The limiting factors of quantum-cutting efficiency of ytterbium-doped lead halide perovskites: dimeric and monomeric ytterbium species

Abstract

Ytterbium-doped cesium lead halide perovskites exhibit unusual down-conversion with quantum efficiencies exceeding 100%. This phenomenon has been attributed to quantum cutting (QC) when a perovskite exciton is converted into two Yb³⁺ excitations. However, the mechanism of energy transfer from the perovskite to the Yb³⁺ ions is still debated, and it is still unclear what processes limit the conversion efficiency. Here, Yb³⁺-doped CsPbX₃ (X: Cl⁻, Br⁻, or both) perovskite samples prepared by different techniques were investigated. Electron paramagnetic resonance (EPR) measurements were applied to assess ytterbium incorporation into the perovskite matrix and ultrafast time-resolved fluorescence, together with microscopic studies, to evaluate the QC efficiency limiting factors. It is demonstrated that two types of ytterbium species are formed, depending on the concentration of Yb³⁺. The photoluminescence (PL) of ytterbium shows a biexponential decay at low concentrations, attributed to the presence of monomeric and dimeric ytterbium species. The fast decay component is attributed to monomeric Yb³⁺ ions, which do not perform QC and have a low PL yield. This component disappears when the nominal concentration of ytterbium increases to about 5% and the PL of dimeric species dominates. The PL decay becomes faster again when the ytterbium concentration exceeds about 10%, due to migration limited excitation quenching. These two processes are important QC efficiency limiting factors.