Cesium Lead Halide Perovskite Nanocrystals Prepared by Anion Exchange for Light-Emitting Diodes

Cesium lead halide perovskite nanocrystals (NCs) have emerged as a promising emitter for lighting and display technologies. However, the iodide-containing CsPb(Br/I)(3) and CsPbI3 NCs suffer from chemical and phase degradation to nonfunctional orthorhombic (delta) phase. To address this challenge, we developed a facile synthetic protocol for halide exchange in pristine CsPbBr3 NCs. The protocol involves change in the nucleophilicity (or basicity) of the halide ions in aqueous-organic media which acts as the driving force to facilitate the halide exchange. Our method avoids the use of additives for anion solubilization, which otherwise induces chemical instability and quench the emission. The halide-exchanged CsPb(Br/I)(3) and CsPbI3 NCs showed enhanced structural and optical properties, such as high phase stability and emission quantum yields up to 94.2%, as compared to the NCs (of same composition) obtained from direct synthesis. In addition, we demonstrated fabrication of light-emitting diodes (LED) based on halide-exchanged CsPb(Br/I)(3) and CsPbI3 NCs. The devices showed peak external quantum efficiency (EQE) of 1.9% with a peak wavelength of 670 nm and a low luminance turn-on voltage of 2.5 V.