Novel block copolymers, poly(3-hexylthiophene)-b-poly(ethylene oxide) (P3HT-b-PEO) were synthesized via Suzuki coupling reaction of P3HT and PEO homopolymers. The copolymers were characterized by NMR, gel permeation chromatography, differential scanning calorimeter, and UV-vis measurements. A series of devices based on the block copolymers with a fullerene derivative were evaluated after thermal or solvent annealing. The device using P3HT-b-PEO showed higher efficiency than using P3HT blend after thermal annealing. Phase-separated structures in the thin films of block copolymer blends were investigated by atomic force microscopy to clarify the relationship between morphologies constructed by annealing and the device performance. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 2645-2652, 2011