Optically Probing Field-Dependent Charge Dynamics in Non-Fullerene Organic Photovoltaics with Small Interfacial Energy Offsets

Current non-fullerene organic photovoltaics (OPVs) can achieve high photocurrent generation yields with small donor/acceptor energy offsets. However, the effect of varying energy offset on the charge dynamics in these systems is not fully understood. Here, we use optical spectroscopy to probe charge dynamics in three model non-fullerene systems with varying energy offsets. We find that systems with marginal and negative energy offsets show low photocurrent generation yields and significant radiative recombination from the exciton state. Improved photocurrent generation and suppressed exciton emission is observed when the device is under reverse bias. Our results indicate that photoexcitation of these systems leads to bound charge pairs at the donor/acceptor interface which tend to recombine via the singlet exciton state unless their separation is facilitated by external electric fields. Our results shed light on the charge separation and recombination dynamics in non-fullerene OPVs with small donor/acceptor energy offsets.