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Title: Carbon nanotube thin film electrodes and magneto-optical spectroscopy of perovskite single crystals and thin films
Author: Dollmann, Markus
ISNI:       0000 0004 9356 2829
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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With each and every passing year, weather patterns seem to become less predictable and more chaotic due to changes to worldwide climate patterns. Greenhouse gas-free energy production is a boon to the aim of preventing further rises in global temperature increases. This thesis concerns itself with one major aspect of green energy, namely photovoltaics, and some materials with great potential for application in photovoltaics, namely single-walled carbon nanotubes (SWNT), and perovskites. SWNT have been known for almost thirty years by now, but their intrinsic fondness of bundling up due to their 2pz-orbitals has to date prevented any large scale application of them. Functionalising of SWNT offers a solution to this problem, either by dispersing them with surfactants in water, or by polymer functionalisation. Here, the latter method was applied to the problem, and the efficacy of deposition from solvents more advantageous than water is shown, achieving values of transmission and sheet resistance close to the necessary values for application in solar cells. Next, an inorganic contender for solar cell applications, Cs2AgBiBr6, is characterised by magneto-optical studies. With these, a direct and indirect gap are identified at 2.85 eV and 1.31eV. Moreover, a strongly Stokes shift influenced AgBi anti-site defect colour centre at 2.25 eV is shown to be part of a self-trapped exciton at low temperatures. Thereafter, the optical properties between 1.4 K and 370 K in conjunction with room temperature x-ray diffraction measurements of MAPbI3 single crystals and thin films grown/annealed at different temperatures are used to explain the double emission structure visible in photo- luminescence and reflectance. A dynamic Rashba effect, likely due to spontaneous alignment of the methylammonium-ions, causes a direct and indirect gap, where the former dominates the surface and the latter dominates the bulk of the samples. The remaining chapter is a study of the polaronic influence on the transitions observed for MAPbBr3 and MAPbI3. In comparison between high field, high energy, and low field, low energy, an almost two-fold difference in exciton binding energy, and an increase in effective mass can be observed due to exciton/lattice interactions in comparison to the established high field literature values. Interestingly, for both perovskites studied, single crystals show more favourable values, indicating that device application improvements could be possible by making thin films more single crystal-like.
Supervisor: Nicholas, Robin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Condensed Matter Physics ; Semiconductor Physics ; Material Sciences