Trap-Mediated Sensitization Governs Near-Infrared Emission from Yb 3+ -Doped Mixed-Halide CsPbCl x Br 3- x Perovskite Nanocrystals.

Understanding the photosensitization mechanisms in Yb 3+ -doped perovskite nanocrystals is crucial for developing their anticipated photonic applications. Here, we address this question by investigating near-infrared photoluminescence of Yb 3+ -doped mixed-halide CsPbCl x Br 3- x nanocrystals as a function of temperature and revealing its strong dependence on the stoichiometry of the host perovskite matrix. To explain the observed experimental trends, we developed a theoretical model in which energy transfer from the perovskite matrix to Yb 3+ ions occurs through intermediate trap states situated beneath the conduction band of the host. The developed model provides an excellent agreement with experimental results and is further validated through the measurements of emission saturation at high excitation powers and near-infrared photoluminescence quantum yield as a function of the anion composition. Our findings establish trap-mediated energy transfer as a dominant photosensitization mechanism in Yb 3+ -doped CsPbCl x Br 3- x nanocrystals and open up new ways of engineering their optical properties for light-emitting and light-harvesting applications.