A diblock copolymer poly(3-hexylthiophene)-block-poly(dimethylsiloxane) (P3HT-b-PDMS) was synthesized as a donor material for organic photovoltaic devices. The regioregular poly(3-hexylthiophene) (P3HT) with a bromo-terminal was prepared via Grignard metathesis polymerization, and poly(dimethylsiloxane) (PDMS) with a silane terminal was obtained by anionic polymerization. Finally, the coupling reaction of these homopolymers via hydrosilylation afforded P3HT-b-PDMSs with different lengths of the PDMS segment. Device evaluation of photovoltaic cells based on P3HT-b-PDMS blended with [ 6,6]-phenyl-C-61-butylic acid methyl ester (PCBM) revealed that the incorporation of PDMS segment effectively increase the photocurrent compared to a P3HT/PCBM system. From AFM and SEM observations for P3HT-b-PDMS/PCBM blend films, it was clearly revealed that the phase-separated structure within several tens nanometer scale was obtained, and that the aggregation of PCBM was sufficiently suppressed by the introduction of PDMS. These facts indicate that the PDMS segment is probably assembled into the interface between P3HT and PCBM to relieve the mismatch of the surface free energy. (C) 2016 Elsevier Ltd. All rights reserved.