Solution processable bulk-heterojunction solar cells using a small molecule acceptor

We report a small-molecule electron-acceptor based on 2-vinyl-4,5-dicyanoimidazole [Vinazene™] for use in solution processed organic solar cells. The material has a favourably located LUMO level of −3.6 eV and absorbs strongly in the visible spectrum up to 520 nm—attractive properties compared to the widely used acceptor (6,6)-phenyl-C60-butyric acid methyl ester (PCBM). The Vinazene derivative was blended with a poly(2,7-carbazole) donor—chosen for its complementary absorption range and comparatively high-lying HOMO level of −5.6 eV—and incorporated into bulk heterojunction devices. The influence of the donor/acceptor composition and annealing temperature on device performance were then investigated. The best performing devices exhibited reasonable power conversion efficiencies of 0.75% and open-circuit voltages of more than 1.3 V, substantially higher than previously reported devices using small molecule acceptors.