Two new 5,6-difluorobenzotriazole (FBTA)-oligothiophene copolymers PFBTA-3T and PFBTA-4T, comprising terthiophene (3T) and quaterthiophene (4T) on the backbone, respectively, were successfully synthesized. A new route to synthesize FBTA monomer was established. Polymers PFBTA-3T and PFBTA-4T exhibited good solubility in common organic solvents and good thermal stability. In comparison to poly (3-hexylthiophene), the incorporations of the FBTA as in PFBTA-3T and PFBTA-4T could result in smaller band gaps around 1.83 eV for the two copolymers. The HOMO levels of PFBTA-3T and PFBTA-4T were -5.49 and -5.31 eV, respectively, while their LUMO levels were -3.65 and -3.90 eV, respectively. In field-effect transistors fabricated without high temperature thermal annealing, PFBTA-3T and PFBTA-4T could display hole mobilities of 1.68 x 10(-3) and 1.31 x 10(-2) cm(2) V-1 s(-1), respectively. The mobility for PFBTA-4T is the highest among the reported FBTA-based polymers, suggesting that FBTA is a promising heterocycle to construct polymers with high mobility. Polymer solar cells were also fabricated with PFBTA-3T and PFBTA-4T as the donor and PC61BM as the acceptor. With copolymer: PC61BM = 1:1.5 as the active layers, polymer solar cells showed power conversion efficiencies of 3.0% and 2.51% for PFBTA-3T and PFBTA-4T, respectively. (C) 2014 Elsevier Ltd. All rights reserved.