Trap-Mediated Energy Transfer of Localized Excitons for Efficient Mn2+ Luminescence in Highly Doped Cd-Based Perovskite Nanocrystals

Mn 2+ -doped nanomaterials are attractive for optoelectronic applications due to their characteristic dopant emission enabled by host-to-dopant energy transfer (ET). However, increasing Mn 2+ concentrations typically leads to severe photoluminescence (PL) quenching due to strong Mn 2+ -Mn 2+ interactions and the formation of nonradiative trap states. Here, we demonstrate that this challenge can be overcome in Cd-based perovskite nanocrystals (NCs) by exploiting localized excitons (LEs) coupled to a fast thermodynamic equilibrium with trap states. Ultrafast transient absorption spectroscopy suggests a rapid LE-trap equilibrium (∼100 ps) at room temperature. Mn 2 + doping further enhances exciton localization and redistributes the exciton population into trap states, which serve as intermediates for Dexter-type energy transfer, accelerating Mn 2 + excitation by over an order of magnitude compared to direct energy transfer from LEs. Consequently, efficient and thermally stable Mn 2+ emission is achieved at elevated dopant concentrations. The PL quantum yield exceeds that of the undoped NCs by more than 12-fold and retains 70% of its maximum value even at 20% Mn 2+ concentration in Cs 2 CdCl 4 :Mn 2+ NCs.