In this work, polymer solar cells are fabricated based on the blend of PTB7-Th: PC 71 BM by using a mixed solvent additive of 1,8-diiodooctane and N-methyl pyrrolidone to optimize the morphology of the blend. A high power conversion efficiency (PCE) of 10.8% has been achieved with a simple conventional device. In order to deeply investigate the influence of the mixed solvent additives on the morphology and device performance, the variations of the molecular packing and bulk morphology of the blend film cast from ortho-dichlorobenzene with single or binary solvent additives are measured. Although all the blend films exhibit similar domain size and nanoscale phase separation, the blend film processed with mixed solvent additive shows the highest domain purity, resulting in the least bimolecular recombination, relatively high J(sc) and FF, and hence enhanced PCE. Therefore, the best photovoltaic performance with the V-oc of 0.82 V, J(sc) of 19.1 mA cm(-2), FF of 69.1%, and PCE of 10.8% are obtained for the device based on the blend with binary solvent additive treatment.