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Yb3+-doped inorganic metal-halide perovskites (Yb3+:CsPbX3, X = Cl, Br) haverecently been discovered to display highly efficient quantum cutting, in whichthe energy from individual blue or UV photons absorbed by the material isre-emitted in the form of pairs of near-infrared photons by Yb3+ dopants.Experimental photoluminescence quantum yields approaching 200{%} have beenreported. As the first quantum-cutting materials that combine such highphotoluminescence quantum yields with strong, broadband absorption in thevisible, these materials offer unique opportunities for enhancing theefficiencies of solar technologies. Little is known about the fundamentalorigins of this quantum cutting, however. Here, we describevariable-temperature and time-resolved photoluminescence studies ofYb3+:CsPbCl3 in two disparate forms - colloidal nanocrystals and macroscopicsingle crystals. Both forms show very similar spectroscopic properties,demonstrating that quantum cutting is an intrinsic property of the Yb3+:CsPbX3composition itself. Diverse Yb3+ speciation is observed in both forms bylow-temperature photoluminescence spectroscopy, but remarkably, quantum cuttingis dominated by the same specific Yb3+ species in both cases. Time-resolvedphotoluminescence measurements provide direct evidence of the previouslyhypothesized intermediate state in the quantum-cutting mechanism. Thisintermediate state mediates relaxation from the photogenerated excited state ofthe perovskite to the emissive excited state of Yb3+, and hence is of criticalmechanistic importance. At room temperature, this intermediate state ispopulated within a few picoseconds and has a decay time of only ~ 7 ns in bothnanocrystalline and single-crystal Yb3+:CsPbCl3. The mechanistic implicationsof these observations are discussed.

Yb3+ speciation and energy-transfer dynamics in quantum-cutting Yb3+-doped CsPbCl3 perovskite nanocrystals and single crystals