Alloying Bi-Doped Cs 2 Ag 1- x Na x InCl 6 Nanocrystals with K + Cations Modulates Surface Ligands Density and Photoluminescence Efficiency.

We show how, in the synthesis of yellow-emissive Bi-doped Cs 2 Ag 1- x Na x InCl 6 double perovskite nanocrystals (NCs), preventing the transient formation of Ag 0 particles increases the photoluminescence quantum yield (PLQY) of the NCs from ∼30% to ∼60%. Calculations indicate that the presence of even a single Ag 0 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 BiCl 6 →AgCl 6 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.