Hole Localization and Ultrafast Electron Transfer Dynamics in CuInS₂/Organic Molecule Composite

Exploring the dynamics of photoexcited electrons and holes in green I-III-VI ternary quantum dots (QDs) is of utmost necessity to design and improve their functionality and applicability in devices. Here, we have studied the dynamics of photoexcited nascent carriers in CuInS₂/ZnS (CISZ) core-shell QDs as potential excited state electron donors using picosecond time-resolved emission and femtosecond transient absorption (fsTA) spectroscopy techniques. The emission decay measurements conducted in the 460-660 nm wavelength range allowed us to distinguish between emission due to electron-hole recombination at conduction (CB) and valence bands (VB), respectively, and due to recombination of the CB electrons and holes at the self-trapped states (Cu¹⁺ centers). The excited state electron transfer (ET) was studied by attaching an anthraquinone-2-carboxylic acid (AQ) electron acceptor to the QDs. The ET from the CB of CISZ QD to AQ was confirmed by the fsTA studies, which revealed the formation of a characteristic anion (AQ^-) band at 600 nm. The ET time constant was estimated to be 1.5 ps in ideal one-to-one CISZ-AQ complex, and the charge recombination takes place with time constant >5 ns at delay times beyond the fsTA Instruments reach. We propose that such a long lifetime of the charge-separated state is achieved after hole localization at the (Cu¹⁺) defect, which effectively decouples the hole at the QD and the electron at AQ.