Revealing Exciton and Metal-Ligand Conduction Band Charge Transfer Absorption Spectra in Cu-Zn-In-S Nanocrystals

Copper indium sulfide quantum dots (QDs) have attracted substantial attention in recent years due to environmental issues and diverse applications. We report the synthesis and characterization of copper-zinc-indium-sulfide (CZIS) QDs and CZIS treated with excess Zn2+ at different temperatures, denoted here as CZIS/ZnS 100 and CZIS/ZnS 200. Zn2+ can diffuse into the lattice by an exchange-cation reaction, replacing Cu+ and In3+. We employed transient absorption (TA) spectroscopy to study the role of Zn2+ in the lattice. The data were treated by global analysis, which yielded the decay-associated spectra (DAS). Through DAS and secondorder derivative absorption spectra, we could for the first time isolate the excitonic contribution from the metal-ligand conduction band charge transfer (MLCBCT) absorption in the TA spectrum, finding the hole localization lifetime by its spectral and dynamical features. The hole localization lifetime increases from 0.3 to 1.7 ps by increasing the Zn2+ concentration. We also measured the electron-trapping constants to be dozens of picoseconds and nonradiative recombination larger than 1 ns. Finally, we concluded that suppression of the electron-trapping rate is not the only process responsible for increasing the photoluminescence quantum yield (PLQY); however, suppression of this process is important since it is the first step of the nonradiative recombination pathway. The detailed mechanism was proposed, and our results suggest that the introduction of Zn2+ in the lattice of copper indium sulfide (CIS) QDs could be beneficial for charge extraction.