Evolution of Yb³⁺ Speciation in Cl^-/Br^-- and Yb³⁺/Gd³⁺-Alloyed Quantum-Cutting Lead-Halide Perovskite Nanocrystals

Ytterbium-doped all-inorganic lead-halide perovskites (Yb3+:CsPb(Cl1-xBrx)(3)) generate near-infrared photoluminescence (PL) quantum yields exceeding 100% by quantum cutting. Experimental and computational studies have suggested complex dopant speciation in these materials arising from the formation of lattice defects needed to compensate for the excess charge of Yb3+ relative to Pb2+, but the relationship between quantum cutting and such speciation is still poorly understood. Here, we use cryogenic photoluminescence spectroscopy and density functional theory-assisted kinetic Monte Carlo simulations to investigate changes in Yb3+ speciation induced by anion (Cl- vs Br-) and trivalent-dopant (Yb3+ vs Gd3+) alloying in CsPb(Cl1-xBrx)(3) (0.00 <= x <= 1.00) perovskite nanocrystals (NCs). The experimental results reveal nonstatistical distributions of Yb-Cl and Yb-Br bonds in Yb3+:CsPb(Cl1-xBrx)(3) NCs. Monte Carlo simulations reproduce the experimental trends well and predict thermodynamic favorability for Yb3+ dopants to retain Cl- coordination, even in the presence of high lattice Br- concentrations. For a given lattice composition (e.g., CsPbCl3), low-temperature PL spectra reveal that the relative populations of three dominant Yb3+ species change substantially with Gd3+ codoping. These results further show that quantum cutting is largely insensitive to these differences in Yb3+ speciation, ruling out any "magic" configuration of Yb3+ ions and defects. These results are discussed in relation to the microscopic prerequisites for the concerted sensitization of two Yb3+ ions during quantum cutting. Overall, these findings highlight the mechanistic robustness of Yb3+:CsPb(Cl1-xBrx)(3) quantum cutting in lead-halide perovskites and provide deeper insight into the inner workings of this unique phenomenon.