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%.

Highly efficient vacuum-deposited CsPbBr 3 perovskite light-emitting diodes (PeLEDs) are demonstrated by introducing a separate polyethylene oxide (PEO) passivation layer. A CsPbBr 3 film deposited on the PEO layer via thermal co-evaporation of CsBr and PbBr 2 exhibits an almost 50-fold increase in photoluminescence quantum yield intensity compared to a reference sample without PEO. This enhancement is attributed to the passivation of interfacial defects of the perovskite, as evidenced by temperature-dependent photoluminescence measurements. However, direct application of PEO to an LED device is challenging because of the electrically insulating nature of PEO. This issue is solved by doping PEO layers with MgCl 2 . This strategy results in an enhanced luminance and external quantum efficiency (EQE) of up to 6887 cd m −2 and 7.6%, respectively. To the best of our knowledge, this is the highest EQE reported to date among vacuum-deposited PeLEDs.

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