Mr. Xuewen Zhang Profile

Xuewen Zhang

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Proceedings Article | 1 December 1997 Paper

Efficient light-emitting diodes from blends of conjugated polymers

Xuejun Zhang, Ashok Shetty, Samson Jenekhe

Proceedings Volume 3148, (1997) https://doi.org/10.1117/12.295542

KEYWORDS: Electroluminescence, Polymers, Absorption, Binary data, Excitons, Luminescence, Light emitting diodes, Energy transfer, Quantum efficiency, Thin films

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Electroluminescent devices from binary blends of conjugated polyquinolines are fabricated and used to systematically investigate the mechanisms of efficient electroluminescence (EL) in multicomponent conjugated polymer system. The roles of energy transfer, excited state complex formation, charge transport and trapping, miscibility and phase separation, and spatial confinement on EL efficiency can be directly probed by a judicious choice of binary blend components. Large enhancement of EL efficiency and device brightness was observed in some conjugated polymer blend systems compared to the component homopolymers. For example, binary blends of poly(2,2'-(2,5-thienylene)-6-6'-bis(4- phenylquinoline)) and poly(2,2'-(biphenylene)-6,6'- (4-phenylquinoline)) are found to exhibit EL quantum efficiency of up to a factor of 30 enhancement at 200 cd/m² luminance levels compared to the components. EL enhancement in this and other polymer blend system is shown by electric field-modulated photoluminescence spectroscopy and other experiments to be due to improved radiative electron-hole recombination efficiency facilitated by spatial confinement of excitons in the blends.

Proceedings Article | 9 October 1997 Paper

Efficient xerographic photoreceptors from conjugated polymers and polymer blends

Xuejun Zhang, Anita Alanko, Samson Jenekhe

Proceedings Volume 3144, (1997) https://doi.org/10.1117/12.290246

KEYWORDS: Polymers, Quantum efficiency, Absorption, Binary data, Excitons, Diffusion, Electronics, Interfaces, Polymer thin films, Optoelectronics

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Bilayer xerographic photoreceptors in which (pi)-conjugated polymers and binary conjugated polymer blends are used as the charge generation layer have been fabricated, evaluated, and shown to be highly efficient. Devices incorporating poly(1,4-phenylene benzobisthiazole), poly(2,5-pyridylene benzobisthiazole), poly(l ,4-phenylenebisvinylene benzobisthiazole), poly(1 ,4-(2-hydroxy)phenylene benzobisthiazole), and poly(benzimidazobenzophenanthroline ladder) as the charge generation layer and a layer of tris(p-tolyl)amine (TTA) dispersed in polycarbonate as the charge-transport layer showed good photosensitivities (6-18 ergs/cm²), good dark decay characteristics (2-10 V/s at surface potentials of 400- 600 V), and high charge generation quantum efficiencies (20-50% at ~10⁶ V/cm). Photoreceptors using binary blends of conjugated polymers had enhanced spectral range of photosensitivity (300-700 nm) and significantly enhanced quantum yield for charge photogeneration relative to the component conjugated polymers. Photocarrier generation in these bilayer photoreceptors is extrinsic in nature and is mediated by exciplex formation between the conjugated polymer and TTA at the bilayer interface. Observed nanoscale size effects, in which the photoreceptor performance (quantum efficiency, photosensitivity) is enhanced with decreasing size of the charge generation layer, provide a means of tuning the photoconductive properties of optoelectronic devices based on conjugated polymers. A model derived from Onsager's 1934 theory was used to estimate the primary quantum yield of ion-pairs and ion-pair separation distance in the bilayer photoreceptors to be 0.21-0.62, and 4-6 angstroms, respectively.

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