Defect Tolerant Quantum Cutting in Mechanosynthesized Ytterbium-Doped Cesium Lead Chloride Perovskites

Ytterbium doped cesium lead halide materials exhibit a property known as quantum cutting which allows for greater than 100% photoluminescent quantum yields (PLQYs). The local atomic structure of the defects responsible for these properties and the effectiveness of the doping for producing the desired PLQYs is not readily discerned using techniques requiring long-range order. In this work we prepared 2.5, 5, 10, and 20% Yb³⁺ doped CsPbCl₃ powders using mechanosynthesis under distinct stoichiometric ratio conditions and characterized the defect incorporation and its effects on local atomic disorder using solid-state nuclear magnetic resonance (SSNMR) spectroscopy. We then correlate our observations to the observed PLQYs for each of the prepared samples. All samples prepared were found to be in an orthorhombic phase and no lattice shrinking was observed upon increased Yb³⁺ doping. An increase in doping concentrations was accompanied by a decrease in ¹³³Cs NMR spin-lattice relaxation times <i>T</i> ₁ consistent with a paramagnetic relaxation enhancement effect induced by Yb³⁺ incorporation into the perovskite lattice. Through a comparison of synthesis methods, PLQY and NMR <i>T</i> ₁ parameters we found that incorporated defects favorable for PLQY in mechanosynthesized samples are more likely to form in the presence of excess lead and excess chloride ions. The maximum PLQY values obtained for each set of samples correlated with <i>T</i> ₁ parameters in the range of 13 to 35 s. In addition, we found that the observed PLQY in 5% doped samples was optimized after 1 to 2 h of interval grinding in stainless steel jars. Further grinding beyond 2 h led to a reduction in particle size below 1 μm as well as a reduction in PLQY and spin relaxation times.