Alloying Bi-Doped Cs2Ag1–xNaxInCl6 Nanocrystals with K+ Cations Modulates Surface Ligands Density and Photoluminescence Efficiency

We show how, in the synthesis of yellow-emissive Bi-doped Cs2Ag1–xNaxInCl6 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.