Exciton 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. Here, we applied nonadiabatic molecular dynamics simulations to study the charge transfer dynamics in both donor:Y6 and donor:C60 crystalline systems. Based on our simulations, we proposed a five-step charge transfer process in nonfullerene systems, which is consistent with experimental findings. We found that the efficient exciton dissociation with a small energy offset can be attributed to the charge redistribution occurring on both the polymer and the Y6 backbones, driven by the interactions between the donor polymer and the Y6 molecule. This mechanism significantly reduces the Coulomb attraction in the local excitions; however, it 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.