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

Ytterbium-doped all-inorganic lead-halide perovskites (Yb 3+:CsPb(Cl 1– x Br x ) 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 Yb 3+ relative to Pb 2+, 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 Yb 3+ speciation induced by anion (Cl – vs Br – ) and trivalent-dopant (Yb 3+ vs Gd 3+ ) alloying in CsPb(Cl 1– x Br x ) 3 (0.00 ≤ x ≤ 1.00) perovskite nanocrystals (NCs). The experimental results reveal nonstatistical distributions of Yb–Cl and Yb–Br bonds in Yb 3+:CsPb(Cl 1– x Br x ) 3 NCs. Monte Carlo simulations reproduce the experimental trends well and predict thermodynamic favorability for Yb 3+ dopants to retain Cl – coordination, even in the presence of high lattice Br – concentrations. For a given lattice composition (e.g., CsPbCl 3 ), low-temperature PL spectra reveal that the relative populations of three dominant Yb 3+ species change substantially with Gd 3+ codoping. These results further show that quantum cutting is largely insensitive to these differences in Yb 3+ speciation, ruling out any “magic” configuration of Yb 3+ ions and defects. These results are discussed in relation to the microscopic prerequisites for the concerted sensitization of two Yb 3+ ions during quantum cutting. Overall, these findings highlight the mechanistic robustness of Yb 3+:CsPb(Cl 1– x Br x ) 3 quantum cutting in lead-halide perovskites and provide deeper insight into the inner workings of this unique phenomenon.