Non-Markovian quantum-classical ratchet for ultrafast long-range electron-hole separation in condensed phases

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.