Emissive Bi-Doped Double Perovskite Cs₂Ag_1–xNa_xInCl₆ Nanocrystals

We report the composition-dependent optical properties of Bi-doped Cs₂Ag_1–xNa_xInCl₆ nanocrystals (NCs) having a double perovskite crystal structure. Their photoluminescence (PL) was characterized by a large Stokes shift, and the PL quantum yield increased with the amount of Na up to ∼22% for the Cs₂Ag_0.4Na_0.6InCl₆ stoichiometry. The presence of Bi³⁺ dopants was crucial to achieve high PL quantum yields (PLQYs) as nondoped NC systems were not emissive. Density functional theory calculations revealed that the substitution of Ag⁺ with Na⁺ leads to localization of AgCl₆ energy levels above the valence band maximum, whereas doping with Bi³⁺ creates BiCl₆ states below the conduction band minimum. As such, the PL emission stems from trapped emission between states localized in the BiCl₆ and AgCl₆ octahedra, respectively. Our findings indicated that both the partial replacement of Ag⁺ with Na⁺ ions and doping with Bi³⁺ cations are essential in order to optimize the PL emission of these systems.