Compositional Tuning of Carrier Dynamics in Cs₂Na_1–<i>x</i>Ag_<i>x</i>BiCl₆ Double-Perovskite Nanocrystals

Abstract

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-<i>x</i> Ag _<i>x</i> BiCl₆ alloy NCs with <i>x</i> ranging from 0 to 1. Whereas Cs₂NaBiCl₆ NCs were not emissive, Cs₂Na_1-<i>x</i> Ag _<i>x</i> BiCl₆ 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-<i>x</i> Ag _<i>x</i> BiCl₆ 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.