  In organic photovoltaic systems, a photogenerated molecular exciton in the
donor domain dissociates into a hole and an electron at the donor/acceptor
heterojunction, and subsequently separate into free charge carriers that can be
extracted as photocurrents. The recombination of the once-separated electron
and hole is a major loss mechanism in photovoltaic systems, which controls
their performance. Hence, efficient photovoltaic systems need built-in ratchet
mechanisms, namely, ultrafast charge separation and retarded charge
recombination. In order to obtain insight into the internal working of the
experimentally observed ultrafast long-range charge separation and protection
against charge recombination, we theoretically investigate a potential ratchet
mechanism arising from the combination of quantum delocalization and its
destruction by performing numerically accurate quantum-dynamics calculations on
a model system. It is demonstrated that the non-Markovian effect originating
from the slow polaron formation strongly suppresses the electron transfer
reaction back to the interfacial charge-transfer state stabilized at the
donor/accepter interface and that it plays a critical role in maintaining the
long-range electron--hole separation.
