Thieno[3,2-b]thiophene-substituted benzo[1,2-b:4,5-b]dithiophene donor units (TTBDT) serve as novel promising building blocks for donor-acceptor (D-A) copolymers in organic photovoltaic cells. In this study, a new D-A type copolymer (PTTBDT-TPD) consisting of TTBDT and thieno[3,4-c]pyrrole-4,6-dione (TPD) is synthesized by Stille coupling polymerization. A PTTBDT-TPD analog consisting of TTBDT and alkylthienyl-substituted BDT (PTBDT-TPD) is also synthesized to compare the optical, electrochemical, morphological, and photovoltaic properties of the polymers. Bulk heterojunction photovoltaic devices are fabricated using the polymers as p-type donors and [6,6]-phenyl C-71-butyric acid methyl ester (PC71BM) as the n-type acceptor. The power conversion efficiencies of the devices fabricated using PTTBDT-TPD and PTBDT-TPD are 6.03 and 5.44%, respectively. The difference in efficiency is attributed to the broad UV-visible absorption and high crystallinity of PTTBDT-TPD. The replacement of the alkylthienyl moiety with thieno[3,2-b]thiophene on BDT can yield broad UV-visible absorption due to extended -conjugation, and enhanced molecular ordering and orientation for organic photovoltaic cells. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3608-3616