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Title: Electronic processes in metal halide perovskites
Author: Wright, Adam
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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Metal halide perovskites are a class of materials which have recently achieved remarkable success in solar cells. The rapid rate of improvement in the efficiencies of perovskite-based devices has however often outpaced understanding of the electronic processes occurring within these semiconductor materials. This thesis is concerned with providing insight into the factors influencing electron motion and recombination in a series of hybrid metal halide perovskites, principally using photoluminescence spectroscopy. Such knowledge is essential for the development of higher-performance perovskite-based devices. The temperature dependence of emission line broadening is investigated in the four commonly-studied formamidinium and methylammonium perovskites, FAPbI3, FAPbBr3, MAPbI3 and MAPbBr3, and it is discovered that scattering from longitudinal optical phonons via the Fröhlich interaction is the dominant source of electron-phonon coupling near room temperature, with scattering off acoustic phonons negligible. Energies for the interacting longitudinal optical phonon modes are determined to be 11.5 and 15.3 meV, and Fröhlich coupling constants of approximately 40 and 60 meV are extracted for the lead iodide and bromide perovskites, respectively. The low-temperature photoluminescence of FAPbI3 is studied, and both a power-law time dependence in the emission intensity, and an additional low-energy emission peak which exhibits an anomalous relative Stokes shift, are observed. Using a rate-equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from the existence of an exponential trap-density tail with characteristic energy scale of ≈3 meV. In this high-quality hybrid perovskite, trap states therefore comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated e.g. with elemental vacancies). These first two studies confirm that hybrid perovskites may be viewed as classic semiconductors, using a band-structure picture. Finally, the relationship between the structural and optical properties of the double perovskite Cs2AgBiBr6 is examined. Temperature-dependent time-resolved photoluminescence measurements reveal the presence of a 'fast' and a 'slow' carrier decay component, The appearance of the former component correlates with the structural phase-transition to the low-temperature tetragonal phase, indicating that it may originate from the formation of tetragonal twin domains.
Supervisor: Herz, Laura ; Johnston, Michael Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available