The power conversion efficiency of organic solar cells based on donoracceptor blends is governed by two competing processes, extraction and recombination of charge carriers. Both processes are strongly dependent on the charge carrier mobility. Using microscopic effective medium simulation, we demonstrate that the best device efficiencies are achieved in the mobility range . For lower mobilities lead to losses by bimolecular recombination and higher mobilities lead to a reduction of the open circuit voltage, and consequently a drop in efficiency