  We propose a model to explain the reduced bimolecular recombination rate
found in state-of-the-art bulk heterojunction solar cells. When compared to the
Langevin recombination, the experimentally observed rate is one to four orders
of magnitude lower, but gets closer to the Langevin case for low temperatures.
Our model considers the organic solar cell as device with carrier concentration
gradients, which form due to the electrode/blend/electrode device
configuration. The resulting electron concentration under working conditions of
a solar cell is higher at the cathode than at the anode, and vice versa for
holes. Therefore, the spatially dependent bimolecular recombination rate,
proportional to the local product of electron and hole concentration, is much
lower as compared to the calculation of the recombination rate based on the
extracted and thus averaged charge carrier concentrations. We consider also the
temperature dependence of the recombination rate, which can for the first time
be described with our model.
