A series of donor-acceptor low-bandgap conjugated polymers, that is, HThmBT (m = 3, 6, 9, 12, 15), composed of regioregular 3-hexylthiophene segments and 2,1,3-benzothiadiazole units, were synthesized through the Stille coupling polymerization to optimize the pi-conjugation length of the polymer and the intramolecular charge transfer (ICT) effect in the polymer backbone. The polymers had relatively low optical bandgaps ranging from 1.6 to 1.72 eV. Among these polymers, HTh6BT exhibited the best device performance with a power conversion efficiency (PCE) of 1.6%. Moreover, despite being based on thiophene, HTh6BT exhibited a high-open circuit voltage (V-OC) of over 0.8 V because of its low high occupied molecular orbital (HOMO) energy level. These results provided an effective strategy for designing and synthesizing low-bandgap conjugated polymers with broad absorption ranges and well-balanced energy levels. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4875-4883, 2010