Alkyl side chain impact on the charge transport and photovoltaic properties of benzodithiophene and diketopyrrolopyrrole-based copolymers

To investigate the side chain effect on the photovoltaic performance of conjugated copolymers with alternating electron push-pull structures, three alternating copolymers (O-HD, BO-BO, and PU-O) of benzodithiophene and dithienyldiketopyrrolopyrrole were designed and synthesized. They were nomenclated based on the side chains on the benzodithiophene (BDT) and the diketopyrrolopyrrole (DPP) units, which are octyl (O) and 2-hexyldecyl (HD); 3-butyloctyl (BO), and 2-butyloctyl (BO): and 3-pentylundecyl (PU) and octyl (O) groups, respectively. The total C number of the side chains in each repeat unit was kept at 48 to control the dilute effect. The solubility, optical, and electrochemical properties, and crystalline structure of the polymers were depended on the combination of these linear or branched alkyl chains. Thin film transistor (TFT) characterization showed that PU-O had the best hole mobility up to 1.6 × 10^-3 cm²V^-1 s^-1. The best photovoltaic performance was observed from O-HD with power conversion efficiency (PCE) up to 4.1%. However, it only showed a modest hole mobility of 3.8 × 10^-4 cm²V^-1 s^-1, about 4-fold lower than PU-O. This dramatically different performance of these polymers for TFT and photovoltaic devices was explained by the interaction at the interface of the polymer electron donor and the PCBM acceptor domains. © 2011 American Chemical Society.