Ultrafast Transient Electroabsorption Illuminates Additive Effects for Enhancing Non-fullerene Photovoltaic Devices

Additives are crucial in optimizing organic photovoltaic (OPV) performance, yet their influence on intrinsic photophysical properties remains underexplored. Here we use sub-55 fs transient absorption spectroscopy to extract the transient electroabsorption (TEA) response─a sensitive and contactless probe of electronic coupling─in Y6 films processed with 0.5% 1,8-diiodooctane (DIO) or 1-chloronaphthalene (CN). The DIO-treated film exhibits a primarily first-derivative-like signal, whereas the CN-treated film displays a second-derivative-like response, indicating enhanced delocalized charge transfer character and stronger core-core interactions. Additionally, the TEA rise time in CN-treated films is an order of magnitude shorter than in DIO-treated ones (0.2 ps vs 2 ps). Importantly, these TEA dynamics quantitatively mirror charge generation rates in PM6:Y6 blends, establishing TEA as a predictive metric of OPV performance. Our results uncover a direct nanomorphology-dynamics-function relationship and offer a powerful framework for rational additive design and selection in next-generation OPVs.