The effect of doping carrier transporting materials being used for organic light emitting diodes was investigated in bulk-heterojunction photovoltaic devices composed of poly(3-hexylthiophene) (P3HT) and a perylene bisimide (PBI) derivative. We found that the doping of 4,4'-bis(carbazol-9-yl)biphenyl (CBP) can improve the short circuit current and fill factor with a doping concentration of 2.5wt%, resulting in an increase of power conversion efficiency from 0.14% to 0.28%. This effect is mainly attributable to the improvement of miscibility between P3HT and PBI.