An extremely low bandgap donor-acceptor copolymer has been designed and synthesized as the donor material of the polymer solar cells via Stille coupling reaction. The donor-acceptor alternating structure consisted of 4,4-diethylhexyl-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) donor unit and 2,3-bis(2-ethylhexyl)thieno[3,4-b]pyrazine (TP) acceptor unit. Since both units have attached branched alkyl chains, the polymer was well dissolved in common organic solvents. UV-vis spectrum of the polymer film exhibited a panchromatic absorption ranging from 280 to 1285 nm and a low bandgap of 1.20 eV. Compared to that in solution, solid-state UV-vis absorption spectrum of the polymer showed a strong bathochromic shift, indicating more efficient pi-stacking and stronger intermolecular interactions. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymer were estimated to be 5.07 and 3.87 eV, respectively. Based on the ITO/PEDOT:PSS/Polymer:PC61BM/Al device structure, the power conversion efficiency (PCE) under the illumination of AM 1.5 (100 mW/cm(2)) was 0.149%. The effects of annealing temperature (100-200 degrees C) for 30 min on the device performance were studied as well. It was found that PCE of 0.264% could be acquired under the annealing condition at 175 degrees C for 30 min. The improved device efficiency under the optimal condition was confirmed by the higher light harvest in UV-vis spectra, the enhanced quenching of photoluminescence (PL) emission, and the increase in external quantum efficiency. (C) 2012 Elsevier B.V. All rights reserved.