Polarized local excitons assist charge dissociation in Y6-based nonfullerene organic solar cells: a nonadiabatic molecular dynamics study

Y6-based nonfullerene organic solar cells (OSCs) have achieved an outstanding power conversion efficiency (PCE) of over 19% due to the low energy loss and high exciton dissociation efficiency with a small energy offset. However, the exciton dissociation mechanism is still under debate. It is unclear why a small energy offset can lead to efficient exciton dissociation in nonfullerene systems, but causes significant charge recombination in fullerene ones. Most of the previous theoretical studies have focused on the static properties of donor-acceptor heterojunctions, while neglecting excited electron dynamics and nonadiabatic effects. Here, we applied nonadiabatic molecular dynamics simulations to study the charge transfer dynamics in both donor:Y6 and donor:C60 crystalline systems. We found that thermal effects do not significantly influence the exciton dissociation in the NT-4T-FF:Y6 system, which aligns with experimental observations. Based on our simulations, we identified a five-step charge transfer process in nonfullerene systems. While previous studies suggested electrostatic interfacial fields from non-fullerene small molecule acceptors, our research reveals that strong donor-acceptor interactions primarily affect the local exciton states rather than the ground state. Consequently, the polarized local excitons play a key role in reducing the Coulomb attraction between electrons and holes, thus facilitating exciton dissociation with a small energy offset. In contrast, this mechanism is not observed in fullerene OSC systems. Our findings provide a fundamental basis for the further development of novel OSC materials with the potential for achieving even higher PCE. Our study underscores the pivotal significance of charge polarization in local-exciton states for exciton dissociation in polymer/Y6 organic solar cells with a small energy offset. This phenomenon is not observed in polymer/fullerene systems.