A new series of low-bandgap copolymers based on electron-accepting thieno[3,4-b]pyrazine (TPZ) and different electron-donating aza-heteroaromatic units, such as carbazole (CZ), dithieno[3,2-b: 2',3'-d]pyrrole (TPR) and dithieno[3,2-b:2',3'-e] pyridine (TPY), have been synthesized by Suzuki or Stille coupling polymerization. The resulting copolymers were characterized by NMR, elemental analysis, gel permeation chromatography, thermogravimetric analysis, and differential scanning calorimetry. UV-vis absorption and cyclic voltammetry measurements show that TPZ-based copolymer with TPR has the best absorption due to the strongest intramolecular charge transfer effect and smallest bandgap. The basic electronic structure of D-A model compounds of these copolymers were also studied by density functional theory (DFT) calculations. The conclusion of calculation agreed also well with the experimental results. The polymer solar cells (PSCs) based on these copolymers were fabricated with a typical structure of ITO/PEDOT:PSS/copolymer: PC(71)BM/Ca/Al under the illumination of AM 1.5G, 100 mW cm(-2). The performance results showed that TPZ-based copolymer with TPR donor segments showed highest efficiency of 1.55% due to enhanced short-circuit current density. The present results indicate that good electronic, optical, and photovoltaic properties of TPZ-based copolymers can be achieved by just fine-tuning the structures of aza-heteroaromatic donor segments for their application in PSCs. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4458-4467, 2011