(Invited) Role of the Energy Offset in the Charge Separation at the Donor: Acceptor Interface

Abstract

The trade-off between the short-circuit current density and open-circuit voltage has been one of the largest bottlenecks for further improving the device performance of organic solar cells (OSCs). Therefore, understanding the limit to which the energy offset can be reduced and the physics underlying the trade-off relationship is crucial. This study provides a threshold energy that can ensure high charge photogeneration efficiencies for Y-series nonfullerene acceptor-based OSCs and discusses the role of the energy offset in charge separation. In this study, we used transient absorption spectroscopy to track the time evolution of electroabsorption caused by electron-hole pairs generated at donor:acceptor interfaces. We found that an insufficient energy offset led to not only slow charge transfer at the donor:acceptor interfaces, but also inefficient long-range spatial dissociation of the charge transfer states. Our findings highlight the importance of overcoming the trade-off between fill factor and open-circuit voltage for further improving the power conversion efficiency.