Mechanistic Insights Into Quantum-Cutting in Yb³⁺-Doped CsPbCl₃ Nanocrystals

Yb³⁺-doped CsPbCl₃ nanocrystals exhibit an exceptionally efficient quantum-cutting, converting single UV photons into a pair of near-infrared photons with a photoluminescence quantum yield approaching 200%, offering broad opportunities for optoelectronic and photonic applications. Recent studies highlighted the importance of Yb³⁺-induced defect states in mediating energy transfer from the host to Yb³⁺ multiplets. However, the persistent observation of thermally activated Yb³⁺ emission even when the defect-state energy lies well above the threshold for quantum-cutting remains intriguing and unresolved. Based on temperature-dependent emission measurements, we establish that the thermal activation of Yb³⁺ emission is universal and introduce temperature-dependent rate equations that quantitatively reproduce all experimental observations. The detailed analyses demonstrate that the observed thermal activation is the combined effect of the thermally activated defect state population and the presence of 15 phonon-assisted multiphonon processes, presenting a phonon bottleneck at low temperatures, and thereby suppressing the dissipation of excess energy to lattice phonons. Furthermore, photon-correlation analyses, performed here for the first time to probe the quantum nature of the Yb³⁺ emission in CsPbCl₃ nanocrystals, unambiguously establish the decoherence of the emitted photons due to the large time separations between the cooperative excitation of Yb³⁺ ions (∼ns) and their long-lived emission (∼2.7 ms).