CuInS2 quantum dots have been studied in a broad range of applications, but despite this, the fine details of their charge carrier dynamics remain a subject of intense debate. Two of the most relevant points of discussion are the hole dynamics and the influence of Cu:In synthesis stoichiometry. It has been proposed that Cu-deficiency leads to the formation of Cu2+, affecting the localization of holes into Cu defects. Importantly, it is precisely these confined hole states that are used to explain the interesting photoluminescence properties of CuInS2 quantum dots. We use static X-ray spectroscopy to show no evidence for a measurable amount of native Cu2+ states in Cu-deficient samples (above 20%). Instead, the improved properties of these samples are explained by an increase of crystallinity, reducing the concentration of mid-gap states. Furthermore, to understand the charge carrier dynamics, herein, we employ ultrafast optical transient absorption and fluorescence up-conversion spectroscopies in combination with ultrafast X-ray absorption spectroscopy using a hard X-ray free electron laser. We demonstrate that in nonpassivated samples, holes are transferred from Cu atoms on subpicosecond time scales. Finally, we observe that Cu-deficient samples are more robust against photothermal effects at higher laser fluences. This is not the case for the Cu-rich sample, where heating effects on the structure are directly observed.
Keywords: CuInS2 quantum dots; X-ray absorption; XFEL; charge carrier dynamics; hole trapping; transient absorption.