Four polythiophene derivatives including regiorandom polymers P1, P2, and P3 and a regioregular polymer P4, containing a phenyl side chain with electron-withdrawing carbonyl groups such as an ester and a ketone at the 3-position of the thiophene ring, were synthesized by Stille coupling reaction. Bulk-heterojunction polymer solar cells (PSCs) based on these polymers as p-type semiconductors and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) were fabricated, and their photovoltaic performances were evaluated for the first time. The PSC devices based on the regioregular polymer P4:PCBM=1:2 (w/w) exhibited a high-open-circuit voltage (V-oc) of 0.943 V because of the low-lying highest occupied molecular orbit energy level of P4. The short - stacking distance (0.355 nm) in the parallel direction to the substrate and face-on rich orientation were observed by the grazing incidence wide-angle X-ray scattering experiment, which might reflect higher J(sc) and FF values of the P4:[6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) PSC device than others. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 875-887