The effect of an additive, 4-cyano-4'-pentylbiphenyl (5CB), on the performance of organic photovoltaic cells (OPVs) has been demonstrated. The addition of 3wt% 5CB to a P3HT [poly(3-hexylthiophene)]:PCBM [[6,6]-phenyl-C61-butyric acid methyl ester] blend film resulted in an improvement of both fill factor (FF) and short circuit current (J(sc)), and hence increased the overall power conversion efficiency by over 22% compared to that of reference cell without 5CB. Carrier mobility studies and optical measurements revealed that the increase in FF in the OPV with 5CB was attributed to more balanced ratio of hole and electron mobility, resulting from the enhanced crystallization of the P3HT chains, which can increase hole mobility. This ordering of P3HT also led to an increase in J(sc).