Cette thèse concerne la réalisation des films minces d’oxydes et l’étude de leurs propriétés physiques pour les cellules photovoltaïques (PV) et les modules thermoélectriques. Dans une première partie, les propriétés de l’oxyde de titane TiOx (1,45<x<2) sont mises en évidence pour une utilisation en tant qu’oxyde transparent conducteur optiquement actif à disposer en face avant des cellules PV ou, comme couche de couplage optique à intercaler entre le métal réflecteur et la couche absorbante d’une cellule PV. Les couches sont déposées par ablation laser pulse (PLD). Cette méthode permet d’obtenir des couches stoechiométriques ou déficitaires en oxygène grâce au contrôle de la pression d’oxygène pendant le dépôt. Les couches sont dopées par Nb pour un gain en conductivité électrique et/ou par Nd pour la conversion des photons UV en photons du Proche IR. Les films d’une part, isolants, transparents et luminescents ou d’autre part, conducteurs et absorbants ont été obtenus. La présence de polarons et/ou de bipolarons dans les couches TiO₁,₄₅₋₁,₆₀ explique la discontinuité observée sur leurs courbes de thermoconductivité. Une seconde partie du manuscrit concerne la thermoelectricité ou il est question de modifier les propriétés des cobaltites de calcium pour la conversion en énergie électrique des gradients de température faibles, centres autour de 300-365 K. Le contrôle de la concentration en oxygène des films a permis d’obtenir les phases polymorphes CaxCoO₂, Ca₃Co₄O₉, et Ca₃Co₄O₆,₄₋₆,₈ présentant des comportements semiconducteurs ou métalliques en fonction de la température de dépôt. Les films Ca₃Co₄O₆,₄₋₆,₈ montrent de faibles résistivités (3,8-6 mΩ.cm) et des coefficients de Seebeck élevés (S) ≥ 1000 μV/K qui doivent être confirmes pour que de tels films soient utilisés dans les thermogénérateurs. / This thesis concerns the realization of oxide thin films and the study of their properties for photovoltaic or thermoelectric devices. In the first part, the TiOx properties are studied for use as an optically active transparent conductive oxide to put in front of the PV cells or, as optical coupling layer to interpose between the metal reflector and the absorbent layer of a PV cell. The layers are deposited by pulsed laser deposition (PLD). This method allows to get stoichiometric or oxygen deficient layers by controlling the oxygen partial pressure during the growth. The layers are doped with Nb to enhance electrical conductivity and/or with Nd for the conversion of Ultra-Violet photons to Near Infra-Red photons. Insulating and transparent layers, luminescent layers or conducting and absorbent layers are obtained. The TiO₁,₄₅₋₁,₆₀ films show polaronic or bipolaronic conductivity and exhibited the jump of electrical conductivity with jump height and temperature depending on the nature of the dopants. A second part of the manuscript concerns thermoelectricity in which the properties of cobalt calcium oxide are modulated for an efficient conversion of low temperature gradients centered at 300-365K. The control of the oxygen concentration of films allows to obtain the polymorphic phases CaxCoO₂,Ca₃Co₄O₉ and Ca₃Co₄O₆,₄₋₆,₈ having metallic or semiconducting behavior depending on the deposition temperature. The Ca₃Co₄O₆,₄₋₆,₈ films show high Seebeck coefficients (S) ≥ 1 000 μV/K and low electrical resistivity (3.8 to 6 mΩ.cm). Such interesting values have to be confirmed by additional experiments in order to be used as thermoelectric films.

To support the growing global demand for energy, new sustainable solutions are needed both economically and environmentally. Thermoelectric waste heat recovery and energy harvesting could contribute by increasing industrial process efficiency, as well as powering stand-alone devices, microgenerators, and small body appliances.The structural complexity of quaternary chalcogenide materials provides an opportunity for engineering defects and disorder, to modify and possibly improve specific properties. Cu2ZnSnS4 (CZTS, often kesterite), valued for the abundance and non-toxicity of the raw materials, seems particularly suited to explore these possibilities, as it presents several structural defects and polymorphic phase transformations. The aim of this doctoral work is to systematically investigate the effects of structural polymorphism, disorder, and defects on the thermoelectric properties of CZTS, with particular emphasis to their physical origin. A remarkable case is the order-disorder transition of tetragonal CZTS, which is found responsible for a sharp enhancement in the Seebeck coefficient due to a flattening and degeneracy of the electronic energy bands. This effect, involving a randomization of Cu and Zn cations in certain crystallographic planes, is verified in bulk and thin film samples, and applications are proposed to exploit the reversible dependence of electronic properties on disorder. Low-temperature mechanical alloying is instead discovered stabilizing a novel polymorph of CZTS, which disordered cubic structure is studied in detail, and proposed deriving from sphalerite-ZnS. The total cation disorder in this compound provides an uncommon occurrence in thermoelectricity: a concurrent optimization of Seebeck coefficient, electrical and thermal conductivity. These findings, besides providing new and general understanding of CZTS, can cast light on profitable mechanisms to enhance the thermoelectric performance of semiconducting chalcogenides, as well as delineate alternative and fruitful applications.

Diese Arbeit demonstriert die vollständige thermoelektrische Charakterisierung einzelner einkristalliner Bismuttellurid- und Silbernanodrähte und deren anschließende lokale strukturelle und chemische Charakterisierung mittels analytischer Transmissionselektronenmikroskopie. Die lokale strukturelle, chemische und morphologische Charakterisierung entlang der Nanodrähte trägt essentiell zum Verständnis des thermoelektrischen Transportes bei und bestätigt die Korrelation zwischen Oberflächen- und den thermoelektrischen Eigenschaften. Für durchmesservariierte Bismuttelluridnanodrähte wird der Einfluss der Morphologie auf die Wärmeleitfähigkeit bei Raumtemperatur quantifiziert. Im Vergleich zu einem glatten Referenznanodraht zeigt der durchmesservariierte Nanodraht gleicher Zusammensetzung und Kristallorientierung eine Reduktion der Wärmeleitfähigkeit um 55 %. Diese Reduktion kann durch Phononenrückstreuung an der eingekerbten Oberfläche erklärt werden. Die elektrische Leitfähigkeit und der Seebeckkoeffizient der Bismuttelluridnanodrähte deuten auf einen topologischen Oberflächenzustand hin. Für Silbernanodrähte werden die elektrische Leitfähigkeit und die Wärmeleitfähigkeit im Temperaturbereich von 1,4 K bis 300 K gemessen. Mit fallender Temperatur steigt die relative Reduktion der Wärmeleitfähigkeit im Verhältnis zur elektrischen Leitfähigkeit stärker, sodass die Lorenzzahl die klassische Wiedemann-Franz-Relation nicht erfüllt und eine Funktion der Temperatur darstellt. Der Temperaturverlauf der Lorenzzahl der Silbernanodrähte entspricht der 1938 von Makinson aufgestellten Theorie für hochreine Metalle und ist im Tieftemperaturbereich um bis zu zwei Größenordnungen zum Sommerfeldwert reduziert. / This work demonstrates the full thermoelectric characterisation of individual single crystalline bismuth telluride and silver nanowires and their subsequent local structural and chemical characterisation via analytical transmission electron microscopy along the whole nanowires. Therefore, the correlation between the structure, in particular the surface morphology, and the thermoelectric transport properties is unambiguously shown. For diameter varied bismuth telluride nanowires the influence of the morphology on the thermal conductivity is quantified at room temperature. The diameter varied nanowire shows a reduction of 55 % with respect to the smooth nanowire of the same chemical composition and structural orientation. This reduction can be explained by phonon backscattering at the indents. The electrical conductivity and the Seebeck coefficient indicate the presence of a topological surface state. For silver nanowires the electrical and thermal conductivity are determined in the temperature range between 1.4 K and 300 K. With decreasing temperature the relative reduction of the thermal conductivity is higher than the reduction of the electrical conductivity resulting in a temperature-dependent Lorenz number, so that the classical Wiedemann-Franz relation is not fulfilled. The temperature characteristic of the silver nanowires'' Lorenz number is in agreement with the theory Makinson established for highly pure metals in 1938 and is reduced by two orders of magnitude with respect to the Sommerfeld value in the low temperature regime.

Increasing interest in an effciency enhancement of existing energy sources led to an extended research in the field of thermoelectrics. Especially skutterudites with their high power factor (electric conductivity times Seebeck coefficient squared) are suitable thermoelectric materials. However, a further improvement of their thermoelectric properties is necessary. The relatively high thermal conductivity can be decreased by introducing loosely bound guest ions, whereas atom substitution or nanostructuring (as thin films) could yield an increased power factor. The present work proves the feasibility to deposit single phase skutterudite thin films by MBE technique. In this regard CoSby and FeSby film series were deposited with three different methods: i) codeposition at elevated temperatures, ii) codeposition at room temperature followed by post-annealing, and iii) modulated elemental reactant method. The structural and thermoelectric properties of these films were investigated by taking the thermal stability of the film and the substrate properties into account. Compared to the stoichiometric Sb content of skutterudites of 75 at.%, a small excess of Sb is necessary for achieving single phase skutterudite films. It was found, that the deposited single phase CoSb3 films reveal bipolar conduction (and therefore a low Seebeck coefficient), whereas FeSb3 films show p-type conduction and very promising power factors at room temperature. The need of substrates with a low thermal conductivity and a suitable thermal expansion coefficient is also demonstrated. A high thermal conductivity influences the measurements of the Seebeck coefficient and the obtained values will be underestimated by thermal shortening of the film by the substrate. If the thermal expansion coefficient of film and substrate differ strongly from each other, crack formation at the film surface was observed. Furthermore, the realization of controlled doping by substitution as well as the incorporation of guest ions was successfully shown. Hence, this work is a good starting point for designing skutterudite based thin film structures. Two successful examples for such structures are given: i) a thickness series, where a strong decrease of the resistivity was observed for films with a thickness lower than 10nm, and ii) a FexCo1-xSb3 gradient film, for which the gradient was maintained even at an annealing temperature of 400°C.:Contents 1 Introduction 2 Nanostructured thermoelectric materials 2.1 Thermoelectric materials and ZT 2.2 Recent developments in improving ZT in thin films 3 Thermoelectric transport theory 3.1 Electronic transport coefficients 3.2 Lattice thermal conductivity 4 Skutterudites as promising thermoelectric material 4.1 CoSb3 4.1.1 Structural properties of skutterudites 4.1.2 Band structure of CoSb3 and density of states 4.1.3 Thermoelectric properties of CoSb3 4.1.4 Synthesis of CoSb3 thin films 4.2 FeSb3 4.2.1 Structural and thermoelectric properties of FeSb3 thin films 4.2.2 Synthesis of FeSb3 thin films 5 Experimental methods 5.1 Basic methods for structural characterization 5.2 Electric characterization: Resistivity and Hall measurements using van der Pauw geometry 5.3 Thermoelectric characterization (Seebeck coefficient) 5.4 Thermal characterization methods 6 Deposition of skutterudite thin films 6.1 Deposition chamber and deposition parameters 6.2 Deposition methods 6.3 Composition control of skutterudite films 7 Control of structural properties by the used deposition method 7.1 Structural properties of CoSb3 thin films 7.1.1 Crystallization characteristics of CoSb3 films 7.1.2 Comparison of films deposited with different deposition methods 7.1.3 Influence of different deposition parameters on the film properties 7.2 Structural properties of FeSb3 thin films 7.2.1 Crystallization behaviour 7.2.2 Structural properties of post-annealed Fe-Sb films prepared by codeposition 7.2.3 Influence of the heating rate on the film properties 8 CoSb3 and FeSb3 composition series 8.1 CoSby composition series 8.1.1 Films deposited at elevated temperatures 8.1.2 Annealed films 8.2 FeSby composition series 9 Influence of various substrates on the film properties 9.1 Substrate influence on the film morphology 9.2 Substrate influence on thermoelectric properties and measurements 10 FexCo1-xSb3 - controlled doping by substitution of Co with Fe 10.1 Properties of codeposited FexCo1-xSb3 films 10.2 Properties of FexCo1-xSb3 films deposited via MERM 11 Filled CoSb3 thin films 12 Examples for nanostructured thin film approaches 12.1 CoSb3 thickness series 12.2 FexCo1-xSb3 gradient films 13 Summary and Outlook