In this work cadmium telluride thin film photovoltaic devices have successfully been produced using a novel closed-field magnetron sputtering technique. This technique offers the possibility of producing cells in an all-in-one vacuum process with the potential to provide a new lower cost production route. The sputtered cadmium telluride layers were characterised in detail using a range of advanced microscopy based techniques both in the as deposited and after the cadmium chloride treated state, a treatment that is necessary to produce a working cell. In the as deposited condition the cadmium telluride layer was seen to have a fine-grained columnar structure containing a high density of stacking faults. After the cadmium chloride treatment these grains recrystallized and the new grains were equiaxed with a much lower density of intragranular defects. Similar effects were also observed in samples prepared using close space sublimation. To understand this recrystallization behaviour during the cadmium chloride treatment, the key treatment parameters were systematically varied. Chemical analysis in Scanning Transmission Electron Microscopy (STEM) showed that chlorine travelled down the cadmium telluride grain boundaries and accumulated adjacent to the cadmium telluride/cadmium sulphide interface. This interface is where the cadmium telluride grains were found to recrystallise first during interrupted cadmium chloride treatments. The nature of the stacking faults was examined using High Resolution Transmission Electron Microscopy (HR-TEM). This showed that in localised regions up to one plane of atoms per sequence was missing based on the expected zinc blende structure. This changed the packing of the atoms such that a local change in crystal structure occurred. This local change in phase was successfully mapped using Electron Backscatter Diffraction in planar section produced using Focused Ion Beam milling. This was subsequently studied in more detail using Transmission Electron Backscatter Diffraction in the Scanning Electron Microscope, where the intra-granular arrangement of the phases was observed. HR-TEM was used to quantitatively measure the linear defects in the cadmium telluride layer after thermal annealing with and without the cadmium chloride present. This showed that annealing alone resulted in only a modest reduction in the density of linear defects and grain recrysallisation only occurred in the presence of cadmium chloride. Cadmium magnesium telluride (CMT) was successfully grown epitaxially onto the cadmium telluride as an electron reflector layer to improve cell performance. During deposition the cell experienced high temperatures and this caused the stacking faults to return in a cell that had been previously cadmium chloride treated. This resulted in a reduction in cell efficiency, providing another link between linear defects and a degradation in cell performance.

La mission Darwin, un projet d'interférométrie spatiale initié par l'ESA, nécessite l'utilisation de filtres modaux fonctionnant dans la gamme spectrale [6-20 µm]. Dans le cadre de ce travail, nous proposons la réalisation de filtres modaux basés sur des guides d'onde « tout tellurures » obtenus par la méthode d'empilement et de gravure. L'originalité de ce travail réside dans le fait que les guides réalisés sont de grandes dimensions (couches épaisses et profondeurs de gravure importantes), pour satisfaire aux exigences du projet. La première étape a donc consisté à choisir une méthode de dépôt qui permette d'obtenir des couches épaisses. La co-évaporation thermique a ainsi été mise en place et les paramètres de dépôt optimisés. Des couches d'épaisseur pouvant atteindre 17 µm, de bonne qualité (adhérentes, amorphes, denses et homogènes), transparentes de 6 à 20 µm et d'indice de réfraction contrôlé ont pu être obtenues. La gravure physique réactive (RIE) de ces couches, en utilisant un mélange gazeux CHF3/O2/Ar, a constitué la deuxième partie de ce travail. L'obtention de marches de profondeur pouvant dépasser 10 µm, présentant des profils de gravure de qualité, a été démontrée. Les différents guides d'onde IR réalisés ont été caractérisés optiquement après préparation de leurs faces d'entrée et de sortie. L'observation d'un bon confinement de la lumière sur un banc de guidage à λ = 10,6 µm et l'obtention d'un taux de réjection de 10-3 sur un banc d'interférométrie annulante nous ont permis de confirmer que les guides d'onde à base de couches tellurures et réalisés par la méthode d'empilement et de gravure constituaient une solution de choix en tant que filtres modaux pour l'interférométrie spatiale. / The Darwin mission, an interferometric spatial project initiated by ESA, requires modal filters being able to work in the whole spectral range [6-20 µm]. In the framework of this work, we propose the realization of modal filters based on waveguides obtained by stacking and etching chalcogenide films. The originality of this work lies in the fact that the realized waveguides have large dimensions (thick films and deep etching), to satisfy the project requirements. The first step consisted in choosing the deposition method which allows obtaining thick films. The thermal co-evaporation was setting up and the deposition parameters were optimized. Films with thickness which can reach 17 µm, of good quality (adhesive, amorphous, dense and homogeneous), transparent from 6 to 20 µm and with controlled refractive index were obtained. The physical reactive etching of these films, by using a gas mixture CHF3/O2/Ar, constituted the second part of this work. The obtaining of deep rib which can exceed 10 µm, presenting etching profiles of good quality was demonstrated. The elaborated IR waveguides were optically characterized after preparation of their entrance and exit faces. The observation of light confinement on a guiding bench at λ = 10.6 µm and the obtaining of a rejection rate of 10-3 on a nulling interferometry bench allowed confirming that the waveguides based on the stacking and etching of telluride films was a choice solution as modal filters for the spatial interferometry.

Topological insulators (TIs) are a new state of quantum matter that has fundamentally challenged our knowledge of insulator and metals. They are insulators in the bulk, but metallic on the edge. A TI is characterized by a so-called topological invariant. This characteristic integer number is associated to every mapping between two topological spaces and can be defined for an electronic system on the lattice. Due to the bulk-edge correspondence a non-trivial value leads to topologically protected edge states. To get insight into the electronic characteristics of these edge/surface states, however, an effective continuum theory is needed. Continuum models are analytical and are also able to model transport. In this thesis we will address the suitability of continuum low-energy theories to describe the topological characteristics of TIs. The models which are topologically well-defined are called topological k.p Hamiltonians. After introducing a necessary background in chapter 1 and 2, we will discuss in the methodological chapter 3 the strategies that have to be taken into account to allow for studying topological surface states. In chapter 4 we will study two different model classes associated to a spherical basis manifold. Both have an integer topological invariant, but one shows a marginal bulk-edge correspondence. In chapter 5 we will study a different continuum theory where the basis manifold corresponds to a hemisphere. We then apply all these ideas to a time-reversal invariant TI -- uniaxially strained Mercury Telluride (HgTe). We determine the spin textures of the topological surface states of strained HgTe using their close relations with the mirror Chern numbers of the system and the orbital composition of the surface states. We show that at the side surfaces with $C_{2v}$ point group symmetry an increase in the strain magnitude triggers a topological phase transition where the winding number of the surface state spin texture is flipped while the four topological invariants characterizing the bulk band structure are unchanged. In the last chapter we will give a summary.

Spectral computed tomography with energy-resolving detectors has a potential to improve the detectability of images and correspondingly reduce the radiation dose to patients by extracting and properly using the energy information in the broad x-ray spectrum. A silicon photon-counting detector has been developed for spectral CT and it has successfully solved the problem of high photon flux in clinical CT applications by adopting the segmented detector structure and operating the detector in edge-on geometry. The detector was evaluated by both the simulation and measurements. The effects of energy loss and charge sharing on the energy response of this segmented silicon strip detector with different pixel sizes were investigated by Monte Carlo simulation and a comparison to pixelated CdTe detectors is presented. The validity of spherical approximations of initial charge cloud shape in silicon detectors was evaluated and a more accurate statistical model has been proposed. A photon-counting energy-resolving application specific integrated circuit (ASIC) developed for spectral CT was characterized extensively by electrical pulses, pulsed laser and real x-ray photons from both the synchrotron and an x-ray tube. It has been demonstrated that the ASIC performs as designed. A noise level of 1.09 keV RMS has been measured and a threshold dispersion of 0.89 keV RMS has been determined. The count rate performance of the ASIC in terms of count loss and energy resolution was evaluated by real x-rays and promising results have been obtained. The segmented silicon strip detector was evaluated using synchrotron radiation. An energy resolution of 16.1% has been determined with 22 keV photons in the lowest flux limit, which deteriorates to 21.5% at an input count rate of 100 Mcps mm−2. The fraction of charge shared events has been estimated and found to be 11.1% for 22 keV and 15.3% for 30 keV. A lower fraction of charge shared events and an improved energy resolution can be expected by applying a higher bias voltage to the detector. / <p>QC 20121123</p>

This thesis aims to investigate structural, optical and photoelectrochemical behavior of CdS nanowires and their heterojunctions with CdTe and polypyrrole nanowires. In the first part, CdS nanowires have been synthesized via electrochemical template-based route. It has been observed that synthesis conditions, such as bias voltage and deposition time, affect the morphology, optical and photoelectrochemical characteristics of CdS nanowires. Depending on the deposition time, length of the CdS nanowires changed from 100-200 nm to 3-4 m. Also the diameter of the nanowires increased with increasing the deposition time. Structure of the CdS nanowires has been confirmed by X-ray diffraction spectrometry and EDX analysis. Phototelectrochemical performances of the CdS nanowires have been changed dramatically with bias voltage and deposition time.In the second part of this thesis, CdTe nanostructures have been deposited on CdS nanowires. Change in optical and photoelectrochemical behavior of CdS nanowires after CdTe deposition has been investigated. Organic semiconductors and their composites with inorganic materials have been gaining attention due to tunable optical, electrical and magnetic properties. Also, ease of fabrication techniques, and therefore, low cost made these materials attractive for lots of applications including photovoltaic devices and flexible electronics. In the last part of this thesis, heterojunctions of CdS and Polypyrrole (Ppy) nanowires have been synthesized. Like CdS/CdTe heteronanostructures, first the CdS nanowires have been electrochemically deposited in anodized alumina template and then Ppy has been successfully deposited on CdS nanowires. In order to investigate the effects of polypyrrole synthesis conditions on CdS/Ppy heteronanostructures, CdS nanowire synthesis conditions have been kept constant. It has been observed that morphology and photoelectrochemical behavior of the Ppy nanowires has been affected from Ppy synthesis conditions. The photoelectrochemical performance changes of CdS/Ppy heteronanostructures have been also investigated in this part.

abstract: Achieving high efficiency in solar cells requires optimal photovoltaics materials for light absorption and as with any electrical device—high-quality contacts. Essentially, the contacts separate the charge carriers—holes at one terminal and electrons at the other—extracting them to an external circuit. For this purpose, the development of passivating and carrier-selective contacts that enable low interface defect density and efficient carrier transport is critical for making high-efficiency solar cells. The recent record-efficiency n-type silicon cells with hydrogenated amorphous silicon (a-Si:H) contacts have demonstrated the usefulness of passivating and carrier-selective contacts. However, the use of a-Si:H contacts should not be limited in just n-type silicon cells. In the present work, a-Si:H contacts for crystalline silicon and cadmium telluride (CdTe) solar cells are developed. First, hydrogen-plasma-processsed a-Si:H contacts are used in n-type Czochralski silicon cell fabrication. Hydrogen plasma treatment is used to increase the Si-H bond density of a-Si:H films and decrease the dangling bond density at the interface, which leads to better interface passivation and device performance, and wider temperature-processing window of n-type silicon cells under full spectrum (300–1200 nm) illumination. In addition, thickness-varied a-Si:H contacts are studied for n-type silicon cells under the infrared spectrum (700–1200 nm) illumination, which are prepared for silicon-based tandem applications. Second, the a-Si:H contacts are applied to commercial-grade p-type silicon cells, which have much lower bulk carrier lifetimes than the n-type silicon cells. The approach is using gettering and bulk hydrogenation to improve the p-type silicon bulk quality, and then applying a-Si:H contacts to enable excellent surface passivation and carrier transport. This leads to an open-circuit voltage of 707 mV in p-type Czochralski silicon cells, and of 702 mV, the world-record open-circuit voltage in p-type multi-crystalline silicon cells. Finally, CdTe cells with p-type a-Si:H hole-selective contacts are studied. As a proof of concept, p-type a-Si:H contacts enable achieving the highest reported open-circuit voltages (1.1 V) in mono-crystalline CdTe devices. A comparative study of applying p-type a-Si:H contacts in poly-crystalline CdTe solar cells is performed, resulting in absolute voltage gain of 53 mV over using the standard tellurium contacts. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2018

[pt] O objetivo do presente trabalho é a deposição de filmes de telureto de cádmio a partir de duas fontes de materiais, Cd e Te, com base no uso de diagramas de potenciais termodinâmicos para avaliar as condições de deposição. Em especial, o método proposto permite avaliar a influência de contaminantes gasosos, tais como, oxigênio, sobre as fases condensadas. O método também pode ser aplicado para a deposição de outros compostos que sejam mais estáveis que os elementos que os compõem. O processamento utilizado na deposição utiliza uma técnica alternativa onde as temperaturas de fonte e de substrato são as mesmas. / [en] The objective of the present work is deposition of Cadmiun Telurides films from two sources of materials, Cd and Te, on the basis of the use of diagrams of thermodynamic potentials to evaluate the deposition conditions. In special, the considered method allows to evaluate the influence of gaseous contaminantes, such as, oxygen, on the condensed phases. The method can be applied for the deposition of other compounds that are more stable than the constituent elements. The equipment used in the deposition uses an alternative technique where the temperatures of source and substrate are the same ones.

As existing energy sources have been depleting at a fast pace, thermoelectric (TE) materials have received much attention in recent years because of their role in clean energy generation and conversion. Thermoelectric materials hold promise in terrestrial applications such as waste heat recovery. Bismuth selenide (Bi2Se3), lead telluride (PbTe), skutterudites CoSb3, and Bi-Sb alloys are among the widely investigated thermoelectric materials. Synthesis of above mentioned thermoelectric materials in nanostructured form and their characterization were investigated. Highly crystalline Bi2Se3, undoped and indium (In) doped PbTe, unfilled and ytterbium (Yb) filled CoSb3 nanomaterials were synthesized using hydrothermal/solvothermal technique and Ca-doped Bi-Sb alloy was synthesized using ball milling method. The mechanism of indium doping to the PbTe matrix was investigated using X-ray diffraction, laser-induced breakdown spectroscopy (LIBS) and a first principle calculation. It was found that indium doping, at a level below 2%, is substitution on Pb site. The effects of the amount of sodium borohydride (NaBH4) as the reducing agent and the annealing treatment on the phase transition of CoSb3 were investigated. It was found that a sufficient amount of NaBH4 along with the specific annealing condition was needed for the formation of pure phase CoSb3. Thermoelectric properties of Bi2Se3 and Ca-doped Bi85Sb15 were also investigated. A lower thermal conductivity and a higher Seebeck coefficient were achieved for a Bi2Se3 sample prepared in dimethyl formamide (DMF) at 200ºC for 24 h as compared to bulk Bi2Se3. The decrease in thermal conductivity can be attributed to the increased phonon scattering at the interfaces of the nanostructures and at the grain boundaries in the bulk nanocomposite. The increase in the Seebeck coefficient of Bi2Se3 nanostructures is likely the result of the quantum confinement of the carriers in nanostructures. The effect of calcium doping on Bi85Sb15 nanostructures were investigated. It was found that 2% calcium doped Bi-Sb alloy showed the best TE efficiency due to the enhanced power factor and reduced thermal conductivity.

Topological insulators (TIs) are a new state of quantum matter that has fundamentally challenged our knowledge of insulator and metals. They are insulators in the bulk, but metallic on the edge. A TI is characterized by a so-called topological invariant. This characteristic integer number is associated to every mapping between two topological spaces and can be defined for an electronic system on the lattice. Due to the bulk-edge correspondence a non-trivial value leads to topologically protected edge states. To get insight into the electronic characteristics of these edge/surface states, however, an effective continuum theory is needed. Continuum models are analytical and are also able to model transport. In this thesis we will address the suitability of continuum low-energy theories to describe the topological characteristics of TIs. The models which are topologically well-defined are called topological k.p Hamiltonians. After introducing a necessary background in chapter 1 and 2, we will discuss in the methodological chapter 3 the strategies that have to be taken into account to allow for studying topological surface states. In chapter 4 we will study two different model classes associated to a spherical basis manifold. Both have an integer topological invariant, but one shows a marginal bulk-edge correspondence. In chapter 5 we will study a different continuum theory where the basis manifold corresponds to a hemisphere. We then apply all these ideas to a time-reversal invariant TI -- uniaxially strained Mercury Telluride (HgTe). We determine the spin textures of the topological surface states of strained HgTe using their close relations with the mirror Chern numbers of the system and the orbital composition of the surface states. We show that at the side surfaces with $C_{2v}$ point group symmetry an increase in the strain magnitude triggers a topological phase transition where the winding number of the surface state spin texture is flipped while the four topological invariants characterizing the bulk band structure are unchanged. In the last chapter we will give a summary.

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