Long-Lived Dark Excited States Enable Efficient Charge Generation in Photocatalytic Organic Nanoparticles

Organic semiconductor nanoparticles (NP's) have recently emerged as promising photocatalysts for efficient and sustainable hydrogen production owing to their exceptional performance and cost-effectiveness compared with traditional wide-bandgap metal oxides. However, further advancements are often hindered by their limited excited-state lifetimes, which do not match the kinetics of the catalytic processes. In this study, a comprehensive investigation of the feasibility of utilizing long-lived dark excited states, i.e., triplets and charge-transfer-like excitons (CTE), in non-fullerene acceptors is performed to boost the photogeneration of charges in photocatalytic NP's, bypassing the conventional photoconversion process involving short-lived excited species. Two distinct photophysical routes are investigated: cold charge generation from loosely bound CTEs and direct charge transfer from highly populated triplet states. Ultrafast spectroscopic studies show that both routes demonstrate slow yet efficient polaron generation from their triplet excited states, even without the aid of heterojunctions. Additionally, single-component and heterojunction NP's exhibit significantly different ambipolar mobilities, and donor materials are required to achieve balanced charge transport. These findings highlight the potential of utilizing triplet excited states to overcome the limitations of organic photocatalysts in the development of highly efficient and scalable hydrogen evolution systems.