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Title: Electronic characterisation of earth-abundant sulphides for solar photovoltaics
Author: Whittles, T. J.
ISNI:       0000 0004 6496 3877
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2017
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This thesis explores the electronic characterisation of materials for use as absorber layers within photovoltaic solar cells: an attractive solution to the energy crisis. Primarily, XPS was used to characterise CuSbS₂, Cu₃BiS₃, SnS, and Cu₂ZnSnS₄. All of these materials can be classified as earth-abundant: an important factor when considering materials that are both readily available and environmentally friendly. To varying degrees, all of these materials are established as potential absorber layers, with reports of successful, albeit low-efficiency devices. Also, the literature is often scant with regards to more fundamental characterisation of these materials, specifically in terms of how the underlying electronic structure affects properties that are pertinent to solar cells. Where XPS is utilised, it often lacks rigour, and is obfuscated by the complexities of the spectra. Work is then presented with two aims. First, to use a combination of high-quality XPS measurements and density of states calculations, in order to give insight into the electronic structure of the materials. Second, to present the full potential of XPS, as applied to solar absorbers, and how this technique complements others that are used for characterisation. Without exception, each material demonstrated natural band positions that make them unsuitable for use with established solar cell technologies. That is, a low IP (4.71–5.28 eV), resulting in a large CBO (0.5–0.85 eV) with CdS, for example. This offers some explanation to the poor efficiencies. These properties were found to be a consequence of the bonding nature of the valence and conduction bands, differing to the conventional absorber materials because of the influence of lone-pair electrons or other second-cation states. The power of XPS was demonstrated for application to absorber materials. The presence, effects, and formation pathways of contamination of the materials were elucidated using XPS, showing how the presence of such can act adversely to the performance of cells, especially with regards to oxidation, and also how these factors have been overlooked in the past. When coupled with other, complementary techniques, it has the ability to aid in the phase identification of a grown material, and also to help determine the presence of unwanted phases, which cause detriment to the device. For these applications, the methods, fitting procedures and analysis considerations detailed in this thesis should be followed. Research interest in these materials should be maintained based on the findings, with new approaches to cell design being aided by the characterisation methods developed here.
Supervisor: Dhanak, V. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral