Dual substitution at 4,9-positions of carbazole in donor-π-acceptor copolymer enhances performance of bulk-heterojunction organic solar cells

Several carbazole-2,7-diyl units with alkoxy substituents at the 4-position in addition to branched (1-alkyl)alkyl substituents at the 9-position were developed and combined with difluorinated dithienylbenzothiadiazole by using a Suzuki coupling reaction to synthesize donor-pi-acceptor-type copolymers. X-ray diffraction analyses of these copolymers revealed that the 4,9-disubstituted carbazole unit contributed to form a balanced interdigitated structure, which resulted in smaller at-stacking distances (4.0-3.5 angstrom) of the polymer backbone, a narrower bandgap (2.0-1.8 eV), and higher hole mobilities (4.2 x 10(-4) to 2.9 x 10(-3) cm(2) V-1 s(-1)) than those of the carbazole copolymer without the 4-substituent (PCDT2FBT). A bulk heterojunction organic photovoltaic device with a configuration of ITO/PEDOT:PSS/polymer:PC70BM/LiF/Al exhibited a power conversion efficiency of 5.46%, a short-circuit current density of 12.4 mA cm(-2), a fill factor of 0.58, and an open-circuit voltage of 0.76 V. The photovoltaic performance was much higher than that of PCDT2FBT. These results demonstrate that incorporation of the side chains into the 4,9-positions of carbazole is a novel approach to obtain self-organizing carbazole polymers, enabling improvement in light absorption, hole mobility, and photovoltaic performance. (C) 2016 Elsevier Ltd. All rights reserved.