A new low-band-gap thieno[3,4-c]pyrrole-4,6-dione-based copolymer, PBDTTPD, has been designed and synthesized. PBDTTPD is soluble in chloroform or o-dichlorobenzene upon heating and shows a broad absorption in the visible region. The HOMO and LUMO energy Levels were estimated to be at -5.56 and -3.75 eV, respectively. These electrochemical measurements fit well with an optical bandgap of 1.8 eV. When blended with PC71BM, this polymer demonstrated a power conversion efficiency of 5.5% in a bulk-heterojunction photovoltaic device having an active area of 1.0 cm(2).