A novel conjugated alternating copolymer (PCzDBSe) based on N-9'-heptadecanyl-2,7-carbazole and 5,5(4',7'-di-2-thienyl-2',1',3'-benzoselenadiazole) was synthesized by Suzuki polycondensation. The polymer reveals excellent thermal stabilities with the decomposition temperature (5% weight loss) of 390 degrees C and the glass-transition temperature of 140 degrees C. The absorption peaks of the polymer are located at 412 and 626 nm, respectively, while the absorption onset is extended to 716 nm, which is 56 nm red-shifted as compared with its analogue, poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3 '-benzothiadiazole)] (PCDTBT). The HOMO and LUMO levels of the polymer were estimated to be -5.28 and -3.55 eV, respectively, with an optical bandgap of 1.73 eV. The hole mobility of PCzDBSe as deduced from a solution-processed organic field effect transistor (OFET) was found to be 3.9 x 10(-4) cm(2) V-1 s(-1). Polymer solar cells (PSCs) based on the blends of PCzDBSe and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) with a weight ratio of 1:4 were fabricated. Under AM 1.5 (AM, air mass), 100 mW cm(-2) illumination, the devices were found to have an open-circuit (V-OC) of 0.75 V, a short-circuit current density (J(sc)) of 7.23 mA cm(-2), a fill factor (FF) of 45% and a power conversion efficiency (PCE) of 2.58%. The primary results indicate that 5,5-(4',7'-di-2-thienyl-2',1',3'-benzoselenadiazole) is a promising unit for low bandgap polymer for polymer solar cells. (C) 2010 Elsevier Ltd. All rights reserved.