Long-Lived Charge Carriers in Organic Semiconductor Blends Upon Surface Contact With Water

Abstract

Organic semiconductor materials are attracting interest for their tunable energy levels and are used in organic photovoltaics and photocatalysts for hydrogen production. Photocatalysts are typically employed as nanoparticles dispersed in water, making interactions with surrounding water molecules a key factor in photophysical studies. In fact, long-lived charge carriers are observed in nanoparticles but not in thin films, and this may be due to surrounding water, yet the detailed mechanism remains unclear. In this study, we focus on the specific surface area and investigate how water affects charge carrier dynamics within the system. To this end, we prepare films of 35 nm (high specific surface area) and 130 nm (bulk-like) thickness, and compare the effects of water on charge carrier dynamics in these systems using transient absorption spectroscopy. We find that water prolongs the lifetime of charge carriers in 35 nm films, whereas its effect is negligible in 130 nm films. This result suggests that the impact of water is confined to the surface rather than being widespread over the entire film. We therefore conclude that only the charge carriers near the surface of thin films or nanoparticles have prolonged lifetimes due to the influence of the surrounding water.