Direct Electron Transfer Enables Highly Efficient Dual Emission Modes of Mn²⁺-Doped Cs₂Na_1-AgBiCl₆ Double Perovskites

Double perovskites with bright emission, low toxicity, and excellent stability have drawn considerable attention. Herein, we report the hydrothermal synthesis of Mn²⁺-doped Cs₂Na_1-AgBiCl₆ double perovskites that exhibit dual emission modes. Introducing Ag⁺ ions to Cs₂NaBiCl₆ samples enables a bright self-trapped exciton (STE) emission in orange-red color, whereas Mn²⁺ dopants induce a yellow-orange emission. Importantly, Mn²⁺ doping into Cs₂Na_1-AgBiCl₆ double perovskites with an indirect bandgap enables a high photoluminescence quantum yield of 49.52 ± 2%. Density functional theory calculations reveal that bringing Ag⁺ ions into Cs₂NaBiCl₆ can localize wave function to the [AgCl₆]^5- octahedron and convert dark transitions to bright STE transitions. Moreover, the 3d orbitals of Mn²⁺ dopants hybridize with Bi-6p and Cl-3p orbitals at the conduction band minimum, resulting in direct electron transfer from the host to Mn²⁺ and a significant increase in photoluminescence efficiency. These results shed light on the optical physical process of Mn²⁺-doped systems, providing useful information for further improvement of the photoluminescence efficiency of double perovskites.