We have quantitatively characterized the phase separation of poly(3-hexylthiophene) (P3HT):C61-butyric acid methyl ester (PCBM) blend films and studied the effect of phase separation of photoactive layer on the performance of polymer solar cells. A mixed solvent of dichlorobenzene and chlorobenzene was adopted to prepare the P3HT:PCBM blend films, and a series of blend films with different morphologies were obtained by adjusting both the volume ratio of dichlorobenzene to chlorobenzene and the solvent annealing time. The surface morphology and phase distribution were measured by atomic force microscopy (AFM). The interface length between the domains of donor and acceptor, which is extracted from the AFM phase image of blend film, was used to quantitatively characterize the phase separation of photoactive layer. It was found that the short-circuit current density (J(SC)) of bulk heterojunction solar cells is proportional to the interfacial area of two phases, while it has a negligible effect on the open-circuit voltage. These results indicate that the larger interfacial area of donor and acceptor phases is beneficial to the exciton dissociation and carrier transport resulting in a significant increase of J(SC) and power conversion efficiency. (C) 2015 Elsevier B.V. All rights reserved.