Kavanagh, Seán R.; (2024) Accurately Modelling Point Defects in Semiconductors: The Case of CdTe. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In the realm of solid-state materials science, chemistry and physics intertwine. The characterisation and manipulation of materials at the atomic level, in particular, is our primary route to achieving fundamental breakthroughs in technological capabilities. Atomic-scale defects dictate the functional properties and applications of most solid materials, ranging from doping in semiconductors, active sites in catalysts, charge-carrier recombination and efficiency in solar photovoltaics, conductivity in mixed ionic-electronic conductors (such as those in batteries) and beyond. The typically dilute concentration of defects, despite their major impact on macroscopic properties, renders their experimental characterisation extremely challenging however. Thus, theoretical methods represent the primary avenue for the investigation of point defects at the atomic-scale, with computational predictions of defect behaviour (and resultant impact) often being compared to experimental measurements of global properties, such as carrier concentrations, solar cell efficiency, ionic conductivity or catalytic activity. This thesis describes my doctoral studies on modelling point defects in solids using ab-initio density functional theory (DFT), primarily focused on the technologically-relevant solar photovoltaic material; cadmium telluride (CdTe). In Chapter 1, I discuss the mechanisms by which point defects in solids affect key material applications, primarily focusing on solar photovoltaic technologies, before discussing experimental approaches to characterise these species. I also introduce solar photovoltaic (PV) technologies; their underlying physical principles and the current challenges in the field, and finish by giving an overview of current and historical research on CdTe thin-film solar cells. Next, the physical and mathematical theories underpinning the electronic structure methods used in this work are presented in Chapter 2, followed by a discussion of some of the computational methodologies for accurate defect modelling in Chapter 3. Chapter 4 focuses on the initial work of my PhD in London, analysing the behaviour of cadmium vacancies (VCd) in CdTe. A rapid recombination mechanism involving a metastable defect state is identified, with these observations being the inspiration behind my following work on global structure optimisation for defects. The work described in Chapter 5 was mostly performed during my time in the Max-Planck- Institut für Eisenforschung in Düsseldorf, Germany in late 2021, where I visited the group of Dr. Christoph Freysoldt for a collaboration looking at the potential impact of metastable defects on recombination in photovoltaic materials from a general perspective. The cul- mination of this research visit was a case study of tellurium interstitials (Tei) in CdTe, which exhibit an unusual thermal-excitation recombination cycle via metastable defect states — discussed in Chapter 5. In Chapter 6 the focus shifts from in-depth characterisations of single, key point defects in CdTe, to characterising the overall defect chemistry and thermodynam- ics. The predicted defect concentrations, doping, compensation mechanisms and expected carrier lifetimes effects are analysed, with the effect of stoichiometry, doping polarity and growth temperatures discussed. Finally, I conclude with a summary and discussion of future research directions in Chapter 7. In addition to providing key insights on the specific behaviour and impacts of defects in CdTe, I hope that this thesis will serve as a useful guide for key considerations in the accurate modelling of defects in solids (both methodological and analytical), as well as illustrating important research directions for advancing computational accuracy and capability for defect investigations.

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