We have systematically investigated the morphology of poly(3-hexylthiophene: P3HT) and [6,6]-phenyl C(61)-butyic acid methyl ester (PCBM) blend films prepared through a slow growth apparoch as a function of PCBM loading in the blends for bulk heterojunction solar cells. In addition to the electrical characterisitcs, the molecular ordering and crystallinity of the polymers was examined by using the UV-vis spectroscopy and grazing incidence X-ray diffraction. From the images of confocal laser scan microscopy, we were also able to in-situ monitor the phase separation of the P3HT:PCBM blends during the process of solvent evaporation. Finally, the conductive atomic force microscopy was utilized to probe the spatial distribution of the local current across the photoactive layer prepared with various P3HT:PCBM ratios. From the experimental results, we found that the distribution of PCBM molecules in the P3HT matrix became homogeneous after the solvent annealing process, even though the distribution of the PCBM molecules was not even in the beginning. Therefore, the higher device efficiency could be attributed to the better thin-film morphology of the polymer blends. (C) 2010 Elsevier B.V. All rights reserved.