The short circuit current density (Jsc) of polymer solar cells is strictly related to the absorption of the blend film. Recently it has been shown that the use of PCBM as electron acceptor can improve the current output of such devices because C70 derivatives have a stronger and broader absorption compared to C60 ones. The aim of this work is to study the influence of the fullerene on the optical behaviour of the photoactive blend film of a polymer solar cell. We have determined the optical constants of a P3HT:PCBM blend film and studied their variation as a function of the annealing temperature. Afterward, we simulated the optical absorption of the active layer inside the device structure and calculated the maximum achievable Jsc with the aim to correlate the variation of the optical constants to the device output current. We compared this value with that one obtained using a P3HT:PCBM blend.