Layered BiOI single crystals capable of detecting low dose rates of X-rays

Detecting low dose rates of X-rays is critical for making safer radiology instruments, but is limited by the absorber materials available. Here, we develop bismuth oxyiodide (BiOI) single crystals into effective X-ray detectors. BiOI features complex lattice dynamics, owing to the ionic character of the lattice and weak van der Waals interactions between layers. Through use of ultrafast spectroscopy, first-principles computations and detailed optical and structural characterisation, we show that photoexcited charge-carriers in BiOI couple to intralayer breathing phonon modes, forming large polarons, thus enabling longer drift lengths for the photoexcited carriers than would be expected if self-trapping occurred. This, combined with the low and stable dark currents and high linear X-ray attenuation coefficients, leads to strong detector performance. High sensitivities reaching 1.1  × 103 μC Gyair−1 cm−2 are achieved, and the lowest dose rate directly measured by the detectors was 22 nGyair s−1. The photophysical principles discussed herein offer new design avenues for novel materials with heavy elements and low-dimensional electronic structures for (opto)electronic applications

Static and Dynamic Disorder in Emerging Optoelectronic Materials

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

Prediction and realisation of high mobility and degenerate p-type conductivity in CaCuP thin films

Phosphides are interesting candidates for hole transport materials and p-type transparent conducting applications, capable of achieving greater valence band dispersion than their oxide counterparts due to the higher lying energy and increased size of the P 3p orbital. After computational identification of the indirect-gap semiconductor CaCuP as a promising candidate, we now report reactive sputter deposition of phase-pure p-type CaCuP thin films. Their intrinsic hole concentration and hole mobility exceed 1e20 cm-3 and 35 cm2/Vs at room temperature, respectively. Transport calculations indicate potential for even higher mobilities. Copper vacancies are identified as the main source of conductivity, displaying markedly different behaviour compared to typical p-type transparent conductors, leading to improved electronic properties. The optical transparency of CaCuP films is lower than expected from first principles calculations of phonon-mediated indirect transitions. This discrepancy could be partly attributed to crystalline imperfections within the films, increasing the strength of indirect transitions. We determine the transparent conductor figure of merit of CaCuP films as a function of composition, revealing links between stoichiometry, crystalline quality, and opto-electronic properties. These findings provide a promising initial assessment of the viability of CaCuP as a p-type transparent contact