Determining the Chemical Origin of the Photoluminescence of Cesium-Bismuth-Bromide Perovskite Nanocrystals and Improving the Luminescence via Metal Chloride Additives

We report the synthesis of Bi-based lead-free halide perovskite nanocrystals (NCs) via a ligand-assisted reprecipitation (LARP) method. Detailed chemical analysis of the synthesized Cs-Bi-Br NCs, which are commonly called stoichiometric Cs₃Bi₂Br₉, revealed that the actual composition of the NCs was extremely Cs deficient. Photoluminescence (PL) spectra from the Cs-deficient Cs-Bi-Br NCs and BiBr₃ NCs were nearly identical except for a higher emission intensity with Cs, which suggested that the chemical origin of the PL of the Cs-Bi-Br NCs was BiBr₃, and the inclusion of a few atomic percentages of Cs improved the PL intensity. Further improvements in the emissive property of the Cs-Bi-Br NCs were achieved by Cl surface passivation, which was mediated by transition metal chloride additives, namely, FeCl₃, MnCl₂, and NiCl₃, in the precursor solutions. Dispersive Raman spectroscopy studies suggested that the role of the transition metal in the salt additives was to facilitate the donation of Cl ions to the growing NCs during the synthesis. Additionally, a combined incorporation of methylammonium chloride and FeCl₃ significantly enhances the PL quantum yield compared to pristine Cs-Bi-Br NCs by a 7.5 times increase from 2 to 15%.