The effect of meta-substituted or para-substituted phenyl as side chains on the performance of polymer solar cells

Two donor-acceptor alternative copolymers (P1 and P2) with carbazole group as the donor unit and 2,3-diphenylquinoxaline group as the acceptor unit were designed and synthesized by Suzuki polycondensation reaction. Alkoxy groups were linked to meta-position or para-position of phenyl rings, which were attached on the 2,3-position of quinoxaline group to investigate the effect of the alkoxy position at phenyl rings on the resulting polymer optical, electrochemical and photovoltaic properties. GIWAXS experiments revealed that para-substituted polymer P2 possesses closer packing properties in the solid state. The absorption spectra of para-substituted polymer P2 also displayed larger red-shift than that of meta-substituted polymer P1 when it went from solution to the film, which was beneficial to improve the hole mobilities and short circuit current density (J(sc)). When used as the donor materials, devices based on P2:PC71BM exhibited a PCE of 3.33%, which was obviously higher than that based on P1: PC71BM. Our results revealed that the slightly structure change of polymer has a significant influence on the photovoltaic properties of polymer solar cells and adopting para-substituted phenyl ring as the side groups gave superior photovoltaic performance in donor materials based on carbazole and quinoxaline. (C) 2016 Published by Elsevier B.V.