Spin dependent polaron pair dissociation at charge transfer states enables low driving force for nonfullerene organic photovoltaic system

Abstract

A desirable driving force or dissociation energy greater than 0.300 eV is thought to be the prerequisite for an efficient dissociation of exciton in the organic bulk heterojunction photovoltaic system. This criterion has confronted the challenge in some nonfullerene acceptors (NFAs) based solar cells. Factors that govern the dissociation are still under debate. In this work, it is demonstrated that a large photocurrent can be produced by a NFA organic blend with a negligible driving force (0.070 eV) and a small dielectric constant (ε = 4). By evaluating the magnetic field dependent photocurrent density and photoluminescence, we postulate that the spin-dependent polaron pair dissociation at charge transfer states due to a significant singlet generation may prove critical for the photocurrent production. The driving force that originates from the energy offset may play a negligible role in the exciton dissociation.