Two polythiophene derivatives with electron-donating alkylthio side chains, poly[(3-hexylthio)thiophene] (P3HST) and poly[(3-hexylthio)thiophene-a It-thiophene] (P3HST-co-Th) have been synthesized and characterized. Both P3HST and P3HST-co-Th show broader absorption peaks than poly(3-hexylthiophene). Meanwhile, the alkylthio side chains decrease the HOMO energy level of the polymers, which benefits the higher open circuit voltage of the polymer solar cells (PSCs) based on the polymer as donor. PSCs have been fabricated with the polymers as donor and [6,6]-phenyl C61 butyric acid methyl ester as acceptor (1: 1, w/w). The devices based on P3HST and P3HST-co-Th show an open circuit voltage of 0.63 V, and a power conversion efficiency of 0.34% and 0.5%, respectively, under the illumination of AM1.5, 80 mW.cm(-2).