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Static and Dynamic Disorder in Emerging Optoelectronic Materials



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Bravic, Ivona 


The digital revolution, which commenced with the invention of the germanium based transistor in 1947, provided an unprecedented boost in semiconductor research and manufacturing. Advances in the synthesis of pure semiconductors of first and second generation including silicon, germanium and gallium arsenide has not only opened doors to novel electronic technologies but also facilitated the design and manufacturing of so-called optoelectronic devices.

The resulting scientific narrative that emerging materials are bearers of novel technologies has led to the discovery of manifold new semiconducting material types which significantly differ from the above materials in structure and arising charge-carrier species. It remains the task of current researchers to extend the concepts and theories of light-matter interactions and design to novel materials including Van der Waals and two-dimensional materials, organic semiconductors and to all-inorganic as well as hybrid perovskites.

This work looks into some of these novel materials, focusing on the role of structure as well as static and dynamic disorder on the optoelectronic properties using first-principles electronic structure theory. We first investigate the III-V semiconductor boron arsenide, which exhibits similar absorption features to those of silicon but has a much higher room-temperature thermal conductivity. We employ density-functional theory (DFT) combined with finite differences to study, how dynamic disorder impacts its optoelectronic properties at operating temperatures of photovoltaic (PV) devices. We show, that electron-phonon coupling and electron-electron correlation have a strong impact on the temperature-dependence of the band gap, while it remains fairly robust with respect to thermal expansion. Additionally, we find that the absorption coefficient at the indirect absorption onset is six times higher than that of silicon, leading to a higher absorption cross-section and to potentially interesting PV applications. We then look into two chemically related Van der Waals materials, namely bismuth triiodide (BiI3) and bismuth oxyiodide (BiOI), that exhibit promising optoelectronic properties for PV applications. Here we use a combination of DFT and many-body theory together with finite differences as well as transient spectroscopy to show, that BiI3 is an intrinsically poor semiconductor for photovoltaics due to its strongly bound photogenerated electron-hole pair, prohibiting charge carrier separation and high charge-carrier densities. In contrast, the photoexcited carriers in BiOI are delocalised within the Van der Waals layer and, despite exhibiting strong carrier-phonon coupling, their delocalisation remains intact. The low absorption coefficient at the direct absorption onset is a result of a symmetry-forbidden optical transition and combined with nonradiative decay channels at room temperature, these properties make BiOI a rather poor material for PV devices as well. Instead, we illustrate, that its charge-carrier features make BiOI a suitable X-ray detector material.

Lastly, we study the impact of static, rather than dynamic disorder in the form of chemical doping in the all-inorganic lead-halide perovskite CsPbX3 (X = Cl, Br) on its band gap and its band dispersions using DFT. We propose, that the chemical disorder in this system created by B-site substitution improves the optoelectronic properties required for efficient light-emitting diodes (LED).

The overarching goal of this thesis is to find intuitive explanations to photophysical phenomena occurring in these novel materials related to structure and disorder, creating ideas about rational design of novel semiconducting materials with desirable optoelectronic properties for innovative and more environmentally sustainable technologies.





Monserrat, Bartomeu


Computational Materials Science


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge