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 sub-picosecond timescales for several parameter combinations, leading to design principles for enhancing charge separation in multi-chromophore systems.

We simulate subpicosecond charge separation in two donor–acceptor molecular dyads. Charge separation dynamics is described using a quantum master equation, with parameters of the dyad Hamiltonian obtained from density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations and the rate of energy dissipation estimated from Ehrenfest-TDDFT molecular dynamics simulations. We find that higher-energy charge transfer states must be included in the dyad Hamiltonian in order to obtain agreement of charge separation rates with the experimental values. Our results show that efficient and irreversible charge separation involves both coherent electron transfer from the donor excited state to higher-energy unoccupied states on the acceptor and incoherent energy dissipation that relaxes the dyad to the lowest energy charge transfer state. The role of coherence depends on the initial excited state, with electron delocalization within Hamiltonian eigenstates found to be more important than coherence between eigenstates. We conclude that ultrafast charge separation is most likely to occur in donor–acceptor dyads possessing dense manifolds of charge transfer states at energies close to those of Frenkel excitons on the donor, with strong couplings to these states enabling partial delocalization of eigenstates over acceptor and donor.