  Organic Photovoltaic devices (OPVs) are becoming adequately cost and energy
efficient to be considered a good investment and it is, therefore, especially
important to have a concrete understanding of their operation. We compute
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
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 100meV 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. We
find that "cold" excitons follow the free energy curve defined at the operating
temperature of the device. Our results provide a unifying picture linking
various proposed mechanisms for charge separation.
