Four new polymers containing a benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic imide (DIITN) unit including the homopolymer and three donor-acceptor copolymers were designed, synthesized, and characterized. For these copolymers, DIITN unit with low bandgap was selected as an electron acceptor, whereas 5,5'-(2,7-bisthiophen-2-yl)-9-(2-decyltetradecyl)-9H-carbazole), 5,5'-(3,3'-di-n-octylsilylene-2,2'-bithiophene), and 5,5'-(2,7-bisthiophen-2-yl-9,9-bisoctyl-9H-fluoren-7yl) were chosen as the electron donor units to tune the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels of the copolymers for better light harvesting. These polymers exhibit extended absorption in the visible and near-infrared range and are soluble in common organic solvents. The relative low lying HOMO of these polymers promises good air stability and high open-circuit voltage (V(oc)) for photovoltaic application. Bulk heterojunction solar cells were fabricated by blending the copolymers with [6,6]-phenyl-C61-butyric acid methyl ester or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The best power conversion efficiency of 1.6% was achieved under simulated sunlight AM 1.5G (100 mW/cm(2)) from solar cells containing 20 wt % of the fluorene copolymer poly[5,5'-(2,7-bisthiophen-2-yl-9,9-bisoctyl-9H-fluoren-7-yl)-alt-2,9-(benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic imide)] and 80 wt % of PC71BM with a high open-circuit voltage (V(oc)) of 0.84 V. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 250-260, 2012