Comparison of thiophene- and selenophene-bridged donor–acceptor low band-gap copolymers used in bulk-heterojunction organic photovoltaics

We report a detailed comparison of absorption spectroscopy, electrochemistry, DFT calculations, field-effect charge mobility, as well as organic photovoltaic characteristics between thiophene- and selenophene-bridged donor–acceptor low-band-gap copolymers. In these copolymers, a significant reduction of the band-gap energy was observed for selenophene-bridged copolymers by UV-visible absorption spectroscopy and cyclic voltammetry. Field-effect charge mobility studies reveal that the enhanced hole mobility of the selenophene-bridged copolymers hinges on the solubilising alkyl side chain of the copolymers. Both cyclic voltammetry experiments and theoretical calculations showed that the decreased band-gap energy is mainly due to the lowering of the LUMO energy level, and the raising of the HOMO energy level is just a secondary cause. These results are reflected in a significant increase of the short circuit current density (JSC) but a slight decrease of the open circuit voltage (VOC) of their bulk-heterojunction organic photovoltaics (BHJ OPVs), of which the electron donor materials are a selenophene-bridged donor–acceptor copolymer: poly{9-dodecyl-9H-carbazole-alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl) benzo[c][1,2,5]-thiadiazole} (pCzSe) or poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b′]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pBDTSe), or a thiophene-bridged donor–acceptor copolymer: poly{9-dodecyl-9H-carbazole-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pCzS) or poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b′]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pBDTS); the electron acceptor material is [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Judging from our device data, the potential Se–Se interactions of the selenophene-bridged donor–acceptor copolymers, which is presumably beneficial for the fill factor (FF) of BHJ OPVs, is rather susceptible to the device fabrication conditions.