  Yb3+-doped inorganic metal-halide perovskites (Yb3+:CsPbX3, X = Cl, Br) have
recently been discovered to display highly efficient quantum cutting, in which
the energy from individual blue or UV photons absorbed by the material is
re-emitted in the form of pairs of near-infrared photons by Yb3+ dopants.
Experimental photoluminescence quantum yields approaching 200{%} have been
reported. As the first quantum-cutting materials that combine such high
photoluminescence quantum yields with strong, broadband absorption in the
visible, these materials offer unique opportunities for enhancing the
efficiencies of solar technologies. Little is known about the fundamental
origins of this quantum cutting, however. Here, we describe
variable-temperature and time-resolved photoluminescence studies of
Yb3+:CsPbCl3 in two disparate forms - colloidal nanocrystals and macroscopic
single crystals. Both forms show very similar spectroscopic properties,
demonstrating that quantum cutting is an intrinsic property of the Yb3+:CsPbX3
composition itself. Diverse Yb3+ speciation is observed in both forms by
low-temperature photoluminescence spectroscopy, but remarkably, quantum cutting
is dominated by the same specific Yb3+ species in both cases. Time-resolved
photoluminescence measurements provide direct evidence of the previously
hypothesized intermediate state in the quantum-cutting mechanism. This
intermediate state mediates relaxation from the photogenerated excited state of
the perovskite to the emissive excited state of Yb3+, and hence is of critical
mechanistic importance. At room temperature, this intermediate state is
populated within a few picoseconds and has a decay time of only ~ 7 ns in both
nanocrystalline and single-crystal Yb3+:CsPbCl3. The mechanistic implications
of these observations are discussed.
