Synergistic Halide- and Ligand-Exchanges of All-Inorganic Perovskite Nanocrystals for Near-Unity and Spectrally Stable Red Emission

Abstract

All-inorganic perovskite nanocrystals (NCs) of CsPbX3 (X = Cl, Br, I) have emerged as the promising candidates for displays due to their excellent properties including wide color gamut, narrow emission bandwidth and high photoluminescence quantum yield (PLQY). However, the pure red perovskite NCs are commonly prepared by mixing halide ions, and the migration and segregation of halide ions will result in vacancy defects and spectral instabilities. Herein, we demonstrate an approach to prepare mixed-halide perovskite NCs with stable pure red emission and high PLQY by simultaneous halide-exchange and ligand-exchange. The green CsPbBr3 NCs were firstly synthesized by the ligand assisted reprecipitation (LARP) method at room temperature. And then ZnI2 was introduced as the iodine source to transform CsPbBr3 NCs to CsPbBrxI3-x NCs by anion exchange. In addition to providing iodine ions, ZnI2 can also act as the inorganic ligand anchored on the perovskite NCs to passivate surface defects and prevent ion migration, leading to the suppression of non-radiative losses and halide segregation. Thus, the luminescence properties of the resultant CsPbBrxI3-x NCs are strongly dependent on the ZnI2 content in the precursor solution. By regulating the ZnI2 dominated exchange process, the red CsPbBrxI3-x NCs with organic/inorganic hybrid ligands achieve near-unity PLQY with a stable emission peak at 640 nm, matching the requirement of the Rec. 2020 standard. Consequently, we demonstrate that the spectrally stable and efficient red CsPbBrxI3-x NCs can combine with the pristine green CsPbBr3 NCs to construct white light-emitting diodes with high color gamut. Our work presents a facile ion exchange strategy to prepare spectrally stable mixed-halide perovskite NCs that can approach the photoluminescence efficiency limit for display or lighting applications.

Keywords: perovskite nanocrystals, ligand assisted reprecipitation, halide-exchange, ligand-exchange, pure red emission