Novel bromine-functionalized photocrosslinkable low-bandgap copolymers, PBDTTT-Br25 and PBDTTT-Br50, are synthesized via Stille cross-coupling polymerization for the purpose of stabilizing the film morphology in polymer solar cells (PSCs). Photocrosslinking of PBDTTT-Br25 and PBDTTT-Br50 copolymers dramatically improves the solvent resistance of the active layer without disrupting the molecular ordering and charge transport, which is confirmed by the insolubility of the films washed by organic solvents and by their thermal behavior. As a result, the formation of large aggregations of fullerene is suppressed in polymer:fullerene blend films even after prolonged thermal annealing, and the stability of the device is enhanced when compared with cells based on noncrosslinkable PBDTTT. The power conversion efficiency of the PSCs based on PBDTTT-Br25 and PBDTTT-Br50 reaches 5.17% and 4.48%, respectively, which is improved obviously in comparison with that (4.26%) of the PSCs based on the control polymer PBDTTT. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3123–3131