Poly(3-alkylthiophene) formed regular, layered arrays and showed liquid crystalline structures in the solid state. To elucidate the nanophase segregation in poly(3-alkylthiophene)/fullerene blends, X-ray diffraction, atomic force microscopy, scanning electron microscopy, and polarized optical microscopy were conducted. The effect of side-chain influence on the photovoltaic properties of solution-processed poly(3-alkylthiophene) /fullerene blends has been studied in this paper. Side chains investigated here include butyl, hexyl, and dodecyl groups. It shows that the photovoltaic performance is strongly affected by the length of side groups, which alters the morphology of the photoactive layer due to changes in solvent solubility, miscibility, and fullerene diffusion rate in the blends. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3461139] All rights reserved.