Polymers consisting of benzo[1,2-b:4,5-b]dithiophene and thieno[3,4-b]thiophene units (PTB-based polymers), either fully or partially containing 4-fluorophenyl pendants, are synthesized as electron donor materials for inverted-type polymer solar cells (PSCs). The influence of the 4-fluorophenyl pendant content on the thermal and optical properties, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), the hole mobilities, and photovoltaic performances are investigated. As the 4-fluorophenyl pendant content increased, the HOMO and LUMO of the polymers were deepened proportionally and the open-circuit voltages of the PSCs improved. Incorporation of 4-fluorophenyl pendants into the polymers also affected the crystallinity, orientation, and compatibility with [6,6]-phenyl-C-61-butyric acid methyl ester in the active layers, leading to nonlinearities in the short-circuit current densities, and fill factors. The incorporation of an appropriate number of 4-fluorophenyl pendants enhanced the power conversion efficiencies of the PSC devices from 2.25 to 3.96% for identical device configurations. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1586-1593