Synergetic impact of energy transfer and site preference for enhanced emission in Mn-doped Cs2AgInCl6 double perovskite

Strategic doping in halide double perovskites, with structure A2M(I)M′(III)X6, has shown great potential in improving their emission properties. While the site preference of monovalent or trivalent metal dopants is on the expected lines, the same is not true for bivalent dopants, like Mn2+, etc. In this Letter, we employ positron annihilation lifetime spectroscopy measurements along with density functional theory (DFT) calculations to address the preferred doping site of Mn2+ in the Cs2AgInCl6 double perovskite structure. Our results conclusively reveal that the preferred substitution site of Mn2+ is Ag+ for the most stable configuration, and the overall decrease in the average lifetime of the positrons indicates an excess of electrons after doping. Furthermore, temperature-dependent photoluminescence measurements reveal a negative thermal quenching of the Mn2+ emission, attributed to effective energy transfer from self-trapped excitons to Mn2+, explained using a two-term Arrhenius equation. Such efficient exciton-dopant energy transfer is crucial, as it bridges the host excitonic states with dopant emission, thereby maximizing luminescence efficiency.