  The charge carrier mobility is a key parameter for the organic bulk
heterojunction solar cell efficiency. It was recently shown that the interplay
charge carrier transport and recombination, both depending on electron and hole
mobilities, leads to a point of maximum power conversion efficiency at a finite
mobility. Changes of bulk and surface recombination rate, however, can strongly
influence this behavior. These processes were previously not considered
adequately, as surface recombination velocities of infinity were implicitly
assumed or bulk recombination parameters not discussed in detail. In this
manuscript, using a macroscopic effective medium simulation, we consider how a
reduced bulk recombination process in combination with finite surface
recombination velocities affect the power conversion efficiency. Instead of a
maximum efficiency at a specific charge carrier mobility, we show that with
realistic assumptions and passivated surfaces the efficiency is increased
further, saturating only at higher mobilities. Thus, a mobility optimisation is
more important for the solar cell performance then previously shown.
