How quasi-free holes and electrons are generated in organic photovoltaic interfaces

Abstract

We introduced a minimal model of the donor–acceptor interface encountered in organic solar cells to explain the efficient generation of free charges in these systems and we investigated the nature of charge transfer states formed by a delocalized exciton in the donor component. Contrarily to the generally accepted view excitons do not generate strongly bound hole-electron pairs, but relatively delocalized charge transfer states with energy very close to the energy of free holes and electrons. These states are kinetically more accessible from the exciton state and very close in structure to the free hole and electron states. The most relevant molecular parameter that affects the rate of exciton dissociation is the electronic coupling between donor orbitals and acceptor orbitals, i.e. the band widths of the donor and acceptor materials. Moreover, and to some surprise, we find that the process of charge separation is mostly a purely electronic process and a very similar physics can be described neglecting the role of nuclear degrees of freedom altogether.