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. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

without contamination of PEO-BE which can be removed by reprecipitation in methanol. The polymer structure was confirmed by 1 H and 13 C NMR spectra. The signal assignable to PEO was observed at 3.6 ppm in 1 H NMR spectrum of P3HT-b-PEO. In addition, the signals derived from phenylene ring at the junction point shifted from that of PEO-BE. These results clearly indicate that PEO segment was introduced at the end of P3HT. The weight ratio of P3HT and PEO segments was estimated as 93.1:6.9 from integral ratio of the signals. Differential scanning calorimetric (DSC) analysis of the polymer also revealed that the formation of block copolymer was confirmed by the presence of T m s for PEO and P3HT segments at around 25 and 220 °C, respectively.