Alloying Bi-Doped Cs₂Ag_1-NaInCl₆ Nanocrystals with K⁺ Cations Modulates Surface Ligands Density and Photoluminescence...

We show how, in the synthesis of yellow-emissive Bi-doped Cs₂Ag_1-NaInCl₆ double perovskite nanocrystals (NCs), preventing the transient formation of Ag⁰ particles increases the photoluminescence quantum yield (PLQY) of the NCs from ∼30% to ∼60%. Calculations indicate that the presence of even a single Ag⁰ species on the surface of a NC introduces deep trap states. The PL efficiency of these NCs is further increased to ∼70% by partial replacement of Na⁺ with K⁺ ions, up to a 7% K content, due to a lattice expansion that promotes a more favorable ligands packing on the NC surface, hence better surface passivation. A further increase in K⁺ lowers the PLQY, due to both the activation of nonradiative quenching channels and a lower oscillator strength of the BiCl₆→AgCl₆ transition (through which PL emission occurs). The work indicates how a deeper understanding of parameters influencing carrier trapping/relaxation can boost the PLQY of double perovskites NCs.