The effect of electrical annealing treatment under different reverse bias on the performance of bulk heterojunction photovoltaic cells based on poly(3-hexylthiophene): 6,6-phenyl-C61-butyric acid methyl ester is explored. After electrical annealing at -6 V, the polymer solar cell exhibits a 7.77 mA/cm(2) short circuit current density, a 0.53 fill factor, and a 0.63 V open circuit voltage. A corresponding efficiency of 2.59% was achieved. In comparison, solar cell without electrical annealing exhibits power conversion efficiency of 2.37%. This enhanced efficiency is attributed to the modified orientation of the polymer chains inside the photoactive layer that increases the mobility of charge carriers. In addition, after electrical annealing, the Al cathode will penetrate further into the active layer increases the contact area and reduces the contact resistance. This Al penetration was confirmed by the depth profile of atomic concentration in the X-ray photoemission spectroscopy. (C) 2012 Elsevier B.V. All rights reserved.