A series of donor--acceptor (D--A) conjugated copolymers (PBDT-AT, PDTS-AT, PBDT-TT, and PDTS-TT), based on benzo[1,2-b:4,5-c']dithiophene-4,8-dione (BDD) acceptor unit with benzodithiophene (BDT) or dithienosilole (DTS) as donor unit, alkylthiophene (AT) or thieno[3,2-b]thiophene (TT) as conjugated -bridge, were designed and synthesized for application as donor materials in polymer solar cells (PSCs). Effects of the donor unit and -bridge on the optical and electrochemical properties, hole mobilities, and photovoltaic performance of the D--A copolymers were investigated. PSCs with the polymers as donor and PC70BM as acceptor exhibit an initial power conversion efficiency (PCE) of 5.46% for PBDT-AT, 2.62% for PDTS-AT, 0.82% for PBDT-TT, and 2.38% for PDTS-TT. After methanol treatment, the PCE was increased up to 5.91%, 3.06%, 1.45%, and 2.45% for PBDT-AT, PDTS-AT, PBDT-TT, and PDTS-TT, respectively, with significantly increased FF. The effects of methanol treatment on the photovoltaic performance of the PSCs can be ascribed to the increased and balanced carrier transport and the formation of better nanoscaled interpenetrating network in the active layer. The results indicate that both donor unit and -bridge are crucial in designing a D--A copolymer for high-performance photovoltaic materials. (c) 2014 Wiley Periodicals, Inc.