Carrier Dynamics in Nonfullerene Acceptor Organic Photovoltaics through Ultrafast Spectroscopy.

Abstract

Nonfullerene acceptors (NFAs) have transformed organic photovoltaics (OPVs) by enabling high efficiencies at small energetic offsets, but rational progress demands a mechanistic view of photoinduced processes across femtosecond-microsecond windows. This review synthesizes ultrafast spectroscopy─transient absorption, pump-push-probe, and two-dimensional electronic spectroscopy─to reveal how NFA molecular design and nanoscale structure govern exciton generation and transport, hybridization of local exciton-charge transfer states, delocalized singlet excitons, hot/endothermic charge separation, and geminate vs nongeminate recombination. Recent studies demonstrate how quadrupole-driven interfacial electrostatics, molecular crystallinity, and energetic disorder contribute to near-barrierless charge separation, long-range delocalization, and strongly non-Langevin carrier dynamics. The review concludes with a roadmap for spatiotemporally resolved, operando studies that integrate ultrafast probes with microscopy to directly correlate local structure with charge carrier dynamics in working NFA-based OPVs.