Ultrafast energy transfer beyond the Förster approximation in organic photovoltaic blends with non-fullerene acceptors

Recent studies on organic photovoltaic (OPV) systems have highlighted the critical role of energy transfer in excited-state dynamics. This process has traditionally been explained through the model of long-range Förster resonance energy transfer (FRET). In this study, we demonstrate a donor-to-acceptor short-range energy transfer (SRET) mechanism in OPV blends with non-fullerene acceptors, extending beyond the Förster approximation. This SRET occurs as a two-step process mediated by interfacial excitations with mixed charge-transfer and local excitation features. We further validate this model through studies on planar heterojunctions, precisely controlling the thickness of interlayers. These findings underscore the short-range interactions in regulating the donor-to-acceptor energy transfer in OPV blends, suggesting that SRET should be considered alongside FRET and charge-transfer processes for device optimizations.