We report the synthesis, characterization, microphase separation, field-effect charge transport, and photovoltaic properties of regioregular poly(3-hexylthiophene)-b-poly(3-cyclohexylthiophene) (P3HT-b-P3cHT). Two compositions of P3HT-b-P3cHT (HcH63 and HcH77) were synthesized with weight-average molecular weights of 155,500 and 210,800 and polydispersity indices of 1.45 and 1.57, respectively. Solvent-casted HcH77 was found to self-assemble into nanowires with a width of 12.5 +/- 0.9 nm and aspect ratios of 50-120, as observed by TEM imaging. HcH77 and HcH63 annealed 280 degrees C were observed by small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) to be microphase-separated with characteristic length scales of 17.0-21.7 nm. The microphase-separated domains were shown to be crystalline with interlayer backbone (100) d-spacings of 1.69 and 1.40 nm, which correspond to the P3HT and P3cHT blocks, respectively. Field-effect transistors fabricated from P3HT-b-P3cHT thin films showed a mobility of holes (0.0019 cm(2)/VS) which is independent of thermal annealing. Bulk heterojunction solar cells based on HcH77/fullerene (PC71BM) blend thin films had a maximum power conversion efficiency of 2.45% under 100 mW/cm(2) AM1.5 solar illumination in air. These results demonstrate that all-conjugated block copolymers are suitable semiconductors for applications in field-effect transistors and bulk heterojunction solar cells. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 614-626, 2010