Re-excitation dynamics of a “cold” charge transfer state at organic donor/acceptor interfaces

By employing a femtosecond electric pump pulse, we theoretically investigate the re-excitation dynamics of a “cold” charge transfer (CCT) state at organic donor/acceptor (D/A) interfaces. It is demonstrated that a relaxed CCT state can be pushed to different “hot” CT (HCT) states via experiencing electron (HCT1 state) and/or hole (HCT2 state) higher-energy transitions, where the transition modes and probabilities are primarily determined by the pulse energy. Without the assistance of a charge driving field, both the two HCT states relax to the initial CCT state through different internal conversion processes, whose dynamics are clearly clarified in this work. However, after a driving field is applied, we find that both of the HCT states can be dissociated into free charges before their relaxations. In particular, the HCT2 state is very easily dissociated compared to the HCT1 state, as well as the CCT state, due to the more delocalized hole charge distribution along the donor. In addition, by enhancing the pulse intensity, we can further improve the hole delocalization along the donor so that the pulsed HCT2 state is more favorable to be dissociated. This work underlines the importance of charge delocalization for the interfacial charge dynamics, including both the internal conversion and charge separation, mediated by different intermediate HCT states in organic solar cells.