Compositional Tuning of Carrier Dynamics in Cs₂Na_1–xAg_xBiCl₆ Double-Perovskite Nanocrystals

We devised a hot-injection synthesis to prepare colloidal double-perovskite Cs₂NaBiCl₆ nanocrystals (NCs). We also examined the effects of replacing Na⁺ with Ag⁺ cations by preparing and characterizing Cs₂Na_1–xAg_xBiCl₆ alloy NCs with x ranging from 0 to 1. Whereas Cs₂NaBiCl₆ NCs were not emissive, Cs₂Na_1–xAg_xBiCl₆ NCs featured a broad photoluminescence band at ∼690 nm, Stokes-shifted from the respective absorption by ≥1.5 eV. The emission efficiency was maximized for low Ag⁺ amounts, reaching ∼3% for the Cs₂Na_0.95Ag_0.05BiCl₆ composition. Density functional theory calculations coupled with spectroscopic investigations revealed that Cs₂Na_1–xAg_xBiCl₆ NCs are characterized by a complex photophysics stemming from the interplay of (i) radiative recombination via trapped excitons localized in spatially connected AgCl₆–BiCl₆ octahedra; (ii) surface traps, located on undercoordinated surface Bi centers, behaving as phonon-assisted nonradiative decay channels; and (iii) a thermal equilibrium between trapping and detrapping processes. These results offer insights into developing double-perovskite NCs with enhanced optoelectronic efficiency.