Metal halide perovskite nanocrystals : synthesis, stability, and lead-free alternatives

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2021-05

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Metal halide perovskites are an emerging family of semiconductor materials with excellent optoelectronic properties suitable for applications such as solar cells and light-emitting diodes. The nanocrystals of perovskites are especially versatile due to their tunable size, shape, compositions, and size-dependent band gaps. Perovskite nanocrystals are also ideal candidates as nanoscale building blocks for the formation of superlattice assemblies, potentially giving rise to new properties depending both on the intrinsic characteristics of individual nanocrystals and on their organization in the assembly. Despite the amazing opportunities it offers, perovskite research is always faced with two major challenges, limited stability and lead toxicity. In this dissertation, I presented my efforts in addressing these two challenges in perovskites nanocrystals and their assemblies. Because the instability of nanocrystals is closely related to the synthetic process, we first provided a practical guide to the synthesis and purification of perovskite nanocrystals, emphasizing the tips and tricks in obtaining samples with better stability and reproducibility. Then, we probed the structural changes in two prototypical iodine-based perovskite systems, MAPbI₃ and CsPbI₃ nanocrystal superlattices, by in-situ X-ray scattering. The thermal-induced transformations in MAPbI₃ nanocrystals started with a tetragonal-to-cubic perovskite phase transformation at ~60 °C, followed by a chemical decomposition into PbI₂ at ~90 °C. The nanocrystal superlattice disintegrated simultaneously with the degradation of perovskite lattice. When annealed with hexane solvent vapor, CsPbI₃ nanocrystals didn’t change atomic structure, but their superlattice changed from a tetragonal lattice to a semi-disordered structure. The annealing process can be reversed by evaporating the solvent. Considering the limited stability of MAPbI₃ and CsPbI₃ nanocrystals, I designed and obtained mixed A-site Cs₁₋ₓMAₓPbI₃ nanocrystals by post-synthetic cation exchange. Cs₀.₅₅MA₀.₄₅PbI₃ nanocrystals showed enhanced thermal stability than parent MAPbI₃ and CsPbI₃ nanocrystals, suggesting that compositional alloying can be an effective strategy for obtaining more stable perovskite nanocrystals. Finally, due to the concerns with lead toxicity, Cs₂AgBiBr₆ double perovskite nanocrystals are developed as promising lead-free alternatives. We explored the roles of ligands in the synthesis of Cs₂AgBiBr₆ nanocrystals, and found that acid-to-amine ratio affects nanocrystal morphology and reaction yield. Only oleylamine remains bonded to nanocrystals after purification, and oleic acid can be replaced completely by an alkyl phosphinic acid in the reaction. Cs₂AgBiBr₆ nanocrystals were more thermally stable than lead-based perovskites, but their colloidal stability needs to be improved.

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