Focused on phase separation and morphologies of poly(3-hexylthiophene):[6,6]-phenyl C(61) butyric acid methyl ester (P3HT:PCBM) active layers, we studied the effect of preparation conditions of the active layer on photovoltaic performance by changing concentration of P3HT:PCBM in the solvent. The performances of the cells varied depending on concentration of P3HT:PCBM (1:1 ratio by weight) in solvent even with the same thickness. The P3HT:PCBM active layer is prepared in cell structure of ITO/PEDOT/P3HT:PCBM/Al by changing spin-coating speed with different concentrations (1, 2 and 3 wt%) in chlorobenzene. Here, it was found that both the P3HT:PCBM concentrations and spin-coating conditions affected the crystalline structure formation, interchain interaction, morphology and phase separation during drying process of solvent and subsequent annealing. (C) 2009 Elsevier B.V. All rights reserved.