Investigating the effect of solvent on the fabrication of photo-active layers in bulk heterojunction organic solar cells based on poly(3-hexylthiophene) and [6,6]-phenyl C(61)-butyric acid methyl ester, we found the short-circuit current density (J(SC)) and fill factor (FF) of a cell with a photo-active layer made using materials dissolved in a mixture of o-dichlorobenzene with chloroform to be slightly higher than those of a cell made using the same materials dissolved only in chlorobenzene. Evaluating the absorption spectra, surface morphology, charge mobility, and solubility of photo-active films made using different solvents, we concluded that the higher J(SC) and FF were due to a larger interfacial area between the photo-active materials in the layer. (C) 2008 Elsevier B.V. All rights reserved.