A soluble graphene, which has a one-atom thickness and a two-dimensional structure, is blended with poly(3-hexylthiophene) (P3HT) and used as the active layer in bulk heterojunction (BHJ) polymer photovoltaic cells. Adding graphene to the P3HT induces a great quenching of the photoluminescence of the P3HT, indicating a strong electron/energy transfer from the P3HT to the graphene. In the photovoltaic devices with an ITO/PEDOT:PSS/ P3HT:graphene/LiF/Al structure, the device efficiency increases first and them decreases with the increase in the graphene content. The device containing only 10 wt % of graphene shows the best performance with a power conversion efficiency of 1.1%, an open-circuit voltage of 0.72 V, a short-circuit current density of 4.0 mA cm(-2), and a fill factor of 0.38 under simulated AM1.5G conditions at 100 mW cm(-2) after an annealing treatment at 160 degrees C for 10 min. The annealing treatment at the appropriate temperature (160 degrees C, for example) greatly improves the device performance; however, an annealing at overgenerous conditions such as at 210 degrees C results in a decrease in the device efficiency (0.57%). The morphology investigation shows that better performance can be obtained with a moderate content of graphene, which keeps good dispersion and interconnection. The functionalized graphene, which is cheap, easily prepared, stable, and inert against the ambient conditions, is expected to be a competitive candidate for the acceptor material in organic photovoltaic applications.