Dose-Dependent and Irreversible Photodarkening of InP/ZnSe/ZnS Quantum Dots

Indium phosphide-based quantum dots (QDs) are leading Cd- and Pb-free alternatives for photonic applications spanning 400-920 nm. However, their long-term photostability under prolonged illumination remains poorly understood. In this study, we systematically investigate the optical response of colloidal InP/ZnSe/ZnS core/shell/shell QDs exposed to white light under a dynamic inert atmosphere (glovebox filled with circulating N₂ with O₂ < 0.1 ppm, H₂O < 1.0 ppm). Upon illumination, the QDs exhibit irreversible photodarkening, defined as a reduction in photoluminescence quantum yield with a nearly unchanged absorption spectrum. Photodarkening shows a linear dependence on absorbed photon dose, with photoexcitation deep into the conduction band inducing it more rapidly than resonant photoexcitation. Additionally, prolonged photoexposure leads to an emission redshift and an enhanced delayed-emission component, consistent with the selective photodarkening of individual QDs. Experiments in air and controlled humidity (10% and 90% relative humidity) indicate that photodarkening primarily arises from photooxidation by oxygen, whereas water promotes QD photoetching. This is further corroborated by control experiments in which the QDs are illuminated under dry oxygen (<3 ppm of H₂O) and in a cuvette sealed by fusion under a N₂ atmosphere with <0.1 ppm of O₂ and <1 ppm of H₂O. These experiments show that photodarkening does not occur in QDs contained in the fusion-sealed cuvette, while under dry oxygen the photodarkening rates of the QDs are even faster than in dry air. These findings advance understanding of InP-based QD photodegradation and provide a framework for quantifying their stability under light stress.