Mobility dependent efficiencies of organic bulk heterojunction solar cells: Surface recombination and charge transfer state distribution

Recent simulations of the efficiency of polymer/fullerene solar cells as a function of mobility predicted finite optimum mobilities due to a decrease in open circuit voltage for higher mobilities. We explain this decrease in open circuit voltage with two features of the commonly used model, namely, infinite surface recombination and an integration over a distribution of separation distances of electron and hole in a charge transfer state at the interface between donor and acceptor molecules. Especially, the assumption of a variable electron/hole pair separation at the interface has a considerable influence on the open circuit voltage.