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Organic solar cell efficiencies under the aspect of reduced surface recombination velocities

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.