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Title: Local structure of lead halide perovskites for photovoltaic applications
Author: Liu, Jiaxun
ISNI:       0000 0004 8508 1232
Awarding Body: Queen Mary University of London
Current Institution: Queen Mary, University of London
Date of Award: 2019
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Lead halide perovskites are becoming promising in the field of photovoltaics for their superior power conversion efficiency and simple synthesis. This thesis explores the structure of a wide range of perovskites, including hybrid organic-inorganic and inorganic perovskites using total neutron and X-ray scattering. Local structure analysis of neutron total scattering data indicates that the Pb–I bonds in CsPbI3 are extremely anharmonic, and thus it follows that all near-neighbour bond distributions are significantly asymmetric. The asymmetric distribution of nearest-neighbour Pb–I distances with a long tail, reflects an underlying anharmonic bond potential energy function which is well-represented by the Morse potential. The effect of anharmonicity was examined in the other two inorganic analogues CsPbCl3 and CsPbBr3 but with X-rays instead of neutrons. Local structures of these two materials do not show significant changes above the first phase transition temperature, but there is apparent peak splitting and merging in the diffraction patterns. Similar anharmonicity was observed by analysing the shape of the first Pb–X peak. Rietveld refinement of the structure of the hybrid perovskite MAPbI3 indicates the methylammonium cation remained disordered in both tetragonal and cubic phases and ordered in the low-temperature orthorhombic phase, but traditional crystallographic models are inadequate to describe the rotational disorder of the cation. Orientational distribution analysis of large configurations refined from total scattering data was applied to study the rotational behaviour across temperatures. Moreover, the flexibility of the PbI6 framework and the anharmonicity of the Pb–I is reduced compared to that of CsPbI3, which we attribute to the larger size of the methylammonium cation in the cavity. Additionally, the structural response of MAPbI3 under pressure was studied up to 1.8 GPa. The effect of pressure is comparable to the cooling effect, because the phase boundary in the pressure-temperature diagram is almost a straight line.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: School of Physics and Astronomy