Shell-Free CuInS₂ Nanocrystals with Near-Unity Photoluminescence for Deep-Red LEDs

CuInS₂ quantum dots (CIS-QDs) are promising nontoxic and air-stable materials that can be readily synthesized through a controllable heat-up reaction between metal cation precursors and a sulfur source, enabling tunable photoluminescence (PL) across the visible to near-infrared range. However, their broader application in light-emitting diodes (LEDs) is limited by inefficient radiative recombination and a photoluminescence quantum yield (PLQY) significantly below unity. To address this challenge, we introduce formamidinium acetate (FAAc) into the reactiona common additive in metal halide perovskite precursor solutions. FAAc modulates precursor chemistry by forming complexes that control the size and bandgap of the resulting nanocrystals, without significantly altering their crystal structure. We also find that FAAc regulates the stoichiometry, inducing substantial Cu-(I) deficiency and a corresponding decrease in the lattice work function. These effects, combined with the potential passivation of surface defects by nitrogen-containing byproducts of FAAc decomposition, contribute to a dramatic enhancement of PLQY, from 43% to 94%, and an increase in PL lifetime from 0.2 to 7.2 μs. Proof-of-concept LED devices incorporating FAAc-modified CIS-QDs exhibit bright red emission, demonstrating FAAc as an effective additive for engineering the electroluminescence of CIS-QDs. We propose that this strategy could be extended to other ternary quantum dots to enable high-performance optoelectronic applications.