Poly(3-hexylthiophene) formed gels and showed liquid crystalline structures at high concentrations. The absorption properties of poly(3-hexylthiophene) showed dramatic changes during gelation, which is an indication of strong intermolecular pi-electronic coupling of the ordered self-assembled poly(3-hexylthiophene) gels. The effect of conformational transitions on the photovoltaic properties of solution-processed poly(3-hexylthiophene)/fullerene blends have been studied in this paper. It is shown that the photovoltaic performance is strongly affected by gelation, which alters the morphology of the photoactive layer. Device optimization yields solar cells with a power conversion efficiency of 4.0% under standard test conditions (AM 1.5, 100 mW/cm(2)). (C) 2008 Elsevier B.V. All rights reserved.