In this study, we synthesized a new polymer, PCTDBI, containing alternating carbazole and thiadiazole-benzoimidazole (TDBI) units. This polymer (number-average molecular weight - 25,600 g mol(-1)), which features a planar imidazole structure into the polymeric main chain, possesses reasonably good thermal properties (T-g = 105 degrees C; T-d = 396 degrees C) and an optical band gap of 1.75 eV that matches the maximum photon flux of sunlight. Electrochemical measurements revealed an appropriate energy band offset between the polymer's lowest unoccupied molecular orbital and that of PCBM, thereby allowing efficient electron transfer between the two species. A solar cell device incorporating PCTDBI and PCBM at a blend ratio of 1: 2 (w/w) exhibited a power conversion efficiency of 1.20%; the corresponding device incorporating PCTDBI and PC71BM (1: 2, w/w) exhibited a PCE of 1.84%. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4456-4464, 2010