Photoluminescence Saturation in Quantum-Cutting Yb3+-Doped CsPb(Cl1-xBrx)3 Perovskite Nanocrystals: Implications for Solar Downconversion

Yb3+-doped halide perovskites have recently emerged as extra-ordinarily promising materials for solar spectral downconversion applications because of their extremely high photoluminescence quantum yields of nearly 200%, attributable to a highly efficient picosecond quantum-cutting process. One of the major roadblocks to widespread application of these materials is their photo-luminescence saturation under modest photoexcitation fluences. In this study, we examine the excitation-fluence dependence of Yb3+-doped CsPb (Cl1-xBrx)(3) nanocrystal photoluminescence to develop a quantitative understanding of this saturation. Facile saturation is observed across a multitude of halide and Yb3+ compositions, with specific trends that provide insight into the microscopic mechanism behind this saturation. We show that the data can be simulated well by a kinetic model that introduces a specific new Auger-type cross-relaxation process involving nonradiative energy transfer from photoexcited nanocrystals to Yb3+ ions that are already in their luminescent F-2(5/2) excited state from a previous photoexcitation event. This cross relaxation occurs with a subnanosecond rate constant, allowing it to compete with picosecond quantum cutting when excited-state Yb3+ is accumulated. These results point to specific strategies for ameliorating photoluminescence saturation in this class of materials, one of which is demonstrated experimentally. The proposed strategies provide guidance for future materials development and application efforts involving these materials.