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Title: Fundamental properties, disorder and stability of methylammonium lead halide perovskites for solar cells
Author: Leguy, Aurélien
ISNI:       0000 0004 6423 5378
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Hybrid perovskite semiconductors from the MAPbX3 family (MA= CH3NH3; X = I, Br, Cl) can be used to make efficient ( > 22 %) solar cells despite disorder introduced by solution processing. Much remains to be understood about these materials. Optical constants of MAPbX3 single crystals derived from ellipsometry measurements are assigned to inter-band transitions from electronic structure calculations. These are used to simulate the contributions from different optical transitions to a typical transient absorption spectrum. The ellipsometry measurements are further used to show the reversible formation of CH3NH3PbI2·H2O and/or (CH3NH3)4PbI6·2H2O in single crystals thin films and devices upon exposure of MAPbI3 to water vapour, which is an important degradation pathway. Quasi-elastic neutron measurements allowed the dynamics of MA cations to be probed in the material. The dipolar MA+ reorientate between preferred alignments with a room temperature residence time of ~14 ps. Collective realignment of MA+ to screen a device’s built-in potential could reduce photovoltaic performance. However, the timescale for a domain wall to traverse a typical device is roughly estimated to be ~0.1 – 1 ms, faster than most observed hysteresis in MAPbI3 solar cells. Temperature dependent Raman and terahertz spectroscopy measurements indicate that MA+ reorientations are crucial to the transport properties of the material. Most of the vibrational features in MAPbX3 observed between 50 and 3500 cm-1 are assigned to calculated vibrational modes. The presence of additional peaks in the experimental spectra might be due to mode splitting caused by dynamic effects. The spectral linewidths of MAPbX3 indicate unusually short phonon lifetimes, linked to its low lattice thermal conductivity. This suggests that optical rather than acoustic phonon scattering prevails at room temperature in these materials, limiting charge mobility. These findings highlight the central role of disorder and heterogeneity to the optoelectronic properties of MAPbX3 and its impact on device behaviour and stability.
Supervisor: Barnes, Piers ; Nelson, Jenny Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral