We investigated the effect of annealing sequence on the performance of polymer solar cell. with the composite of poly(3-hexylthiophene) (P3HT) and a C-60 derivative. The post-annealing under the existence of Al metal cap induced a substantial increase in the interfacial area between the active layer and Al cathode and in crystallization of P3HT molecules, resulting in reduction of series resistance and the enhanced photocurrent in the device. Furthermore, the post-annealing brought about the increase of light harvest in active layer and hindered the formation of shunt paths in the donor (D)/acceptor (A) bulk-heterojunction structure compared to the pre-annealing. (c) 2006 Elsevier B.V. All rights reserved.