Unified Theory of Geminate and Bulk Electron−Hole Recombination in Organic Solar Cells

Geminate and bulk charge recombination in organic solar cells are extensively studied because they are major loss processes for cell efficiency. It was recently found that the observed bulk charge recombination rate constants in organic solar cells are more than 3 orders of magnitude smaller than those predicted from Langevin theory. In order to resolve this discrepancy, we develop a unified theory of geminate and bulk charge recombination by taking into account that recombination between an electron and a hole occurs at a nonzero separation with a finite intrinsic recombination rate. On the basis of this theory we successfully explain the observed results on bulk recombination rate constants. We also analyze how recombination between electrons and holes in geminate and bulk recombination phases is affected by various physical parameters, such as the intrinsic recombination rate, the diffusion coefficients of electrons and holes, the Onsager radius, and an external electric field, and show how they are optimized to suppress recombination loss.