Dual Impact of Long‐Lived Localized Carriers Induced by Restricted Hole Transfer in Organic Photovoltaics and Photocatalytic Hydrogen Evolution

ABSTRACT Organic semiconductors are key photoactive materials for solar energy conversion, including organic photovoltaics (OPVs) and photocatalytic hydrogen evolution. Here, we uncover a paradox: materials optimized for efficient charge extraction in OPVs exhibit different behavior when used as photocatalysts in aqueous colloids. To probe this contrast, we study nanoparticles (NPs) prepared from PM6:Y6 and its structural analogue D18:Y5, comparing their photophysical responses across photovoltaic and photocatalytic regimes. We observe an inverse relationship between power conversion efficiency in thin films and hydrogen evolution activity in dispersions. Y5‐based NPs show stronger aggregation, longer exciton lifetimes, and slower hole transfer, indicative of localized charges that promote interfacial redox chemistry. Density functional theory calculations reveal that Y5 possesses more n…