrum. Polarized absorption spectra show different polarization ratios for the various peaks. This polarization behavior is explained by the varying contribution of the charge-transfer transition dipole, which has a direction different from the Frenkel transition dipole. Our model for the exciton band structure is supported by transient emission measurements.
Abstract: Coherent phonon oscillations in quasi-1D organic crystals of MePTCDI (N-N'-dimethylperylene-3,4,9,10-dicarboximide) are directly time-resolved by femtosecond pump-probe experiments. We observe both oscillations caused by intra-molecular vibrations (internal phonons) and, for the first time in a quasi-1D organic system. modulations which are related to coherent lattice phonons (external phonons). From the observed phonon coherence of a few picoseconds, we assign the observed phonon oscillations to the electronic ground state potential. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: We use semiconductor superlattices as a model system for the investigation of Fano resonances. In absorption the excitonic transitions of the Wannier-Stark ladder show the typical asymmetric line shape due to coupling to the continuum of lower-lying transitions. The unique feature of these Fano resonances is that they allow to continuously tune the key parameter - the coupling strength Gamma between the discrete state and the degenerate continuum - by varying the bias voltage. Using this feature, we directly show that the Fano coupling leads to a fast polarization decay. We also investigate the dependence of the Fano parameters on the structure of the superlattice and compare with an extensive theoretical model of the resonances. (C) 2001 Elsevier Science B.V. All rights reserved.
Notes: 9th International Conference on Modulated Semiconductor Structures (MSS9), FUKUOKA, JAPAN, JUL 12-16, 1999
Abstract: We investigate the heteroepitaxial growth of two films of large planar molecules on an inorganic substrate: for the particular system [perylene-tetracarboxylic-dianhydride (PTCDA) on hexa-peri-benzocoronene (HBC) on highly ordered pyrolytic graphite (HOPG)], large domains of several hundred of nanaometers in size with interfaces that are flat and almost defect-free on the molecular scale are observed using scanning tunneling microscopy (STM). The images obtained represent, to the best of our knowledge, the first STM observation of the ordered growth of large planar molecules in a heteroepitaxially stacked film. The mutual lattice alignment and epitaxial relations between the two organic layers are determined from the pronounced moire pattern in the STM images. Beside these findings, the intramolecular contrast of HBC on HOPG is used to determine the molecular orientation of the molecules within the two-dimensional unit cell by STM. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We model the optical constants of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) using a modified Lorentz oscillator model and compare our results to the conventional Lorentz oscillator model. It is found that an adjustable broadening function incorporated in the modified Lorentz oscillator model should be used instead of a Lorentzian broadening function in the conventional oscillator model for modeling successfully the optical constants of organic materials. We attribute this to the assumption that each absorption peak is composed of a large number of individual transitions closely spaced in energy. (C) 2000 Published by Elsevier Science B.V.
Abstract: We report on the systematical investigation of Fano resonances in electrically biased semiconductor superlattices. In such quantum structures, Coulomb inter action couples discrete excitonic states of the Wannier-Stark ladder to continua of lower-lying subbands. In absorption these transitions show the typical asymmetric line shape of a Fano resonance. The unique feature of these Fano resonances is that they allow to continuously tune the key parameter - the coupling strength Gamma between the discrete state and the degenerate continuum - by varying the bias voltage. Using this feature, we directly show that the Fano coupling leads to a fast polarization decay. We also investigate the dependence of the Fano parameters on the structure of the superlattice and compare to an extensive theoretical model of the resonances.
Abstract: A new technique for determining the optical properties of organic thin films is presented. A detailed evaluation of the accuracy of the determined optical constants has been performed, and the best combination of measured values yielding the smallest errors in the index of refraction for realistic experimental uncertainties has been found. The proposed method utilizes the fact that optical constants are smooth continuous functions, which reduces the possibility of encountering multiple solutions. The method consists of two steps. In the first step the optical constants at all wavelengths and the him thickness are determined. In the second step the thickness and the imaginary part of the index of refraction are kept fixed while we reevaluate the real part of the index of refraction by using a different objective function with improved sensitivity to the refractive index. After verifying that the proposed method is capable of an accurate estimation of optical constants, we determine the index of refraction data of vanadyl phthalocyanine in the visible spectral range. (C) 2000 Optical Society of America.
Abstract: We investigate the electronic structure in strongly coupled superlattices over a wide field range. It is shown that the Wannier-Stark localization is followed by a field-induced delocalization which is due to a strong Zener coupling to continuum states. We compare different superlattice structures with high and with low barriers to alternatively investigate the effects of resonant interminiband tunneling and a strong Zener coupling to continuum states. The associated electrical breakdown is quantitatively shown in a detailed analysis of the absorption. A comprehensive theoretical model excellently describes the experimental results. Zener tunneling times are directly addressed by time-resolved experiments.
Notes: 6th International Workshop on Nonlinear Optics and Excitation Kinetics in Semiconductors (NOEKS 2000), MARBURG, GERMANY, APR 10-13, 2000
Abstract: Microscopic photoluminescence (mu-PL) spectroscopy through a photomask with a sub-micron resolution has been performed on In0.15Ga0.85As/GaAs quantum wells (QWs) containing three QWs with the same nominal well width of 30Angstrom but grown at different temperatures. PL from each QW was well resolved demonstrating that the interface abruptness of each QW is different due to the temperature dependence of In surface segregation during epitaxial growth. With decreasing window size down to sub-micron, the PL from QWs grown at higher temperatures showed a more drastic decrease compared with that from QWs grown at lower temperatures. This could be explained by assuming that there are less localized states and/or the in-plane exciton diffusion is enhanced in QWs grown at higher temperatures due to an enhanced smearing of the interfaces in the growth direction and the resultant formation of effectively smooth heterointerfaces.
Abstract: We show how the spatial dynamics of a Bloch oscillating wave packet can directly be measured and manipulated by means of optical techniques. For excitation below and above the Wannier-Stark ladder (WSL) center, the wave packet performs harmonic oscillations, following the predictions of Zener; for excitation near the WSL center the wave packet is in the so-called breathing mode: a symmetric oscillation with virtually zero center of mass motion. How the position and line width of the excitation laser changes the initially excited wave packets and their dynamics will be discussed. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: An investigation is made of the absorption spectra of a GaAs(82 Angstrom)/Al0.07Ga As-0.93(37 Angstrom) superlattice at 10 K in electric fields between 0 and 60 kV/cm. By comparing the experimental absorption spectra with calculations of the energies and oscillator strengths of transitions between states of the Wannier-Stark ladder it is established that in strong electric fields above-barrier states do not form a structureless continuum but a fan of states. Intersection of electric-field-localized electron states with the fan of above-barrier states leads to broadening, splitting, and nonmonotonic changes in the intensity of an intrawell transition band. The additional absorption band observed can be attributed to intrawell transitions between states in the left and right quantum wells of the superlattice. (C) 2000 MAIK "Nauka/Interperiodica".
Abstract: Self-assembled monolayers of decanethiol on Au(111) have been investigated using high resolution scanning tunneling microscopy (STM). After the annealing of the as-adsorbed densely packed c(2 root 3 x 4 root 3)R30 degrees structure and before the formation of a previously described low density (11.5 x root 3) stripe phase three different phases with intermediate molecular surface density have been observed. We present highly resolved and distortion-corrected STM images for both the (7.5 x root 3) stripe phase and the meshlike structure. We describe a mixed (7.5/11.5 x root 3) stripe phase that shows a switching of the stripe arrangement. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We report the realization of organic-organic heteroepitaxy by combining liquid-phase self-assembly with ultrahigh vacuum (UHV) gas-phase molecular beam epitaxy. As a model system, we have used self-assembled monolayers (SAMs) prepared by exposing an Au(111)-mica substrate to a dilute solution of decanethiol in ethanol, with subsequent evaporation of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) dye molecules. The well-known ( root 3 x root 3)R30 degrees superstructure of almost upright standing molecules after chemisorption is replaced by the 11.5 x root 3 (33.2 Angstrom) pin-stripe phase with flat lying molecules when the samples are annealed in UHV and coverage decreases. The deposition of PTCDA induces reordering and displacement in the decanethiol SAM. Additional to the 33.2 Angstrom periodicity, the previously reported 22 Angstrom thiol stripe phase can be observed by scanning tunneling microscopy. Several PTCDA structures are observed: single and double rows of PTCDA that grow along decanethiol stripes on top of the Aat-lying alkane chain groups, as well as densely packed PTCDA monolayer domains embedded into the thiol layer. These exhibit the well-known herringbone structure or a novel square lattice structure. We have also investigated thicker PTCDA islands and observed molecular resolution for a thickness of several monolayers. The structure of the three-dimensional islands can be identified as the alpha bulk modification. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: In this work, we model the optical constants of three organic materials using a modified Lorentz oscillator model. We have analysed the impact of the choice of the objective or merit function, which is employed for model parameter estimation, on the modelling of the optical constants. The objective function for model parameter determination should be chosen in such a manner that discrepancies between the experimental and calculated values for both parts of the index of refraction are minimized at the same time. We have found that the choice of objective function strongly influences the existence of multiple solutions. By combining a global optimization method, an appropriately chosen objective Function and a modified oscillator model, we obtain good agreement with the experimental data for three organic materials.
Abstract: We observe that the oscillatory motion of photoinjected electron-hole pairs in a biased semiconductor superlattice (Bloch oscillation) is accompanied by a coherent quasi-de current that is generated by the interaction of the carriers with the self-induced oscillating field. It is shown that this novel macroscopic quantum effect, which is a coherent analog of the Shapiro effect observed in Josephson junctions, can be controlled by changing the spectral position of the exciting laser pulse, which in turn determines the amplitude and phase of the wave packet oscillations. It is thereby possible to coherently drive the electrons either downwards or upwards in the potential of the static field.
Abstract: We model the optical constants of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) using a modified Lorentz oscillator model and compare our results to the conventional Lorentz oscillator model. It is found that an adjustable broadening function incorporated in the modified Lorentz oscillator model should be used instead of a Lorentzian broadening function in the conventional oscillator model for modeling successfully the optical constants of organic materials. We attribute this to the assumption that each absorption peak is composed of a large number of individual transitions closely spaced in energy. (C) 2000 Published by Elsevier Science B.V.
Abstract: In this work, we model the optical constants of three organic materials using a modified Lorentz oscillator model. We have analysed the impact of the choice of the objective or merit function, which is employed for model parameter estimation, on the modelling of the optical constants. The objective function for model parameter determination should be chosen in such a manner that discrepancies between the experimental and calculated values for both parts of the index of refraction are minimized at the same time. We have found that the choice of objective function strongly influences the existence of multiple solutions. By combining a global optimization method, an appropriately chosen objective Function and a modified oscillator model, we obtain good agreement with the experimental data for three organic materials.
Abstract: We investigate the electronic structure in strongly coupled superlattices over a wide field range. It is shown that the Wannier-Stark localization is followed by a field-induced delocalization which is due to a strong Zener coupling to continuum states. We compare different superlattice structures with high and with low barriers to alternatively investigate the effects of resonant interminiband tunneling and a strong Zener coupling to continuum states. The associated electrical breakdown is quantitatively shown in a detailed analysis of the absorption. A comprehensive theoretical model excellently describes the experimental results. Zener tunneling times are directly addressed by time-resolved experiments.
Abstract: We observe that the oscillatory motion of photoinjected electron-hole pairs in a biased semiconductor superlattice (Bloch oscillation) is accompanied by a coherent quasi-de current that is generated by the interaction of the carriers with the self-induced oscillating field. It is shown that this novel macroscopic quantum effect, which is a coherent analog of the Shapiro effect observed in Josephson junctions, can be controlled by changing the spectral position of the exciting laser pulse, which in turn determines the amplitude and phase of the wave packet oscillations. It is thereby possible to coherently drive the electrons either downwards or upwards in the potential of the static field.
Abstract: We consider the exciton states in quasi-one-dimensional organic crystals with strong orbital overlap between neighboring molecules. In such crystals, the energy difference between the lowest Frenkel exciton and the nearest-neighbor charge-transfer excitons becomes small and their strong mixing determines the nature of the lowest energy states. We discuss these effects for crystalline N,N'-dimethylperylene-3,3,9,10-dicarboximide (MePTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). To model the exciton states, we use a Hamiltonian which includes the mixing of Frenkel excitons with several vibronic levels and charge-transfer excitons. With appropriate fitting parameters, we demonstrate that this model call explain the main features of the low temperature absorption spectra. Polarized absorption spectra of MePTCDI show different polarization ratios for the various absorption peaks. This polarization behavior is discussed as a qualitative proof for the varying contribution of the charge-transfer excitons, which have a transition dipole direction different from that of the Frenkel excitons. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We discuss the spectrum of excitons in quasi-one-dimensional crystals with strong orbital overlap between neighboring molecules. In such crystals, the energy difference between the lowest Frenkel exciton and charge-transfer exciton becomes small and their strong mixing determines the nature of the lowest energy states. We demonstrate here that in finite chains, simultaneously with excitonic 'bulk' states also 'surface' states can appear. The 'surface' states are localized at the end of the chain and can be blue- or red-shifted in comparison with the 'bulk' states. We discuss the contribution of these states to the absorption and fluorescence spectrum of molecular chains. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We investigate the influence of scattering and coherent plasmon coupling on the temporal and spatial dynamics of Bloch-oscillating electrons in a semiconductor superlattice. We demonstrate that the dynamics are strongly influenced due to the scattering processes. For higher carrier densities, coupling to coherent plasmons leads to anharmonic Bloch oscillations since the static bias field is considerably changed by the oscillating carriers.
Abstract: The dissociation processes of excited states in vapor-deposited thin films of N,N'-dimethylperylene-3,4,9,10-dicarboximide (Me-PTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) are investigated in detail by means of electric field induced fluorescence quenching and photoresponse. The decay mechanisms of the two materials are quite different: Time resolved field induced fluorescence quenching on Me-PTCDI shows rate type quenching. Two spectral ranges with different field dependencies and decay times were observed. An efficient dissociation requires external fields of up to 2 x 10(6) V/cm. In PTCDA films, the quenching is of both rate and amplitude type. For this material, two spectral ranges with different field dependencies are recorded. A field induced luminescence enhancement is measured in the range between 520 and 600 nm. The combination of the fluorescence quenching measurement with the relative photoresponse and absolute luminescence quantum yield of Me-PTCDI allows to estimate the decay rate constants of the visible states of Me-PTCDI. (C) 2000 Elsevier Science S.A. All rights reserved.
Abstract: In this article, we present a simple method for the determination of the absolute internal quantum efficiency of thin organic dye layers. The basic idea is the comparison of the luminescence of the film with the reflection of a white diffusive reflectance standard measured at one angle with a simple spectrofluorometer. The method is compared to the procedure of de Mello [J. C. de Mello, H. F. Wittmann and R. H. Friend, Adv. Mater. 9, 230 (1997)], which uses an integrating sphere. As examples, the quantum yields of films of the two perylene derivatives N,N'-dimethylperylene-3,4:9,10-dicarboximide and 3,4,9,10-perylenetetracarboxylic dianhydride, are determined. (C) 2000 American Institute of Physics. [S0021-8979(00)08311-0].
Abstract: We present a systematic study on doping of vanadyl- and zinc-pathalocyanine by a fully fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic films by cosublimation of matrix and dopant. The films are characterized in situ by temperature dependent Seebeck and conductivity measurements. We observe a drastic increase of conductivity and a corresponding shift of the Fermi level towards the valence states with increasing dopant concentration. We thus conclude that doping has the potential of both reducing the series resistance and increasing the photovoltage of organic solar cells. As a first step to exploit this potential, we present two different ways of preparing diodes with rectification ratios in excess of 10(4) using doped phthalocyanines. By adding an undoped interlayer between the contact and the doped layer, we have produced diodes which work already in the strict absence of oxygen and are stable in air. To increase the efficiency of charge carrier generation in photovoltaic cells, we need to use photo active donor-acceptor-heterojunctions. We present here first examples of pn- and pin-type heterojunctions combining p-doped and nominally undoped layers. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We present a study of controlled n-type doping in molecular organic semiconductors: Naphthalenetetracarboxylic dianhydride is doped by cosublimation with the donor molecule bis(ethylenedithio)-tetrathiafulvalene. Electrical parameters are deduced from temperature-dependent measurements of the conductivity and the thermopower for various dopant concentrations. The results are compared to the predictions of a standard model commonly used for crystalline semiconductors. The Fermi level shifts towards the transport level, the conductivity is increased, and the mobility decreases with the doping ratio. (C) 2000 American Institute of Physics. [S0021-8979(00)00607-1].
Abstract: An investigation is made of the absorption spectra of a GaAs(82 Angstrom)/Al0.07Ga As-0.93(37 Angstrom) superlattice at 10 K in electric fields between 0 and 60 kV/cm. By comparing the experimental absorption spectra with calculations of the energies and oscillator strengths of transitions between states of the Wannier-Stark ladder it is established that in strong electric fields above-barrier states do not form a structureless continuum but a fan of states. Intersection of electric-field-localized electron states with the fan of above-barrier states leads to broadening, splitting, and nonmonotonic changes in the intensity of an intrawell transition band. The additional absorption band observed can be attributed to intrawell transitions between states in the left and right quantum wells of the superlattice. (C) 2000 MAIK âNauka/Interperiodicaâ.
Abstract: A new technique for determining the optical properties of organic thin films is presented. A detailed evaluation of the accuracy of the determined optical constants has been performed, and the best combination of measured values yielding the smallest errors in the index of refraction for realistic experimental uncertainties has been found. The proposed method utilizes the fact that optical constants are smooth continuous functions, which reduces the possibility of encountering multiple solutions. The method consists of two steps. In the first step the optical constants at all wavelengths and the him thickness are determined. In the second step the thickness and the imaginary part of the index of refraction are kept fixed while we reevaluate the real part of the index of refraction by using a different objective function with improved sensitivity to the refractive index. After verifying that the proposed method is capable of an accurate estimation of optical constants, we determine the index of refraction data of vanadyl phthalocyanine in the visible spectral range. (C) 2000 Optical Society of America.
Abstract: We show how the spatial dynamics of a Bloch oscillating wave packet can directly be measured and manipulated by means of optical techniques. For excitation below and above the Wannier-Stark ladder (WSL) center, the wave packet performs harmonic oscillations, following the predictions of Zener; for excitation near the WSL center the wave packet is in the so-called breathing mode: a symmetric oscillation with virtually zero center of mass motion. How the position and line width of the excitation laser changes the initially excited wave packets and their dynamics will be discussed. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We report the realization of organic-organic heteroepitaxy by combining liquid-phase self-assembly with ultrahigh vacuum (UHV) gas-phase molecular beam epitaxy. As a model system, we have used self-assembled monolayers (SAMs) prepared by exposing an Au(111)-mica substrate to a dilute solution of decanethiol in ethanol, with subsequent evaporation of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) dye molecules. The well-known ( root 3 x root 3)R30 degrees superstructure of almost upright standing molecules after chemisorption is replaced by the 11.5 x root 3 (33.2 Angstrom) pin-stripe phase with flat lying molecules when the samples are annealed in UHV and coverage decreases. The deposition of PTCDA induces reordering and displacement in the decanethiol SAM. Additional to the 33.2 Angstrom periodicity, the previously reported 22 Angstrom thiol stripe phase can be observed by scanning tunneling microscopy. Several PTCDA structures are observed: single and double rows of PTCDA that grow along decanethiol stripes on top of the Aat-lying alkane chain groups, as well as densely packed PTCDA monolayer domains embedded into the thiol layer. These exhibit the well-known herringbone structure or a novel square lattice structure. We have also investigated thicker PTCDA islands and observed molecular resolution for a thickness of several monolayers. The structure of the three-dimensional islands can be identified as the alpha bulk modification. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: Self-assembled monolayers of decanethiol on Au(111) have been investigated using high resolution scanning tunneling microscopy (STM). After the annealing of the as-adsorbed densely packed c(2 root 3 x 4 root 3)R30 degrees structure and before the formation of a previously described low density (11.5 x root 3) stripe phase three different phases with intermediate molecular surface density have been observed. We present highly resolved and distortion-corrected STM images for both the (7.5 x root 3) stripe phase and the meshlike structure. We describe a mixed (7.5/11.5 x root 3) stripe phase that shows a switching of the stripe arrangement. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We consider the exciton states in quasi-one-dimensional organic crystals with strong orbital overlap between neighboring molecules. In such crystals, the energy difference between the lowest Frenkel exciton and the nearest-neighbor charge-transfer excitons becomes small and their strong mixing determines the nature of the lowest energy states. We discuss these effects for crystalline N,Nâ-dimethylperylene-3,3,9,10-dicarboximide (MePTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). To model the exciton states, we use a Hamiltonian which includes the mixing of Frenkel excitons with several vibronic levels and charge-transfer excitons. With appropriate fitting parameters, we demonstrate that this model call explain the main features of the low temperature absorption spectra. Polarized absorption spectra of MePTCDI show different polarization ratios for the various absorption peaks. This polarization behavior is discussed as a qualitative proof for the varying contribution of the charge-transfer excitons, which have a transition dipole direction different from that of the Frenkel excitons. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We report on the systematical investigation of Fano resonances in electrically biased semiconductor superlattices. In such quantum structures, Coulomb inter action couples discrete excitonic states of the Wannier-Stark ladder to continua of lower-lying subbands. In absorption these transitions show the typical asymmetric line shape of a Fano resonance. The unique feature of these Fano resonances is that they allow to continuously tune the key parameter - the coupling strength Gamma between the discrete state and the degenerate continuum - by varying the bias voltage. Using this feature, we directly show that the Fano coupling leads to a fast polarization decay. We also investigate the dependence of the Fano parameters on the structure of the superlattice and compare to an extensive theoretical model of the resonances.
Notes: 6th International Conference on Optics of Excitons in Confined Systems (OECS-6), ASCONA, SWITZERLAND, AUG 30-SEP 02, 1999
Abstract: We investigate the heteroepitaxial growth of two films of large planar molecules on an inorganic substrate: for the particular system [perylene-tetracarboxylic-dianhydride (PTCDA) on hexa-peri-benzocoronene (HBC) on highly ordered pyrolytic graphite (HOPG)], large domains of several hundred of nanaometers in size with interfaces that are flat and almost defect-free on the molecular scale are observed using scanning tunneling microscopy (STM). The images obtained represent, to the best of our knowledge, the first STM observation of the ordered growth of large planar molecules in a heteroepitaxially stacked film. The mutual lattice alignment and epitaxial relations between the two organic layers are determined from the pronounced moire pattern in the STM images. Beside these findings, the intramolecular contrast of HBC on HOPG is used to determine the molecular orientation of the molecules within the two-dimensional unit cell by STM. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We present a systematic study on doping of vanadyl- and zinc-pathalocyanine by a fully fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic films by cosublimation of matrix and dopant. The films are characterized in situ by temperature dependent Seebeck and conductivity measurements. We observe a drastic increase of conductivity and a corresponding shift of the Fermi level towards the valence states with increasing dopant concentration. We thus conclude that doping has the potential of both reducing the series resistance and increasing the photovoltage of organic solar cells. As a first step to exploit this potential, we present two different ways of preparing diodes with rectification ratios in excess of 10(4) using doped phthalocyanines. By adding an undoped interlayer between the contact and the doped layer, we have produced diodes which work already in the strict absence of oxygen and are stable in air. To increase the efficiency of charge carrier generation in photovoltaic cells, we need to use photo active donor-acceptor-heterojunctions. We present here first examples of pn- and pin-type heterojunctions combining p-doped and nominally undoped layers. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We present a study of controlled n-type doping in molecular organic semiconductors: Naphthalenetetracarboxylic dianhydride is doped by cosublimation with the donor molecule bis(ethylenedithio)-tetrathiafulvalene. Electrical parameters are deduced from temperature-dependent measurements of the conductivity and the thermopower for various dopant concentrations. The results are compared to the predictions of a standard model commonly used for crystalline semiconductors. The Fermi level shifts towards the transport level, the conductivity is increased, and the mobility decreases with the doping ratio. (C) 2000 American Institute of Physics. [S0021-8979(00)00607-1].
Abstract: We discuss the spectrum of excitons in quasi-one-dimensional crystals with strong orbital overlap between neighboring molecules. In such crystals, the energy difference between the lowest Frenkel exciton and charge-transfer exciton becomes small and their strong mixing determines the nature of the lowest energy states. We demonstrate here that in finite chains, simultaneously with excitonic âbulkâ states also âsurfaceâ states can appear. The âsurfaceâ states are localized at the end of the chain and can be blue- or red-shifted in comparison with the âbulkâ states. We discuss the contribution of these states to the absorption and fluorescence spectrum of molecular chains. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: The dissociation processes of excited states in vapor-deposited thin films of N,Nâ-dimethylperylene-3,4,9,10-dicarboximide (Me-PTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) are investigated in detail by means of electric field induced fluorescence quenching and photoresponse. The decay mechanisms of the two materials are quite different: Time resolved field induced fluorescence quenching on Me-PTCDI shows rate type quenching. Two spectral ranges with different field dependencies and decay times were observed. An efficient dissociation requires external fields of up to 2 x 10(6) V/cm. In PTCDA films, the quenching is of both rate and amplitude type. For this material, two spectral ranges with different field dependencies are recorded. A field induced luminescence enhancement is measured in the range between 520 and 600 nm. The combination of the fluorescence quenching measurement with the relative photoresponse and absolute luminescence quantum yield of Me-PTCDI allows to estimate the decay rate constants of the visible states of Me-PTCDI. (C) 2000 Elsevier Science S.A. All rights reserved.
Abstract: We investigate the influence of scattering and coherent plasmon coupling on the temporal and spatial dynamics of Bloch-oscillating electrons in a semiconductor superlattice. We demonstrate that the dynamics are strongly influenced due to the scattering processes. For higher carrier densities, coupling to coherent plasmons leads to anharmonic Bloch oscillations since the static bias field is considerably changed by the oscillating carriers.
Abstract: In this article, we present a simple method for the determination of the absolute internal quantum efficiency of thin organic dye layers. The basic idea is the comparison of the luminescence of the film with the reflection of a white diffusive reflectance standard measured at one angle with a simple spectrofluorometer. The method is compared to the procedure of de Mello [J. C. de Mello, H. F. Wittmann and R. H. Friend, Adv. Mater. 9, 230 (1997)], which uses an integrating sphere. As examples, the quantum yields of films of the two perylene derivatives N,Nâ-dimethylperylene-3,4:9,10-dicarboximide and 3,4,9,10-perylenetetracarboxylic dianhydride, are determined. (C) 2000 American Institute of Physics. [S0021-8979(00)08311-0].
Abstract: In this work, we propose a method for calculating the real and imaginary part of the index of refraction N = n - ik and the film thickness d of absorbing thin films from normal incidence reflectance R and transmittance T measurements. The method determines the film thickness and the index of refraction values at all experimental points simultaneously, utilizing the fact that both the real and imaginary parts of the index of refraction are continuous functions. We test the ability of our method to extract correct d, n, and k values from two test datasets of R and T data generated from known n and k values. One of the test datasets contains random noise up to +/- 1.5% for R and +/- 0.5% for T, emulating random experimental errors. After verifying that our method is capable of accurate and reliable estimation of the index of refraction, we apply it to the determination of optical constants of thin films of the organic dye N,Nâ-dimethylperylene-3,4:9,10-dicarboximide. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: The growth of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) has been characterized on Au(100) from the submonolayer range to multilayer films by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV) and under ambient conditions. Two different structures of PTCDA are found, one of which is the well-known herringbone structure, with the plane of the molecule flat-lying on the substrate surface. This structure is point-online coincident with the Au( 100) lattice. The second structure which is predominant on this surface consists of closely spaced rods, each rod consisting of closely packed PTCDA molecules. We propose that this structure is associated with the (010) bulk plane of the alpha-polymorph of PTCDA. The reason for the different growth behavior compared to PTCDA on Au(111) at the very same growth conditions can be found in the nanoscopic structure of the reconstructed Au(100)(hex) surface. This reconstruction of the Au(100) surface provides trenches along the Au[011] azimuth which act as a specific adsorption site for PTCDA, leading to very low diffusion rates of this molecule following its deposition. The diffusion rate of first-deposited PTCDA was low enough to enable us to image single molecules at room temperature. These findings, in comparison with results on the reconstructed Au(111) surface, show that not only the substrate material and the symmetry, but also the surface structure on the nanoscopic scale can rule the growth of the organic overlayer. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) was deposited using organic molecular beam deposition (OMBD) onto various substrates, i.e. mica(0001), Au(111) layers on mica, and Se-passivated GaAs(100). Layer thicknesses were from 2 to 30 nm. Reflectance and transmittance measurements were performed in order to identify PTCDA absorption features and find suitable laser wavelengths for subsequent Raman investigations. Despite the low thicknesses the Raman spectra reveal strong scattering by the molecular vibrational modes, in particular above 1200 cm(-1). Frequency shifts of various modes in the layers from their values in PTCDA source material may indicate the influence of the substrates. Similar shifts were also observed in infrared spectra of the same materials.
Abstract: We investigate the electronic structure in strongly coupled superlattices over a wide held range. It is shown that the Wannier-Stark localization is followed by a held-induced delocalization which is due to a strong Zener coupling to continuum states. The associated Zener breakdown is quantitatively shown in a detailed analysis of the absorption. A comprehensive theoretical model excellently describes the experimental results. Zener tunneling times are directly addressed with the help of time-resolved experiments. (C) 1999 Published by Elsevier Science B.V. All rights reserved.
Notes: 11th International Conference on Nonequilibrium Carrier Dynamics in Semiconductors (HCIS 11), KYOTO, JAPAN, JUL 19-23, 1999
Abstract: We demonstrate that strain-induced structural modifications in SiGe-based heterostructures can be nondestructively probed by photoluminescence spectroscopy with several different excitation wavelengths. Owing to the indirect band structure of SiGe-based semiconductors, an appropriate choice of excitation energy leads to drastic change of the generation depth of photocarriers and allows to probe different part of the samples in the growth direction. By using this technique, we show that thin strained quantum wells on relaxed SiGe are almost free from threading dislocations and most of them are confined in composition-graded SiGe buffer layer. Furthermore, we show that the accumulation of strain in pure Ge/Si multiple quantum wells causes breakdown of the layer-by-layer growth mode at certain critical number of wells. (C) 1999 American Institute of Physics. [S0021-8979(99)01604-7].
Abstract: The dynamics of the Bloch oscillations in GaAs/Al0.3Ga0.7As superlattices are studied experimentally applying spectrally-resolved four-wave mixing in a self-diffraction geometry. We have found that the interband dephasing rate and the decay rate of the Bloch oscillations have different dependencies on excitation density. We have explored the dynamical features of the Bloch oscillations depending on the number of Wannier-Stark states excited. The conditions for observation of the Bloch oscillations and their harmonics are determined, the destructive role of excitons in the decay of the Bloch oscillations is demonstrated.
Notes: 18th Nordic Semiconductor Meeting, LINKOPING, SWEDEN, JUN 07-10, 1998
Abstract: 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) was deposited using organic molecular beam deposition (OMBD) onto various substrates, i.e. mica(0001), Au(111) layers on mica, and Se-passivated GaAs(100). Layer thicknesses were from 2 to 30 nm. Reflectance and transmittance measurements were performed in order to identify PTCDA absorption features and find suitable laser wavelengths for subsequent Raman investigations. Despite the low thicknesses the Raman spectra reveal strong scattering by the molecular vibrational modes, in particular above 1200 cm(-1). Frequency shifts of various modes in the layers from their values in PTCDA source material may indicate the influence of the substrates. Similar shifts were also observed in infrared spectra of the same materials.
Abstract: The dynamics of the Bloch oscillations in GaAs/Al0.3Ga0.7As superlattices are studied experimentally applying spectrally-resolved four-wave mixing in a self-diffraction geometry. We have found that the interband dephasing rate and the decay rate of the Bloch oscillations have different dependencies on excitation density. We have explored the dynamical features of the Bloch oscillations depending on the number of Wannier-Stark states excited. The conditions for observation of the Bloch oscillations and their harmonics are determined, the destructive role of excitons in the decay of the Bloch oscillations is demonstrated.
Abstract: A short laser pulse exciting semiconductor superlattice induces quantum beats between different excitonic states that in turn lead to a formation of a time-varying coherent wave packet. A real spatial oscillations of the excited wave packet identified quantum beats of the Wannier-Stark states as Bloch oscillations. Using a new technique for measurement of the spatial dynamics of Bloch wave packets in semiconductor superlattices we investigate the features of spatial-temporal dynamics on the initial experimental conditions and the composition of the wave packet.
Abstract: The changing of the four wave mixing spectrum of GaAs/AlGaAs superlattices are investigated theoretically and experimentally. It was shown that under applying the external electric field, which leads to an anticrossing of heavy hole and light hole excitons, the nonharmonic oscillations of the energy position of exciton resonances are detected. The nonharmonicity of these oscillations are caused by the influence of the inhomogeneous broadening of the excitons on the Bloch oscillations.
Abstract: We investigate the electronic structure in strongly coupled superlattices over a wide held range. It is shown that the Wannier-Stark localization is followed by a held-induced delocalization which is due to a strong Zener coupling to continuum states. The associated Zener breakdown is quantitatively shown in a detailed analysis of the absorption. A comprehensive theoretical model excellently describes the experimental results. Zener tunneling times are directly addressed with the help of time-resolved experiments. (C) 1999 Published by Elsevier Science B.V. All rights reserved.
Abstract: We demonstrate that strain-induced structural modifications in SiGe-based heterostructures can be nondestructively probed by photoluminescence spectroscopy with several different excitation wavelengths. Owing to the indirect band structure of SiGe-based semiconductors, an appropriate choice of excitation energy leads to drastic change of the generation depth of photocarriers and allows to probe different part of the samples in the growth direction. By using this technique, we show that thin strained quantum wells on relaxed SiGe are almost free from threading dislocations and most of them are confined in composition-graded SiGe buffer layer. Furthermore, we show that the accumulation of strain in pure Ge/Si multiple quantum wells causes breakdown of the layer-by-layer growth mode at certain critical number of wells. (C) 1999 American Institute of Physics. [S0021-8979(99)01604-7].
Abstract: In this work, we propose a method for calculating the real and imaginary part of the index of refraction N = n - ik and the film thickness d of absorbing thin films from normal incidence reflectance R and transmittance T measurements. The method determines the film thickness and the index of refraction values at all experimental points simultaneously, utilizing the fact that both the real and imaginary parts of the index of refraction are continuous functions. We test the ability of our method to extract correct d, n, and k values from two test datasets of R and T data generated from known n and k values. One of the test datasets contains random noise up to +/- 1.5% for R and +/- 0.5% for T, emulating random experimental errors. After verifying that our method is capable of accurate and reliable estimation of the index of refraction, we apply it to the determination of optical constants of thin films of the organic dye N,N'-dimethylperylene-3,4:9,10-dicarboximide. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: The growth of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) has been characterized on Au(100) from the submonolayer range to multilayer films by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV) and under ambient conditions. Two different structures of PTCDA are found, one of which is the well-known herringbone structure, with the plane of the molecule flat-lying on the substrate surface. This structure is point-online coincident with the Au( 100) lattice. The second structure which is predominant on this surface consists of closely spaced rods, each rod consisting of closely packed PTCDA molecules. We propose that this structure is associated with the (010) bulk plane of the alpha-polymorph of PTCDA. The reason for the different growth behavior compared to PTCDA on Au(111) at the very same growth conditions can be found in the nanoscopic structure of the reconstructed Au(100)(hex) surface. This reconstruction of the Au(100) surface provides trenches along the Au[011] azimuth which act as a specific adsorption site for PTCDA, leading to very low diffusion rates of this molecule following its deposition. The diffusion rate of first-deposited PTCDA was low enough to enable us to image single molecules at room temperature. These findings, in comparison with results on the reconstructed Au(111) surface, show that not only the substrate material and the symmetry, but also the surface structure on the nanoscopic scale can rule the growth of the organic overlayer. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: We have investigated the crystalline structure of PTCDA overlayers on reconstructed Au(111) surfaces by the combination of scanning tunneling microscopy (STM) with in situ reflection high energy electron diffraction (RHEED). Large domains of PTCDA (up to 200 nm) are found on the still reconstructed Au surface. These structurally well-characterized PTCDA samples have been optically examined in detail, mainly by cw- and time-resolved fluorescence. An important finding is the occurrence of a new spectral feature in the emission around a wavelength of 590 nm. Temperature and intensity dependent measurements reveal that the very broad emission around 700 nm (commonly considered to be caused by excimer emission) consists of several sub-bands with different origin.
Abstract: The short-wavelength (400-700 nm) photoluminescence (PL) spectra of SiO2 layers implanted with Si+, Ge+, and Ar+ ions in the dose range 3.2x10(16)-1.2x10(17) cm(-2) are compared. After Ar+ implantation an extremely weak luminescence, which vanishes completely after annealing for 30 min at 400 degrees C or 20 ms at 1050 degrees C, was observed. After implantation of group-IV elements the luminescence intensities were 1 to 2 orders of magnitude higher, and the luminescence remained not only with annealings but it could also increase. The dose and heating dependences of the luminescence show that it is due to the formation of impurity clusters and this process is more likely to be of a percolation than a diffusion character. For both group-IV impurities an intense blue band and a weaker band in the orange part of the spectrum were observed immediately after implantation. The ratio of the excitation and emission energies of the blue luminescence is characteristic of oxygen vacancies in SiO2; its properties are determined by the direct interaction of group-IV atoms. On this basis it is believed that the centers of blue PL are chains of Si (or Ge) atoms embedded in SiO2. The orange luminescence remained after annealings only in the case of Si+ implantation. This is attributed directly to the nonphase precipitates of Si in the form of strongly developed nanometer-size clusters. (C) 1998 American Institute of Physics.
Abstract: The short-wavelength (400-700 nm) photoluminescence (PL) spectra of SiO2 layers implanted with Si+, Ge+, and Ar+ ions in the dose range 3.2x10(16)-1.2x10(17) cm(-2) are compared. After Ar+ implantation an extremely weak luminescence, which vanishes completely after annealing for 30 min at 400 degrees C or 20 ms at 1050 degrees C, was observed. After implantation of group-IV elements the luminescence intensities were 1 to 2 orders of magnitude higher, and the luminescence remained not only with annealings but it could also increase. The dose and heating dependences of the luminescence show that it is due to the formation of impurity clusters and this process is more likely to be of a percolation than a diffusion character. For both group-IV impurities an intense blue band and a weaker band in the orange part of the spectrum were observed immediately after implantation. The ratio of the excitation and emission energies of the blue luminescence is characteristic of oxygen vacancies in SiO2; its properties are determined by the direct interaction of group-IV atoms. On this basis it is believed that the centers of blue PL are chains of Si (or Ge) atoms embedded in SiO2. The orange luminescence remained after annealings only in the case of Si+ implantation. This is attributed directly to the nonphase precipitates of Si in the form of strongly developed nanometer-size clusters. (C) 1998 American Institute of Physics.
Abstract: We use a technique to measure the spatial dynamics of Bloch wave packets in semiconductor superlattices and investigate the dependence of the dynamics on the optical excitation conditions. For excitations well above or below the center of the Wannier-Stark ladder (WSL), the wave packets perform harmonic oscillations following the prediction of Zener; for excitations near the center of the WSL, the wave packets undergo a symmetric oscillation with virtually zero center-of-mass amplitude.
Abstract: We present a novel experimental technique which allows to precisely measure the spatial displacement of Bloch-oscillating electrons in semiconductor superlattices as a function of time: The dipole field caused by the motion of the electrons is traced by small shifts of the Wannier-Stark ladder states. The electron wave packet displacement can then be derived from these shifts with the excitation density as the only free parameter. Using this method, we show that the optically generated electron wave packets perform harmonic oscillations, as predicted by Zener for the semiclassical motion of electrons in 1934. The absolute amplitudes of the wave packets depend inversely on the static field and are close to the values expected from semiclassical theory. (C) 1998 Elsevier Science B.V. All rights reserved.
Abstract: We report the first observation of Fano resonances in biased semiconductor superlattices: The excitonic Wannier-Stark ladder transitions show asymmetric absorption due to the coupling to the continue of lower transitions. In contrast to other known examples of Fano resonances, the Fano coupling can be continuously tuned in this system by changing the static field across the superlattice. The line shapes and their coupling dependence are in excellent agreement with theory. We also investigate the dephasing dynamics of the resonances and observe an increase in dephasing with increasing Fano coupling.
Abstract: The experimental observation of Bloch oscillations in solids was long thought to be out of experimental reach. The invention of the semiconductor superlattice has changed this situation decisively. Transport experiments performed in the past two decades have given indirect evidence for the existence of Bloch oscillations. Recently, it has been possible to directly observe Bloch oscillations in semiconductor superlattices by optical experiments: interband laser excitation of several Wannier-Stark ladder states creates a wave packet which oscillates in time with a characteristic dependence on the static electric field. These oscillations have been observed by several optical methods, both using detection of the interband polarization (four-wave mixing and photorefractive sampling) and intraband polarization (THz emission spectroscopy). The latter experimental technique directly proves the emission of radiation at the Bloch oscillator frequency. Recent experiments have directly measured the displacement of the Bloch wave packet as a function of time, proving the harmonic dependence as predicted by Zener. A number of experiments have addressed other key physical parameters of Bloch oscillations like the damping mechanisms. This article gives a brief review of recent investigations of Bloch oscillations in superlattices, focused on the interband optical experiments performed in the last few years.
Abstract: We investigate the temporal and spatial displacement dynamics of optically excited wave packets in a semiconductor superlattice. We present an experimental technique which measures directly the displacement of the wave packet center-of-mass: the oscillating Bloch wave packets create a microscopic dipole moment which carl be detected using the shift of the Wannier-Stark ladder transition energy as a sensitive field detector. A true spatial oscillation of the photo-excited wave packet. identifies quantum beats of the Wannier-Stark states as: Bloch oscillations. We show that the Bloch wave packet undergoes harmonic spatial motion in accordance with the predictions of Bloch and Zener. The influence of initial experimental conditions on evolution and composition of the wave packet is discussed.
Abstract: We perform detailed time-resolved photoluminescence (TRPL) measurements on coevaporated CuInSe2(CISe) thin films with a lateral gradient in the composition. The dynamics of minority carriers in bare absorber layers is compared with that of CISe/CdS samples and complete solar cells. The observed increase in the carrier lifetimes for the cell-like film structures is explained by surface recombination effects.
Abstract: The photoluminescence (PL) and electroluminescence (EL) properties of Ge-implanted SiO2 films thermally-grown on a Si substrate have been investigated and compared to those of Si-implanted SiO2 films. It is found that the blue-violet PL from both Si and Ge-rich layers reaches a maximum after annealing at 500 degrees C for 30 min. The PL and EL from Ge-implanted SiO2 are distinctly higher than that from Si-implanted layers and well visible for the naked eye. The EL spectrum from the Ge-implanted oxide annealed at 1000 degrees C correlates very well with the PL one and shows a linear dependence on the injected current. The neutral oxygen vacancy is assumed to be responsible for the observed luminescence. In the case of Ge the microstructure after high temperature annealing is studied.
Abstract: We have investigated the crystalline structure of PTCDA overlayers on reconstructed Au(111) surfaces by the combination of scanning tunneling microscopy (STM) with in situ reflection high energy electron diffraction (RHEED). Large domains of PTCDA (up to 200 nm) are found on the still reconstructed Au surface. These structurally well-characterized PTCDA samples have been optically examined in detail, mainly by cw- and time-resolved fluorescence. An important finding is the occurrence of a new spectral feature in the emission around a wavelength of 590 nm. Temperature and intensity dependent measurements reveal that the very broad emission around 700 nm (commonly considered to be caused by excimer emission) consists of several sub-bands with different origin.
Abstract: We show that doping of the transport layers can strikingly improve the properties of organic light emitting diodes (OLEDs). The electroluminescence onset voltage of diodes containing an vanadyl-phthalocyanine (VOPc) hole transport layer intentionally doped with tetrafluorotetracyano-quinodimethan (F-4-TCNQ) is reduced by up to an order of magnitude compared to OLED with undoped VOPc. The improved properties of our devices can be explained by the improved conductivity and better injection for a doped transport layer. (C) 1998 American Institute of Physics.
Abstract: Self-assembled monolayers with low molecular surface density of decanethiol on Au(lll) have been prepared by annealing of the densely packed (root 3 x root 3)R30 degrees structure. High-resolution scanning tunneling microscope (STM) imaging reveals the formation of a 11.5 x root 3 stripe structure where the molecules are aligned along the substrate surface in double lamellae. We can deduce from the images that the molecules bind to specific sites at the Au(lll) surface rather than adsorb as didecane disulfides. For the first time, a lattice structure for an alkanethiol stripe phase can be derived from the obtained real-space data.
Abstract: We report the first observation of Fano resonances in biased semiconductor superlattices: The excitonic Wannier-Stark ladder transitions show asymmetric absorption due to the coupling to the continue of lower transitions. In contrast to other known examples of Fano resonances, the Fano coupling can be continuously tuned in this system by changing the static field across the superlattice. The line shapes and their coupling dependence are in excellent agreement with theory. We also investigate the dephasing dynamics of the resonances and observe an increase in dephasing with increasing Fano coupling.
Abstract: We show that doping of the transport layers can strikingly improve the properties of organic light emitting diodes (OLEDs). The electroluminescence onset voltage of diodes containing an vanadyl-phthalocyanine (VOPc) hole transport layer intentionally doped with tetrafluorotetracyano-quinodimethan (F-4-TCNQ) is reduced by up to an order of magnitude compared to OLED with undoped VOPc. The improved properties of our devices can be explained by the improved conductivity and better injection for a doped transport layer. (C) 1998 American Institute of Physics.
Abstract: We present a novel experimental technique which allows to precisely measure the spatial displacement of Bloch-oscillating electrons in semiconductor superlattices as a function of time: The dipole field caused by the motion of the electrons is traced by small shifts of the Wannier-Stark ladder states. The electron wave packet displacement can then be derived from these shifts with the excitation density as the only free parameter. Using this method, we show that the optically generated electron wave packets perform harmonic oscillations, as predicted by Zener for the semiclassical motion of electrons in 1934. The absolute amplitudes of the wave packets depend inversely on the static field and are close to the values expected from semiclassical theory. (C) 1998 Elsevier Science B.V. All rights reserved.
Notes: 8th International Conference on Modulated Semiconductor Structures, SANTA BARBARA, CALIFORNIA, JUL 14-18, 1997
Abstract: We use a technique to measure the spatial dynamics of Bloch wave packets in semiconductor superlattices and investigate the dependence of the dynamics on the optical excitation conditions. For excitations well above or below the center of the Wannier-Stark ladder (WSL), the wave packets perform harmonic oscillations following the prediction of Zener; for excitations near the center of the WSL, the wave packets undergo a symmetric oscillation with virtually zero center-of-mass amplitude.
Abstract: We investigate the temporal and spatial displacement dynamics of optically excited wave packets in a semiconductor superlattice. We present an experimental technique which measures directly the displacement of the wave packet center-of-mass: the oscillating Bloch wave packets create a microscopic dipole moment which carl be detected using the shift of the Wannier-Stark ladder transition energy as a sensitive field detector. A true spatial oscillation of the photo-excited wave packet. identifies quantum beats of the Wannier-Stark states as: Bloch oscillations. We show that the Bloch wave packet undergoes harmonic spatial motion in accordance with the predictions of Bloch and Zener. The influence of initial experimental conditions on evolution and composition of the wave packet is discussed.
Notes: 5th International Workshop on Nonlinear Optics and Excitation Kinetics in Semiconductors (NOEKS 5), GRAAL MURITZ, GERMANY, SEP 01-05, 1997
Abstract: We investigate the doping of vanadyl-phthalocyanine by a fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic dye films by cosublimation of matrix and dopant. The electrical parameters of the films derived from conductivity and Seebeck measurements show that the results largely follow standard models used to describe the doping of crystalline semiconductors; e.g., a smooth shift of the Fermi level towards the valence states with increasing doping is observed. Other effects, like the superlinear increase of conductivity with the molar doping ratio, need the inclusion of additional effects like percolation. (C) 1998 American Institute of Physics. [S0003-6951(98)00448-3].
Abstract: The photoluminescence (PL) and electroluminescence (EL) properties of Ge-implanted SiO2 films thermally-grown on a Si substrate have been investigated and compared to those of Si-implanted SiO2 films. It is found that the blue-violet PL from both Si and Ge-rich layers reaches a maximum after annealing at 500 degrees C for 30 min. The PL and EL from Ge-implanted SiO2 are distinctly higher than that from Si-implanted layers and well visible for the naked eye. The EL spectrum from the Ge-implanted oxide annealed at 1000 degrees C correlates very well with the PL one and shows a linear dependence on the injected current. The neutral oxygen vacancy is assumed to be responsible for the observed luminescence. In the case of Ge the microstructure after high temperature annealing is studied.
Abstract: The experimental observation of Bloch oscillations in solids was long thought to be out of experimental reach. The invention of the semiconductor superlattice has changed this situation decisively. Transport experiments performed in the past two decades have given indirect evidence for the existence of Bloch oscillations. Recently, it has been possible to directly observe Bloch oscillations in semiconductor superlattices by optical experiments: interband laser excitation of several Wannier-Stark ladder states creates a wave packet which oscillates in time with a characteristic dependence on the static electric field. These oscillations have been observed by several optical methods, both using detection of the interband polarization (four-wave mixing and photorefractive sampling) and intraband polarization (THz emission spectroscopy). The latter experimental technique directly proves the emission of radiation at the Bloch oscillator frequency. Recent experiments have directly measured the displacement of the Bloch wave packet as a function of time, proving the harmonic dependence as predicted by Zener. A number of experiments have addressed other key physical parameters of Bloch oscillations like the damping mechanisms. This article gives a brief review of recent investigations of Bloch oscillations in superlattices, focused on the interband optical experiments performed in the last few years.
Abstract: We investigate the doping of vanadyl-phthalocyanine by a fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic dye films by cosublimation of matrix and dopant. The electrical parameters of the films derived from conductivity and Seebeck measurements show that the results largely follow standard models used to describe the doping of crystalline semiconductors; e.g., a smooth shift of the Fermi level towards the valence states with increasing doping is observed. Other effects, like the superlinear increase of conductivity with the molar doping ratio, need the inclusion of additional effects like percolation. (C) 1998 American Institute of Physics. [S0003-6951(98)00448-3].
Abstract: Self-assembled monolayers with low molecular surface density of decanethiol on Au(lll) have been prepared by annealing of the densely packed (root 3 x root 3)R30 degrees structure. High-resolution scanning tunneling microscope (STM) imaging reveals the formation of a 11.5 x root 3 stripe structure where the molecules are aligned along the substrate surface in double lamellae. We can deduce from the images that the molecules bind to specific sites at the Au(lll) surface rather than adsorb as didecane disulfides. For the first time, a lattice structure for an alkanethiol stripe phase can be derived from the obtained real-space data.
Abstract: The absolute spatial displacement of Bloch-oscillating electrons in semiconductor superlattices is measured as a function of time with a few angstrom resolution using a novel experimental technique: The oscillating Bloch wave packet creates a small dipole field which can be determined using the field shift of the Wannier-Stark ladder transitions as a sensitive detector. The total amplitudes and their dependence on the static electric field are in good agreement with a theory including excitonic effects.
Abstract: The photoluminescence (PL) and electroluminescence (EL) properties of Ge-implanted SiO2 layers thermally grown on a Si substrate were investigated and compared to those of Si-implanted SiO2 films. The PL spectra from Ge-implanted SiO2 were recorded as a function of annealing temperature. It was found that the blue-violet PL from Ge-rich oxide layers reaches a maximum after annealing at 500 degrees C for 30 min, and is substantially more intense than the PL emission from Si-implanted oxides. The neutral oxygen vacancy is believed to be responsible for the observed luminescence. The EL spectrum from the Ge-implanted oxide after annealing at 1000 degrees C correlates very well with the PL one, and shows a linear dependence on the injected current. The EL emission, was strong enough to be readily seen with the naked eye and the EL efficiency was assessed to be about 5 X 10(-4). (C) 1997 American Institute of Physics. [S0003-6951(97)02745-9].
Abstract: The photoluminescence (PL) and electroluminescence (EL) properties of Ge-implanted SiO2 layers thermally grown on a Si substrate were investigated and compared to those of Si-implanted SiO2 films. The PL spectra from Ge-implanted SiO2 were recorded as a function of annealing temperature. It was found that the blue-violet PL from Ge-rich oxide layers reaches a maximum after annealing at 500 degrees C for 30 min, and is substantially more intense than the PL emission from Si-implanted oxides. The neutral oxygen vacancy is believed to be responsible for the observed luminescence. The EL spectrum from the Ge-implanted oxide after annealing at 1000 degrees C correlates very well with the PL one, and shows a linear dependence on the injected current. The EL emission, was strong enough to be readily seen with the naked eye and the EL efficiency was assessed to be about 5 X 10(-4). (C) 1997 American Institute of Physics. [S0003-6951(97)02745-9].
Abstract: The emission behaviour of ultrathin high-quality evaporated dye layers of N,Nâ-dimethylperylene-3,4:9,10-bis-dicarboximide (MePTCDI) is investigated by temperature dependent, steady state and transient photoluminescence. Layers with a nominal thickness much less than a monolayer show monomer emission, while the luminescence of thicker samples is dominated by excimer-type transitions. The fluorescence dynamics of the system are studied using a streak camera with a temporal resolution better than 2 ps. We compare our results with the measurements on the luminescence dynamics of crystalline alpha-perylene [1-3].
Notes: 1996 International Conference on luminescence and Optical Spectroscopy of Condensed Matter (ICL 96), PRAGUE, CZECH REPUBLIC, AUG 18-23, 1996
Abstract: The absolute spatial displacement of Bloch-oscillating electrons in semiconductor superlattices is measured as a function of time with a few angstrom resolution using a novel experimental technique: The oscillating Bloch wave packet creates a small dipole field which can be determined using the field shift of the Wannier-Stark ladder transitions as a sensitive detector. The total amplitudes and their dependence on the static electric field are in good agreement with a theory including excitonic effects.
Abstract: The crystal structure of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) grown by organic molecular-beam epitaxy on reconstructed Au(111) surfaces is investigated by scanning tunneling microscopy and reflection high-energy electron diffraction. Large domains of PTCDA (up to 200 nm) are found on the still reconstructed Au surface. The reconstruction can be used to precisely determine the lattice orientation of the PTCDA layers. Three different orientations with respect to the gold substrate were observed. All of them can be explained in terms of a point-on-line growth model that was originally proposed for explaining organic epitaxy results on graphite.
Abstract: We have investigated the charge transport properties of vapor-deposited thin organic films, using the Seebeck effect for determining conduction type and Fermi energy and the field effect to measure mobility and total charge carrier density. We show that the combination of both techniques gives a complete picture of the electrical properties of the films.
Abstract: We perform a detailed analysis of the radiative recombination processes in CuInSe2 thin films grown by multisource physical vapor deposition. The photoluminescence and photoluminescence excitation spectra are investigated as a function of stoichiometry, temperature and excitation intensity. Using samples with a large composition gradient, we are able to obtain a coherent picture of the optical transitions in the films. The broad-band photoluminescence spectrum typical for In-rich films breaks into a number of well-defined emission lines in Cu-rich CulnSe(2). At low temperatures, emission peaks due to free-exciton, bound-exciton, and free-to-bound recombination are identified in Cu-rich films. The spectra of In-rich films tend to be dominated by donor-acceptor transitions. From the optical spectra, exciton ionization energies and the temperature dependence of the band gap are determined. The observed optical transitions are related to intrinsic defects. (C) 1997 American Institute of Physics.
Abstract: The emission behaviour of ultrathin high-quality evaporated dye layers of N,N'-dimethylperylene-3,4:9,10-bis-dicarboximide (MePTCDI) is investigated by temperature dependent, steady state and transient photoluminescence. Layers with a nominal thickness much less than a monolayer show monomer emission, while the luminescence of thicker samples is dominated by excimer-type transitions. The fluorescence dynamics of the system are studied using a streak camera with a temporal resolution better than 2 ps. We compare our results with the measurements on the luminescence dynamics of crystalline alpha-perylene [1-3].
Abstract: Strong blue (around 470 nm) and violet (around 395 nm) photoluminescence (PL) at room temperature (RT) was obtained from thermally-grown SiO2 films on crystalline Si implanted with Si+ and Ge+ ions, respectively. Photoluminescence excitation (PLE) spectroscopy measurements indicate maximum PL at 248 nm (for Si+) and 242 nm (for Ge+). The blue PL intensity was investigated as a function of subsequent furnace and flash lamp annealing. The results obtained are interpreted in terms of the excess atoms introduced in the SiO2 network.
Abstract: Bloch oscillations in semiconductor superlattices have recently been observed using optical techniques. However, it remained to show that the optically observed quantum beats of the Wannier-Stark ladder are connected with a true spatial oscillation of the wave packet. We present an experimental technique which measures directly the displacement of the wave packet center-of-mass: The oscillating Bloch wave packets create a microscopic dipole moment which can be determined using the shift of the Wannier-Stark ladder transition energy as a sensitive dipole field detector. We show that the wave packet undergoes harmonic spatial motion in accordance with the predictions of Bloch and Zener.
Abstract: We perform a detailed analysis of the radiative recombination processes in CuInSe2 thin films grown by multisource physical vapor deposition. The photoluminescence and photoluminescence excitation spectra are investigated as a function of stoichiometry, temperature and excitation intensity. Using samples with a large composition gradient, we are able to obtain a coherent picture of the optical transitions in the films. The broad-band photoluminescence spectrum typical for In-rich films breaks into a number of well-defined emission lines in Cu-rich CulnSe(2). At low temperatures, emission peaks due to free-exciton, bound-exciton, and free-to-bound recombination are identified in Cu-rich films. The spectra of In-rich films tend to be dominated by donor-acceptor transitions. From the optical spectra, exciton ionization energies and the temperature dependence of the band gap are determined. The observed optical transitions are related to intrinsic defects. (C) 1997 American Institute of Physics.
Abstract: Bloch oscillations in semiconductor superlattices have recently been observed using optical techniques. However, it remained to show that the optically observed quantum beats of the Wannier-Stark ladder are connected with a true spatial oscillation of the wave packet. We present an experimental technique which measures directly the displacement of the wave packet center-of-mass: The oscillating Bloch wave packets create a microscopic dipole moment which can be determined using the shift of the Wannier-Stark ladder transition energy as a sensitive dipole field detector. We show that the wave packet undergoes harmonic spatial motion in accordance with the predictions of Bloch and Zener.
Notes: 5th International Meeting on Optics of Excitons in Confined Systems (OECS 5), GOTTINGEN, GERMANY, AUG 10-14, 1997
Abstract: We have investigated the charge transport properties of vapor-deposited thin organic films, using the Seebeck effect for determining conduction type and Fermi energy and the field effect to measure mobility and total charge carrier density. We show that the combination of both techniques gives a complete picture of the electrical properties of the films.
Abstract: The crystal structure of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) grown by organic molecular-beam epitaxy on reconstructed Au(111) surfaces is investigated by scanning tunneling microscopy and reflection high-energy electron diffraction. Large domains of PTCDA (up to 200 nm) are found on the still reconstructed Au surface. The reconstruction can be used to precisely determine the lattice orientation of the PTCDA layers. Three different orientations with respect to the gold substrate were observed. All of them can be explained in terms of a point-on-line growth model that was originally proposed for explaining organic epitaxy results on graphite.
Abstract: Strong blue (around 470 nm) and violet (around 395 nm) photoluminescence (PL) at room temperature (RT) was obtained from thermally-grown SiO2 films on crystalline Si implanted with Si+ and Ge+ ions, respectively. Photoluminescence excitation (PLE) spectroscopy measurements indicate maximum PL at 248 nm (for Si+) and 242 nm (for Ge+). The blue PL intensity was investigated as a function of subsequent furnace and flash lamp annealing. The results obtained are interpreted in terms of the excess atoms introduced in the SiO2 network.
Notes: 10th Biennial Conference on Insulating Films on Semiconductors (INFOS 97), STENUNGSUND, SWEDEN, JUN 10-14, 1997
Abstract: We show that the Kelvin probe method (KPM) is well suited for ia situ measurements of the Fermi level of organic vapor-deposited dye layers. The method works well even for high resistivity materials, is nondestructive. and does not need any top contacts. First results for zinc-phthalocyanine (ZnPc) and perfluorinated ZnPc (ZnPc-F-16) are presented. The KPM results show that a 2.5% admixture of ZnPc-F-16 to a ZnPc matrix leads to a 0.2 eV lowering of the Fermi level of ZnPc, i.e., ZnPc-F-16 has acceptorlike properties relative to ZnPc. (C) 1996 American Institute of Physics.
Abstract: Thermal SiO2 films have been implanted with Si+ ions using double-energy implants (200 + 100 keV) at a substrate temperature of about -20 degrees C to total doses in the range 1.6 x 10(16)-1.6 x 10(17) cm(-2) followed by short-time thermal processing, in order to form a Si nanostructure capable of yielding blue photoluminescence (PL). The intensity and the peak position of the PL band have been investigated as a function of ion dose, manner of heat treatment, anneal time and anneal temperature. For the formation of blue PL emitting centres, optimum processing conditions in terms of excess Si concentration and overall thermal budget are mandatory. The nature of the observed blue emission is discussed.
Abstract: We present detailed measurements of carrier lifetimes in CuInSe2 thin films. Time-resolved photoluminescence measurements are performed on polycrystalline samples grown with a lateral gradient in the Cu/In ratio, thus enabling continuous evaluation of lifetime dependence on him composition. Luminescence at two distinct spectral positions is observed: A high energy emission, attributed to free carrier or free exciton recombination (depending: on composition), that decays extremely fast with a lifetime of tens of picoseconds to a few ns; a lower energy emission from defect related recombination that decays much more slowly, with typical lifetimes greater than tens of nanoseconds. We compare these results with results from CuInGaSe2 films. Radiative coefficients and minority carrier lifetimes are obtained from the data. (C) 1996 American Institute of Physics.
Abstract: We have investigated the coherent dynamics of wave packets in semiconductor superlattices with an applied electric field. In particular, we have performed four-wave mixing experiments with time-resolving of the diffracted beam. The temporal behavior of the signal shows the oscillation dynamics of the Bloch wave packet. Additionally, the delayed rise of the signals clearly demonstrates the influence of many-body effects due to Coulomb coupling in the nonlinear optical response of the system. We show that the influence of the many-body effects can be tuned by changing the static applied field: for low field (small exciton binding energy), the FWM signal rises fast; for high field (large exciton binding energy), the signal rises slowly. The experimental results are well described by theory.
Abstract: Experiments are reported which explore the possibility of using low-temperature, multiple-energy Si+ ion implantation into thin SiO2 films on Si and subsequent short-time thermal processing to form silicon nanostructures capable of yielding a high-intensity emission in the short-wavelength part of the visible spectrum. A room-temperature short-wavelength PL band of high intensity was found after double implantation with energies of 200 and 100 keV at a temperature of -20 degrees C to a total dose of 4.8x10(16) cm(-2) (atomic concentration about 2x10(21) cm(-3)) and subsequent furnace annealing at 400 degrees C for 0.5 h in forming gas or by flash lamp annealing at 1050 degrees C for 20 ms. (C) 1996 American Institute of Physics.
Abstract: We investigate high-quality evaporated dye layers of N,N'-dimethylperylene-3,4:9,10-bis-discarboximide of varying thicknesses, using temperature-dependent luminescence. Layers with a nominal thickness much less than a monolayer show monomer emission similar to that of dilute solutions. With increasing layer thickness, the luminescence is dominated by excimer transitions. The excimer linewidth decreases markedly with decreasing temperature. We show that the aggregation of molecules can be controlled by variation of the surface roughness. (C) 1996 Optical Society of America
Abstract: The progress in growth and understanding of semiconductor heterostructures made the realization of artificial materials like the semiconductor superlattices possible. In the last few years, optical experiments have unambiguously proven that Bloch oscillations exist in semiconductor superlattices. In this paper, the key experiments and their contributions to the understanding of Bloch oscillations are briefly discussed. Then, open questions and possible experiments to answer them are discussed.
Abstract: We investigate high-quality evaporated dye layers of N,Nâ-dimethylperylene-3,4:9,10-bis-discarboximide of varying thicknesses, using temperature-dependent luminescence. Layers with a nominal thickness much less than a monolayer show monomer emission similar to that of dilute solutions. With increasing layer thickness, the luminescence is dominated by excimer transitions. The excimer linewidth decreases markedly with decreasing temperature. We show that the aggregation of molecules can be controlled by variation of the surface roughness. (C) 1996 Optical Society of America
Abstract: The progress in growth and understanding of semiconductor heterostructures made the realization of artificial materials like the semiconductor superlattices possible. In the last few years, optical experiments have unambiguously proven that Bloch oscillations exist in semiconductor superlattices. In this paper, the key experiments and their contributions to the understanding of Bloch oscillations are briefly discussed. Then, open questions and possible experiments to answer them are discussed.
Notes: Nobel Symposium 99 on Heterostructures in Semiconductors, ARILD, SWEDEN, JUN 04-08, 1996
Abstract: Thermal SiO2 films have been implanted with Si+ ions using double-energy implants (200 + 100 keV) at a substrate temperature of about -20 degrees C to total doses in the range 1.6 x 10(16)-1.6 x 10(17) cm(-2) followed by short-time thermal processing, in order to form a Si nanostructure capable of yielding blue photoluminescence (PL). The intensity and the peak position of the PL band have been investigated as a function of ion dose, manner of heat treatment, anneal time and anneal temperature. For the formation of blue PL emitting centres, optimum processing conditions in terms of excess Si concentration and overall thermal budget are mandatory. The nature of the observed blue emission is discussed.
Notes: Symposium 1 on New Trends in Ion Beam Processing of Materials, at the E-MRS 96 Spring Meeting, STRASBOURG, FRANCE, JUN 04-07, 1996
Abstract: We have investigated the coherent dynamics of wave packets in semiconductor superlattices with an applied electric field. In particular, we have performed four-wave mixing experiments with time-resolving of the diffracted beam. The temporal behavior of the signal shows the oscillation dynamics of the Bloch wave packet. Additionally, the delayed rise of the signals clearly demonstrates the influence of many-body effects due to Coulomb coupling in the nonlinear optical response of the system. We show that the influence of the many-body effects can be tuned by changing the static applied field: for low field (small exciton binding energy), the FWM signal rises fast; for high field (large exciton binding energy), the signal rises slowly. The experimental results are well described by theory.
Notes: 7th International Conference on Modulated Semiconductor Structures (MSS-7), MADRID, SPAIN, JUL 10-14, 1995
Abstract: We present detailed measurements of carrier lifetimes in CuInSe2 thin films. Time-resolved photoluminescence measurements are performed on polycrystalline samples grown with a lateral gradient in the Cu/In ratio, thus enabling continuous evaluation of lifetime dependence on him composition. Luminescence at two distinct spectral positions is observed: A high energy emission, attributed to free carrier or free exciton recombination (depending: on composition), that decays extremely fast with a lifetime of tens of picoseconds to a few ns; a lower energy emission from defect related recombination that decays much more slowly, with typical lifetimes greater than tens of nanoseconds. We compare these results with results from CuInGaSe2 films. Radiative coefficients and minority carrier lifetimes are obtained from the data. (C) 1996 American Institute of Physics.
Abstract: Experiments are reported which explore the possibility of using low-temperature, multiple-energy Si+ ion implantation into thin SiO2 films on Si and subsequent short-time thermal processing to form silicon nanostructures capable of yielding a high-intensity emission in the short-wavelength part of the visible spectrum. A room-temperature short-wavelength PL band of high intensity was found after double implantation with energies of 200 and 100 keV at a temperature of -20 degrees C to a total dose of 4.8x10(16) cm(-2) (atomic concentration about 2x10(21) cm(-3)) and subsequent furnace annealing at 400 degrees C for 0.5 h in forming gas or by flash lamp annealing at 1050 degrees C for 20 ms. (C) 1996 American Institute of Physics.
Abstract: The photoluminescence of CulnSe(2) thin films grown by multisource physical vapor deposition is investigated over a wide composition range with a specially designed sample. Measurements as a function of excitation intensity, and temperature are correlated with the Cu/In ratio x of the films ranging from 1.47 less than or equal to x less than or equal to 0.46. All the Cu-rich samples (x>1.0) show well-defined peak positions independent of the composition. In the slightly In-rich region (x=0.90) where the best absorber layers for high efficiency CuInSe2 solar cells are expected, two broad emission lines are ascribed to donor-acceptor pair transitions. For very In-rich samples (x<0.67), a blue shift of the dominant transition is observed. (C) 1996 American Institute of Physics.
Abstract: The photoluminescence of CulnSe(2) thin films grown by multisource physical vapor deposition is investigated over a wide composition range with a specially designed sample. Measurements as a function of excitation intensity, and temperature are correlated with the Cu/In ratio x of the films ranging from 1.47 less than or equal to x less than or equal to 0.46. All the Cu-rich samples (x>1.0) show well-defined peak positions independent of the composition. In the slightly In-rich region (x=0.90) where the best absorber layers for high efficiency CuInSe2 solar cells are expected, two broad emission lines are ascribed to donor-acceptor pair transitions. For very In-rich samples (x<0.67), a blue shift of the dominant transition is observed. (C) 1996 American Institute of Physics.
Abstract: We show that the Kelvin probe method (KPM) is well suited for ia situ measurements of the Fermi level of organic vapor-deposited dye layers. The method works well even for high resistivity materials, is nondestructive. and does not need any top contacts. First results for zinc-phthalocyanine (ZnPc) and perfluorinated ZnPc (ZnPc-F-16) are presented. The KPM results show that a 2.5% admixture of ZnPc-F-16 to a ZnPc matrix leads to a 0.2 eV lowering of the Fermi level of ZnPc, i.e., ZnPc-F-16 has acceptorlike properties relative to ZnPc. (C) 1996 American Institute of Physics.
Abstract: The polarization and population dynamics of resonantly generated excitons in GaAs/AlGaAs quantum well structures are investigated using transient three-pulse four-wave mixing. The decay dynamics show strongly nonexponential behavior for both variation of the delay between the first two pulses reflecting the polarization dynamics and variation of the delay between the second two pulses, which reflects the population dynamics. A comparison with a theoretical model shows that the dynamics of both polarization and population are mainly determined by frequency fluctuations and not by scattering processes or by recombination. The characteristic time of the fluctuations T-C has to be energy-dependent to fully describe the results.
Abstract: We show that a detailed understanding of the influence of the exciton-exciton interaction on the non-linear excitonic response in a GaAs/AlGaAs superlattice can be obtained by changing the external applied field. In time-resolved four-wave mixing this implies a change from a third-order polarization signal dominated by phase space occupation to a Coulomb interaction-induced signal when the bias is increased at high excitation densities. The experimental data are compared with theoretical results based on the numerical solution of the semiconductor Bloch equations.
Abstract: The polarization and population dynamics of resonantly generated excitons in GaAs/AlGaAs quantum well structures are investigated using transient three-pulse four-wave mixing. The decay dynamics show strongly nonexponential behavior for both variation of the delay between the first two pulses reflecting the polarization dynamics and variation of the delay between the second two pulses, which reflects the population dynamics. A comparison with a theoretical model shows that the dynamics of both polarization and population are mainly determined by frequency fluctuations and not by scattering processes or by recombination. The characteristic time of the fluctuations T-C has to be energy-dependent to fully describe the results.
Notes: 4th International Workshop on Nonlinear Optics and Excitation Kinetics in Semiconductors (NOEKS IV), GOSEN, GERMANY, NOV 06-10, 1994
Abstract: We show that a detailed understanding of the influence of the exciton-exciton interaction on the non-linear excitonic response in a GaAs/AlGaAs superlattice can be obtained by changing the external applied field. In time-resolved four-wave mixing this implies a change from a third-order polarization signal dominated by phase space occupation to a Coulomb interaction-induced signal when the bias is increased at high excitation densities. The experimental data are compared with theoretical results based on the numerical solution of the semiconductor Bloch equations.
Notes: IV International Conference on Optics of Excitons in Confined Systems, CORTONA, ITALY, AUG 28-31, 1995
Abstract: Interdigital heterobarrier metal-semiconductor-metal (MSM) structures are proposed as ultrafast photodetectors of visible radiation. Vertical photocarrier drift in MSM diode structures provides the simplest way of reducing the active region of a detector to the submicron size and of shortening considerably the response time without a significant loss of the radiation coupling-in efficiency. External electrooptic sampling was used to determine the subpicosecond (half-amplitude duration 0.6 ps) electrical response of a detector built into a copolanar microwave energy transmission line. Under a bias voltage of 1 V and for an optical excitation energy of 10 pJ per pulse the change in the photodiode voltage amounted to 40% of the bias voltage, which makes these diode structures sufficiently efficient and extremely fast detectors of optical radiation.
Abstract: We study Bloch oscillations (BO) in GaAs/AlGaAs superlattices with miniband widths of 19 and 38 meV at various lattice temperatures. In the time domain, we use transient degenerate four-wave mixing (FWM), while simultaneously monitoring the Wannier-Stark ladder (WSL) in the frequency domain by transmission spectroscopy. Spectrally resolved FWM measurements prove unambiguously that the oscillations in the decay of the FWM signal are due to quantum interference as expected for a wavepacket performing the Bo
Notes: 8th International Conference on Hot Carriers in Semiconductors, OXFORD, ENGLAND, AUG 16-20, 1993
Abstract: The coherent excitation of excitonic wavepackets in AlGaAs/GaAs superlattices is investigated in a comparative study employing three different time-resolved optical techniques. The experimental results obtained in time-resolved four-wave mixing (FWM), THz emission spectroscopy and transmissive electro-optic sampling (TEOS) provide the final proof of the existence of Bloch oscillations in the THz range. Each of these measurements provides complementary information on these oscillations. As a result, Bloch oscillations appear as a special case of quantum interference involving the coherent superposition of Wannier-Stark states.
Abstract: We investigate the intersubband relaxation in GaAs quantum wells at room temperature using differential transmission spectroscopy with subpicosecond time resolution. The dynamics of the subband populations are derived from the experimentally observed reduction of oscillator strength of the corresponding exciton transitions. The obtained electron intersubband scattering times are 160 fs for relaxation from the second to the first subband in a 15-nm quantum well and 170 fs for relaxation from the third to the first subband in a 20-nm structure. These times are significantly shorter than those deduced from previous experiments and from some theoretical studies.
Abstract: We detect the coherent submillimetre-wave radiation from electron Bloch oscillations in a biased GaAs/AlGaAs semiconductor superlattice by time-resolved terahertz (THz) spectroscopy. The emitted THz radiation is tunable from 0.3 to 4 THz. The emission from Bloch oscillations is observable for temperatures as high as 100 K.
Abstract: We study Bloch oscillations (BO) in GaAs/AlGaAs superlattices with miniband widths of 19 and 38 meV at various lattice temperatures. In the time domain, we use transient degenerate four-wave mixing (FWM), while simultaneously monitoring the Wannier-Stark ladder (WSL) in the frequency domain by transmission spectroscopy. Spectrally resolved FWM measurements prove unambiguously that the oscillations in the decay of the FWM signal are due to quantum interference as expected for a wavepacket performing the Bo
Abstract: We have studied the spatial dynamics of coherently excited wave packets of charge carriers in a semiconductor superlattice structure by both time-resolved THz spectroscopy and degenerate four-wave mixing. In the Wannier-Stark bias regime, we observe electron Bloch oscillations with a frequency that depends linearly on the electric bias field and can be tuned between a few hundred GHz and 5 THz.
Abstract: We investigate the temporal evolution and the density dependence of exciton bleaching in GaAs/AlxGa1-xAs quantum wells. We find that the carrier-induced reduction of exciton oscillator strength is not influenced by carrier cooling after creation of hot carriers with femtosecond laser pulses. Excitation density experiments suggest that either phase-space filling or Coulomb screening dominates the exciton bleaching, depending on the carrier density.
Abstract: We investigate carrier capture and carrier transport in the InGaAs/InP material system by luminescence spectroscopy with femtosecond time resolution. Comparative studies are performed on samples of different well width, barrier width and gallium mole fraction of the InGaAs layers. The investigations focus on excitation conditions that are comparable to those for semiconductor laser operation. Firm data on carrier dynamics are presented for these conditions. We find that the overall transfer rates of electrons and holes are similar and independent of well width. Furthermore, the transfer times show a linear dependence on barrier width. From experimental and model calculation results we derive some guidelines for the design of high-frequency laser devices.
Abstract: The dynamics of photoexcitations in conjugated polymers and polymer blends is studied by means of time resolved luminescence spectroscopy. After excitation into the vibronic progression, the luminescence rises quasi-instantaneously. The luminescence decay depends strongly on the detection energy and the polymer concentration. The results are explained within a hopping model.
Notes: 1993 International Conference on Luminescence (ICL 93), STORRS, CT, AUG 09-13, 1993
Abstract: The coherent excitation of excitonic wavepackets in AlGaAs/GaAs superlattices is investigated in a comparative study employing three different time-resolved optical techniques. The experimental results obtained in time-resolved four-wave mixing (FWM), THz emission spectroscopy and transmissive electro-optic sampling (TEOS) provide the final proof of the existence of Bloch oscillations in the THz range. Each of these measurements provides complementary information on these oscillations. As a result, Bloch oscillations appear as a special case of quantum interference involving the coherent superposition of Wannier-Stark states.
Abstract: Interdigital heterobarrier metal-semiconductor-metal (MSM) structures are proposed as ultrafast photodetectors of visible radiation. Vertical photocarrier drift in MSM diode structures provides the simplest way of reducing the active region of a detector to the submicron size and of shortening considerably the response time without a significant loss of the radiation coupling-in efficiency. External electrooptic sampling was used to determine the subpicosecond (half-amplitude duration 0.6 ps) electrical response of a detector built into a copolanar microwave energy transmission line. Under a bias voltage of 1 V and for an optical excitation energy of 10 pJ per pulse the change in the photodiode voltage amounted to 40% of the bias voltage, which makes these diode structures sufficiently efficient and extremely fast detectors of optical radiation.
Abstract: We present an experimental and theoretical study of the dephasing of heavy-hole excitons in a 70-angstrom GaAs/AlxGa1-xAs multiple-quantum-well structure. We measure the time-integrated three-pulse four-wave-mixing signal of heavy-hole excitons in the quantum-well sample. We model the third-order nonlinear optical response incorporating the effects of a stochastic modulation of the transition frequency. We find that the dephasing of the nonlinear polarization is mainly the result of a spectral-migration process of the excitons and, to a lesser extent, due to a homogeneous dephasing process. The time dependence of the four-wave-mixing signal is very well described by theory if the spectral-migration process is assumed to be energy dependent.
Abstract: We investigate carrier capture and carrier transport in the InGaAs/InP material system by luminescence spectroscopy with femtosecond time resolution. Comparative studies are performed on samples of different well width, barrier width and gallium mole fraction of the InGaAs layers. The investigations focus on excitation conditions that are comparable to those for semiconductor laser operation. Firm data on carrier dynamics are presented for these conditions. We find that the overall transfer rates of electrons and holes are similar and independent of well width. Furthermore, the transfer times show a linear dependence on barrier width. From experimental and model calculation results we derive some guidelines for the design of high-frequency laser devices.
Abstract: We have studied the spatial dynamics of coherently excited wave packets of charge carriers in a semiconductor superlattice structure by both time-resolved THz spectroscopy and degenerate four-wave mixing. In the Wannier-Stark bias regime, we observe electron Bloch oscillations with a frequency that depends linearly on the electric bias field and can be tuned between a few hundred GHz and 5 THz.
Notes: 6th International Conference on Modulated Semiconductor Structures, GARMISCH PARTENKIR, GERMANY, AUG 23-27, 1993
Abstract: We present an experimental and theoretical study of the dephasing of heavy-hole excitons in a 70-angstrom GaAs/AlxGa1-xAs multiple-quantum-well structure. We measure the time-integrated three-pulse four-wave-mixing signal of heavy-hole excitons in the quantum-well sample. We model the third-order nonlinear optical response incorporating the effects of a stochastic modulation of the transition frequency. We find that the dephasing of the nonlinear polarization is mainly the result of a spectral-migration process of the excitons and, to a lesser extent, due to a homogeneous dephasing process. The time dependence of the four-wave-mixing signal is very well described by theory if the spectral-migration process is assumed to be energy dependent.
Abstract: The dynamics of photoexcitations in conjugated polymers and polymer blends is studied by means of time resolved luminescence spectroscopy. After excitation into the vibronic progression, the luminescence rises quasi-instantaneously. The luminescence decay depends strongly on the detection energy and the polymer concentration. The results are explained within a hopping model.
Abstract: We investigate the temporal evolution and the density dependence of exciton bleaching in GaAs/AlxGa1-xAs quantum wells. We find that the carrier-induced reduction of exciton oscillator strength is not influenced by carrier cooling after creation of hot carriers with femtosecond laser pulses. Excitation density experiments suggest that either phase-space filling or Coulomb screening dominates the exciton bleaching, depending on the carrier density.
Abstract: We investigate the intersubband relaxation in GaAs quantum wells at room temperature using differential transmission spectroscopy with subpicosecond time resolution. The dynamics of the subband populations are derived from the experimentally observed reduction of oscillator strength of the corresponding exciton transitions. The obtained electron intersubband scattering times are 160 fs for relaxation from the second to the first subband in a 15-nm quantum well and 170 fs for relaxation from the third to the first subband in a 20-nm structure. These times are significantly shorter than those deduced from previous experiments and from some theoretical studies.
Abstract: We detect the coherent submillimetre-wave radiation from electron Bloch oscillations in a biased GaAs/AlGaAs semiconductor superlattice by time-resolved terahertz (THz) spectroscopy. The emitted THz radiation is tunable from 0.3 to 4 THz. The emission from Bloch oscillations is observable for temperatures as high as 100 K.
Notes: 8th International Conference on Hot Carriers in Semiconductors, OXFORD, ENGLAND, AUG 16-20, 1993
Abstract: Ultrafast relaxation processes in poly(p-phenylenevinylene) and its oligomers are investigated using femtosecond luminescence spectroscopy. A quasi-instantaneous luminescence rise and the absence of luminescence near the excitation energy indicate very rapid vibronic relaxation. The subsequent transient redshift of the spectra is attributed to ultrafast energy relaxation of optical excitations within an inhomogeneously broadened density of states.
Abstract: Femtosecond time-resolved luminescence spectroscopy is used as a novel tool to study the ultrafast transport in interdigitated metal-semiconductor-metal Schottky diodes. The time-resolved luminescence signals of GaAs-MSM diodes depend strongly on bias voltage and carrier density. In these experiments, field-induced transfer to indirect valleys is directly observed in the time domain for the first time.
Abstract: We have analyzed ultrathin (5-10 monolayers) ln1-xGaxAs/InP (0.17 less-than-or-equal-to x less-than-or-equal-to 1) quantum wells grown by low-pressure metalorganic vapour phase epitaxy using optical spectroscopy and transmission electron microscopy. We find, for all compositions, evidence for a complex interface structure at both the lower and the upper interface. Both originate from interdiffusion of arsenic and phosphorus at growth time. The influence of the interface structure on the optical transition energies is discussed in detail.
Abstract: We present a detailed investigation of the dynamics of laser-excited carriers in YBa2Cu3Ox thin films with various oxygen contents x. We observe a Fermi level shift due to oxygen reduction and determine the energy gap between the Fermi level and the upper Hubbard band. The electron-phonon coupling constant lambda is determined as a function of transition temperature.
Abstract: We directly detect the coherent electromagnetic radiation originating from Bloch oscillations of charge carriers in an electrically biased semiconductor superlattice structure. The oscillation frequency can be tuned with the applied bias field from 0.5 THz to more than 2 THz, the detection limit of our measurement system.
Abstract: Ultrafast relaxation processes in poly(p-phenylenevinylene) and its oligomers are investigated using femtosecond luminescence spectroscopy. A quasi-instantaneous luminescence rise and the absence of luminescence near the excitation energy indicate very rapid vibronic relaxation. The subsequent transient redshift of the spectra is attributed to ultrafast energy relaxation of optical excitations within an inhomogeneously broadened density of states.
Abstract: We report on the non-squared composition profiles of three series of thin (L(Z)<30 angstrom) InGaAs/InP quantum wells grown with interruption sequences at, both, the lower and the upper interfaces. First is a series of LM (lattice matched) samples with nominal thicknesses ranging from 0 to 8 monolayers. Second is a series of samples with a constant thickness of 5 monolayers and gallium compositions ranging from 0.13 to 0.73. Third are two samples with thickness L(Z)=30 angstrom and gallium compositions x=0.47 (LM) and x=0.73 (gallium rich). Comparing spectrometric data collected at 2 K with high-resolution transmission electron microscope (HR-TEM) pictures, we find that the lower interface morphology (InP/InGaAs) can be easily probed from excitonic absorption and luminescence spectra. Nothing similar is found for the higher interface (InGaAs/InP), even if a considerable amount of interface roughness is resolved from HR-TEM. We show that this upper interface roughness originates from unperfected 2-dimensional growth kinetics of InGaAs.
Notes: 3rd International Conference on Optics of Excitons in Confined Systems, MONTPELLIER, FRANCE, AUG 30-SEP 02, 1993
Abstract: We have analyzed ultrathin (5-10 monolayers) ln1-xGaxAs/InP (0.17 less-than-or-equal-to x less-than-or-equal-to 1) quantum wells grown by low-pressure metalorganic vapour phase epitaxy using optical spectroscopy and transmission electron microscopy. We find, for all compositions, evidence for a complex interface structure at both the lower and the upper interface. Both originate from interdiffusion of arsenic and phosphorus at growth time. The influence of the interface structure on the optical transition energies is discussed in detail.
Notes: SYMP ON DIAGNOSTIC TECHNIQUES FOR SEMICONDUCTOR MATERIALS ANALYSIS AND FABRICATION PROCESS CONTROL, AT THE 1992 SPRING CONF OF THE EUROPEAN MATERIALS RESEARCH SOC, STRASBOURG, FRANCE, JUN 02-05, 1992
Abstract: Since the early proposal that InGaAs/InGaAs strained-layer superlattices (SLSs) should constitute a new class of optoelectronic materials, very little work has been done to master the growth conditions and optimize the resulting superlattice properties. In this work, we present the results of a preliminary investigation of SLSs grown by low-pressure metal-organic vapour phase epitaxy. Two different series of samples were grown to check independently the effect of well thickness and barrier composition. In both cases, in order to conserve one layer nominally lattice-matched to InP, the wells had a standard composition In0.53Ga0.47As.
Notes: SYMP ON MATERIALS FOR OPTOELECTRONIC DEVICES, OEICS ( OPTOELECTRONICS INTEGRATED CIRCUITS ) AND PHOTONICS, STRASBOURG, FRANCE, MAY 04-07, 1993
Abstract: A combination of second-harmonic generation (SHG) and electrical characterization of plasma processed SiO2/Si(111) structures has enabled us to separate two different effects that contribute to interfacial electrical quality. Thermally-grown oxides, prepared at 850-degrees-C, exhibit considerable compressive strain in the bulk oxide film. In contrast, there is significantly less build-up of intrinsic strain in plasma-processed SiO2/Si Structures. As a result, the driving force for stress relaxation is much smaller compared to that of thermally-grown structures. On the other hand, interfacial relaxation, as monitored by the SHG technique, involves changes in the local bonding of Si- and 0-atoms at the SiO2/Si interface. This relaxation occurred at approximately 900-degrees-C and was sufficient to optimize the electrical properties of plasma-deposited structures.
Abstract: The formation of minibands is demonstrated in photocurrent experiments on shallow In0.53Ga0.47As/In0.40Ga0.60As superlattices grown by low pressure metal-organic vapor phase epitaxy. Field-dependent variations of the spectral shape are attributed to Wannier-Stark-localization. Both type I transitions between electrons and heavy holes and type-II transitions involving light holes confined in the In0.40Ga0.60As layers are observed and identified by comparison with theoretical calculations of the superlattice electronic transitions.
Notes: 3rd International Conference on Optics of Excitons in Confined Systems, MONTPELLIER, FRANCE, AUG 30-SEP 02, 1993
Abstract: The transient development of electric-field distributions in a biased GaAs film after low density optical excitation is determined by measurements of Franz-Keldysh modulations with a time resolution of 100 fs. The experimental results are compared with theoretical calculations. The transient field is calculated with a drift-diffusion model. Our calculation of the dielectric function of GaAs includes the Coulomb coupling and the electric field. The resulting optical transmission changes are calculated with a transfer-matrix method. The theory predicts a modification of the Franz-Keldysh modulation due to the nonuniform field, in quantitative agreement with the experimental observations.
Abstract: We have analysed the interface structure of ultrathin (5-10 monolayers) In1-xGaxAs/InP (0.17 less-than-or-equal-to x less-than-or-equal-to 1) quantum wells grown by low pressure, metallo-organic vapour phase epitaxy, using optical spectroscopy and transmission electron microscopy. For all compositions, we find evidence for a complex interface structure originating from intermixing of arsenic and phosphorus during growth. The effect of the interface structure on the optical transition energies is compared with calculations based on the envelope function model.
Abstract: A combination of second-harmonic generation (SHG) and electrical characterization of plasma processed SiO2/Si(111) structures has enabled us to separate two different effects that contribute to interfacial electrical quality. Thermally-grown oxides, prepared at 850-degrees-C, exhibit considerable compressive strain in the bulk oxide film. In contrast, there is significantly less build-up of intrinsic strain in plasma-processed SiO2/Si Structures. As a result, the driving force for stress relaxation is much smaller compared to that of thermally-grown structures. On the other hand, interfacial relaxation, as monitored by the SHG technique, involves changes in the local bonding of Si- and 0-atoms at the SiO2/Si interface. This relaxation occurred at approximately 900-degrees-C and was sufficient to optimize the electrical properties of plasma-deposited structures.
Abstract: Since the early proposal that InGaAs/InGaAs strained-layer superlattices (SLSs) should constitute a new class of optoelectronic materials, very little work has been done to master the growth conditions and optimize the resulting superlattice properties. In this work, we present the results of a preliminary investigation of SLSs grown by low-pressure metal-organic vapour phase epitaxy. Two different series of samples were grown to check independently the effect of well thickness and barrier composition. In both cases, in order to conserve one layer nominally lattice-matched to InP, the wells had a standard composition In0.53Ga0.47As.
Abstract: Recently the capability of photomodulated spectroscopic ellipsometry for complete non-destructive optical characterization of GaAs/AlGaAs multiple quantum well (MQW) samples was demonstrated. This technique is particularly well suited for identifying higher quantized transitions and determining sample parameters, such as well thickness, band offset and aluminium content. In this paper modulated ellipsometry is applied to MQW and superlattice samples in the AlGaAs/GaAs material system. The results are compared with calculations of the anisotropic dielectric function based on the solution of Schrodinger's equation for the confined state energies and wavefunctions. Franz-Keldysh-like features in the E0 region of the barrier material are included in the model. The results are compared with photoluminescence, photoreflectance, and conventional spectroscopic ellipsometry and sensitivity limits of modulated spectroscopic ellipsometry are given.
Abstract: The formation of minibands is demonstrated in photocurrent experiments on shallow In0.53Ga0.47As/In0.40Ga0.60As superlattices grown by low pressure metal-organic vapor phase epitaxy. Field-dependent variations of the spectral shape are attributed to Wannier-Stark-localization. Both type I transitions between electrons and heavy holes and type-II transitions involving light holes confined in the In0.40Ga0.60As layers are observed and identified by comparison with theoretical calculations of the superlattice electronic transitions.
Abstract: We report on the non-squared composition profiles of three series of thin (L(Z)<30 angstrom) InGaAs/InP quantum wells grown with interruption sequences at, both, the lower and the upper interfaces. First is a series of LM (lattice matched) samples with nominal thicknesses ranging from 0 to 8 monolayers. Second is a series of samples with a constant thickness of 5 monolayers and gallium compositions ranging from 0.13 to 0.73. Third are two samples with thickness L(Z)=30 angstrom and gallium compositions x=0.47 (LM) and x=0.73 (gallium rich). Comparing spectrometric data collected at 2 K with high-resolution transmission electron microscope (HR-TEM) pictures, we find that the lower interface morphology (InP/InGaAs) can be easily probed from excitonic absorption and luminescence spectra. Nothing similar is found for the higher interface (InGaAs/InP), even if a considerable amount of interface roughness is resolved from HR-TEM. We show that this upper interface roughness originates from unperfected 2-dimensional growth kinetics of InGaAs.
Abstract: The formation of minibands is demonstrated in photocurrent experiments on shallow In0.53Ga0.47As/In0.40Ga0.60 superlattices grown by low-pressure metal-organic vapor-phase epitaxy. Field-dependent variations of the spectral shape are attributed to Wanner-Stark localization. Both type-I transitions between electrons and heavy holes and type-II transitions involving light holes confined in the In0.40Ga0.60As layers are observed and distinguished by their characteristic field dependence.
Abstract: Si(111) wafers, cut at angles between 0-degrees and 5-degrees in the [112BAR] direction, were prepared by a standard RCA cleaning process followed by a rinse in HF/NH4F solutions with different pH. values. Si-SiO2 structures were formed on these surfaces by a two-step low-temperature plasma-assisted oxidation/deposition process, and compared with interfaces formed by thermal oxidation, followed by rapid thermal annealing. The Si(111)-SiO2 interface, subjected to predeposition rinse in a 40 wt. % NH4F solution, displayed a midgap interface trap density D(it) of approximately 4 X 10(10) cm-2 eV-1. The D(it) values increased systematically up to approximately 2 X 10(11) cm-2 eV-1 as the pH of the final rinse was decreased. Using a second-harmonic generation pumped at 1053 nm, the influence of off-axis orientation and surface structure of Si (111) surfaces was examined. The surface structure was modified by thermal oxidation at 850-degrees-C, annealing at temperatures in the range of 900-1100-degrees-C, and removing the oxide in a HF solution. Changes in the characteristic rotational anisotropy are analyzed using harmonic functions reflecting the onefold and threefold symmetry of vicinal Si(111) surfaces, and are compared with changes in the thermal oxide D(it) values on annealing.
Abstract: Second-harmonic generation (SHG) on vicinal Si(111) surfaces, approximately 4-degrees off axis, was studied by varying the chemical bonding state of the surface atoms. 0-terminated surfaces were prepared by thermal oxidation, plasma deposition, and chemical oxidation. H-terminated surfaces were prepared by removing the oxide layers and subjecting the Si surfaces to RCA cleans terminated by rinses in dilute HF solutions. SHG experiments were performed in the s-and p-polarization modes, using the 1053 nm emission from a Nd:YLF laser. An angular anisotropy, characteristic of vicinal Si(111) surfaces, and including a distinctive SHG feature polarized in the [112BAR] direction of the surface steps, was enhanced by the presence of an oxide layer, independent of how the Si-SiO2 interface was prepared. We show for the first time that H-terminated Si surfaces exhibited a similar angular anisotropy, and that the SHG signal polarized in the [112BAR] step direction was reduced, rather than enhanced by the change in the surface bonding chemistry. Using model calculations, we illustrate that this apparent reduction in SHG signal is due to a change in the relative phase of the background and step-induced contributions to the SHG signal.
Abstract: Highly reflecting dielectric multilayer stacks, also designated as Bragg reflectors (BRs) have been fabricated using SiO2 and Si3N4 layers prepared by remote plasma-enhanced chemical-vapor deposition (RPECVD). Departures from the reflectance spectrum of a perfectly periodic bilayer structure with nondispersive dielectric materials results from (i) intrinsic effects associated with dispersion in the dielectric constants of the constituent layers, and (ii) extrinsic effects that are a result of departures from ideal bilayer geometry. Model calculations have shown that extrinsic effects in the optical response of BRs prepared by RPECVD derive from (i) systematic increases in nitride film thickness with increasing stack thickness, and/or (ii) from nonideal optical thicknesses in which the individual layers do not have an optical path length (OPL) equal to one-quarter of the wavelength at the center of the first low photon energy reflectance band, lambda/4, but instead maintain a periodic structure in which the sum of the two OPLs equals lambda/2.
Abstract: We present a detailed investigation of the dynamics of laser-excited quasiparticles in YBa2Cu3O7 thin films below the critical temperature. Reflectivity transients at low temperature trace the generation and recombination behavior of quasiparticles. The quasiparticle cascading and recombination rates are determined by comparison with a detailed nonlinear model of the quasiparticle dynamics based on extended Rothwarf-Taylor equations.
Abstract: The formation of minibands is demonstrated in photocurrent experiments on shallow In0.53Ga0.47As/In0.40Ga0.60 superlattices grown by low-pressure metal-organic vapor-phase epitaxy. Field-dependent variations of the spectral shape are attributed to Wanner-Stark localization. Both type-I transitions between electrons and heavy holes and type-II transitions involving light holes confined in the In0.40Ga0.60As layers are observed and distinguished by their characteristic field dependence.
Abstract: Recently the capability of photomodulated spectroscopic ellipsometry for complete non-destructive optical characterization of GaAs/AlGaAs multiple quantum well (MQW) samples was demonstrated. This technique is particularly well suited for identifying higher quantized transitions and determining sample parameters, such as well thickness, band offset and aluminium content. In this paper modulated ellipsometry is applied to MQW and superlattice samples in the AlGaAs/GaAs material system. The results are compared with calculations of the anisotropic dielectric function based on the solution of Schrodingerâs equation for the confined state energies and wavefunctions. Franz-Keldysh-like features in the E0 region of the barrier material are included in the model. The results are compared with photoluminescence, photoreflectance, and conventional spectroscopic ellipsometry and sensitivity limits of modulated spectroscopic ellipsometry are given.
Notes: 1ST INTERNATIONAL CONF ON SPECTROSCOPIC ELLIPSOMETRY, PARIS, FRANCE, JAN 11-14, 1993
Abstract: Highly reflecting dielectric multilayer stacks, also designated as Bragg reflectors (BRs) have been fabricated using SiO2 and Si3N4 layers prepared by remote plasma-enhanced chemical-vapor deposition (RPECVD). Departures from the reflectance spectrum of a perfectly periodic bilayer structure with nondispersive dielectric materials results from (i) intrinsic effects associated with dispersion in the dielectric constants of the constituent layers, and (ii) extrinsic effects that are a result of departures from ideal bilayer geometry. Model calculations have shown that extrinsic effects in the optical response of BRs prepared by RPECVD derive from (i) systematic increases in nitride film thickness with increasing stack thickness, and/or (ii) from nonideal optical thicknesses in which the individual layers do not have an optical path length (OPL) equal to one-quarter of the wavelength at the center of the first low photon energy reflectance band, lambda/4, but instead maintain a periodic structure in which the sum of the two OPLs equals lambda/2.
Notes: 39TH NATIONAL SYMP OF THE AMERICAN VACUUM SOC, CHICAGO, IL, NOV 09-13, 1992
Abstract: We have analysed the interface structure of ultrathin (5-10 monolayers) In1-xGaxAs/InP (0.17 less-than-or-equal-to x less-than-or-equal-to 1) quantum wells grown by low pressure, metallo-organic vapour phase epitaxy, using optical spectroscopy and transmission electron microscopy. For all compositions, we find evidence for a complex interface structure originating from intermixing of arsenic and phosphorus during growth. The effect of the interface structure on the optical transition energies is compared with calculations based on the envelope function model.
Notes: 1ST WORKSHOP ON EXPERT EVALUATION AND CONTROL OF COMPOUND SEMICONDUCTOR MATERIALS AND TECHNOLOGIES : SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY ( EXMATEC 92 ), ECULLY, FRANCE, MAY 19-22, 1992
Abstract: Si(111) wafers, cut at angles between 0-degrees and 5-degrees in the [112BAR] direction, were prepared by a standard RCA cleaning process followed by a rinse in HF/NH4F solutions with different pH. values. Si-SiO2 structures were formed on these surfaces by a two-step low-temperature plasma-assisted oxidation/deposition process, and compared with interfaces formed by thermal oxidation, followed by rapid thermal annealing. The Si(111)-SiO2 interface, subjected to predeposition rinse in a 40 wt. % NH4F solution, displayed a midgap interface trap density D(it) of approximately 4 X 10(10) cm-2 eV-1. The D(it) values increased systematically up to approximately 2 X 10(11) cm-2 eV-1 as the pH of the final rinse was decreased. Using a second-harmonic generation pumped at 1053 nm, the influence of off-axis orientation and surface structure of Si (111) surfaces was examined. The surface structure was modified by thermal oxidation at 850-degrees-C, annealing at temperatures in the range of 900-1100-degrees-C, and removing the oxide in a HF solution. Changes in the characteristic rotational anisotropy are analyzed using harmonic functions reflecting the onefold and threefold symmetry of vicinal Si(111) surfaces, and are compared with changes in the thermal oxide D(it) values on annealing.
Notes: 20TH ANNUAL CONF ON THE PHYSICS AND CHEMISTRY OF SEMICONDUCTOR INTERFACES, WILLIAMSBURG, VA, JAN 25-29, 1993
Abstract: We present a detailed investigation of the dynamics of laser-excited quasiparticles in YBa2Cu3O7 thin films below the critical temperature. Reflectivity transients at low temperature trace the generation and recombination behavior of quasiparticles. The quasiparticle cascading and recombination rates are determined by comparison with a detailed nonlinear model of the quasiparticle dynamics based on extended Rothwarf-Taylor equations.
Abstract: The transient development of electric-field distributions in a biased GaAs film after low density optical excitation is determined by measurements of Franz-Keldysh modulations with a time resolution of 100 fs. The experimental results are compared with theoretical calculations. The transient field is calculated with a drift-diffusion model. Our calculation of the dielectric function of GaAs includes the Coulomb coupling and the electric field. The resulting optical transmission changes are calculated with a transfer-matrix method. The theory predicts a modification of the Franz-Keldysh modulation due to the nonuniform field, in quantitative agreement with the experimental observations.
Abstract: Second-harmonic generation (SHG) on vicinal Si(111) surfaces, approximately 4-degrees off axis, was studied by varying the chemical bonding state of the surface atoms. 0-terminated surfaces were prepared by thermal oxidation, plasma deposition, and chemical oxidation. H-terminated surfaces were prepared by removing the oxide layers and subjecting the Si surfaces to RCA cleans terminated by rinses in dilute HF solutions. SHG experiments were performed in the s-and p-polarization modes, using the 1053 nm emission from a Nd:YLF laser. An angular anisotropy, characteristic of vicinal Si(111) surfaces, and including a distinctive SHG feature polarized in the [112BAR] direction of the surface steps, was enhanced by the presence of an oxide layer, independent of how the Si-SiO2 interface was prepared. We show for the first time that H-terminated Si surfaces exhibited a similar angular anisotropy, and that the SHG signal polarized in the [112BAR] step direction was reduced, rather than enhanced by the change in the surface bonding chemistry. Using model calculations, we illustrate that this apparent reduction in SHG signal is due to a change in the relative phase of the background and step-induced contributions to the SHG signal.
Notes: 39TH NATIONAL SYMP OF THE AMERICAN VACUUM SOC, CHICAGO, IL, NOV 09-13, 1992
Abstract: We present a detailed investigation of the dynamics of laser-excited carriers in YBa2Cu3Ox thin films with various oxygen contents x. We observe a Fermi level shift due to oxygen reduction and determine the energy gap between the Fermi level and the upper Hubbard band. The electron-phonon coupling constant lambda is determined as a function of transition temperature.
Abstract: Femtosecond time-resolved luminescence spectroscopy is used as a novel tool to study the ultrafast transport in interdigitated metal-semiconductor-metal Schottky diodes. The time-resolved luminescence signals of GaAs-MSM diodes depend strongly on bias voltage and carrier density. In these experiments, field-induced transfer to indirect valleys is directly observed in the time domain for the first time.
Abstract: We directly detect the coherent electromagnetic radiation originating from Bloch oscillations of charge carriers in an electrically biased semiconductor superlattice structure. The oscillation frequency can be tuned with the applied bias field from 0.5 THz to more than 2 THz, the detection limit of our measurement system.
Abstract: The optical response of hot carriers in III-V compounds is modeled within a density matrix formalism. As a starting point for the calculation of the luminescence, a four-point electron-hole density matrix has to be used. The band structure is described by a realistic model including all hole subbands and GAMMA-, L, and X-valleys of the conduction band. The theory is compared in detail with recent time-resolved and spectrally resolved luminescence up-conversion results. Satisfactory agreement can be obtained even with a rather simple exponential relaxation ansatz for the energies of internally thermalized electron and hole distributions. Arguments are given that the detected luminescence intensity must be distinguished from the photon generation rate.
Abstract: Simultaneous excitation of heavy- and light-hole quantum-well excitons with linearly polarized ultrashort laser pulses results in oscillating four-wave-mixing and pump-probe signals. These are elliptically polarized, except for exactly parallel and perpendicular incident polarizations, for which they are also linearly polarized. In the latter case, the heavy- and light-hole components are in phase or out of phase, respectively.
Abstract: We present a detailed study of coherent tunneling oscillations of an optically excited electronic wavepacket in a semiconductor double quantum-well structure. The oscillation of the wavepacket between the two wells is traced by time-resolved pump-probe spectroscopy as well as time-resolved degenerate four-wave mixing. The experimental observations are compared with a simple theoretical model. The motion of the wavepacket is studied as a function of both an external electric field and the lattice temperature. While the oscillation frequency shows the expected dependence on electric field, it increases with temperature at fixed electric field when perfectly delocalized electronic states exist at low temperature.
Abstract: We report the study of optical dephasing of Wannier-Stark ladder excitations in a semiconductor superlattice by means of transient degenerate four-wave mixing. We observe pronounced modulations of the signal with a time period varying linearly with the electric field. The time period,is found to equal the temporal periodicity of Bloch oscillations, in agreement with theory. In addition, we find that the dephasing time decreases with increasing localization of the Wannier-Stark states, which is attributed to carrier escape out of the lowest miniband.
Abstract: We directly observe the electromagnetic radiation emitted by electrons coherently oscillating between the two wells of a semiconductor coupled-quantum-well structure. Using time-resolved coherent detection of the submillimeter-wave radiation from these spatial charge oscillations, we trace up to fourteen oscillations at 1.5 THz before phase relaxation destroys the coherence of the oscillating wave packet. In addition to the oscillatory electromagnetic signal, we also observe an instantaneous signal from electric-field-induced optical rectification in the semiconductor structure.
Abstract: The transfer of electrons and holes from barriers to wells is investigated in strained In1-xGaxAs/InP multiple quantum wells by time-resolved luminescence upconversion with 300 fs time resolution. The transfer times are in the range of a few ps and independent of the Ga content. The investigation of Ga-rich structures allows to observe directly the hole transfer.
Abstract: We discuss tunnelling processes in double quantum well structures in two limiting cases. Firstly, we report the coherent oscillations of a photogenerated electronic wavepacket between the wells. The THz oscillations are traced by an optical pump-probe as well as by four-wave-mixing spectroscopy. A comparison with theory shows that the two experiments give complementary information about the various relaxation processes. Secondly, we discuss hole tunnelling in weakly coupled wells as an example of resonant tunnelling in the limit of strong scattering. The experimental and theoretical results show that the resonant tunnelling process is considerably slowed down by the scattering and relaxation processes.
Abstract: We present time-resolved measurements of the relaxation of photoexcited holes in n-doped GaAs and InP with 100 fs time resolution. The band-edge luminescence is predominantly controlled by the hole dynamics, when the optical excitation density is much lower than the doping density. Information on the electron-hole scattering and hole-phonon scattering rates is obtained by varying the doping density. Thermal equilibrium between hot holes and lattice is found to be established on a subpicosecond time scale. The energy-loss rate of holes is dominated by optical-deformation-potential scattering.
Abstract: We study the dynamics of excitons in GaAs/AlGaAs quantum wells using time-resolved self-diffracted four-wave-mixing. We discuss two topics: First, we show that the decay of the diffracted signal is different for parallel and perpendicular polarization of the incident laser pulses, and second, we study the dephasing dynamics of a wavepacket in a coupled quantum well structure.
Abstract: We report unambiguous experimental evidence for electron Bloch oscillations: The transient four-wave mixing signal of a biased semiconductor superlattice shows a periodic modulation with a time constant expected from Bloch oscillation theory. The oscillation frequency can be tuned over 400% with the applied electric field. The electron performs up to four oscillation cycles before the coherence is lost.
Abstract: We perform a complete nondestructive optical characterization of a GaAs/AlxGa1-xAs multiple-quantum-well (MQW) sample. Modulated ellipsometry, a combination of spectroscopic ellipsometry and photomodulation spectroscopy, is used as an experimental technique. The electric field is modulated via photoexcitation of electron-hole pairs in the p-i-n structure. The field-induced modification in the dielectric function of the multiple quantum well is derived in the spectral range from the GaAs to the AlxGa1-xAs band pp. We show that the modulated ellipsometry technique is particularly well suited to characterize higher quantized transitions. The experimental results are compared to an extensive theoretical model of the MQW dielectric function, based on a transfer-matrix solution of the Schrodinger equation. The effective-mass dispersion of electrons and light holes are treated by the Kane model, and the excitonic transitions are introduced by oscillator functions. A detailed comparison of the experimental spectra and this theory allows a very precise determination of the sample parameters, such as well thickness, barrier thickness, and aluminum content.
Abstract: We have analysed ultrathin (5-10 monolayers) In1-xGaxAs/InP (0.17 less-than-or-equal-to x less-than-or-equal-to 1) quantum wells grown by low-pressure metal organic vapour phase epitaxy using optical spectroscopy and transmission electron microscopy. We find, for all compositions, evidence for a complex interface structure at both the lower and the upper interface. Both originate from interdiffusion of arsenic and phosphorus at growth time. The influence of the interface structure on the optical transition energies is discussed in detail.
Abstract: For self electro-optic effect device applications, multiple quantum well modulator devices in the material system InGaAs/InP are studied. The applied experimental techniques are differential electrotransmission and photoluminescence with and without electric field. The optoelectronic properties of the modulators, including transport and recombination processes, are studied in the experiments. The theory describes the electric field dependence of respectively the confined state energies, overlap of electron-hole wavefunctions, the dielectric constants epsilon1 and epsilon2 of the multiple quantum well material and the differential electrotransmission spectra.
Abstract: We investigate the carrier-transfer dynamics in In1-xGaxAs/InP heterostructures using luminescence upconversion spectroscopy with 300-fs time resolution. Carrier transport across the InP barriers to the quantum wells and the final carrier capture into confined states of the quantum wells are separated in a comparative study of samples of different well and barrier thicknesses. The transport of carriers across the barrier material to the quantum wells is found to be almost instantaneous for barrier thicknesses up to 30 nm. We obtain capture-time constants of about 800 fs for electrons and 200 fs for holes. The capture times are virtually independent of well width. We develop a formalism to calculate the carrier transfer time from the barriers to the quantum wells for given well and barrier thicknesses. An upper limit of modulation frequencies set by this transfer process of the order of 300 GHz for devices with thin barriers is derived.
Abstract: We review our observations of quantum beats of excitons in quantum wells arising from (i) heavy hole-light hole splitting, (ii) exciton splitting due to fluctuations in the well-width of quantum wells, and (iii) free and bound excitons, using the techniques of time resolved four-wave-mixing (FWM) and pump-and-probe (PP). We also discuss quantum beats in appropriately biased asymmetric double quantum wells, representing the first observation of coherent oscillations of an electronic wavepacket in a solid, and report on the observation of unusual polarization dependence in the FWM experiments.
Abstract: We discuss recent experimental observations of the spatial dynamics of photoexcited wavepackets in semiconductor heterostructures. In particular, coherent oscillation of a wavepacket in coupled double quantum well structures are traced with time-resolved transmission and four-wave-mixing experiments. The oscillation period and modulation can be tuned with an electric field. The experimental results are compared to a theory based on the third-order nonlinear response of a three-level system.
Abstract: We present a detailed study of coherent tunneling oscillations of an optically excited electronic wavepacket in a semiconductor double quantum-well structure. The oscillation of the wavepacket between the two wells is traced by time-resolved pump-probe spectroscopy as well as time-resolved degenerate four-wave mixing. The experimental observations are compared with a simple theoretical model. The motion of the wavepacket is studied as a function of both an external electric field and the lattice temperature. While the oscillation frequency shows the expected dependence on electric field, it increases with temperature at fixed electric field when perfectly delocalized electronic states exist at low temperature.
Abstract: We directly observe the electromagnetic radiation emitted by electrons coherently oscillating between the two wells of a semiconductor coupled-quantum-well structure. Using time-resolved coherent detection of the submillimeter-wave radiation from these spatial charge oscillations, we trace up to fourteen oscillations at 1.5 THz before phase relaxation destroys the coherence of the oscillating wave packet. In addition to the oscillatory electromagnetic signal, we also observe an instantaneous signal from electric-field-induced optical rectification in the semiconductor structure.
Abstract: We discuss tunnelling processes in double quantum well structures in two limiting cases. Firstly, we report the coherent oscillations of a photogenerated electronic wavepacket between the wells. The THz oscillations are traced by an optical pump-probe as well as by four-wave-mixing spectroscopy. A comparison with theory shows that the two experiments give complementary information about the various relaxation processes. Secondly, we discuss hole tunnelling in weakly coupled wells as an example of resonant tunnelling in the limit of strong scattering. The experimental and theoretical results show that the resonant tunnelling process is considerably slowed down by the scattering and relaxation processes.
Notes: 7TH INTERNATIONAL CONF ON HOT CARRIERS IN SEMICONDUCTORS ( HCIS-7 ), NARA, JAPAN, JUL 01-05, 1991
Abstract: We present time-resolved measurements of the relaxation of photoexcited holes in n-doped GaAs and InP with 100 fs time resolution. The band-edge luminescence is predominantly controlled by the hole dynamics, when the optical excitation density is much lower than the doping density. Information on the electron-hole scattering and hole-phonon scattering rates is obtained by varying the doping density. Thermal equilibrium between hot holes and lattice is found to be established on a subpicosecond time scale. The energy-loss rate of holes is dominated by optical-deformation-potential scattering.
Abstract: We review our observations of quantum beats of excitons in quantum wells arising from (i) heavy hole-light hole splitting, (ii) exciton splitting due to fluctuations in the well-width of quantum wells, and (iii) free and bound excitons, using the techniques of time resolved four-wave-mixing (FWM) and pump-and-probe (PP). We also discuss quantum beats in appropriately biased asymmetric double quantum wells, representing the first observation of coherent oscillations of an electronic wavepacket in a solid, and report on the observation of unusual polarization dependence in the FWM experiments.
Notes: 5TH INTERNATIONAL CONF ON MODULATED SEMICONDUCTOR STRUCTURES ( MSS-5 ), NARA, JAPAN, JUL 08-12, 1991
Abstract: We investigate the carrier-transfer dynamics in In1-xGaxAs/InP heterostructures using luminescence upconversion spectroscopy with 300-fs time resolution. Carrier transport across the InP barriers to the quantum wells and the final carrier capture into confined states of the quantum wells are separated in a comparative study of samples of different well and barrier thicknesses. The transport of carriers across the barrier material to the quantum wells is found to be almost instantaneous for barrier thicknesses up to 30 nm. We obtain capture-time constants of about 800 fs for electrons and 200 fs for holes. The capture times are virtually independent of well width. We develop a formalism to calculate the carrier transfer time from the barriers to the quantum wells for given well and barrier thicknesses. An upper limit of modulation frequencies set by this transfer process of the order of 300 GHz for devices with thin barriers is derived.
Abstract: We study the dynamics of excitons in GaAs/AlGaAs quantum wells using time-resolved self-diffracted four-wave-mixing. We discuss two topics: First, we show that the decay of the diffracted signal is different for parallel and perpendicular polarization of the incident laser pulses, and second, we study the dephasing dynamics of a wavepacket in a coupled quantum well structure.
Abstract: We report the study of optical dephasing of Wannier-Stark ladder excitations in a semiconductor superlattice by means of transient degenerate four-wave mixing. We observe pronounced modulations of the signal with a time period varying linearly with the electric field. The time period,is found to equal the temporal periodicity of Bloch oscillations, in agreement with theory. In addition, we find that the dephasing time decreases with increasing localization of the Wannier-Stark states, which is attributed to carrier escape out of the lowest miniband.
Abstract: We perform a complete nondestructive optical characterization of a GaAs/AlxGa1-xAs multiple-quantum-well (MQW) sample. Modulated ellipsometry, a combination of spectroscopic ellipsometry and photomodulation spectroscopy, is used as an experimental technique. The electric field is modulated via photoexcitation of electron-hole pairs in the p-i-n structure. The field-induced modification in the dielectric function of the multiple quantum well is derived in the spectral range from the GaAs to the AlxGa1-xAs band pp. We show that the modulated ellipsometry technique is particularly well suited to characterize higher quantized transitions. The experimental results are compared to an extensive theoretical model of the MQW dielectric function, based on a transfer-matrix solution of the Schrodinger equation. The effective-mass dispersion of electrons and light holes are treated by the Kane model, and the excitonic transitions are introduced by oscillator functions. A detailed comparison of the experimental spectra and this theory allows a very precise determination of the sample parameters, such as well thickness, barrier thickness, and aluminum content.
Abstract: We report unambiguous experimental evidence for electron Bloch oscillations: The transient four-wave mixing signal of a biased semiconductor superlattice shows a periodic modulation with a time constant expected from Bloch oscillation theory. The oscillation frequency can be tuned over 400% with the applied electric field. The electron performs up to four oscillation cycles before the coherence is lost.
Abstract: Simultaneous excitation of heavy- and light-hole quantum-well excitons with linearly polarized ultrashort laser pulses results in oscillating four-wave-mixing and pump-probe signals. These are elliptically polarized, except for exactly parallel and perpendicular incident polarizations, for which they are also linearly polarized. In the latter case, the heavy- and light-hole components are in phase or out of phase, respectively.
Abstract: The optical response of hot carriers in III-V compounds is modeled within a density matrix formalism. As a starting point for the calculation of the luminescence, a four-point electron-hole density matrix has to be used. The band structure is described by a realistic model including all hole subbands and GAMMA-, L, and X-valleys of the conduction band. The theory is compared in detail with recent time-resolved and spectrally resolved luminescence up-conversion results. Satisfactory agreement can be obtained even with a rather simple exponential relaxation ansatz for the energies of internally thermalized electron and hole distributions. Arguments are given that the detected luminescence intensity must be distinguished from the photon generation rate.
Abstract: We have analysed ultrathin (5-10 monolayers) In1-xGaxAs/InP (0.17 less-than-or-equal-to x less-than-or-equal-to 1) quantum wells grown by low-pressure metal organic vapour phase epitaxy using optical spectroscopy and transmission electron microscopy. We find, for all compositions, evidence for a complex interface structure at both the lower and the upper interface. Both originate from interdiffusion of arsenic and phosphorus at growth time. The influence of the interface structure on the optical transition energies is discussed in detail.
Notes: 22ND EUROPEAN SOLID STATE DEVICE RESEARCH CONF ( ESSDERC 92 ), LOUVAIN, BELGIUM, SEP 14-17, 1992
Abstract: For self electro-optic effect device applications, multiple quantum well modulator devices in the material system InGaAs/InP are studied. The applied experimental techniques are differential electrotransmission and photoluminescence with and without electric field. The optoelectronic properties of the modulators, including transport and recombination processes, are studied in the experiments. The theory describes the electric field dependence of respectively the confined state energies, overlap of electron-hole wavefunctions, the dielectric constants epsilon1 and epsilon2 of the multiple quantum well material and the differential electrotransmission spectra.
Notes: 22ND EUROPEAN SOLID STATE DEVICE RESEARCH CONF ( ESSDERC 92 ), LOUVAIN, BELGIUM, SEP 14-17, 1992
Abstract: The transfer of electrons and holes from barriers to wells is investigated in strained In1-xGaxAs/InP multiple quantum wells by time-resolved luminescence upconversion with 300 fs time resolution. The transfer times are in the range of a few ps and independent of the Ga content. The investigation of Ga-rich structures allows to observe directly the hole transfer.
Abstract: We present results on the picosecond dynamics of spin relaxation and thermalization of excitons in GaAs quantum wells. The spin relaxation time constant is almost-equal-to 50 ps for resonant excitation and less than 10 ps for nonresonant excitation, considerably shorter than those for free electrons in the bulk. Photoexcited cold, nonthermal excitons thermalize in almost-equal-to 50 ps at 10 K and less than 10 ps at 35 K. Exciton-acoustic phonon energy exchange rates are determined from these measurements.
Abstract: We present the first study of the dynamics of an extended electronic wave packet in a solid. The wave packet is created in a GaAs/AlGaAs double-quantum-well structure by ultrashort pulse excitation. We observe the oscillatory motion of the wave packet between the two wells by using time-resolved degenerate four-wave-mixing and pump-and-probe spectroscopy.
Abstract: We investigate coherent exciton transitions in GaAs/Al(x)Ga1-xAs quantum wells using subpicosecond four-wave-mixing spectroscopy. In a first part, we show that the four-wave-mixing lineshape deviates considerably from earlier predictions when the detailed structure of the excitons is taken into account, and we study the density dependence and excitation energy dependence in detail. In a second part, we discuss the observation of quantum beats due to the coherent superposition of different excitonic transitions and show how the analysis of the lineshape can yield information about relaxation processes.
Abstract: The cooling of an electron-hole plasma in bulk n- and p-type doped GaAs is calculated without assuming complete thermalization between electrons and holes. Electron-hole scattering and nonequilibrium optical phonons are included in the simulation. In n-doped GaAs, we show that at low excitation density (i.e., when the photogenerated plasma density is small compared to the donor density), the electrons are at first colder than the holes but heat up quickly and subsequently stay close to the hole temperature. At low density in p-doped GaAs, however, the holes stay cold and the electron cooling is mainly controlled by electron-hole scattering.
Abstract: We investigate the properties of coherent exciton states in GaAs/Al(x)Ga(1-x)As quantum wells using the four-wave-mixing technique with subpicosecond time resolution. In the first part of this work, we show that many-body interactions in a solid manifest themselves in a line shape of the four-wave-mixing signal that is substantially different from the predictions of the generally used simple two-level model. In particular, the coherent interaction of the quasiparticles leads to a signal at negative delay times, in agreement with recent theoretical predictions. In the second part, we report in detail about experiments where energetically close exciton transitions are excited coherently with a short laser pulse. The quantum or polarization interference leads to the observation of quantum beats in the decay of the diffracted signal. These results demonstrate that quantum-beat spectroscopy can be applied to study intrinsic excitations in solids.
Abstract: The cooling of an electron-hole plasma in bulk n- and p-type doped GaAs is calculated without assuming complete thermalization between electrons and holes. Electron-hole scattering and nonequilibrium optical phonons are included in the simulation. In n-doped GaAs, we show that at low excitation density (i.e., when the photogenerated plasma density is small compared to the donor density), the electrons are at first colder than the holes but heat up quickly and subsequently stay close to the hole temperature. At low density in p-doped GaAs, however, the holes stay cold and the electron cooling is mainly controlled by electron-hole scattering.
Abstract: We investigate coherent exciton transitions in GaAs/Al(x)Ga1-xAs quantum wells using subpicosecond four-wave-mixing spectroscopy. In a first part, we show that the four-wave-mixing lineshape deviates considerably from earlier predictions when the detailed structure of the excitons is taken into account, and we study the density dependence and excitation energy dependence in detail. In a second part, we discuss the observation of quantum beats due to the coherent superposition of different excitonic transitions and show how the analysis of the lineshape can yield information about relaxation processes.
Abstract: We present results on the picosecond dynamics of spin relaxation and thermalization of excitons in GaAs quantum wells. The spin relaxation time constant is almost-equal-to 50 ps for resonant excitation and less than 10 ps for nonresonant excitation, considerably shorter than those for free electrons in the bulk. Photoexcited cold, nonthermal excitons thermalize in almost-equal-to 50 ps at 10 K and less than 10 ps at 35 K. Exciton-acoustic phonon energy exchange rates are determined from these measurements.
Abstract: We investigate the properties of coherent exciton states in GaAs/Al(x)Ga(1-x)As quantum wells using the four-wave-mixing technique with subpicosecond time resolution. In the first part of this work, we show that many-body interactions in a solid manifest themselves in a line shape of the four-wave-mixing signal that is substantially different from the predictions of the generally used simple two-level model. In particular, the coherent interaction of the quasiparticles leads to a signal at negative delay times, in agreement with recent theoretical predictions. In the second part, we report in detail about experiments where energetically close exciton transitions are excited coherently with a short laser pulse. The quantum or polarization interference leads to the observation of quantum beats in the decay of the diffracted signal. These results demonstrate that quantum-beat spectroscopy can be applied to study intrinsic excitations in solids.
Abstract: We present the first study of the dynamics of an extended electronic wave packet in a solid. The wave packet is created in a GaAs/AlGaAs double-quantum-well structure by ultrashort pulse excitation. We observe the oscillatory motion of the wave packet between the two wells by using time-resolved degenerate four-wave-mixing and pump-and-probe spectroscopy.
Abstract: The lasing threshold of a microcavity is mainly determined by the spontaneous emission factor 13, which is inversely proportional to the mode volume V-c. We demonstrate an experimental way to decrease the mode volume via lateral structuring of the microcavity. This redistributes both number and density of the transversal cavity modes, which increases the amplitude of the internal electromagnetic field. Our samples are microcavities with an active layer of variable thickness (0.2 to 2 mu m) made of tris-(8-hydroxyquinoline) aluminium (Alq3) doped with 4-(dicyanomethylene)-2- methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). With thermal coevaporation through a shadow mask, this layer is structured into an array of photonic boxes square-shaped microcavities with an area of 5x5 mu m(2). Using a microscope objective, we record the spatial distribution of the cavity transmission spectra with submicron resolution. The modes of the photonic boxes show a clear discretization, which is due to the multidimensional optical confinement. Under selective excitation of the DCM molecules via a focused pulsed laser (532 nm, 1.5 ns, 2 kHz, circle divide approximate to 3 mu m), we record the spatially and spectrally resolved emission of single photonic boxes. The laser pulse energy is varied to obtain input-output curves of the cavity modes. At an excitation energy of similar to 30 pJ, we observe superlinear growth as well as a, spectral narrowing of the emission from the lowest energy), mode of a single photonic box. For this lasing transition, we determine a spontaneous emission factor beta of approximate to 0.01.
Notes: Conference on Organic Optoelectronics and Photonics III, Strasbourg, FRANCE, APR 07-10, 2008
Abstract: We report on a series of organci solar cells based on heterojunctions of oligothiophene derivatives with varying chain length and C60 fullerences. Devices are based on either p-i-n or p-i-i structure. In the first the intrinsic photovoltaic active layer is sandwiched between a p-type and n-type doped organic wide-gap layer for hole and electron transport respectively. In the latter the electron transport layer is replaced by a thin layer of wide-gap material as exciton blocker. Through optimization of transport and absorber layers we are able to reach in devices with single heterojunctions an open circuit voltage V-oc of about 1V, a short circuit current density J(sc) of about 5.6mA/cm(2) and a fill factor FF above 50% under an AM1.5 illumination with 1000W/m(2). However, still only a small part of the available solar spectrum is used. Thus, based on these materials stacked solar cells have been made to further improve the light absorption. The thickness of each layer is optimized using optical simulations to match the currents delivered by each of the solar cells in the stack. Through the incorporation of a very efficient recombination zone between the stacked solar cells the resulting V-oc nearly reaches the sum of the V-oc of the two serially connected solar cells.
Notes: Conference on Photonics for Solar Energy Systems II, Strasbourg, FRANCE, APR 07-08, 2008
Abstract: Alternatives for replacing the expensive ITO are explored and Poly(ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) is introduced as one possibility. We present the first small-molecule organic solar cells employing only PEDOT:PSS as transparent electrode. Solar cells on glass and on flexible plastic foil were prepared, using a p-doped hole transporting material, zinc phthalocyanine (ZnPc) and C-60 as donor-acceptor heterojunction, and an exciton blocking layer. Different methods to structure the PEDOT:PSS electrodes were investigated and are presented. As proof of principle, non-optimized prototype cells with efficiencies of over 0.7% on glass and 0.9% on flexible plastic foil substrate were obtained.
Notes: Conference on Photonics for Solar Energy Systems II, Strasbourg, FRANCE, APR 07-08, 2008
Abstract: In this contribution, we present simulation results of an emitter doped phosphorescent OLED. An alternative method for the determination of correct electrical input parameters is proposed. The exciton profile from the electrical simulation is used to simulate angle dependent emission spectra. Comparison with experimental data demonstrates the feasibility of the underlying physical models.
Notes: International Symposium of the Society-for-Information-Display (SID 2008), Los Angeles, CA, MAY 18-23, 2008
Abstract: The application of organic materials as solid state lasers critically relies on a low lasing threshold. We investigate the characteristics of emission from an organic vertical cavity surface emitting laser. The microcavity studied here consists of two highly reflective distributed Bragg reflectors enclosing a wedge-shaped active layer of Alq(3):DCM. Lasing of the DCM molecules is induced via two different pump regimes, either exciting Alq3 at a wavelength of 400 nm or pumping directly into the absorption band of DCM at 532 nm. By a variation of the pump beam position with respect to the microcavity surface, we demonstrate a continuous wavelength tuning in the organic microcavities in a range of 55 nm. The continuously variable Cavity thickness allows us to study the thickness dependence of the input-output characteristics in a single sample. These data are obtained at a certain emission wavelength, A, close to the maximum of the gain spectrum, for a number of cavity thicknesses, which correspond to different multiples of lambda/2. For a decreasing thickness of the active layer, one-dimensional optical confinement is expected to result in an increased spontaneous emission factor. On the other hand, the loss rate through the mirrors increases with decreasing thickness resulting in a, minimum threshold value for an active layer thickness of approximately 3/2 lambda. This lower threshold limit; is set by nonradiative losses as well as residual absorption.
Notes: Conference on Organic Optoelectronics and Photonics III, Strasbourg, FRANCE, APR 07-10, 2008
Abstract: Organic light-emitting diodes (OLEDs) based on small-molecule materials are currently developed for applications in flat panel displays and general lighting sources. However, a large number of efficient deep blue emitters still suffer from rather fast degradation and thus, requires further improvement. The aim of the present work is to gain a fundamental understanding of the intrinsic degradation processes causing the low stability of blue OLED emitters. For this purpose we study the photoluminescence (PL) degradation instead of the most often investigated electroluminescence (EL) degradation to separate electrically and optically generated effects. We show a newly developed PL lifetime measurement system which allows the study of degradation processes under the influence of either electron or hole currents. Using this set-up we demonstrate the veri high PL stability of the highly efficient blue singlet emitter 2,2â,7,7â-tetrakis(2,2-diphenylvinyl)spiro-9,9â-bifluorene (Spiro-DPVBi) under electron and hole currents and compare this to the lifetime of OLEDs using the same emitter material.
Notes: Conference on Organic Optoelectronics and Photonics III, Strasbourg, FRANCE, APR 07-10, 2008
Abstract: A way to reach highly efficient and stable red bottom emission organic light emitting diodes (OLEDs) is the use of doped transport layers, charge and exciton blockers, and phosphorescent emitter materials to combine low operating voltage and high quantum yield. We will show how efficiency and lifetime of such devices can be further increased. In our contribution, we report on highly efficient red p-i-n type organic light emitting diodes using an iridium-based electrophosphorescent dye, Ir(MDQ)(2)(acac), doped in (alpha-NPD as host material. By proper adjustment of the hole blocking layer, the device performance may be enhanced to 20% external quantum efficiency at an operation voltage of 2.4 V and a brightness of 100 cd/m(2). At the same time, a power efficiency of 37.5 lm/W is reached. The quantum efficiency is well above previously reported values for this emitter. We attribute this high efficiency to a combination of a well-adjusted charge carrier balance in the emission layer and a low current density needed to reach a certain luminance due to the use of doped transport layers. High chemical stability of the blocker material assures a long device lifetime of 32.000 hours at 1.000 cd/m(2) initial luminance.
Notes: Conference on Organic Optoelectronics and Photonics III, Strasbourg, FRANCE, APR 07-10, 2008
Abstract: The currently starting technical exploitation of organic electronic devices requires a deep understanding of ageing and degradation mechanisms. In addition to extrinsically caused ageing processes, such as the penetration of oxygen and water in organic layers and subsequent (electro)chemical reactions, further degradation channels exist in such devices, which are based on intrinsic chemical reactions of the materials used in the devices. At this time, we know the degradation mechanisms of only few organic materials applied in organic light emitting devices (OLEDs). To detect specific reaction products, we introduced laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS), a method which allows to distinguish between desired and undesired compounds in thin film organic devices. We use LDI-TOF-MS to detect the degradation products of different Iridium based emitter materials like lr(MDQ)(2)acac (red emitter) and FIrpic (light blue) in dc driven OLEDs and adapted test sample structures. Due to the dissociation behaviour of some Ir complexes and the ability of their fragments to form complexes with several hole blocking materials, the degradation mechanisms of the devices can be understood in terms of such chemical complex formation between the emitter molecules and neighbouring materials. On the other hand, the knowledge about these mechanisms can be used to select the right combination of materials for the benefit of long-living devices as we will show at the end of this work.
Notes: Conference on Organic Optoelectronics and Photonics III, Strasbourg, FRANCE, APR 07-10, 2008
Abstract: In this study a high efficient p-i-n type orange organic light emitting diode (OLED) is presented. It is based on doped charge transport layers to realize low operating voltage and emitting layer which consists of alpha-NPD(4,4-bis [N-(1-naphtyl)-N-phenylamino]biphenyl) and Iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) as a host and a phosphorescence dye dopant respectively. Organic layers arc vacuum-sublimed on ITO-coated glass substrates in vertical inline deposition tool, and aluminium is deposited directly on organic layer by DC magnetron sputtering to form a cathode. Since sputter deposition of top electrode is known to damage organic layers and degrade,OLED performance, various sputter process parameters are selected and applied for cathode formations, and the OLEDs are characterized by means of I-V-L measurements. The OLED characteristics are evaluated with the plasma factors based on sputter process parameters in order to explain the damage sources from sputtering process. The characteristics of OLEDs that cathodes are deposited by sputtering and evaporation are compared. The fabricated OLED which has the lowest damage level exhibits almost comparable result to the OLED that the cathode is deposited by evaporation. The OLED shows good performances of driving voltage of 4.25 V and luminous efficacy of 7.77 lm/W and current efficiency of 10.68 cd/A at 1000cd/m(2).
Notes: Conference on Organic Optoelectronics and Photonics III, Strasbourg, FRANCE, APR 07-10, 2008
Abstract: To reach higher performances in organic solar cells, each layer has to be optimised with respect to its purpose. In the case of a p-i-n structured solar cell, the layers are the absorber system, the doped electron and hole transport layers, and the bottom and top contacts. This work focuses oil the investigation and characterisation of the transparent hole transport materials PV-TPD, PV-TPDoM, Di-NPB, and MeO-Spiro-TPD, as used in organic p-i-n solar cells. The motivation is to replace the hole transport, material MeO-TPD, which has been used so far despite its morphological instability at elevated temperatures, with an energetically and morphologically more suitable material. The hole, transport materials were investigated for dopability, hole mobility,. absorption, reflection, cyclic voltammetry, and glass transition temperature. Further specific material properties were determined with simplified structures, e.g. m-i-p diodes, and the standard solar cells, consisting of the fullerene C-60 as acceptor and ZnPc as the donor material. The Di-NPB has turned out to be the best choice with respect to its intrinsic properties and device parameters. The deep lying HOMO, the high hole mobility of mu = 1.9 . 10(-4) cm(2)/Vs, the morphological stability of T-g = 158 degrees C, and the excellent results of the C-60:ZnPc bulk heterojunction solar cell inakes the Di-NPB highly suitable for replacement of the MeO-TPD in organic solar cells.
Notes: Conference on Organic Optoelectronics and Photonics III, Strasbourg, FRANCE, APR 07-10, 2008
Abstract: The transparent electron transport material NTCDA (1,4,5,8-naphthalenetetracarboxylic dianhydride) was examined in order to find a suitable substitute for C-60 which is today often used in small molecular organic solar cells as transport layer. Due to its wide band gap, NTCDA does not absorb in the visible range and is furthermore exciton blocking. By doping with AOB (acridine orange base), its conductivity was raised to about 1.10(-4)S/cm. It can therefore simultaneously be used as electron transport material and optical spacer in p-i-n type solar cells, leading to power conversion efficiencies of tip to 2.83%. Additionally, an investigation of the surface morphology using AFM was performed.
Notes: Conference on Organic Optoelectronics and Photonics III, Strasbourg, FRANCE, APR 07-10, 2008
Abstract: We investigate the optical properties of laterally confined organic microcavities. For preparation, a shadow mask deposition technique is used, which results in laterally shaped organic dots with micrometer size as the cavity layer. To describe the steady state electric field distribution in these structures, a simplified box model is applied. The complex mode structure measured in transmission and photoluminescence is simulated by transfer matrix calculations and a Fourier transform of the internal electric field distribution.
Notes: 7th International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD â07), Newark, DE, SEP 24-27, 2007
Abstract: We have measured the input-output curve of an organic vertical-cavity surface-emitting laser (OVCSEL), made of Alq3:DCM over more than 2 orders of magnitude beyond threshold. The characteristics cannot be explained by the standard theory of lasing, which uses a single spontaneous emission factor, beta. By assuming an inhomogeneously broadened distribution of 0 values we are able to qualitatively model the observed input-output curve.
Notes: 7th International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD â07), Newark, DE, SEP 24-27, 2007
Abstract: We present a novel organic light emitting device concept for white light generation with the potential for 100% ? 0 internal quantum efficiency, which employs fluorescent blue and phosphorescent green and orange emitters. Due to its high triplet energy the intrinsically non-radiative triplet excitons of the fluorescent blue emitter can still be harvested for liaht emission by letting them diffuse to the phosphor-containing emission layers. Thus, all electrically generated excitons can be used for light emission without the need for phosphorescent blue emitters, which suffer from stability problems. We demonstrate the high potential of this concept in a device achieving 57.6 lm W-1 total external power efficiency at 100 cd m(-2) (20.3% external quantum efficiency) and 37.5 lm W-1 (14.4%) at an illumination revelant brightness of 1,000 cd m(-2), and a high color rendering index of 86.
Notes: Conference on Organic Light Emitting Materials and Devices XI, San Diego, CA, AUG 26-29, 2007
Abstract: Displays based on organic light-emitting diodes (OLED) have rapidly developed and are commercially available since some time. However, in order to achieve large market penetration in new segments like lighting and optoelectronic, it is generally expected that the current status of the field has to advance in terms of manufacturing cost and integration possibilities. OLED devices with electrically doped transport layers show low operating voltage, high efficiency and long lifetime. In this paper we demonstrate that the concept of p- and n-type electrical doping can be applied under manufacturing conditions on the worldwide first vertical in-line fabrication setup for large area lighting applications. An in-line-manufactured highly efficient white-OLED-system will be presented. The driving of large area lighting tiles defines the resulting OLED lifetime and efficiency. In this paper we will present first results on the driving of large area lighting panels. Beside the lighting application the integration of highly efficient OLEDs for optoelectronic applications is an opportunity for innovative new applications. Microdisplays, integrated optocoupler and light barriers are few examples for the potential of OLEDs in optoelectronic applications. We will present results regarding the integration of highly efficient top-emitting PIN OLEDS (TM) for optoelectronic applications.
Notes: Conference on Light-Emitting Diodes - Research, Manufacturing, and Applications XI, San Jose, CA, JAN 24-25, 2007
Abstract: Recently., we have demonstrated an open circuit voltage of 1.0V and a power conversion efficiency of 3.4% in thin film solar cells, utilizing a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor and C-60 as acceptor [1]. Stimulated by this result, we systematically study the energy and electron transfer processes taking place at the oligothiophene:fullerene heterojunction along a homologous series of these oligothiophenes. The heterojunction is modified by tuning the HOMO level using different oligothiophene chain lengths, while the LUMO level is essentially fixed by the choice of the acceptor-type end-groups (dicyanovinyl) attached to the oligothiophene. We study electron transfer at the heterojunction to C-60 using photoinduced absorption. The observed transitions are unambiguously identified by TD-DFT calculations. With increasing the effective energy gap of the donor-acceptor pair, charge carrier dissociation following the photoinduced electron transfer is eventually replaced by recombination into the triplet state, which alters the photovoltaic operation conditions. Therefore, the optimum open-circuit voltage of a solar cell is a trade-off between an efficient charge separation at the interface and a maximized effective gap. We conclude that values between 1.0 and 1.1 V for the open-circuit voltage in our solar cell devices present an optimum, as higher voltages were only achieved with concomitant losses in charge separation efficiency.
Notes: Conference on Organic Photovoltaics VIII, San Diego, CA, AUG 28-30, 2007
Abstract: An OLED for lighting applications was successfully demonstrated on a 150mm x 150mm substrate with ITO anode. Additionally, ZnO:Al (ZAO) as an alternative transparent electrode for OLEDs was fabricated and applied to highly efficient p-i-n type green OLEDs and a white OLED for lighting applications. These devices showed similar performance as ITO anode based OLEDs.
Notes: International Symposium of the Society-for-Information-Display (SID 2007), Long Beach, CA, MAY 20-25, 2007
Abstract: First-Generation OLEDs were employing bottom-emitting structures with limited efficiency and rather high operating voltage. We demonstrate a number of novel approaches which allow realizing high-efficiency, OLEDs with flexible configuration and on low-cost substrates. Electrically doped transport layers play a key role to achieve these favorable properties: First, we discuss monochromic and white pin-OLED structures with very high efficiency. Then, we show top-emitting OLEDs with doped transport layers which allow to easily, optimize the optical outcoupling. Finally, we report on pin-OLED with extremely long life times.
Notes: International Symposium of the Society-for-Information-Display (SID 2007), Long Beach, CA, MAY 20-25, 2007
Abstract: Standard organic light emitting diodes (OLEDs) are usually bottom-emitting, i.e. they emit light through a transparent and electrically conductive substrate. Usually, indium tin oxide (ITO) is used for this purpose. However, as indium is a very expensive metal, replacing it is of vital interest for cheap OLED mass production, especially when it comes to lighting applications. We suggest the use of a polymer instead of ITO, carrying out both hole transport and injection. In contrast to conventional approaches, which use a conductive polymer on top of ITO as smoothening and hole injection layer, we employ solely a highly conductive polymer in combination with an OLED comprising doped charge transport layers. This allows us to renounce the ITO layer underneath. We use a new, highly conductive formulation of PEDOT:PSS, called Baytron (R) PH 500, with a conductivity of typically 500 S/cm, providing a smooth and electrically well-conductive substrate for the OLED stack. The use of such a polymeric injection layer and of a doped small-molecule OLED stack results in a low operating voltage of the devices. The charge transport layers of the OLED consist of MeO-TPD (N,N,Nâ,Nâ-tetrakis(4-methoxyphenyl)-benzidine) doped with a low percentage of F-4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane) for the hole transport layer and of Bphen (4,7-diphenyl-1,10-phenanthroline) co-evaporated with Caesium for the electron transport layer. We demonstrate both fluorescent and phosphorescent monochromic OLEDs based on Baytron (R) PH 500 which achieve good efficiencies. The OLEDs made on Baytron (R) PH 500 are compared with devices made on an ITO anode. Although the polymer possesses a somewhat lower conductivity than ITO, efficient devices can be fabricated. For example, using the blue emitter Spiro-DPVBi (2,2â,7,7â-tetrakis(2,2-diphenylvinyl)spiro-9,9â-bifluorene), we achieve an efficiency of up to 5.1 cd/A. As another example, we discuss green OLEDs based on the triplet emitter Ir(ppy)(3) (fac tris(2-phenylpyridine) iridium) doped in a wide gap material. In this case, even a higher efficiency than on ITO is reached: 62 cd/A at a luminance of 100 cd/m(2), corresponding to an increase in external quantum efficiency by 15% as compared to ITO.
Notes: Conference on Organic Optoelectronics and Photonics II, Strasbourg, FRANCE, APR 03-06, 2006
Abstract: We present a study of time-resolved transmission and emission properties of optically anisotropic planar micro-cavity structures. The structures consist of lambda/4-layers Of SiO2 and TiO2 for the dielectric mirrors and a cavity layer of either SiO2 or the organic dye composite AlQ(3)/DCM. For the SiO2 cavity, we observe a polarization splitting at normal incidence leading to terahertz oscillations of transmitted coherent light. The polarization splitting is explained by an optical anisotropy of the dielectric layers caused by the fabrication process. We apply an up-conversion setup for temporally and spectrally resolved transmission measurements and obtain a corresponding beating of 1.25 THz. Time resolved measurements yield a Q-value of 1600, corresponding to a cavity photon lifetime of 0.65 ps. We explain our observations with a transfer-matrix model and introduce a Fourier-transform based analytical algorithm. The cavity filled with the organic dye composite can act as an organic microcavity laser. The birefringence of the distributed Bragg reflectors leads to lasing in two perpendicularly polarized modes. Investigations of the ultrafast dynamics of this laser system show a phase coupling of the two laser modes leading to the generation of a terahertz optical beat. The oscillation frequency can be widely tuned by variations in the fabrication process.
Notes: Conference on Optoelectronic Devices - Physics, Fabrication, and Application III, Boston, MA, OCT 01-02, 2006
Abstract: We report on white organic light emitting diodes with three stacked emitter layers comprising the fluorescent blue emitter Spiro-DPVBi, the phosphorescent green emitter system TCTA:Ir(ppy)(3) and the phosphorescent red emitter system NPB:Ir(MDQ)(2)(acac). A thin additional layer of mixed TCTA and TPBi separates the fluorescent and phosphorescent emitting regions, simultaneously confining excitons efficiently and letting electrons and holes easily pass. Furthermore, phosphorescence quenching by Dexter transfer to the non-radiative! triplet state of Spi.ro-DPVBi is suppressed. Devices were optimized to get color coordinates very close to the warm white standard illuminant A. Best devices have a current efficiency of 13.8 cd/A, CIE color coordinates of (0.45, 0.42), and a color rendering index of 91 at a brightness of 1000 cd/m(2). Due to the use of electrically doped charge transport layers, the voltage needed for 1000 cd/m(2) was only 3.0 V, which leads to a power efficiency of 14.4 lm/W assuming Lambertian emission.
Notes: Conference on Organic Optoelectronics and Photonics II, Strasbourg, FRANCE, APR 03-06, 2006
Abstract: We report on the experimental observation of polarization splitting and terahertz oscillations in transmission and laser emission from optically anisotropic microcavities. A guest-host composite of tris-(8-hydroxyquinoline) aluminium (Alq(3)) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran(DCM) serves as active laser material. The anisotropy is attributed to oblique columnar structures in the distributed Bragg reflector mirrors of our microcavity, resulting from sample fabrication. A splitting of 0.2 nm occurs in the laser emission from an organic vertical cavity surface emitting laser at a wavelength of 612.6nm, and a splitting of 2.5mn is obtained from a sample for Ti-Sapphire laser transmission at 781 nm. Split modes are perpendicularly polarized. An upconversion setup allows temporally resolved studies of transmission and emission behavior, showing an oscillation at a frequency of 1.25THz in transmission, and 0.18THz in emission, respectively. The temporal behavior of laser emission is modelled by a set of rate-equations and extended to account for the resulting oscillations. Our observations suggest that a phase-coupling mechanism between both occuring modes is present in the laser emission from our microcavity.
Notes: Conference on Organic Optoelectronics and Photonics II, Strasbourg, FRANCE, APR 03-06, 2006
Abstract: In this contribution, we discuss several research results which have contributed to the vision of a broad application of organic light emitting diodes in displays and lighting. We discuss the factors which determine the efficiency of OLEDs. First, we briefly discuss work on the controlled molecular doping of organic semiconductors, having the same advantageous influence on the device properties than in inorganic semiconductor devices, in particular in reducing the operating voltage. By comparison with a simple theory, we show that the voltages achieved with doped OLEDs are already very close to the thermodynamic limit. We then show, with the example of red phosphorescent OLEDs, how many properties of the OLED are improved by doping.
Notes: Conference on Organic Optoelectronics and Photonics II, Strasbourg, FRANCE, APR 03-06, 2006
Abstract: OLED devices with electrically doped transport layers show low operating voltage, high efficiency and long lifetime [3, 5, 7]. This paper demonstrates that the concept of p- and n-type electrical doping can be applied under manufacturing conditions. It shows that handling of dopants, adjustment of doping concentrations, and preparation of p-i-n type OLED stacks is possible with the worldwide first vertical in-line set-up. An in-line-manufactured highly efficient RGB-OLEDsystem is presented.
Notes: Conference on Organic Optoelectronics and Photonics II, Strasbourg, FRANCE, APR 03-06, 2006
Abstract: To realize organic solar cells with high performance, we developed a novel way of stable n-doping using cationic dyes in electron transport materials. In our approach, the volatile donors are created in-situ from stable precursor compounds. Using the cationic dye pyronin B (PyB) as a model precursor, we carried out conductivity and field effect measurements to characterize the properties of doped naphtalene tetracarboxylic dianhydride (NTCDA) thin film. The results show a strong increase in n-type conductivity. Combined FTIR, UV/VIS/NIR and mass spectroscopic measurements suggest the formation of leuco pyronin B during sublimation of pyronin B chloride, and a subsequent charge transfer between dopant and matrix providing free electrons, which increase the n-type conductivity.
Notes: Conference on Organic Optoelectronics and Photonics, Strasbourg, FRANCE, APR 28-30, 2004
Abstract: We demonstrate high-efficiency organic light-emitting diodes (OLEDs) by incorporating a double emission layer (D-EML) into p-i-n-type cell architecture. The D-EML comprises two layers with ambipolar transport characteristics, both doped with the green phosphorescent dye tris(phenylpyridine)iridium [Ir(ppy)(3)]. The first EML features a bipolar, but predominantly hole transporting host material, 4,4â,4â-tris(N-carbazolyl)-triphenylamine (TCTA), while the second EML is made of an exclusively electron transporting host, e.g. 3-phenyl-4-(1â-naphthyl)-5-phenyl-1,2,4-triazole (TAZ); with a weak hole transport capability arising from hopping between dopant sites. The D-EML system of two bipolar layers leads to an expansion of the exciton generation region. Due to its self-balancing character, it avoids accumulation of charge carriers at any interface. Thus, a power efficiency of approximately 77 lm/W and an external quantum efficiency of 19.3% are achieved at 100 cd/m(2) at an operating voltage of only 2.65V. More importantly, the efficiency decays only weakly with increasing brightness and a power efficiency of 50 lm/W is still obtained even at 4,000 cd/m(2).
Notes: Conference on Organic Optoelectronics and Photonics, Strasbourg, FRANCE, APR 28-30, 2004
Abstract: We present a novel n-type doping technique for organic semiconductors using the metal complex bis(terpyridine)ruthenium as a strong donor. Owing to its low oxidation potential, the reduced neutral form of the donor complex allows an electron transfer to the matrix. This enables n-type conduction that has been seldom reported in metallophthalocyanine systems doped with organic compounds. The n-type zinc-phthalocyanine layers are characterized by the conductivity and the field-effect measurements. By sequential coevaporation of p- and n-doped layers, we have prepared the first stable and reproducible organic homojunction of zinc-phthalocyanine. The diode exhibits surprisingly high built-in voltage attractive e.g. for organic solar cell applications. The temperature dependence of the current-voltage characteristics does not follow the standard Shockley theory of pn-junctions. We explain the behavior of the ideality factor and the saturation current by deviations from the classical Einstein relation at low temperatures.
Notes: Conference on Organic Optoelectronics and Photonics, Strasbourg, FRANCE, APR 28-30, 2004
Abstract: The operating voltage of organic light emitting diodes (OLEDs) is important for the power consumption of active or passive matrix displays since it influences both the power consumption of the OLED itself and the power consumption of the driver circuitry. We have shown that very low operating voltages can be achieved in small-molecule OLED by intentional electrical n- and p-type doping. Even more important than the reduction of the voltage is the fact that doping of the charge carrier transport layers improves charge injection, making it basically independent on the actual contact work-ftmctions. Organic light emitting diodes (OLEDs) with electrically doped transport layers show significantly improved properties: For instance, we have achieved a brightness of 100cd/m(2) already at a voltage of 2.55V (based on a simple singlet emitter system), well below previous results for undoped small-molecule devices. With phosphorescent emitter dopants, high quantum and power efficiency of OLEDs with doped transport layers can be achieved: operating voltages and current efficiencies of 3.1V and 44cd/A (corresponding to approx. 44lm/Watt at 100cd/m(2)) are reported here. Inverted and fully transparent devices with parameters comparable to standard bottom-emitting OLED have been demonstrated as well.
Notes: Conference on Organic Light-Emitting Materials and Devices VII, San Diego, CA, AUG 04-06, 2003
Abstract: We present highly efficient, low-voltage multilayer organic light emitting diodes based on the phosphorescent emitter tris(2-phenylpyridine) iridium (Ir(ppy)(3)). The phosphor is doped into various wide gap electron-transport or hole-transport host materials embedded in between doped transport layers. We use 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped N,N,Nâ,Nâ-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) as p-type hole-injection and transport layer, while cesium (Cs) and 4,7-diphenyl-1,10-phenanthroline (Bphen) are co-evaporated for the n-type doped electron transport layer. Sandwiched between these two transport layers, we insert one or two emission layers. This p-i-n structure results in efficient carrier injection from both contacts into the doped transport layers and low ohmic losses. Thus, lower operating voltages are obtained compared to conventional undoped OLEDs. By doping Ir(ppy)(3) into a double layer structure of predominantly electron and hole transporting hosts, a power efficiency of 70 lm/W and an external quantum efficiency of 19.5% is achieved at 100 cd/m(2) (2.95V). Besides this, the efficiency decays only weakly with increasing current density (or brightness). A quantum efficiency of 13.5% is still obtained at a current density of 100 mA/m(2) with a luminance around 50,000 cd/m(2). This improvement can be attributed mainly to the fact that we prevent any charge accumulation at hole or electron blocking layers and spread the generation region to both sides of the interface between the two parts of the emission layer. Moreover, losses due to non-radiative decay of triplet excitons diffusing into regions without the phosphorescent dye are avoided.
Notes: 8th Conference on Organic Light-Emitting Materials and Devices, Denver, CO, AUG 02-04, 2004
Abstract: We investigate the dynamics of a Bloch-oscillating wave packet in the presence of strong coupling to delocalized above barrier states, using time-resolved intraband polarization-sensitive measurements. At a threshold electric field, the resonance of localized and delocalized states causes a quantum beating which is observed as a revival in the interband polarization. Our numerical simulation visualizes the spatial wave packet decomposition and reformation.
Notes: 10th International Symposium on Nanostructures, ST PETERSBURG, RUSSIA, JUN 17-21, 2002
Abstract: The optical properties of an electrically biased semiconductor superlattice are strongly influenced by coupling to other bands. As these coupling mechanisms depend on the strength of the applied electric field, we find a distinct variation in transition line broadening, dephasing time and spatial extension of the wavefunction. The fundamental Bloch-oscillating transport in a periodic structure is drastically modified and exhibits a strong damping with a surprising revival of polarization coherence which reappears on a picosecond scale.
Notes: 3rd International Conference on Particle Physics Beyond the Standard Model, Oulu, FINLAND, JUN 02-07, 2002
Abstract: Conventional organic light-emitting diodes (OLEDs) having electrically doped carrier injection and transport layers with high conductivity exhibit extremely low driving voltages, which is due to the formation of radical anions and cations and ohmic contacts at the electrode interfaces. We report here an inverted OLEDs with indium-tin-oxide (ITO) bottom contact as cathode for electron injection. The device comprise an intrinsic 8-tris-hydroxyquinoline aluminum (Alq(3)) emission layer sandwiched in between n- and p-doped charge transport layers.
Notes: Symposium on Organic and Polymeric Materials and Devices-Optical, Electrical and Optoelectronic Properties held at the 2002 MRS Spring Meeting, SAN FRANCISCO, CA, APR 01-05, 2002
Abstract: We demonstrate an efficient organic electroluminescent devices with p-i-n structure. Anamorphous starburst, 4,4â,4â-tris(3-methylphenylphenylamino)triphenylamine doped with a strong organic acceptor, tetrafluoro-tetracyanoquinodimethane serves as the p-type hole transport layer, and 4,7-diphenyl-1,10-phenanthroline doped with Li as the n-type electron transport layer. A breakthrough is achived in the performances of device based on pure 8-trishydroxyquinoline as an emitter: 100 cd/m(2) at 2.52V, 1,000cd/m(2) at 2.9V and the maximum luminance and efficiency reache 66, 000cd/m(2) and 5.25 cd/A, respectively. The efficiency can be kept above 3 cd/A in a very large luminance region from 100 to 55,000 cd/m(2).
Notes: Conference on Organic Photonic Materials and Devices IV, SAN JOSE, CA, JAN 21-25, 2002
Abstract: Organic light emitting diodes generally suffer from higher operating voltages compared to inorganic ones. This limits their application in passive or active driven displays based on OLED-technology. As was previously shown by our group, p-type doping of the hole injection and transport layer of an organic light emitting diode (OLED) by co-evaporation of a matrix and an acceptor molecule leads to lower operating voltages of the device. In OLEDs using doped transport layers, the use of a proper buffer layer between the doped layer and the light emission layer is essential to yield a high current efficiency and a low operating voltage at the same time. In order to further enhance the device efficiency, we apply here the doping concept to OLEDs with a light emission layer which is doped with a fluorescent dye. This approach proves that doping of the transport layer is able to improve the optoelectronic properties of already highly efficient OLEDs. The doped hole injection and hole transport layer is a Starburst layer p-type-doped with tetrafluoro-tetracyano-quinodimethane (F-4-TCNQ). As blocking layer, a diamine (TPD) is used. The emitter layer consists of quinacridone (QAD) doped alum in um-tris-(8-hydroxy-quinolate) (Alq(3)). Holes are injected from untreated ITO, electrons via a lithium-fluoride (LiF)/aluminum cathode combination. For this OLED layer sequence, we achieved a luminance of 100cd/m(2) in forward direction at the lowest operating voltage reported for completely non-polymeric OLEDs (3.2-3.4V) with a current efficiency of around 10 cd/A.
Notes: Conference on Organic Light-Emitting Materials and Devices V, SAN DIEGO, CA, JUL 30-AUG 01, 2001
Abstract: We observe that the periodic motion of Bloch-oscillating carriers in semiconductor superlattices is accompanied by a quasi-DC current: The photo-injected electron hole pairs interact with their self-induced oscillating field mediated by the nonequilibrium electron-hole plasma. This novel quantum effect is a coherent anolog of the Shapiro effect in Josephson junctions. The direction of the motion can be controlled by the laser excitation.
Notes: 25th International Conference on the Physics of Semiconductors (ICPS25), OSAKA, JAPAN, SEP 17-22, 2000
Abstract: A comprehensive study of controlled n- and p-type doping by co-evaporation of various molecular organic materials is presented. Despite different microscopic mechanisms, the conductivity increase, carrier density and Fermi level shift of the doped films determined by various electrical characterization techniques correspond largely to the behavior observed in classical inorganic semiconductors. We further show that these doping concepts can be successfully applied for devices such as low-voltage organic LED.
Notes: 25th International Conference on the Physics of Semiconductors (ICPS25), OSAKA, JAPAN, SEP 17-22, 2000
Abstract: The influence of scattering and plasmon coupling on temporal and spatial dynamics of Bloch oscillations in a semiconductor superlattice is investigated using optical techniques. The oscillations axe strongly modified due to the scattering processes: The dependence of amplitude on k-space composition of the wave packet is modified due to a renormalization of the energy scale. For higher carrier densities, plasmon coupling causes anharmonic Bloch oscillations.
Notes: 25th International Conference on the Physics of Semiconductors (ICPS25), OSAKA, JAPAN, SEP 17-22, 2000
Abstract: The exciton structure of crystalline MePTCDI (N-Nâ-dimethylperylene-3,4,9,10-dicarboximide) is modeled by a one-dimensional Hamiltonian, which includes the interactions between Frenkel excitons; with several vibronic levels and charge-transfer excitons. Using appropriate fitting parameters, which are verified by quantum chemical calculations, this model can explain the main features of the low temperature absorption spectrum. Polarized absorption spectra show different polarization ratios for the various peaks. This polarization behavior is explained by the varying contribution of the charge-transfer transition dipole, which has a direction different from the Frenkel transition dipole. Our model for the exciton band structure is supported by transient emission measurements.
Notes: 4th International Conference on Excitonic Processes in Condensed Matter (EXCON2000), OSAKA, JAPAN, AUG 22-25, 2000
Abstract: Microscopic photoluminescence (mu-PL) spectroscopy through a photomask with a sub-micron resolution has been performed on In0.15Ga0.85As/GaAs quantum wells (QWs) containing three QWs with the same nominal well width of 30Angstrom but grown at different temperatures. PL from each QW was well resolved demonstrating that the interface abruptness of each QW is different due to the temperature dependence of In surface segregation during epitaxial growth. With decreasing window size down to sub-micron, the PL from QWs grown at higher temperatures showed a more drastic decrease compared with that from QWs grown at lower temperatures. This could be explained by assuming that there are less localized states and/or the in-plane exciton diffusion is enhanced in QWs grown at higher temperatures due to an enhanced smearing of the interfaces in the growth direction and the resultant formation of effectively smooth heterointerfaces.
Notes: 26th International Symposium on Compound Semiconducors, BERLIN, GERMANY, AUG 22-26, 1999
Abstract: The exciton structure of crystalline PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N-Nâ-dimethylperylene-3,4,9, 10-dicarboximide) is modeled by a one-dimensional Hamiltonian, which includes interactions between Frenkel excitons with several vibronic levels and charge transfer excitons. With appropriate fitting parameters, which are verified by quantum chemical calculations, this model can explain the main features of the low temperature absorption spectra. Polarized absorption spectra of MePTCDI show different polarization ratios for the various peaks. This polarization behavior is explained by the varying contribution of the CT transition dipole, which has a direction different from the Frenkel transition dipole.
Notes: NATO Advanced Research Workshop on Multiphoton and Light Driven Multielectron Processes in Organics: New Phenomena, Materials and Applications, MENTON, FRANCE, AUG 26-31, 1999
Abstract: The changing of the four wave mixing spectrum of GaAs/AlGaAs superlattices are investigated theoretically and experimentally. It was shown that under applying the external electric field, which leads to an anticrossing of heavy hole and light hole excitons, the nonharmonic oscillations of the energy position of exciton resonances are detected. The nonharmonicity of these oscillations are caused by the influence of the inhomogeneous broadening of the excitons on the Bloch oscillations.
Notes: 10th International Symposium on Ultrafast Phenomena in Semiconductors (10-UFPS), VILNIUS, LITHUANIA, AUG 31-SEP 02, 1998
Abstract: A short laser pulse exciting semiconductor superlattice induces quantum beats between different excitonic states that in turn lead to a formation of a time-varying coherent wave packet. A real spatial oscillations of the excited wave packet identified quantum beats of the Wannier-Stark states as Bloch oscillations. Using a new technique for measurement of the spatial dynamics of Bloch wave packets in semiconductor superlattices we investigate the features of spatial-temporal dynamics on the initial experimental conditions and the composition of the wave packet.
Notes: 10th International Symposium on Ultrafast Phenomena in Semiconductors (10-UFPS), VILNIUS, LITHUANIA, AUG 31-SEP 02, 1998
Abstract: We perform detailed time-resolved photoluminescence (TRPL) measurements on coevaporated CuInSe2(CISe) thin films with a lateral gradient in the composition. The dynamics of minority carriers in bare absorber layers is compared with that of CISe/CdS samples and complete solar cells. The observed increase in the carrier lifetimes for the cell-like film structures is explained by surface recombination effects.
Notes: 11th International Conference on Ternary and Multinary Compounds (ICTMC-11), SALFORD, ENGLAND, SEP 08-12, 1997
Abstract: We investigate the spatial displacement dynamics of optically excited wave packets in semiconductor superlattices. A short laser pulse exciting semiconductor superlattice induces quantum beats between different excitonic states that in turn leads to formation of a time-varying coherent wave packet. The real space oscillation of the excited wave packet identifies these quantum beats of the Wannier-Stark states as Bloch oscillations: We present an experimental technique which measures directly the displacement of the wave packet center-of-mass. The oscillating Bloch wave packets create a microscopic dipole moment which can be detected using the shift of the Wannier-Stark ladder transition energy as a sensitive field detector. We show that the Bloch wave packet undergoes harmonic spatial motion, Proing for the first time the predictions of Bloch and Zener. The influence of an experimental conditions on displacement of the Bloch wave packet is discussed.
Notes: Conference on Ultrafast Phenomena in Semiconductors II, SAN JOSE, CA, JAN 28-29, 1998
Notes: Symposium A on Semiconductor Materials for Optoelectronic Devices, OEICs and Photonics, at the 1993 E-MRS Spring Conference, STRASBOURG, FRANCE, MAY 04-07, 1993
Abstract: We discuss recent experimental observations of the spatial dynamics of photoexcited wavepackets in semiconductor heterostructures. In particular, coherent oscillation of a wavepacket in coupled double quantum well structures are traced with time-resolved transmission and four-wave-mixing experiments. The oscillation period and modulation can be tuned with an electric field. The experimental results are compared to a theory based on the third-order nonlinear response of a three-level system.
Notes: Plenary Lectures of the 1992 Spring-Meeting of the Solid State Physics Division of the German Physical Society, REGENSBURG, GERMANY, MAR 16-20, 1992
