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 negative electrostatic potential, facilitating charge localization favorable for hydrogen evolution. D18 also exhibits higher photoluminescence quantum yield and reduced non‐radiative recombination relative to PM6. Molecular dynamics simulations show that D18 forms stronger donor–acceptor interactions, stabilizing localized charges for proton reduction, whereas Y6 displays more higher acceptor–acceptor coordination, enabling efficient charge extraction in blends. Collectively, these findings establish the pivotal role of long‐lived, localized carriers in governing the charge dynamics of organic semiconductors and offer design principles for reconciling their dual photovoltaic and photocatalytic functionalities.