Mechanism for the Extremely Efficient Sensitization of Yb³⁺Luminescence in CsPbCl₃ Nanocrystals

Rare earth ion (RE³⁺)-doped inorganic CsPbX₃ (X = Cl or Cl/Br) nanocrystals have been presented as promising materials for applications in solar-energy-conversion technology. An extremely efficient sensitization of Yb³⁺ luminescence in CsPbCl₃ nanoparticles (NCs) was very recently demonstrated where quantum cutting is responsible for the performance of photoluminescence quantum yields over 100% ( T. J. Milstein, et al. Nano Letters 2018 , 18 , 3792 ). In the present work, based on the cubic phase of inorganic perovskite, we seek to obtain atom-level insight into the basic mechanisms behind these observations in order to boost the further development of RE³⁺-doped CsPbX₃ NCs for optoelectronics. In our calculations of cubic crystal structure, we do not find any energy level formed in the middle of the band gap, which disfavors a mechanism of stepwise energy transfer from the perovskite host to two Yb³⁺ ions. Our work indicates that the configuration with "right-angle" Yb³⁺-V_Pb-Yb³⁺ couple is most likely to form in Yb³⁺-doped CsPbCl₃. Associated with this "right-angle" couple, the "right-angle" Pb atom with trapped excited states would localize the photogenerated electrons and act as the energy donor in a quantum cutting process, which achieves simultaneous sensitization of two neighboring Yb³⁺ ions.