One-Pot Synthesis of Luminescent Ag–Cu–Ga–S/ZnS Quantum Dots Bridging the Cyan Gap for Ultrahigh-Color-Rendering White-Light-Emitting Diodes

I–III–VI semiconductor quantum dots (QDs) are gaining attention as ecofriendly, compositionally tunable alternatives to cadmium-based QDs. However, precise control over emission wavelength and strong luminescence remains challenging, as QDs often suffer from poor surface passivation and suppressed radiative recombination. These limitations have hindered their photoluminescence efficiency and emission control across the visible spectrum. White-light-emitting diodes (WLEDs), widely used in solid-state lighting, typically combine blue-LED chips with yellow phosphors like YAG:Ce³⁺; however, this approach limits spectral coverage and color rendering. Specifically, achieving bright and stable cyan emission remains critical for addressing the well-known “cyan gap” and significantly enhancing spectral completeness. In this study, a facile one-pot synthesis was developed to prepare quaternary AgCuGaS/ZnS (ACGS/ZnS) core/shell QDs. Optimization of the Ag/Cu stoichiometry and ZnS shelling enabled bright cyan emission with a photoluminescence quantum yield (PLQY) of 56%. Moreover, optimized QDs exhibited excellent stability under ambient conditions. Notably, the QD–poly(methyl methacrylate) composite showed superstability under 85% relative humidity at 85 °C, highlighting its suitability for practical color-conversion applications. The optimized cyan-emitting ACGS/ZnS QDs were integrated into WLEDs, effectively bridging the cyan gap (∼480–520 nm) in conventional YAG:Ce³⁺-based devices. Remarkably, the fabricated WLED demonstrated significantly enhanced color rendering, achieving an ultrahigh color-rendering index (Ra = 96). These findings highlight the strong potential of ACGS/ZnS QDs as efficient cyan-emitting components for constructing full-spectrum WLEDs with enhanced color-rendering performance.