Colloidal quantum dots (CQDs) are potentially low-cost, solution-processable semiconductors which are endowed, through their nanoscale dimensions, with strong absorption, band gap tunability, high dielectric constants and enhanced stability. CQDs are contenders as a standalone PV technology as well as a potential back layer for augmenting established photovoltaic (PV) technologies, such as Si. However, owing to their small size (ca. few nanometers), CQDs are prone to surface trap states that inhibit charge transport and threaten their otherwise wonderful optoelectronic properties. Surface traps have also, indirectly, impeded scalable and industry-compatible fabrication of these solar cells, as all of the reports, to date, have relied on spin-coating with sophisticated and tedious ligand exchange schemes, some of which need to be performed in low humidity environments. In this thesis, we posit that an in-depth understanding of the process-structure-property-performance relationship in CQDs can usher in fresh insights into the nature and origin of surface traps, lead to novel ways to mitigate them, and finally help achieve scalable fabrication. To this end, we probe the CQD surfaces and their interactions with process solvents, linkers, and ambient environment employing a suite of spectroscopic techniques. These fundamental insights help us develop facile chemical and physical protocols to mitigate surface traps such as solvent engineering, remote molecular doping, and oxygen doping, directly leading to better-performing solar cells. Our efforts finally culminate in the realization of >10% efficient, air-stable CQD solar cells scalably fabricated in an ambient environment of high, uncontrolled R.H. (50-65%). As-prepared solar cells fabricated in high humidity ambient conditions are found to underperform, however, an oxygen-doping recipe is devised to mitigate the moisture-induced surface traps and recover device performances. Importantly, these solar cells are fabricated at coating speeds of >15 m min-1 with roll-to-roll compatible techniques such as blade and bar coating requiring 1/25th the CQD material consumed by the standard spin-coated devices, overcoming the two major challenges of manufacturability and scalability faced by CQD PV.

No description available.

Tato diplomová práce se zabývá dotováním grafenu nízkoenergiovými elektrony. Na křemíkový substrát pokrytý vrstvou SiO2 jsou pomocí litograficky vyrobené masky nadeponované kovové kontakty z titanu a zlata. Grafen vyrobený pomocí metody depozice z plynné fáze je přenesen na substrát a slouží jako vodivé spojení kovových elektrod, které vytvářejí kolektor a emitor. Na křemík je ze spodu přivedeno napětí, které tak vytváří spodní hradlo. Takto vytvořený grafenový tranzistor je ozařován nízkoenergiovými elektrony, které mění dotování grafenu. Z polohy maxima v závislosti odporu grafenu na hradlovém napětí lze vyčíst typ dotování. Toto maximum udává napětí, při kterém Fermiho meze grafenu prochází Diracovým bodem v pásové struktuře grafenu. Velikost hradlového napětí, primární energie elektronového svazku a proud svazku jsou tři parametry, které mají velký vliv na změny dotování. Při ozařování transistoru dochází ke změně typu dotování právě tehdy, když odpor grafenu v závislosti na hradlovém napětí dosáhne maxima. Vývoj této změny je zkoumán pro různé energie a proudy primárního svazku v závislosti na hradlovém napětí i v čase. Typ dotování je také prozkoumán při zastavení ozařování v různých fázích smyčky hradlového napětí. Dopování grafenu nízkoenergiovými elektrony je popsáno v teoretickém modelu.

This diploma thesis is focused on fabrication of graphene field-effect transistors, characterisation of their transport properties and investigation of low-energy electron beam influence on the devices' properties under UHV conditions. The theoretical part of this work describes graphene fabrication methods, options of graphene transfer onto the substrates for graphene field-effect transistor manufacture. Furthermore, model of graphene doping via electrostatic interaction or photon, resp. electron beam exposition is explained. The experimental part of this work consist of manufacture of the graphene field-effect transistor in order to examine the change of its transport properties induced by doping of the graphene via low-energy electron beam exposition.

The presented diploma thesis is focused on the preparation and properties of transparent polycrystalline ceramic materials based on Al2O3. Theoretically, the most important technological aspects of the processing of these materials are presented. Detailed attention is given to transparent Al2O3 polycrystalline ceramics doped with rare earth elements. The influence of microstructural parameters on the optical properties (represented by RIT) is investigated on Al2O3 samples doped and codoped with dysprosium, terbium and chromium. A significant effect of the average grain size on the light transmittance of all samples is observed. The highest RIT = 55 % (measured by a laser beam with a wavelength of 632,8 nm) was achieved by an optimized preparation process for a sample doped with 0,05 at. % of dysprosium. For all samples photoluminescent properties are also analysed. The photoluminescent emission spectra correspond to the activation of doping elements. In case of the terbium and chromium codoped samples, the differences in the activation of individual dopants depending on different excitation wavelengths were demonstrated, resulting in different colour emissions for different excitation wavelengths.

This diploma thesis deals with the preparation of doped piezoceramic materials based on BaTiO3 using electrophoretic deposition. Five rare earth oxides, i.e. Er2O3, Dy2O3, Eu2O3, Tb407 and CeO2, were used as dopants in amounts of 1, 3, and 5 wt. %. The prepared deposits were evaluated in terms of preparation methodology, high temperature dilatometry, X-ray diffraction analysis, relative density, mean grain size, hardness and fractographic analysis. The study of dilatometric curves described the sintering behavior and its changes at different material compositions. X-ray diffraction analysis revealed a tetragonal phase in all samples; the tetragonality of the BaTiO3 crystalline cell decreased with dopant content. By a suitable choice of dopant, it was possible to significantly increase the relative density of sintered samples, their hardness and at the same time prevent the samples from coarsening of the microstructure during heat treatment. A relative density up to 98 %, a mean grain size below 1 m and a hardness of over 10 GPa were achieved. Analysis of the fracture surfaces revealed that the fracture mode was transcrystalline for the most of studied materials; only the samples doped with cerium dioxide had fracture surfaces with both transcrystalline and intercrystalline fracture modes. Based on the obtained results, a suitable composition of the material for the intended use in a layered piezoceramic harvester was identified, which, in addition to the BaTiO3 layers, consists of functionally-protective Al2O3 and ZrO2 layers.

This master‘s thesis deals with study of basic physical, microstructural and mechanical properties of doped piezoceramic materials based on BaTiO3 prepared by electrophoretic deposition. The dopants used were rare earth oxides, i.e. Eu2O3, Er2O3, CeO2, Dy2O3 and Tb4O7 in amounts 1, 3 and 5 wt. %. The influence of dopants and their amount on density, phase composition, mean grain size, hardness, elastic modulus, fracture toughness, and flexural strength was examined. Suitable dopant choice enabled decrease in mean grain size and increase in relative density, hardness, elastic modulus, fracture toughness and flexural strength of sintered specimens. Relative densities up to 99 %, mean grain size below 1 m, hardness up to 13,1 GPa, elastic modulus up to 199 GPa, fracture toughness above 1 MPa·m1/2 and flexural strength above 115 MPa were achieved.

Confiner la lumière dans un volume réduit d'absorbeur photovoltaïque offre de nouvelles voies pour les cellules solaires à haute rendement. Ceci peut être réalisé en utilisant des nanostructures pour le piégeage optique ou des nanofils de semi-conducteurs. Dans une première partie, nous présentons la conception et la fabrication de cellules solaires ultra-minces (205 nm) en GaAs. Nous obtenons des résonances multiples grâce à un miroir arrière nanostructuré en TiO2/Ag fabriqué par nanoimpression, résultant en un courant de court-circuit élevé de 24,6 mA/cm². Nous obtenons le record d’efficacité de 19,9%. Nous analysons les mécanismes des pertes et nous proposons une voie réaliste vers un rendement de 25% en utilisant un absorbeur de GaAs de 200 nm d'épaisseur seulement. Dans une deuxième partie, nous étudions les propriétés de nanofils en GaAs crûs sur substrats Si et nous explorons leur potentiel comme absorbeur photovoltaïque. Un dopage élevé est souhaité dans les cellules solaires à nanofils en jonction coeur-coquille, mais la caractérisation à l'échelle d'un nanofil unique reste difficile. Nous montrons que la cathodoluminescence (CL) peut être utilisée pour déterminer les niveaux de dopage de GaAs de type n et p avec une résolution nanométrique. Les semi-conducteurs III-V de type n présentent une émission décalée vers le bleu, à cause du remplissage de la bande de conduction, tandis que les semi-conducteurs de type p présentent une émission décalée vers le rouge due à la réduction du gap. La loi de Planck généralisée est utilisée pour fitter tout le spectre et ainsi évaluer quantitativement le niveau de dopage. Nous utilisons également la polarimétrie de CL pour déterminer sélectivement les propriétés de phases wurtzite/zinc-blende d'un nanofil unique. Nous montrons enfin des cellules solaires fonctionnelles à nanofils de GaAs. Ces travaux ouvrent des perspectives vers une nouvelle génération de cellules photovoltaïques. / Confining sunlight in a reduced volume of photovoltaic absorber offers new directions for high-efficiency solar cells. This can be achieved using nanophotonic structures for light trapping, or semiconductor nanowires. First, we have designed and fabricated ultrathin (205 nm) GaAs solar cells. Multi-resonant light trapping is achieved with a nanostructured TiO2/Ag back mirror fabricated using nanoimprint lithography, resulting in a high short-circuit current of 24.6 mA/cm². We obtain the record 1 sun efficiency of 19.9%. A detailed loss analysis is carried out and we provide a realistic pathway toward 25% efficiency using only 200 nm-thick GaAs absorber. Second, we investigate the properties of GaAs nanowires grown on Si substrates and we explore their potential as active absorber. High doping is desired in core-shell nanowire solar cells, but the characterization of single nanowires remains challenging. We show that cathodoluminescence (CL) mapping can be used to determine both n-type and p-type doping levels of GaAs with nanometer scale resolution. n-type III-V semiconductor shows characteristic blueshift emission due to the conduction band filling, while p-type semiconductor exhibits redshift emission due to the dominant bandgap narrowing. The generalized Planck’s law is used to fit the whole spectra and allows for quantitative doping assessment. We also use CL polarimetry to determine selectively the properties of wurtzite and zincblende phases of single nanowires. Finally, we demonstrate successful GaAs nanowire solar cells. These works open new perspectives for next-generation photovoltaics.

Ce travail s’est articulé autour de la modulation des propriétés électroniques du graphène. Un des objectifs visés étant la conception d’électrodes transparentes pour des applications photovoltaïques. Différentes techniques de dopage ont été utilisées pour la modulation du travail de sortie (WF) et de la mobilité électronique comme l’incorporation d’azote in-situ lors de la croissance, l’incorporation d’azote ex-situ par acide nitrique et acide aurique. Diverses techniques de caractérisation ont été employées notamment la microscopie à force atomique AFM/CPAFM, la spectroscopie Raman, la spectroscopie photoélectronique (XPS et UPS), les mesures de transport électrique par effet Hall et effet de champ. Ces techniques nous ont permis de déterminer l’homogénéité, la qualité cristalline, la variation de densité de charges électronique, la résistance électrique et la mobilité électronique des différents matériaux intrinsèques et dopés. Par ailleurs, nous avons montré qu’il était possible de moduler le travail de sortie du graphène CVD en déposant par PECVD du silicium amorphe dopé N ou P sans endommager le graphène. Cette approche présente un intérêt particulier pour la substitution de l’ITO par le graphène en tant qu’électrode transparente. Les mesures de transport électronique ont mis en évidence un transfert de charges à l’interface de l’hétérojonction graphène/silicium amorphe. Cette variation dépend non seulement du type du dopage du silicium amorphe mais aussi de la cristallinité de ce dernier, ainsi peut-on espérer réduire la résistivité d’une électrode pour cellule photovoltaïque. / This work was structured around the modulation of the electronic properties of graphene obtained via the CVD growth on copper substrate and/or the graphitization of the carbon atoms in the SiC substrate. One of the objectives was the design of electrodes (front or rear) for photovoltaic cells, among other applications. Different doping techniques have been implemented for modulating the work function (WF) and the electron mobility i.e. the incorporation of nitrogen in-situ during the growth, ex-situ incorporation by nitric acid and/or nano gold colloids (AuCl3). In this work, various characterization techniques were employed including atomic force microscopy (AFM), Raman spectroscopy, photoelectron spectroscopy (XPS and UPS), electrical transport measurements by Hall and field effect. These techniques have enabled us to determine the homogeneity , thecrystalline quality of the material, the carrier density, the electrical resistance and the electron mobility of different intrinsic and doped samples. Furthermore, we showed that it is possible to modulate the WF graphene by fabricating a heterostructure composed of PECVD amorphous silicon doped N or P deposited onto the graphene. This approach is of particular interest for replacement of ITO with graphene as transparent electrode. This result was confirmed by the study detailed spectra of the XPS and Raman vibrational states. The electronic transport measurements showed a charge transfer at the interface of the heterojunction graphene/amorphous silicon. The variation observed depends not only on the type of doping of the amorphous silicon but also on the crystallinity of the latter. This approach can readily be adapted to photovoltaic devices.

Materials that are light weight, low cost and have high hydrogen storage capacity are essential for on-board vehicular applications. Some reversible complex hydrides are alanates and amides but they have lower capacity than the DOE target (6.0 wt %) for 2010. High capacity, light weight, reversibility and fast kinetics at lower temperature are the primary desirable aspects for any type of hydrogen storage material. Borohydride complexes as hydrogen storage materials have recently attracted great interest. Understanding the above parameters for designing efficient complex borohydride materials requires modeling across different length and time scales. A direct method lattice dynamics approach using ab initio force constants is utilized to calculate the phonon dispersion curves. This allows us to establish stability of the crystal structure at finite temperatures. Density functional theory (DFT) is used to calculate electronic properties and the direct method lattice dynamics is used to calculate the finite temperature thermodynamic properties. These computational simulations are applied to understand the crystal structure, nature of bonding in the complex borohydrides and mechanistic studies on doping to improve the kinetics and reversibility, and to improve the hydrogen dynamics to lower the decomposition temperature. A combined theoretical and experimental approach can better lead us to designing a suitable complex material for hydrogen storage. To understand the structural, bulk properties and the role of dopants and their synergistic effects on the dehydrogenation and/or reversible rehydrogenation characteristics, these complex hydrides are also studied experimentally in this work.