Unraveling the Temperature Dependence of Exciton Dissociation and Free Charge Generation in Nonfullerene Organic Solar Cells

Abstract

Organic solar cells based on nonfullerene acceptor molecules show high charge generation yields with negligible driving force at the donor–acceptor (D/A) interface to drive exciton dissociation. Understanding the underlying charge generation dynamics in these material systems is crucial for further development of this technology. Herein, the acceptor exciton dissociation dynamics in these materials is studied using transient optical spectroscopy. The results show that exciton dissociation at the D/A interface takes up to ≈100 ps to complete, and this process is not significantly affected by temperature. A similar timescale for charge transfer (CT) state separation into free electrons and holes is observed at room temperature. But in contrast to the weak temperature dependence of exciton dissociation, the free charge generation rate and yield are significantly reduced at low temperature. This suggests that overcoming the Coulomb barrier for charge separation at the D/A interface is the main reason for the endothermic charge separation observed for these material systems, instead of diffusion-limited exciton dissociation.