In this paper semiempirical quantum chemical methods were used to optimize the ge- ometry and to calculate the LUMO level of different fullerene derivatives used as electron acceptors in bulk heterojunction solar cells. The calculated LUMO levels were successfully correlated with the open circuit voltage of devices realized using poly[2-methoxy-5-(3,7- dimethyloctyloxy)]–1,4-phenylenevinylene as electron donor, showing the possibility to use a theoretical approach to design new acceptor molecules.