Indolo[3,2-b]carbazole-based alternating donor–acceptor copolymers: synthesis, properties and photovoltaic application

Two new donor–acceptor (D–A) type copolymers, poly{5,11-di(1-decylundecyl)indolo[3,2-b]carbazole-3,9-diyl-alt-4,7-dithien-2-yl-2,1,3-benzothiadiazole-5′,5″-diyl} (PIC-DTBT) and poly{5,11-di(1-decylundecyl)indolo[3,2-b]carbazole-3,9-diyl-alt-4,7-di(2,2′-bithien-5-yl)-2,1,3-benzothiadiazole-5′,5″-diyl} (PIC-DT2BT) were synthesized. The polymers have an alternating structure consisting of a large-size fused aromatic ring, indolo[3,2-b]carbazole (IC), as the donor segment and 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) or 4,7-di(2,2′-bithien-5-yl)-2,1,3-benzothiadiazole (DT2BT) as the acceptor segment. The absorption spectra and the electrochemical properties of PIC-DTBT and PIC-DT2BT indicate that these copolymers have suitable energy levels for photovoltaic application. Polymer photovoltaic devices based on blends of the copolymers and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) have a high open-circuit voltage (>0.9 V) under the illumination of AM 1.5 (100 mW cm−2). The higher power conversion efficiency (PCE) of PIC-DT2BT (2.07%) compared to that of PIC-DTBT (1.47%) suggests that DT2BT could be a promising building block for the efficient D–A type copolymers if the low solubility is overcome by the molecular design of the donor segment.