Defect-Driven Ultrafast Carrier Dynamics in Copper-Based Chalcogenide Alloy Quantum Dots

I–III–VI quantum dots (QDs) and their derivatives represent environmentally friendly alternatives that have garnered significant research interest. These QDs exhibit high defect tolerance, and their optical properties are profoundly influenced by defects, though the underlying luminescence mechanisms remain incompletely understood. This study aims to elucidate the impact of varying Cu/In ratios and Zn incorporation on the luminescent properties of copper indium sulfide (CIS) and copper-zinc-indium-sulfide (CIZS) QDs, and to reveal their excited-state carrier dynamics. We synthesized Cu-rich and Cu-deficient CIS QDs using low-reactivity precursors. Subsequently, cation exchange was performed on these QDs using low-reactivity Zn2+ precursors to yield Cu-rich and Cu-deficient CIZS QDs, respectively. Analysis of the composition, structure, and optical properties of the synthesized QDs revealed that Zn2+ preferentially substitutes for In3+ in Cu-rich CIS QDs, leading to an increased Cu/In ratio in the resulting CIZS QDs. Conversely, Zn2+ preferentially substitutes for copper ions in Cu-deficient CIS QDs, resulting in a decreased Cu/In ratio in the CIZS QDs derived from them. Concomitant with the reduction in Cu/In ratio and Zn incorporation, the QDs exhibited significant optical optimization: a blue shift of 173 nm in the photoluminescence (PL) emission wavelength, a 47.6 nm narrowing of the full width at half-maximum (fwhm), a 76 nm reduction in the Stokes shift, and a substantial enhancement of the photoluminescence quantum yield (PL QY) up to 90%. The concentration of Cu vacancies (V′Cu) is identified as the key factor governing the luminescent properties of both CIS and CIZS QDs. The incorporation of Zn2+ improves the local cationic ordering in QDs, which not only promotes efficient hole localization but also inhibits nonradiative recombination channels, so as to promote radiative recombination between delocalized conduction band electrons and localized holes─identified as one of the primary mechanisms for the observed PL enhancement. Multirecombination pathways are analyzed in the as synthesized QDs. This work provides new insights for the precise modulation of defect structures and carrier dynamics in CIZS QDs.