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No description available.

Solar absorber coatings are one of the key components in concentrated solar power (CSP) plants. Currently operating at temperatures up to 565°C and suffering from emissive losses, their energy conversion efficiency could be improved by applying high-temperature stable materials with solar selective properties, i.e. high absorptivity and low emissivity. In this work, the transparent conductive oxide (TCO) SnO2:Ta is developed as a solar selective coating (SSC) for CSP absorbers. Starting with simulations covering basic requirements for SSCs, the deposition process of SnO2:Ta is optimized and extensive optical characterization and modelling are performed. It is shown that upon covering with a SiO2 antireflective layer, a calculated absorptivity of 95% and an emissivity of 30% are achieved for the model configuration of SnO2:Ta on top of a perfect black body (BB). High-temperature stability of the developed TCO up to 800 °C is shown in situ by spectroscopic ellipsometry and Rutherford backscattering spectrometry. The universality of the concept is then demonstrated by transforming silicon and glassy carbon from non-selective into solar selective absorbers by depositing the TCO on top of them. Finally, the energy conversion efficiencies ηCSP of SnO2:Ta on top of a BB and an ideal non-selective BB absorber are compared as a function of solar concentration factor C and absorber temperature TH.

Ultra-thin solar cells that incorporate earth-abundant and non-toxic materials are promising candidates in the endeavor toward sustainable energy harvesting. Methods to counteract the inevitable low absorption of thinner semiconductor layers are of high interest and have raised considerable attention in the research society. In an attempt to increase the absorption of these types of assemblies, optical coupling effects between the localized surface plasmon resonances (LSPR) of disordered Au nanostructures and a Fabry-Pérot cavity were studied using a previously established absorber/spacer/reflector stack. The disordered Au array was fabricated by evaporating a thin Au film on a substrate with a 55 nm SiO2 dielectric spacer and a 100 nm Al reflecting film, followed by thermal annealing. Nominal Au film thicknesses in the range of 5-25 Å and annealing temperatures of 200-500 oC were investigated. In situ spectroscopic ellipsometry measurements during the subsequent atomic layer deposition (ALD) of tin monosulfide (SnS) allowed analysis of how the optical properties of the SnS/Au absorber layer changed as a function of the growing SnS layer thickness. By employing the Transfer Matrix Method with the estimated optical properties from the in situ analysis, the absorptance of the absorber/spacer/reflector stacks was simulated as a function of the spacer thickness, revealing any signs of the characteristic anti-crossing behavior. It was discovered that a nominal Au film thickness of 25 Å, annealed at 450 oC, and coated with a SnS film of ∼13 nm primed toward the π-phase, resulted in strong optical coupling between the cavity mode and the LSPR. The energy difference at the avoided crossing in the specular reflectance measurement gave an estimated Rabi-splitting energy of 537 meV. This corresponded to about 40% of the original LSPR energy, placing itself within the ultra-strong coupling regime. To evaluate the relevance of the thin-layered structure in photovoltaic applications, more advanced computational methods are required to estimate the useful absorption that occurs in the SnS layer. Nevertheless, these results elucidate the realization of strong optical coupling effects between disordered Au nanostructures and a Fabry-Pérot cavity, and further the possibility of using scalable fabrication methods for this type of ultra-thin absorber/spacer/reflector stack.

Im Rahmen dieser Arbeit wurde ein Niedertemperatur Sol-Gel Spin-Coating Verfahren entwickelt, mit dem sich Metalloxide und -fluoride in Form von Nanometer-dicken Schichten abscheiden lassen. Ziel dieses Verfahrens ist die Herstellung von anti- (AR) und hoch reflektierenden (HR) Schichtsystemen, die aus einem alternierenden Aufbau von niedrig brechenden und hoch brechenden Materialien bestehen. Zur Darstellung der Metallfluoride wurde dabei eine neuartige nichtwässrige Sol-Gel Synthese ausgehend von Metallalkoxiden und in Alkoholen einkondensiertem Fluorwasserstoff angewendet. Die abgeschiedenen Schichten, die bei 100 °C getrocknet wurden, sind mit REM, TEM und AFM auf ihre morphologische Struktur, mit EDX und XPS auf ihre chemische Zusammensetzung und mit Ellipsometrie und UV-vis Transmissionsspektroskopie auf ihre optischen Eigenschaften hin untersucht worden. Mit diesem Verfahren gelang die Präparation von homogenen MgF2- und TiO2-Schichten mit geringen Rauheiten (Ra < 2,0 nm) auf Si- und SiO2-Substraten. Die MgF2-Schichten haben einen Brechungsindex von n500 = 1,36. Die TiO2-Schichten weisen einen Brechungsindex von n500 = 2,05 auf. Die Dicken der MgF2- und TiO2-Schichten sind zwischen 25 nm und 500 nm einstellbar. Durch einen alternierenden Aufbau von MgF2- und TiO2-Schichten gelang außerdem erstmalig die Herstellung von Metallfluorid- und Metalloxid-basierten Schichtsystemen über einen Niedertemperatur Sol-Gel Prozess. Mit Hilfe der bestimmten optischen Eigenschaften der MgF2- und TiO2-Einzelschichten wurden einfache AR und HR Designs entwickelt und aufgebaut. Die Transmissionsspektren der Designs und der jeweiligen Schichtsysteme zeigten dabei gute Übereinstimmungen, gleiches galt für die mit Hilfe der Spektralellipsometrie aufgenommenen Reflexionsspektren. In dieser Arbeit konnte erstmals gezeigt werden, dass sich das Niedertemperatur Sol-Gel Verfahren prinzipiell zur Herstellung von einfachen AR und HR Schichtsystemen eignet. / This work deals with the development of a low temperature sol-gel spincoating process for thin films with thicknesses in the nanometer range based on metal oxides and metal fluorides. Optical films such as anti-reflective (AR) or high reflective coatings are of much interest and consist of alternating dielectric layers of low and high refractive index materials. Regarding the general procedure for the metal fluorides a novel nonaqueous sol-gel synthesis starting from metal alkoxides and alcohol-dissolved HF was used. The coatings were dried and calcined at 100 °C. The morphology of these films was characterised with REM, TEM and AFM. EDX and XPS were used to indentify the chemical composition and ellipsometry and UV-vis spectroscopy to determine the optical properties of the films. This new process allows the preparation of homogeneous magnesium fluoride and titanium dioxide layers with low roughness (Ra < 2.0 nm) on Si and SiO2 substrates. The thicknesses of the MgF2 and TiO2 single layers were adjustable between 25 nm and 500 nm depending on the number of coating steps and on the concentration of the used sols. The MgF2 layers had a refractive index of n500 = 1.36 and the TiO2 layers a refraction index of n500 = 2.05. For the first time, an alternating metal fluoride and oxide multilayer system was produced with a low temperature sol-gel method (consisting of MgF2 and TiO2). Based on the determined optical constants of the MgF2 and TiO2 single layers, AR and HR multilayer systems were calculated and fabricated. The transmission spectra of the designs and the corresponding multilayer were in good agreement. Similar results were obtained with the reflection spectra determined with spectral ellipsometry. The presented investigations were able to show that the developed low temperature sol-gel process using the spin-coating technique is suitable for the preparation of simple AR and HR multilayer systems.

Silizium (Si) wird für eine breite Palette von Anwendungen wie z.B. in Solarzellen, Mikroelektronik, Biochips und so weiter eingesetzt. In dieser Arbeit wurden neue Hybridsysteme aus Si und organischen Molekülen, bezüglich der Oberflächenpassivierung des Halbleiters und der resultierenden elektronischen Eigenschaften untersucht. Insbesondere wurden Methyl-Gruppen (CH3 und CD3), Ethynyl-Derivate (H−C≡C-, CH3−C≡C-, und C6H5−C≡C-), sowie Pyrrol und Thiophen aus Grignardlösungen untersucht. Bezüglich Stabilität und Defektkonzentration konnte gezeigt werden, dass organisch modifizierte Si-Oberflächen eine höhere Stabilität an Luft haben als Standard wasserstoffpassivierte Si-Oberflächen und dabei eine nur geringfügig höhere Defektkonzentration aufweisen. Untersuchungen mit Infrarot Spektroskopischer Ellipsometrie (IRSE) und Synchrotron Röntgen Photoemissions Spektroskopie (SXPS) zeigen, dass die Oxidationsrate für Oberflächen mit CH3-Terminierung stark reduziert ist. In der vorliegenden Arbeit gelang es erstmalig mittels IRSE die charakteristische „Umbrella“-Schwingungsmode zu beobachten und SXPS Messungen zeigten die Spin-Orbit-Aufspaltung der Si 2p Emission für CH3-passivierte Si-Oberflächen. Die CH3-Gruppen besitzen einen hohen Grad von Ordnung auf der Si(111)-Oberfläche. Das Aufbringen von Ethynyl-Derivaten führt zu extrem dünnen polymerisierten Schichten auf Si durch elektrochemische Radikaloxidation der C≡C Dreifachbindung. Diese Schichten sind homogen und haften sehr gut an der Si-Oberfläche. Weiterhin konnte gezeigt werden, dass die Abscheidung von Ethynyl-Derivaten vom Typ des Halogenatoms im Grignard-Precursor abhängig ist, wobei Br im Vergleich zu Cl zu geringeren Rekombinationsgeschwindigkeiten an der Polymer/Si-Grenzfläche führen. Eine Änderung der Austrittsarbeit von bis zu 0.5 eV und der Bandverbiegung von bis zu 0.24 eV wurde nach der Abscheidung dieser Moleküle gemessen. Diese elektronischen Eigenschaften hängen linear vom Oberflächendipol ab. / Organic functionalization of silicon (Si) surfaces has received a tremendous interest in the development of organic/semiconductor hetero-structures for plenty of potential applications from microelectronics, molecular electronics, photovoltaics to bio-applications. In this thesis, tailoring of the electronic properties and passivation properties of such organic hetero-structures have been investigated. Direct grafting of organic layers like methyl groups (CH3 and CD3), ethynyl derivatives (H−C≡C-, CH3−C≡C-, and C6H5−C≡C-), and heterocyclic molecules (pyrrole and thiophene) onto Si(111) surfaces have been performed in a one-step electrochemical process by anodic treatment in Grignard electrolytes. Organically modified Si surfaces show low interface recombination rates as measured by photoluminescence technique and reveal also a much better passivation with respect to stability in ambient air than H-terminated Si surfaces. Grafting of ethynyl derivatives and heterocyclic molecules lead to the formation of ultrathin polymeric layers, where the thickness depends on charge flow applied to the Si electrode, while methyl groups lead to a monolayer on Si(111) surfaces. Only a very small amount of oxidation states of Si has been observed by infrared spectroscopic ellipsometry (IRSE) and synchrotron X-ray photoemission spectroscopy (SXPS). For the first time, IRSE and SXPS measurements reveal the “umbrella” vibrational mode characteristic from methyl groups and a well-defined spin-orbit splitting of the Si 2p core level emission, respectively, in the case of methylated Si(111) surfaces. For all ethynyl derivatives, high-resolution SXPS investigations reveal the incorporation of halogen atoms in the organic layers obtained. Thereby, exchanging Br for Cl in the Grignard compound leads to lower interface recombination rates at the polymer/Si interface. A shift in work function and surface band bending of up to 0.5 and 0.24 eV has been observed, respectively. The electronic properties reveal a linear relation between the work function and the surface dipole.

Monolayers of the cationic surfactant hexadecyltrimethylammonium halide (CTAX, where X = F¯, Cl¯, Br¯, and I¯) have been studied at the air-water and oilwater interfaces. At the air-water interface, the effects of the halide counterion and the addition of counterion were investigated. Sum-frequency spectroscopy (SFS), ellipsometry, and surface tensiometry indicated that the counterion changed the efficiency and effectiveness of the surfactant, both decreasing in the order of Br¯> Cl¯>F¯. The addition of salt in the form of 0.1 M KX was found to reduce the cmc but had little effect on the limiting area per molecule attained at the cmc, which increased from 44 Ų for CTAB to 65 Ų for CTAC and ca. 94 Ų for CTAF. Neither SFS nor ellipsometry provided any firm evidence for specific effects of the halide ions on the structure of the surfactant monolayers. For CTAB monolayers in the absence of excess electrolyte, the effect of area per molecule on the sum-frequency (SF) spectra was studied. Mixed monolayers of CTAB and tetradecane at the air-water interface exhibit a first-order phase transition from a conformationally disordered to a conformationally ordered state as the temperature is lowered. The phase transition occurs ca. 11 °C above the bulk melting point of tetradecane. A new experimental arrangement is described for acquiring SF spectra from surfactants at the oil-water interface. The key features of this approach are the stabilisation of a thin oil film between a sapphire prism and an aqueous phase, and the use of total internal reflection to enhance the total signal and discriminate against signals from other interfaces in the system. With this new methodology, the first SF vibrational spectra of surfactant monolayers at an alkane-water interface were obtained. Surface tensiometry was used to characterise the monolayers further. The structure of CTAB monolayers at the hexadecane-water interface was determined by SFS and compared with monolayers of CTAB at the air-water interface. At low concentrations, CTAB/hexadecane showed the expected features in the C-H stretching region, characteristic of a conformationally disordered monolayer. As the bulk concentration approached the critical micelle concentration, the spectra changed to one characteristic of a more ordered, upright conformation. Ellipsometric measurements supported this conclusion. This qualitative structural change is not observed in analogous monolayers at the air-water interface or CCl₄-water interface, or in surfactant solutions in contact with a hydrophobic solid surface.

This thesis deals with the analysis of nano-structured layers deposited by high-frequency magnetron sputtering on the monocrystalline silicon surface. The content of the work focuses on the magnetron sputtering application as an alternative method for passivation and antireflection layers deposition of silicon solar cells. The procedure of pre-deposite silicon surface cleaning by plasma etching in the Ar/H2 gas mixture atmosphere is suggested. In the next step the silicon nitride and aluminum nitride layers with hydrogen content in Ar/N2/H2 gas mixture by magnetron sputtering are deposited. One part of the thesis describes an experimental pseudo-carbide films deposition from a silicon target in the atmosphere of acetylene (C2H2). An emphasis is placed on the research of sputtered layers properties and on the conditions on the silicon-layer interface with the help of the standard as well as special measurement methods. Sputtered layers structure is analyzed by modern X-ray spectroscopy (XPS) and by Fourier infrared spectroscopy (FTIR). Optical ellipsometry and spectrophotometry is used for the diagnostic of the layers optical properties depending upon the wavelength of incident light. A special method of determining the surface lay-out of the charge´s carrier life in the volume and on the surface of silicon is employed to investigate the passivating effects of the sputtered layers.

Dans un monde de plus en plus pollué par l'activité industrielle, la détection des espèces gazeuses nocives dans l'atmosphère est d'une importance essentielle. Le marché des capteurs de gaz est déjà bien développé, avec la présence de diverses technologies et principes de détection, chacune présentant des avantages et des inconvénients intrinsèques. Dans le cadre de cette thèse, un alliage entre deux ou plusieurs technologies de détection typiquement utilisées de façon autonome a été visée, afin d'améliorer les performances globales des systèmes capteurs ainsi formées. A ce fin, nous avons conçu et étudié des dispositifs capteurs basées sur la transduction optique, couplée à un matériau sensible au gaz cible à détecter. Plus précisément, nous avons intégré pour la première fois un matériau catalyseur pouvant accélérer le taux d'oxydation des espèces chimiques (tel le monoxyde de carbone ou l'hydrogène) avec une architecture optique capable d'absorber la chaleur cédée lors de cette oxydation. L'augmentation de température occasionnée est traduite en une variation d'intensité lumineuse constituante le signal de sortie du capteur. Les travaux effectués sur les mesures de la dispersion thermique et chromatique de l'indice de réfraction des matériaux constituant le transducteur optique par des techniques d'optique guidé, ellipsométrie et des techniques photométriques sont présentés. Le sondage par moyen optique des propriétés électriques des matériaux semiconducteurs a également été étudié, y compris les variations de ces propriétés en présence des gaz oxydants, réducteurs et combustibles. / In a world suffering from increasing air pollution due to spiraling industrial activity, the detection of toxic gasses in the atmosphere is of paramount importance. The gas detector market is already well developed, and features a wide variety of detection technologies and techniques, each presenting its own set of intrinsic advantages and drawbacks. In this thesis, a combination of two or more technologies typically used independently has been studied in order to improve the global performances of gas detection systems. To this length, we have conceived and studied detector architectures based upon optical transduction systems, coupled with a material presenting a specific sensitivity to the target gas. More precisely, we have for the first time integrated a catalyst designed to accelerate the oxidation rate of chemical species (such as carbon monoxide or hydrogen) with an optical component capable of absorbing the heat generated by the oxidation reaction. The associated increase in temperature is translated to a variation of the optical intensity comprising the exit signal of the detector. The work carried out measuring the chromatic and temperature dispersion of the refractive index of the materials comprising the optical transduction component by guided mode techniques, ellipsometry and photometric techniques is presented. The optical probing of the electrical properties of semiconductor materials has also been studied, including the variations of these properties following interactions with oxidizing, reducing, or combustible gasses.