Relating free energy and open-circuit voltage to disorder in organic photovoltaic systems

Efficient exciton dissociation into mobile charge carries a crucial factor underscoring the performance of organic polymer-based bulk-heterojunction photovoltaic devices. In this paper, we compute the energies of charge-transfer (CT) states of the model donor-acceptor lattice system with varying degrees of structural disorder to investigate how fluctuations in the material properties affect electron-hole separation. We also demonstrate how proper statistical treatment of the CT energies recovers the experimentally observed "hot" and "cold" exciton dissociation pathways. Using a quantum mechanical model for a model heterojunction interface, we recover experimental values for the open-circuit voltage at 50 and 100 meV of site-energy disorder. We find that energetic and conformational disorder generally facilitates charge transfer; however, due to excess energy supplied by photoexcitation, highly energetic electron-hole pairs can dissociate in unfavorable directions, potentially never contributing to the photocurrent while "cold" excitons follow the free energy curve defined at the operating temperature of the device.