Negative Thermal Quenching in Quantum-Cutting Yb 3+ -Doped CsPb(Cl 1- x Br x ) 3 Perovskite Nanocrystals.

Ytterbium-doped all-inorganic lead-halide perovskites (Yb ³⁺ :CsPb(Cl _{1- x} Br _x ) 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 Yb ³⁺ :CsPb(Cl _{1- x} Br _x ) ₃ NCs also show extremely strong negative thermal quenching of the Yb ³⁺ 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 Yb ³⁺ ions (acceptor). In the Yb ³⁺ :CsPbBr ₃ limit, this spectral overlap goes to zero at 5 K, such that only single-Yb ³⁺ sensitization requiring massive phonon emission occurs. At room temperature, Yb ³⁺ PL in this composition is enhanced ∼135-fold by thermally activated quantum cutting, highlighting the extreme efficiency of quantum cutting relative to single-Yb ³⁺ sensitization. These results advance the fundamental mechanistic understanding of quantum cutting in doped perovskites, with potential ramifications for solar and photonics technologies.