Nonradiative Triplet Loss Suppressed in Organic Photovoltaic Blends with Fluoridated Nonfullerene Acceptors.

Abstract

In organic photovoltaic (OPV) blends, photogenerated excitons dissociate into charge-separated electrons and holes at donor/acceptor interfaces. The bimolecular recombination of spin-uncorrelated electrons and holes may cause nonradiative loss by forming the low-lying triplet excited states (T1) via the intermediate charge-transfer triplet states. Here, we show that such a spin-related loss channel can be suppressed in the OPV blends with fluorinated nonfullerene acceptors (NFAs). By combining ultrafast optical spectroscopy and triplet sensitization measurements, the T1 states at the acceptors have been observed to generate from the charge-separated electrons and holes in the OPV blends with a same polymer donor and two sets of NFAs with and without fluorination. The triplet formation is largely suppressed and the lifetime of charge carrier is markedly prolonged in the blends with fluorinated NFAs. The fluorination effect on the charge dynamics can be ascribed to the modified energy alignment between the triplet excited states of charge-transfer and locally excited characters as supported by quantum chemical computation. Our findings explain the mechanism responsible for the improved photocurrent generation in the OPV blends with fluorinated NFAs, suggesting that manipulating the energy landscape of triplet excited states is a promising strategy for further optimizing OPV devices.