Synthesis, Surface Engineering and Stability of CsPbX3 (X= Cl, Br, I) and BaZrS3 Perovskite Nanocrystals

Abstract

Colloidal CsPbX3 (X= Cl, Br, I) perovskite nanocrystals have emerged as a new class of optoelectronic material. One of the major challenges now is to improve their stability. One of the major emphasis of my thesis is to improve the stability of CsPbX3 nanocrystals by understanding and optimizing their surface chemistry. Thereafter, we introduced a new BaZrS3 chalcogenide perovskite nanocrystal, which is stable, environmentally benign and optoelctronically active. At first, we have explored the challenges and opportunities in synthesizing CsPbX3 nanocrystals, followed by elucidation of important physical parameters like band edge energies and optical transition probabilities. Subsequently, a variety of experimental and theoretical techniques were used to understand the nature of binding of organic ligands on the surface of CsPbX3 nanocrystals. Relying on the understanding of this surface chemistry, we developed two approaches to encapsulate CsPbX3 nanocrystals, improving their stability. In the first approach, CsPbX3 nanocrystals were capped with the PbSO4-oleate which prevents unwanted anion exchange between CsPbX3 nanocrystals with different X anion. Therefore, films of PbSO4-Oleate capped CsPbI3 nanocrystals were coated on another film of PbSO4-oleate CsPbBr3 (or any other X) nanocrystals by electrophoretic deposition method, without the exchange of halide ions between the two films. This suppression of anion exchange in films allowed us to tune the band gap of different films, achieving optically pumped white light emitting diode. The second surface coating approach was to prepare CsPbBr3/ZnS core/shell type nanostructure, where the crystalline shell enhances the moisture/water stability of samples. Lastly, we have explored a new kind perovskite nanocrystal, namely BaZrS3 nanocrystals. This sample belongs to the family of chalcogenide that promises better stability and is environmentally benign. Surface of BaZrS3 nanocrystals were decorated appropriately achieving solution processed thin films with reasonably good carrier mobility. Such thin films are required optoelectronic device fabrication.