  In organic photovoltaics, the mechanism by which free electrons and holes are
generated overcoming the Coulomb attraction is a currently much debated topic.
To elucidate this mechanism at a molecular level, we carried out a combined
electronic structure and quantum dynamical analysis that captures the
elementary events from the exciton dissociation to the free carrier generation
at polymer/fullerene donor-acceptor heterojunctions. Our calculations show that
experimentally observed efficient charge separations can be explained by a
combination of two effects: First, the delocalization of charges which
substantially reduces the Coulomb barrier, and second, the vibronically hot
nature of the charge transfer state which promotes charge dissociation beyond
the barrier. These effects facilitate an ultrafast charge separation even at
low-band-offset heterojunctions.
