Separation of geminate charge-pairs at donor–acceptor interfaces in disordered solids

Using an extension of the Onsager model of exciton dissociation in the bulk of a disordered solid, we determine the separation probability of geminate electron–hole pairs following exciton dissociation by charge-transfer at a donor–acceptor interface. The donor–acceptor interface confines the electron and hole in their respective layers, leading to charge separation with a probability that is significantly higher than predicted by bulk theory. The model explains the surprisingly high carrier collection efficiencies observed in organic donor–acceptor photovoltaic cells, and correctly predicts the voltage-dependence of the photocurrent which differs substantially from that of conventional inorganic semiconductor p–n junctions and may ultimately limit the power conversion efficiency of organic donor–acceptor photovoltaic cells.