Negative Thermal Quenching in Quantum-Cutting Yb3+-Doped CsPb(Cl1-Br)3 Perovskite Nanocrystals.Ytterbium-doped all-inorganic lead-halide perovskites (Yb3+:CsPb(Cl1-Br)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-Br)3 NCs 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 x shows 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).  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. 
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