A numerical device model was introduced that consistently describes the current-voltage characteristics of polymer/fullerene bulk heterojunction solar cells. Bimolecular recombination and temperature and field dependent generation mechanism of free charges were also presented. It was demonstrated that in poly based solar cells space-charge effects only play a minor role, leading to a relatively constant electric field in the device. Furthermore, at short-circuit conditions only 7% of all free carriers are lost due to bimolecular recombination. The model predicts that an increased hole mobility together with a reduction of the acceptor strength of 0.5 eV will lead to a maximum attainable efficiency of 5.5% in the PPV/PCBM-based solar cells.