Abstract

Y6-based nonfullerene organic solar cells (OSCs) have achieved an outstanding power conversion efficiency (PCE) of over 18% due to the low energy loss and efficient exciton dissociation with a very small energy offset. However, the exciton dissociation mechanism is under debate. It is still unclear why such small energy offset can result in sufficient exciton dissociation in nonfullerene systems but will lead to overwhelming charge recombination in fullerene systems. Here, we applied first-principles methods to study the charge transfer dynamics in both donor:Y6 and donor:C60 crystalline systems. Based on Ehrenfest molecular dynamics simulations, we proposed a five-step charge transfer process in nonfullerene system. We found that even with a small energy offset, the charge redistribution on both polymer backbone chain driven by Y6-induced dipole moment and Y6 backbone due to its intrinsic dipole moment can largely reduce Coulomb attraction of electron-hole pairs and thus facilitate exciton dissociation. Our study demonstrates that the dipole-driven charge redistribution and delocalization provide the driving force in Y6-based OSCs, which are missing in fullerene OSCs. Our study paves the way for developing next-generation OSC materials.