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Title: Solution processed nickel oxide for photovoltaics applications
Author: Xu, Shengda
ISNI:       0000 0004 9357 1506
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2020
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Metal oxides are of great importance to the research and application of semiconductor devices. Solution processed metal oxides, especially nickel oxide, have been applied extensively as the interlayers in solar cells. The aim of this thesis was to develop understanding to the post annealing process of solution processed nickel oxide (s-NiOx). Perovskite solar cells were fabricated to investigate the optimised post annealing temperature for s-NiOx in perovskite photovoltaic applications. Owing to its unique electronic structure, nickel oxide has been extensively studied in the past decades. Literatures related to this work were reviewed in the beginning (Chapter 1), followed by a summary of experimental methods (Chapter 2). The experimental work began by investigating the decomposition routine of the sol-gel precursor (Chapter 3). The major decomposition event was found to end at 350 °C, but 250 °C appeared to be as sufficient to form nickel oxide nano-crystallites. At 250 °C, the resulting s-NiOx was embedded in organic residuals, and its disordered nature was further elucidated by the Urbach edge in optical characterisations. With increasing temperatures, the width of the band tail states was reduced, and overall demonstrated a stronger p-type character. During thermal annealing, the changes in s-NiOx composition was investigated by X-ray photoelectron spectroscopy (XPS, Chapter 4). s-NiOx oxygen 1s and nickel 2p core level spectra were studied by spectral peak fittings, from which the oxygen to nickel atomic ratio was found to be greater than one overall. Analysis of Auger LVV spectra further revealed the existence of nickel oxide at 250 °C. With understanding of the annealing process, a series of perovskite solar cells were made to suggest an optimised annealing temperature for s-NiOx (Chapter 5). Although s-NiOx crystallinity and p-type character were enhanced with temperature, 250 °C was found to produce the best solar cell performance, which was attributed to the lower series resistance of the device. The understanding of the annealing process developed in this work has implications to the temperature selection in post annealing of other solution processed metal oxide interlayers in solar cell applications. Additionally, aiming to further optimise solar cell performance, optical modelling was employed to suggest an ideal thickness for the nickel oxide interlayer and perovskite active layer (Chapter 6). Together with experimental results, it demonstrated that the purposely designed active layer thickness is a practical method to improve the solar cell short circuit current density (Jsc) output. General concluding remarks and suggestions to further works were discussed in the end (Chapter 7).
Supervisor: McLachlan, Martyn Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral