Core-Localized Cu Dopants Pin Red Emission in Multinary Ag-Based Quantum Dots

Alloy-disordered I-III-VI quantum dots often trade spectral stability for efficiency, limiting photon-transport devices through reabsorption. Here, we confine Cu(I) dopants inside Ag-In-Ga-S cores during GaS_x overgrowth, verified by Cu-valence fingerprints and quantitative elemental mapping showing predominantly core-enriched Cu distribution, and obtain red emission spectrally pinned with a constant Stokes shift (∼140 meV) across growth and maturation. Single-dot spectroscopy resolves symmetric Lorentzian lines down to ∼62 meV, showing that the broad ensemble band is dominated by population inhomogeneity rather than an intrinsically broad dopant transition. First-principles calculations identify substitutional Cu_Ag as a low-energy defect forming an acceptor-like, Cu-S p-d hybridized valence-edge manifold, rationalizing the pinning. The resulting dots deliver photoluminescence quantum yields up to 85% and enhance luminescent solar concentrators to an optical efficiency of 7.33% by mitigating reabsorption.