Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives [C60]PCBM and [C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2‐[{7‐(9,9‐di‐n‐propyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazol‐4‐yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.
n-Type organic semiconductors are important for a range of optoelectronic applications including organic photovoltaic devices, light-emitting diodes, and field effect transistors (FETs). In spite of this clear motivation there has been significantly less development of n-type compounds relative to p-type systems. We have developed a simple, small molecule n-type material, 2-[(7-{9,9-di-n-propyl-9Hfluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (K12), that can be processed either by spin-coating from solution or evaporation in vacuum. The thermal properties of K12 enable the film morphology to be controlled at easily accessible temperatures allowing the charge mobility to be tuned over two orders of magnitude. The electron mobility in the films was found to be independent of the initial processing conditions (solution or evaporation).
We report an n-type organic semiconductor [2-({7-(9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (herein referred to as K12) for use in organic field-effect transistors (OFETs). K12 can be processed by spin-coating from solution or by vacuum deposition, organizing into highly orientated microcrystalline structures at modest (75 °C) annealing temperatures. OFETs with n-octyltrichlorosilane or hexamethyldisilazane monolayers, or poly(propylene-co-1-butene) (PPCB) modified dielectric surfaces were prepared. The mobility, ON/OFF ratio, threshold voltage, and current hysteresis were found to be dependent on the thermal history of the film and surface onto which it was deposited. The highest OFET mobility achieved was 2.4×10−3 cm2/V s, for spin-coated films with a PPCB modified silicon dioxide dielectric.
In this paper, we demonstrate the fabrication of top-contact silicon shadow masks for organic field effect transistors (OFETs) using plasma deep reactive ion etching (DRIE). Over 50 parallel and interdigitated finger contact masks of 30 μm thickness have been created on a single silicon wafer, with lengths spanning from 6.5 to 60 μm and channel widths varying from 1000 to 50 000 μm. Unlike all other mask fabrication techniques to date, these shadow masks are inexpensive, reusable, have nanoscopically sharp edges and can be made with precise (nanoscale) control over various sizes and shapes. Because a large number of these masks can be made at the same time, they can act as a platform for researchers studying new organic materials and OFET structures. Top contact OFETs have been successfully fabricated using these masks with performances comparable if not superior to those made with standard lithography.
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