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Ytterbium-doped all-inorganic lead-halide perovskites (Yb3+:CsPb(Cl1–xBrx)3) show broadband absorption and exceptionally high near-infrared photoluminescence quantum yields, providing opportunities for solar spectral shaping to improve photovoltaic power conversion efficiencies. Here, we report that Yb3+:CsPb(Cl1–xBrx)3NCs also show extremely strong negative thermal quenching of the Yb3+luminescence, with intensities at room temperature >100 times those at 5 K for some compositions. Analysis of this temperature dependence as a function of xshows that it stems from thermally activated quantum cutting related to the temperature dependence of the spectral overlap between the PL of the perovskite (donor) and the simultaneous-pair absorption of two Yb3+ions (acceptor). In the Yb3+:CsPbBr3limit, this spectral overlap goes to zero at 5 K, such that only single-Yb3+sensitization requiring massive phonon emission occurs. At room temperature, Yb3+PL in this composition is enhanced ∼135-fold by thermally activated quantum cutting, highlighting the extreme efficiency of quantum cutting relative to single-Yb3+sensitization. These results advance the fundamental mechanistic understanding of quantum cutting in doped perovskites, with potential ramifications for solar and photonics technologies.

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)3NCs. 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)3quantum cutting in lead-halide perovskites and provide deeper insight into the inner workings of this unique phenomenon.

CsPb(Cl1–xBrx)3(0 ≤ x≤ 1) nanocrystals and thin films doped with a series of trivalent rare-earth ions (RE3+= Y3+, La3+, Ce3+, Gd3+, Er3+, Lu3+) have been prepared and studied using variable-temperature and time-resolved photoluminescence spectroscopies. We demonstrate that aliovalent (trivalent) doping of this type universally generates a new and often-emissive defect state ca. 50 meV inside the perovskite band gap, independent of the specific RE3+dopant identity or of the perovskite form (nanocrystals vs thin films). Chloride-to-bromide anion exchange is used to demonstrate that this near-band-edge photoluminescence shifts with changing band-gap energy to remain just below the excitonic luminescence for all compositions of CsPb(Cl1–xBrx)3(0 ≤ x≤ 1). Computations show that this shift stems from the effect of the changing lattice dielectric constants on a shallow defect-bound exciton. Microscopic descriptions of this dopant-induced near-band-edge state and its relation to quantum cutting in Yb3+-doped CsPb(Cl1–xBrx)3are discussed.

Hydrothermal methods are used for the first time to synthesize distinct crystallographic stoichiometries within the potassium-lutetium-fluoride phase diagram for applications in solid-state laser refrigeration. Four crystalline phases were synthesized hydrothermally and doped with 10% Yb(III) ions, namely, orthorhombic K2LuF5(space group Pnma), trigonal KLuF4(space group P3121), orthorhombic KLu2F7(space group Pna21), and cubic KLu3F10(space group Fm3̅m), with each phase exhibiting unique microcrystalline morphologies. Among the four phases, the most significant cooling was observed for the KLuF4phase, which showed an overall refrigeration of 8.6 ± 2.1 K below room temperature. Laser refrigeration for KLuF4was measured by observing both the eigenfrequencies of optomechanical cantilevers in vacuum and also the Brownian dynamics of optically trapped microcrystals in water. Cooling was also observed for the first time for the K2LuF5phase in vacuum based on measurements of the mean luminescence wavelength of Yb(III) ions. Cooling was not observed with the other two phases.