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We compare organic donor/acceptor heterojunction photovoltaic devices based on copper phthalocyanine (CuPc) as the donor and [6,6]-phenyl C60 butyric acid methyl ester (PCBM) or buckminsterfullerene (C60) as the acceptor. The effect of the two acceptors on open-circuit voltage and short-circuit current density is studied. Improved interface properties, optimal energy level matching, and more balanced charge transport are believed to be the reasons for better performance of CuPc/PCBM heterojunction device, in terms of photocurrent and fill factor. The energy conversion efficiency of the devices under AM1.5G solar illumination at 130 mW/cm2 is improved to 1.18%, up from 0.74%. [ABSTRACT FROM AUTHOR]

We demonstrate bilayer source-drain (S-D) electrodes for organic thin film transistors (OTFT). The bilayer consists of a transition metal oxide (MoO3,WO3, or V2O5) layer and a metal layer. The metal oxide layer, directly contacting the organic semiconducting layer, serves as the charge-injection layer. The overcoated metal layer is responsible for the conduction of charge carriers. We found that the metal oxide layer coupled between pentacene and metal layers played an important role in improving the field-effect transistor characteristics of OTFTs. Devices with the bilayer S-D electrodes showed enhanced hole-injection compared to those with only metal electrode. High field-effect mobility of 0.4 cm2/V s and on/off current ratios of 104 were obtained in the pentacene based TFTs using the bilayer S-D electrodes at a gate bias of -40 V. The improvement is attributed to the reduction in the contact barrier and the prevention of metal diffusion into the organic layer and/or unfavorable chemical reaction between the organic layer and the metal electrode. [ABSTRACT FROM AUTHOR]

We report organic solar cells fabricated with small-molecule organic semiconductor tetracene/C60 heterojunction as the photoactive layer. The external power conversion efficiency of the devices under AM 1.5 solar illumination at 100 mW/cm2 (1 sun) is 2.3±0.5% with relatively high open-circuit voltage (Voc=0.58±0.06 V) compared to most of the other small-molecular donor-acceptor (D-A) heterojunction solar cells reported so far. Using atomic force microscopy and x-ray diffraction we found that tetracene thin films consist of submicron-sized grains with rough surface and well defined molecular order. Therefore, using high mobility polycrystalline tetracene thin films for D-A heterojunction devices dramatically increases area of tetracene and C60 interface for exciton diffusion to reduce the recombination. [ABSTRACT FROM AUTHOR]