Quantum-chemical approach to electronic coupling: Application to charge separation and charge recombination pathways in a model molecular donor - Acceptor system for organic solar cells

We have developed a new computational method to evaluate in a general way the electronic coupling between electronic states of any spin multiplicity. This method is especially relevant to the description of charge-separation and charge-recombination processes in molecular donor-acceptor complexes; in such instances, the electronic states can correspond, for example, to a locally excited initial state and a charge-separated final state. Here, we describe our approach in a simple way by deriving the excited states of the donor and acceptor moieties with Zerner's intermediate neglect of differential overlap method coupled to a configuration interaction scheme involving single and double electron excitations. In order to increase the efficiency of the calculations, the same Hartree-Fock molecular orbital basis set is employed to compute the electronic states of both neutral and charged configurations. We illustrate the application of the present approach by computing the rates of charge generation and charge recombination in a model phthalocyanine/perylene- tetracarboxylicdiimide donor/acceptor molecular system. The role of triplet channels is underlined.