UC Berkeley

In organic materials, exciton dissociation into free charges requires overcoming an electron-hole Coulomb interaction that exceeds the thermal energy and may still be large after charge transfer at a donor/acceptor interface. We analyze the factors affecting efficiency of charge separation and subsequent removal of electrons and holes from such a donor-acceptor interface and suggest strategies for optimizing these processes. Energy transfer, charge separation, and charge transfer in the vicinity of the donor-acceptor interface are studied within a common theoretical framework based on a quantum master equation, for a model system with realistic excitation energies and electronic couplings. We find that enhancing the efficiency of both charge transfer from the donor to the acceptor and of charge removal from the donor-acceptor interface requires an intricate balance between the extent of electronic delocalization throughout the material and rates of energy dissipation. For very large exciton binding energies, cascade charge separation in systems with more than one donor and one acceptor species, such as molecular polyads, is found to greatly facilitate the dissociation of geminate pairs. Our calculations predict charge separation on subpicosecond timescales for several parameter combinations, leading to design principles for enhancing charge separation in multichromophore systems.