In this study the morphology of spin-casted films of polymers blended with [6,6]-phenyl C-61-butyric acid methyl ester (PCBM) has been studied. It was found that the lateral structure formation in the films is favored by rapid solvent evaporation and strong polymer-PCBM repulsion. The formation of homogeneous films is favored by slow evaporation and weak polymer-PCBM repulsion. The effect of solvent evaporation rate is the opposite of what is found for spin-casting polymer-polymer blends. The results can be explained by the kinetics of phase separation and the phase behavior involving limited solubility and crystallization of PCBM.