Synthesis and characterizations of carbazole–isoindigo–carbazole oligomers for photovoltaic application

•Isoindigo (II) was directly coupled to carbazole (C) to give C-II-C.•II was coupled to the 9-phenyl group of C to give P-II-P.•The band gaps of C-II-C and P-II-P were 1.85 and 1.90eV, respectively.•C-II-C is more planar and more conjugated than P-II-P.•C-II-C-based solar cells showed the best power conversion efficiency of 0.44%.