Vipul Gohri

Abstract: We report high brightness and low operating voltage efficient green organic light-emitting diodes (OLEDs) based on silicon complementary metal-oxide semiconductor (CMOS) backplane which can be used in applications such as microdisplays. The small molecule top-emitting OLEDs are based on a fluorescent green emitter accompanied by blocking, doped charge transport layers, and an anode fabricated with standard CMOS processes of a 200 mm integrated circuit (IC) fab. The devices are designed to maximize the efficiency under low operative bias so as to fit the limited voltage budget of the IC. This was done by making optical simulations of the device structure, optimizing the organic layer thicknesses and charge injection in the n and p transport layers. The devices reach a current efficacy of 21.6  cd/A at a luminance of 20,000  cd/m2. The devices exhibit a voltage swing as low as 2.95 V for a contrast ratio of 1000. The optimized devices have a high lifetime of 6000 and 8800 h at 5000  cd/m2. Furthermore, aging inside the emission layer is investigated.

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 (Alq3), 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, xa0;0.26) and a color rendering index of 60 is achieved. Furthermore, the spectral angular dependence of the white device is examined.

Abstract: Magnetic nanoparticles of pure and substituted iron oxides are prepared by single step autocombustion or by wet chemical methods. The nanoparticles prepared by the first process had mixed phase of hematite and maghemite whereas the later essentially gives maghemite phase. XRD patterns and TEM micrographs of the pure and substituted maghemites samples suggest about their monophasic nature and inverse spinel structure. Further, the size of the particles for the above iron oxide samples was found to be in the range of 4 to 30 nm. Saturation magnetization value for the samples was observed to be varying with the type and the amount of substitution. For example, magnetization value initially increased and then decreased for Al- and Mn-substitutions but it continuously decreased for Cr- and Zn-substitutions. Contrary to the saturation magnetization value, the Curie temperature decreased continuously with increased substitutions irrespective of the type of substitutions. Due to higher magnetization value of Mn-substituted maghemite (for x = 0.2, 78 Am2/kg), it has higher heating ability and specific absorption rate compared to Al-substituted maghemite (for x = 0.07, 70 Am2/kg) and pure maghemite (62 Am2/kg).

Abstract: We demonstrate the use of binary colloidal assemblies as lithographic masks to generate tunable Au patterns on SiO(2) substrates with dimensions ranging from micrometers to nanometers. Such patterns can be modified with different chemistries to create patterns with well-defined sites for selective adsorption of proteins, where the pattern size and spacing is adjustable depending on particle choice. In our system, the binary colloidal assemblies contain large and small particles of similar or different material and are self-assembled from dilute dispersions with particle size ratios ranging from 0.10 to 0.50. This allows masks with variable morphology and thus production of chemical patterns of tunable geometry. Finally, the Au or SiO(2) regions of the pattern are surface modified with protein resistant oligoethyleneglycol self-assembled molecules, which facilitates site selective adsorption of proteins into the unmodified regions of the pattern. This we show with fluorescently labeled bovine serum albumin.

Abstract: We developed a 0.61'' diagonal OLED microdisplay dedicated to electronic viewfinders for digital vision systems, e.g. for security or other professional applications. The microdisplay has a very high resolution of 5.4 million subpixels and combines excellent image quality with low power consumption and a 10bit per color digital input. Subpixel pitch is 4.7x4.7μm2. Thanks to the versatile architecture of the underlying ASIC circuit, the device can be easily adapted to different applications and image formats: In the standard full color version, the resulting resolution is 1300 by 1044 pixels (SXGA). In a monochrome version, the resolution is 2600 by 2088 independent pixels, enabling e.g. digital night vision at full 2K by 2K resolution. In addition to this, we developed two- and three color versions of the display that allow to merge high resolution monochrome images e.g.in 2K by 2K resolution with lower resolution images e.g., from an infrared sensor for image fusion or for adding colored graphical overlays.

Abstract: We report high luminance OLED (organic light emitting diode) microdisplay with a high resolution of 5.4 Mpixels and very low operating voltages. The microdisplay is capable of operating at 10,000 cd/m 2 and exhibits a high contrast ratio of 1,000,000:1 and power consumption less than 200 mW.

Abstract: The aim of this study is to analyze and mitigate the voltage drift phenomenon observed in top-emitting organic light emitting diodes (OLED) when driven at constant current. An operating device may experience voltage increase over time due to factors such as interface or bulk material degradation, charge accumulation and formation of trap states. Single-carrier devices were fabricated to understand the contribution to voltage drift from each of these causes. Doping in electron injection layer (4, 7-diphenyl-1,10-phenanthroline or Bphen) and hole injection layer (2,2′,7,7′-tetra(N,N-di-tolyl) amino-spiro-bifluorene or Spiro-TTB) were optimized to obtain ohmic injection contacts. Devices with tris(8-hydroxy-quinoline) aluminium (Alq3) degrade significantly with holes injection and undergo high voltage increase in lifetime test measurements. On the contrary, devices with N,N′-di(naphtalen-1-yl)- N,N′-diphenyl-benzidine (NPB) exhibit an ambipolar charge transport behavior and low voltage drift under both hole and electron injection.