The performance of bulk heterojunction organic solar cells based on Poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-lphenyl-(6,6)C-61 (PCBM) processed from chlorobenzene solution can be enhanced by a thermal treatment of the device. The morphology of films made from 1/2 wt. ratio blends of P3HT and PCBM reveals an unfavorable amorphous phase in the as-produced cells while after annealing a more favorable crystalline phase is obtained. Through variation of the processing solvent a power conversion efficiency of 3.6% is demonstrated without the necessity of an additional annealing step. It is shown that the as-produced active layer already has a crystalline morphology. We argue that the high boiling point of the solvent plays an important role in this by influencing the evaporation speed during deposition of the active layer. Further proof is delivered that indeed slowing down the evaporation speed can beneficially influence the solar cell performance. (c) 2005 Elsevier B.V. All rights reserved.