Low band gap conjugated copolymers containing donor (thiophene)acceptor (quinoxaline, Qx) were synthesized via Stille coupling polymerization. The resulting copolymers were characterized by H-1 NMR, element analysis, GPC, TGA, and DSC. UV-vis spectra indicated that the increase in the content of quinoxaline units increased the interaction strengthen of the polymer main chains and caused a red-shift in the optical absorbance. Cyclic voltammetry was used to estimate energy levels of the lowest unoccupied molecular orbit (LUMO) and the highest occupied molecular orbit (HOMO), and the band gap (E-g) of the copolymers. The basic electronic structures of the copolymers were also studied by density-functional theory (DFT) calculations. Both the experimental and calculation results indicated an increase in the HOMO energy level with increasing the content of quinoxaline units, whereas the corresponding change in the LUMO energy level is much smaller. Polymer photovoltaic cells (PVCs) were fabricated with the structure of ITO/PEDOT:PSS (30 nm)/active layer (80 nm)/Ca (8 nm)/Al(140 nm). The results show that the introduction of a proper amount of electron-acceptor groups in the polymer main chains induces an extension of the absorption spectra and improves the photovoltaic properties of the copolymers. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3399-3408, 2009