Elucidating the Role of Disorder in Charge-Carrier Photoseparation in Organic Solar Cells

Using high-precision simulation calculations and a semianalytical theory, we finally resolve the controversy over the role of energetic disorder in geminate electron-hole photoseparation in organic solids. We demonstrate that in efficient photovoltaic systems, where the charge-carrier separation probability is high, this probability decreases with increasing disorder, both in the absence and presence of a donor-acceptor heterojunction. This results from energy relaxation of the charged particles in the site energy distribution, which effectively deepens the electron-hole Coulomb well. In inefficient photovoltaic systems, the separation probability also initially decreases but turns into an increasing trend at large disorder. We also demonstrate that in the presence of disorder, the geminate charge-pair separation probability is affected by the mismatch between the electron and hole mobilities, even in homogeneous systems. We show how our theoretical results can be practically applied in an analysis of experimental data in organic photovoltaics.