Mechanism of Quantum Cutting in Yb-Doped CsPbCl₃

Yb-doped CsPbCl₃ has been widely studied during the past few decades because of its high photoluminescence quantum yields (due to quantum cutting) and its application as a spectral converter in solar cells. Two controversial mechanisms for the quantum-cutting process have been proposed, both involving a defect state, but with different energy levels and energy-transfer mechanisms. In order to clarify the mechanism of the quantum-cutting process, different Yb configurations in CsPbCl₃ are studied with density-functional theory, the most favorable energetically being two Yb³⁺ ions along with a Pb vacancy distributed in a right-angle configuration. An additional Cl vacancy close to this impurity complex is then shown to be essential to create a shallow defect state that can enable quantum cutting. The important role of the Cl vacancy is also analyzed in quantum cutting in Yb-doped CsPbCl_3(1-Br.