Structural evolution and optical properties of hybrid lead halide perovskites under high pressure and low temperature

Hybrid lead halide perovskites with chemical formula of APbX3 for 3-dimensional (3D) structures, and A2PbX4 for 2-dimensional (2D) structures, where A= [CH3NH3 (MA), NH2CH2=NH2 (FA), C4H9NH3 (BA)], X= [Cl, Br, I], comprise a set of fully corner-sharing inorganic PbX6 octahedra and organic cations at the center for 3D perovskites and corner-sharing sheets of inorganic Pb-X octahedra partitioned by organic cations for 2D perovskites. These materials endow remarkable electronic and photovoltaic properties, exhibiting huge potential application in lasers, light-emitting diodes (LEDs), and solar cells. Exploration to such materials is still at early stage and full assessment of their structures and properties will no doubt further strengthen their understanding and potential applications. High pressure and variable temperature are clean and convenient tools for such investigation as they allow easy access to various structures and interactions among the constituent atoms and molecules. In this thesis, we study the pressure and temperature effects on the structure distortion in the hybrid perovskite family, to address the inorganic PbX6 octahedra tilting and organic cations disorder-ordering, which significantly modify the physical and chemical properties. Chapter 4 deals with the temperature effect on the hydrogen-bonding in 3D MAPbBr3 hybrid perovskite. We demonstrate that the H-bonds in the NH3 end of the MA group shows sequential changes while the H-bonds in the CH3 end only form H-bonding with the Br ions in the orthorhombic phase. High-pressure effect on the MAPbBr3 nanocrystals (NCs) is studied in Chapter 5, where high pressure-induced comminution of NCs and sintering into large thin nanoplates (NPs) are observed for the first time. We present a detailed theoretical simulation to show that the splitting of the NCs is along a crystal plane that involves no breaking of the chemical bonds of the inorganic atoms. In Chapter 6, we report for the first time a systematically high-pressure study of (C4H9NH3)2PbI4 (BAPI) 2D layered perovskite with n=1, where structural transitions cross all pressures are resolved and the structure-property correlation is established.​Doctor of Philosophy (SPMS