Picosecond Quantum Cutting Generates Photoluminescence Quantum Yields Over 100% in Ytterbium-Doped CsPbCl3 Nanocrystals

Recent advances in the ytterbium doping of CsPbX₃ (X = Cl or Cl/Br) nanocrystals have presented exciting new opportunities for their application as downconverters in solar-energy-conversion technologies. Here, we describe a hot-injection synthesis of Yb³⁺:CsPbCl₃ nanocrystals that reproducibly yields sensitized Yb³⁺²F_5/2 → ²F_7/2 luminescence with near-infrared photoluminescence quantum yields (PLQYs) well over 100% and almost no excitonic luminescence. Near-infrared PLQYs of 170% have been measured. Through a combination of synthesis, variable-temperature photoluminescence spectroscopy, and transient-absorption and time-resolved photoluminescence spectroscopies, we show that the formation of shallow Yb³⁺-induced defects play a critical role in facilitating a picosecond nonradiative energy-transfer process that de-excites the photoexcited nanocrystal and simultaneously excites two Yb³⁺ dopant ions, i.e., quantum cutting. Energy transfer is very efficient at all temperatures between 5 K and room temperature but only grows more efficient as the temperature is elevated in this range. Our results provide insights into the microscopic mechanism behind the extremely efficient sensitization of Yb³⁺ luminescence in CsPbX₃ nanocrystals, with ramifications for future applications of high-efficiency spectral-conversion nanomaterials in solar technologies.