This paper describes the influence of the solvent on the morphological evolution and performance of polymer solar cells (PSCs) based on blended films of poly(thiophene-phenylene-thiophene) and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM). The blends are spin-coated with solvents exhibiting various evaporation rates, including o-dichlorobenzene (DCB), chlorobenzene (CB), chloroform (CF), and tetralin. The changing morphologies of these blended films are monitored using atomic force microscopy (AFM) and transmission electron microscopy (TEM). A solvent having a higher boiling point [1,8-octanedithiol (OT)] is also introduced as an additive to further fine-tune the morphology of the bulk heterojunction (BHJ). We demonstrate herein that the morphology of a blend and, hence, the performance of a BHJ device-can be manipulated by controlling the rate of solvent evaporation during film formation. The resulted fine-scale phase separation leads to enhanced performance of such organic photovoltaic devices. The highest power efficiency for our PSCs (5.8%, AM 1.5G irradiation (100 mW/cm(2))) resulted when we use DCB as the solvent with OT as a processing additive.