The energy requirements of our planet will continue to grow with increasing world population and the modernization of currently underdeveloped countries. This will force us to search for environmental friendly alternative energy resources. Solar energy by far provides the largest of all renewable energy resources with an average power of 120 000 TW irradiated from the sun which can be exploited through solar electricity, solar fuel, and biomass. Nanostructured materials have been the subject of extensive research as the building block for construction of solar energy conversion devices for the past decades. The nanostructured materials sometimes have peculiar electrical and optical properties that are often shape and size dependent and are not expected in the bulk phase. Recent research has focused on new strategies to control nanostructured morphologies and compositions of semiconductor materials to optimize their solar conversion efficiency. In this dissertation, we discuss the synthesis and characterizations of one dimensional nanostructured TiO₂ based materials and their solar energy conversion applications. We have developed a solvothermal synthesis method for growing densely packed, vertical, single crystalline TiO₂ rutile nanowire arrays with unprecedented small feature sizes of 5 nm and lengths up to 4.4 [mu]m. Because of TiO₂'s large band gap, the working spectrum of TiO₂ is limited to the ultra violet region with photons shorter than 420 nm. We demonstrate that the active spectrum of TiO₂ can be shifted to ~ 520 nm with incorporation of N via nitridation of TiO₂ nanowires in NH₃ flow. In addition, we demonstrate a synergistic effect involving hydrogenation and nitridation cotreatment of TiO₂ nanowires that further redshift the active spectrum of TiO₂ to 570 nm. The Ta and N co-incorporated TiO₂ nanowires were also prepared and showed significant enhancement in photoelectrochemical performance compared to mono-incorporation of Ta or N. This enhancement is due to fewer recombination centers from charge compensation effects and suppression of the formation of an amorphous layer on the nanowires during the nitridation process. Finally, we have developed hydrothermal synthesis of single crystalline TiO₂ nanoplatelet arrays on virtually all substrates and demonstrated their applications in water photo-oxidation and dye sensitized solar cells. / text

No description available.

Zinc Oxide (ZnO) is one of the most extensively studied semiconductors because of its unique properties, namely, its wide band gap (3.37 eV) and high excitation binding energy (60 meV). These properties make ZnO a promising material for uses in a broad range of applications including sensors, catalysis and optoelectronic devices. The presented research covers a broad spectrum of these interesting nanomaterials, from their synthesis and characterization to their use as photocatalyts. A new synthetic approach for producing morphology controlled ZnO nanostructures was developed using microwave irradiation (MWI). The rapid decomposition of zinc acetate in the presence of a mixture of oleic acid (OAC) and oleylamine (OAM) results in the formation of hexagonal ZnO nanopyramids and ZnO rods of varying aspect ratios. The factors that influence the morphology of these ZnO nanostructures were investigated. Using ligand exchange, the ZnO nanostructures can be dispersed in aqueous medium, thus allowing their use as photocatalysts for the degradation of malachite green dye in water. Photocatalytic activity is studied as a function of morphology; and, the ZnO nanorods show enhanced photocatalytic activity for the degradation of the dye compared to hexagonal ZnO nanopyramids. After demonstrating the catalytic activity of these ZnO nanostructures, various ways to enhance photocatalytic activity were studied by modification of this MWI method. Photocatalytic activity is enhanced through band gap modulation and the reduction of electron-hole recombination. Several approaches were studied, which included the incorporation of Au nanoparticles, N-doping of ZnO, supporting ZnO nanostructures on reduced graphene oxide (RGO), and supporting N-doped ZnO on N-doped RGO. ZnO-based nanostructures were studied systematically through the entire process from synthesis and characterization to their use as photocatalysis. This allows for a thorough understanding of the parameters that impact these processes and their unique photocatalytic properties.

This dissertation discusses the influence of nitrogen doping (N-doping) on the electronic and catalytic properties of carbon nanotubes (CNTs). These properties have been studied using a variety of techniques, in order to both qualitatively and quantitatively analyze the relationship between the nitrogen concentration and observed properties. Chapter 1 provides a general overview of CNTs and N-doping and details some of the previous research from our group. Chapter 2 discusses the assembly and characterization of free-standing electrode mats, which are used in order to understand the intrinsic physicochemical properties of the material without relying on the secondary influence of another conductive support. Raman microscopy, X-Ray photoelectron spectroscopy, scanning and scanning-tunneling electron microscopy, as well as electrochemical methods were all used to demonstrate the viability of the mat electrodes for further experiments. Chapter 3 addresses the examination of a range of nitrogen concentrations in order to better understand the effects of nitrogen concentration on the electrochemical and electrical properties such as the differential capacitance, density of states at the Fermi level (D(E[subscript F])), bulk conductivity and work function. These properties were studied using a variety of techniques, including UV-photoelectron spectroscopy, electrochemical impedance spectroscopy and conductive four point probe. Chapter 4 investigates the inherent catalysis of the nitrogen doped CNTs (N-CNTs) with respect to O2 reduction, and a complex mechanism is proposed. Electrochemical methods such as cyclic and linear sweep voltammetries as well as thermo-gravimetric analysis and gasometric analysis were all employed to determine heterogeneous decomposition rates as well as to detect intermediates of the O₂ reduction reaction. Chapter 5 discusses the electrocatalytic degradation of free cyanide (CN⁻) at the N-CNT mat electrodes. These results both provide further support for the mechanism discussed in Chapter 4, and present the opportunity for a potential application of N-CNTs for environmental purposes. Specifically, spectroscopic and electrochemical methods, in conjunction with theoretical models show both that the presence of CN⁻ does not inhibit O2 reduction, and that it can be effectively converted to cyanate (OCN⁻) at the N-CNT electrodes. Future work involving the assembly and characterization of transparent N-CNT films is discussed in Chapter 6. / text

Le germanium dopé n et contraint en tension est un candidat potentiel pour démontrer un laser sur silicium compatible avec un environnement CMOS. Dans ce travail de thèse, j’ai d’abord développé un formalisme qui permet de calculer le gain optique en fonction de la déformation en tension, du dopage n et de l’injection des porteurs. Une technique de transfert de déformation via le dépôt de couche contrainte de SiN a été optimisée. J’ai réalisé plusieurs types de cavités germanium contraintes sous forme de guides d’onde et de microdisques. Le transfert de déformation a été optimisé par sous-gravure et par une méthode de bi-encapsulation qui a permis d’aboutir à une déformation biaxiale homogène et élevée de l’ordre de 1.5%. L’évaluation des déformations a été confrontée à des simulations par éléments finis, photoluminescence et spectroscopie Raman. L’étude expérimentale et théorique des guides d’onde a montré l’avantage de la direction <100> par rapport à la direction <110> permettant une injection plus efficace de porteurs en centre de zone. L’étude expérimentale des microdisques a permis d’observer des modes de galerie avec un facteur de qualité Q = 1540 à λ = 1940 nm. D’autre part, j’ai mis en évidence par photoluminescence la présence d’un fort dopage de 4×10¹⁹ cm⁻³ dans des couches germanium sur silicium épitaxiées par épitaxie par jets moléculaires utilisant une technique de co-dopage. Une modélisation du gain modal a permis de mettre en exergue l’effet du gradient de déformation dans le volume de la cavité. L’élargissement homogène a été introduit dans la modélisation du gain optique afin de prendre en compte l’impact d’un dopage élevé. / Tensile strained and n-doped germanium is a potential candidate to demonstrate a laser on silicon in a CMOS-compatible environment. In this thesis, I developed a formalism to calculate the optical gain as a function of tensile strain, n-doping and carrier injection. A tensile strain transfer technique via strained SiN layer deposition has been optimized. I realized several types of strained germanium cavities. Tensile strain transfer was optimized by under-etching and a bi-encapsulation technique which allowed to achieve a high and uniform biaxial strain up to 1.5%. The evaluation of strain level was faced with finite elements modeling, photoluminescence and Raman spectroscopy. The experimental and theoretical study of the waveguides showed the advantage of the <100> direction as compared with the <110> direction for more efficient carrier injection at zone center. The experimental study of microdisks allowed us to observe gallery modes with quality factor up to Q = 1540 at λ= 1940 nm. On the other hand, photoluminescence enhancement has shown the presence of a heavy doping of 4×10¹⁹ cm⁻³ in germanium on silicon layers grown by molecular beam epitaxy and using a co-doping technique. Modeling the modal gain helped to emphasize the effect of the strain gradient in the cavity volume. The homogeneous broadening was introduced in the optical gain modeling to take into account the impact of a high doping.

This thesis deals with the characterization and implementation of transparent electron transport materials (ETM) in vacuum deposited p-i-n type organic solar cells (OSC) for substituting the parasitically absorbing standard ETM composed of n-doped C60. In addition to transparency in the visible range of the sun spectrum, the desired material properties include high electron mobility and conductivity, thermal and morphological stability, as well as good energy level alignment relative to the adjacent acceptor layer which is commonly composed of intrinsic C60. In this work, representatives of three different material classes are evaluated with regard to the above mentioned criteria. HATCN (hexaazatriphenylene hexacarbonitrile) is a small discoid molecule with six electron withdrawing nitrile groups at its periphery. It forms smooth thin films with an optical energy gap of 3.3eV, thus being transparent in the visible range of the sun spectrum. Doping with either 5wt% of the cationic n-dopant AOB or 7wt% of the proprietary material NDN1 effectively increases the conductivity to 7.6*10^-6 S/cm or 2.2*10^-4 S/cm, respectively. However, the fabrication of efficient OSC is impeded by the exceptionally high electron affinity (EA ) of approximately 4.8eV that causes the formation of an electron injection barrier between n-HATCN and intrinsic C60 (EA=4.0eV). This work presents a strategy to remove the barrier by introducing doped and undoped C60 intermediate layers, thus demonstrating the importance of energy level matching in a multi-layer structure and the advantages of Fermi level control by doping. Next, a series of six Bis-Fl-NTCDI (N,N-bis(fluorene-2-yl)-naphthalenetetracarboxylic diimide) compounds, which only differ by the length of the alkyl chains attached to the C9 positions of the fluorene side groups, is examined. When increasing the chain length from 0 to 6 carbon atoms, the energy levels remain nearly unchanged: We find EA=3.5eV as estimated from cyclic voltammetry, an ionization potential (IP ) in the range between 6.45eV and 6.63eV, and Eg,opt=3.1eV which means that all compounds form transparent thin films. Concerning thin film morphology, the addition of side chains results in the formation of amorphous layers with a surface roughness <1nm on room temperature glass substrates, and (1.5+/-0.5)nm for deposition onto glass substrates heated to 100°C. In contrast, films composed of the side chain free compound Bis-HFl-NTCDI exhibit a larger surface roughness of (2.5+/-0.5)nm and 9nm, respectively, and are nanocrystalline already at room temperature. Moreover, the conductivity achievable by n-doping is very sensitive to the side chain length: Whereas doping of Bis-HFl-NTCDI with 7wt% NDN1 results in a conductivity in the range of 10^-4 S/cm, the attachment of alkyl chains causes a conductivity which is more than three orders of magnitude smaller despite equal or slightly higher doping concentrations. The insufficient transport properties of the alkylated derivatives lead to the formation of pronounced s-kinks in the jV -characteristics of p-i-n type OSC while the use of n-Bis-HFl-NTCDI results in well performing devices. The last material, HATNA-Cl6 (2,3,8,9,14,15- hexachloro-5,6,11,12,17,18-hexaazatrinaphthylene), exhibits Eg,opt=2.7eV and is therefore not completely transparent in the visible range of the sun spectrum. However, its energy level positions of EA=4.1eV and IP=7.3eV are well suited for the application as ETM in combination with i-C60 as acceptor. The compound is dopable with all available n-dopants, resulting in maximum conductivities of sigma=1.6*10^-6, 3.5*10^-3, and 7.5*10^-3 S/cm at 7.5wt% AOB, Cr2(hpp)4, and NDN1, respectively. Applying n-HATNA-Cl6 instead of the reference ETM n-C60 results in a comparable or improved photocurrent density at an ETM thickness d(ETM)=40nm or 120nm, respectively. At d(ETM)=120nm, the efficiency eta is more than doubled as it increases from eta(n-C60)=0.4% to eta(n-HATNA-Cl6)=0.9% . Optical simulations show that the replacement of n-C60 by n-Bis-HFl-NTCDI, n-HATNA-Cl6, or the previously studied n-NTCDA (naphthalenetretracarboxylic dianhydride) in p-i-n or n-i-p type device architectures is expected to result in an increased photocurrent due to reduced parasitic absorption. For quantifying the gain, the performance of p-i-n type OSC with varying ETM type and thickness is evaluated. Special care has to be taken when analyzing devices comprising the reference ETM n-C60 as its conductivity is sufficiently large to extend the area of the aluminum cathode and thus the effective device area which may lead to distorted results. Overall, the experiment is able to confirm the trends predicted by the optical simulation. At large ETM thickness in the range between 60 and 120nm, the window layer effect of the ETM is most pronounced. For instance, at d(ETM)=120nm, eta(C60) is more than doubled using n-HATNA-Cl6 and even more than tripled using n-Bis-HFl-NTCDI or n-NTCDA. At optimized device geometry the photocurrent gain is slightly less than expected but nonetheless, the efficiency is improved from eta(max)=2.1% for n-C60 and n-HATNA-Cl6 solar cells to eta(max)=2.3, and 2.4% for n-Bis-HFl-NTCDI and n-NTCDA devices, respectively. This development is supported by generally higher Voc and FF in solar cells with transparent ETM. Finally, p-i-n type solar cells with varying ETM are aged at a temperature of 50°C and an illumination intensity of approximately 2 suns. Having extrapolated lifetimes t(80) of 36, 500, and 14000h and nearly unchanged jV-characteristics after 2000h, n-C60 and n-Bis-HFl-NTCDI devices exhibit the best stability. In contrast, n-NTCDA devices suffer from a constant decrease in Isc while n-HATNA-Cl6 solar cells show a rapid dscegradation of both Isc and FF associated with a decomposition of the material or a complete de-doping of the ETM. Here, lifetimes of only 4500h and 445hare achieved.

Ce travail se concentre sur l'ingénierie Intersubband (ISB) des nanofils où nous avons conçu des hétérostructures de GaN / (Al, Ga) N intégrées dans un nanofil GaN pour le rendre optiquement actif dans la région spectrale infrarouge (IR), en utilisant un faisceau moléculaire assisté par plasma épitaxie comme méthode de synthèse. Les transitions ISB se réfèrent aux transitions d'énergie entre les niveaux confinés quantiques dans la bande de conduction de la nanostructure.Un contrôle précis des niveaux élevés de dopage est crucial pour les dispositifs ISB. Par conséquent, nous explorons Ge comme un dopant alternatif pour GaN et AlGaN, pour remplacer le Si couramment utilisé. Nous avons cultivé des couches minces de GaN dopé Ge avec des concentrations de porteurs atteignant 6,7 × 1020 cm-3 à 300 K, bien au-delà de la densité de Mott, et nous avons obtenu des couches minces conductrices AlxGa1-xN dopées Ge avec une fraction molaire Al jusqu'à x = 0,64. Dans le cas de GaN, la présence de Ge n'affecte pas la cinétique de croissance ou les propriétés structurales des échantillons. Cependant, dans des échantillons AlxGa1-xN dopés par Ge avec x> 0,4, la formation de grappes riches en Ge a été observée, avec une baisse de la concentration du porteur.Ensuite, nous avons réalisé une étude comparative du dopage Si vs Ge dans des hétérostructures GaN / AlN pour des dispositifs ISB dans la gamme IR à courte longueur d'onde. Nous considérons les architectures planaire et nanofils avec des niveaux de dopage et des dimensions de puits identiques. Sur la base de cette étude, nous pouvons conclure que les deux Si et Ge sont des dopants appropriés pour la fabrication d'hétérostructures GaN / AlN pour l'étude des phénomènes optoélectroniques ISB, à la fois dans les hétérostructures planaires et nanofils. Dans cette étude, nous rapportons la première observation de l'absorption d'ISB dans des puits quantiques GaN / AlN dopés au Ge et dans des hétérostructures de nanofils GaN / AlN dopés au Si. Dans le cas des nanofils, nous avons obtenu une largeur de ligne d'absorption ISB record de l'ordre de 200 meV. Cependant, cette valeur est encore plus grande que celle observée dans les structures planaires, en raison des inhomogénéités associées au processus de croissance auto-assemblé.En essayant de réduire les inhomogénéités tout en gardant les avantages de la géométrie des nanofils, nous présentons également une analyse systématique de l'absorption de l'ISB dans les micro et nanopillars résultant d'un traitement top-down des hétérostructures planaires GaN / AlN. Nous montrons que lorsque l'espacement du réseau de piliers est comparable aux longueurs d'onde sondées, les résonances des cristaux photoniques dominent les spectres d'absorption. Cependant, lorsque ces résonances sont à des longueurs d'onde beaucoup plus courtes que l'absorption ISB, l'absorption est clairement observée, sans aucune dégradation de son amplitude ou de sa largeur de raie.Nous explorons également la possibilité d'étendre cette technologie de nanofils à des longueurs d'onde plus longues, pour les absorber dans la région IR à mi-longueur d'onde. En utilisant des hétérostructures de nanofils GaN / AlN, nous avons fait varier la largeur du puits GaN de 1,5 à 5,7 nm, ce qui a conduit à un décalage rouge de l'absorption ISB de 1,4 à 3,4 μm. Remplaçant les barrières AlN par Al0.4Ga0.6N, le composé ternaire représente une réduction de la polarisation, ce qui conduit à un nouveau décalage rouge des transitions ISB à 4,5-6,4 um.L'observation de l'absorption de l'ISB dans des ensembles de nanofils nous a motivés pour le développement d'un photodétecteur infrarouge à puits quantiques à base de nanofils. La première démonstration d'un tel dispositif, incorporant une hétérostructure de nanofils GaN / AlN qui absorbe à 1,55 μm, est présentée dans ce manuscrit. / Due to its novel properties nanowires have emerged as promising building blocks for various advanced device applications. This work focuses on Intersubband (ISB) engineering of nanowires where we custom design GaN/(Al,Ga)N heterostructures to be inserted in a GaN nanowire to render it optically active in the infrared (IR) spectral region. ISB transitions refer to energy transitions between quantum confined levels in the conduction band of the nanostructure. All the structures analised in this thesis were synthesized by plasma-assisted molecular beam epitaxy.Precise control of high doping levels is crucial for ISB devices. Therefore, we explored Ge as an alternative dopant for GaN and AlGaN, to replace commonly-used Si. We grew Ge-doped GaN thin films with carrier concentrations of up to 6.7 × 1020 cm−3 at 300 K, well beyond the Mott density, and we obtained conductive Ge-doped AlxGa1-xN thin films with an Al mole fraction up to x = 0.66. In the case of GaN, the presence of Ge does not affect the growth kinetics or structural properties of the samples. However, in Ge doped AlxGa1-xN samples with x > 0.4 the formation of Ge rich clusters was observed, together with a drop in the carrier concentration.Then, we performed a comparative study of Si vs. Ge doping in GaN/AlN heterostructures for ISB devices in the short-wavelength IR range. We considered both planar and nanowire architectures with identical doping levels and well dimensions. Based on this study, we concluded that both Si and Ge are suitable dopants for the fabrication of GaN/AlN heterostructures for the study of ISB optoelectronic phenomena, both in planar and nanowire heterostructures. Within this study, we reported the first observation of ISB absorption in Ge-doped GaN/AlN quantum wells and in Si-doped GaN/AlN nanowire heterostructures. In the case of nanowires, we obtained a record ISB absorption linewidth in the order of 200 meV. However, this value is still larger than that observed in planar structures, due to the inhomogeneities associated to the self-assembled growth process.Trying to reduce the inhomogeneities while keeping the advantages of the nanowire geometry, we also presented a systematic analysis of ISB absorption in micro- and nanopillars resulting from top-down processing GaN/AlN planar heterostructures. We showed that, when the spacing of the pillar array is comparable to the probed wavelengths, photonic crystal resonances dominate the absorption spectra. However, when these resonances are at much shorter wavelengths than the ISB absorption, the absorption is clearly observed, without any degradation of its magnitude or linewidth.We also explore the possibility to extend this nanowire technology towards longer wavelengths, to absorb in the mid-wavelength IR region. Using GaN/AlN nanowire heterostructures, we varied the GaN well width from 1.5 to 5.7 nm, which led to a red shift of the ISB absorption from 1.4 to 3.4 µm. Replacing the AlN barriers by Al0.4Ga0.6N, the reduction of polarization led to a further red shift of the ISB transitions to 4.5-6.4 µm.The observation of ISB absorption in nanowire ensembles motivated us for the development of a nanowire-based quantum well infrared photodetector (NW-QWIP). The first demonstration of such a device, incorporating a GaN/AlN nanowire heterostructure that absorbs at 1.55 µm, is presented in this manuscript.

This thesis deals with the characterization and implementation of transparent electron transport materials (ETM) in vacuum deposited p-i-n type organic solar cells (OSC) for substituting the parasitically absorbing standard ETM composed of n-doped C60. In addition to transparency in the visible range of the sun spectrum, the desired material properties include high electron mobility and conductivity, thermal and morphological stability, as well as good energy level alignment relative to the adjacent acceptor layer which is commonly composed of intrinsic C60. In this work, representatives of three different material classes are evaluated with regard to the above mentioned criteria. HATCN (hexaazatriphenylene hexacarbonitrile) is a small discoid molecule with six electron withdrawing nitrile groups at its periphery. It forms smooth thin films with an optical energy gap of 3.3eV, thus being transparent in the visible range of the sun spectrum. Doping with either 5wt% of the cationic n-dopant AOB or 7wt% of the proprietary material NDN1 effectively increases the conductivity to 7.6*10^-6 S/cm or 2.2*10^-4 S/cm, respectively. However, the fabrication of efficient OSC is impeded by the exceptionally high electron affinity (EA ) of approximately 4.8eV that causes the formation of an electron injection barrier between n-HATCN and intrinsic C60 (EA=4.0eV). This work presents a strategy to remove the barrier by introducing doped and undoped C60 intermediate layers, thus demonstrating the importance of energy level matching in a multi-layer structure and the advantages of Fermi level control by doping. Next, a series of six Bis-Fl-NTCDI (N,N-bis(fluorene-2-yl)-naphthalenetetracarboxylic diimide) compounds, which only differ by the length of the alkyl chains attached to the C9 positions of the fluorene side groups, is examined. When increasing the chain length from 0 to 6 carbon atoms, the energy levels remain nearly unchanged: We find EA=3.5eV as estimated from cyclic voltammetry, an ionization potential (IP ) in the range between 6.45eV and 6.63eV, and Eg,opt=3.1eV which means that all compounds form transparent thin films. Concerning thin film morphology, the addition of side chains results in the formation of amorphous layers with a surface roughness <1nm on room temperature glass substrates, and (1.5+/-0.5)nm for deposition onto glass substrates heated to 100°C. In contrast, films composed of the side chain free compound Bis-HFl-NTCDI exhibit a larger surface roughness of (2.5+/-0.5)nm and 9nm, respectively, and are nanocrystalline already at room temperature. Moreover, the conductivity achievable by n-doping is very sensitive to the side chain length: Whereas doping of Bis-HFl-NTCDI with 7wt% NDN1 results in a conductivity in the range of 10^-4 S/cm, the attachment of alkyl chains causes a conductivity which is more than three orders of magnitude smaller despite equal or slightly higher doping concentrations. The insufficient transport properties of the alkylated derivatives lead to the formation of pronounced s-kinks in the jV -characteristics of p-i-n type OSC while the use of n-Bis-HFl-NTCDI results in well performing devices. The last material, HATNA-Cl6 (2,3,8,9,14,15- hexachloro-5,6,11,12,17,18-hexaazatrinaphthylene), exhibits Eg,opt=2.7eV and is therefore not completely transparent in the visible range of the sun spectrum. However, its energy level positions of EA=4.1eV and IP=7.3eV are well suited for the application as ETM in combination with i-C60 as acceptor. The compound is dopable with all available n-dopants, resulting in maximum conductivities of sigma=1.6*10^-6, 3.5*10^-3, and 7.5*10^-3 S/cm at 7.5wt% AOB, Cr2(hpp)4, and NDN1, respectively. Applying n-HATNA-Cl6 instead of the reference ETM n-C60 results in a comparable or improved photocurrent density at an ETM thickness d(ETM)=40nm or 120nm, respectively. At d(ETM)=120nm, the efficiency eta is more than doubled as it increases from eta(n-C60)=0.4% to eta(n-HATNA-Cl6)=0.9% . Optical simulations show that the replacement of n-C60 by n-Bis-HFl-NTCDI, n-HATNA-Cl6, or the previously studied n-NTCDA (naphthalenetretracarboxylic dianhydride) in p-i-n or n-i-p type device architectures is expected to result in an increased photocurrent due to reduced parasitic absorption. For quantifying the gain, the performance of p-i-n type OSC with varying ETM type and thickness is evaluated. Special care has to be taken when analyzing devices comprising the reference ETM n-C60 as its conductivity is sufficiently large to extend the area of the aluminum cathode and thus the effective device area which may lead to distorted results. Overall, the experiment is able to confirm the trends predicted by the optical simulation. At large ETM thickness in the range between 60 and 120nm, the window layer effect of the ETM is most pronounced. For instance, at d(ETM)=120nm, eta(C60) is more than doubled using n-HATNA-Cl6 and even more than tripled using n-Bis-HFl-NTCDI or n-NTCDA. At optimized device geometry the photocurrent gain is slightly less than expected but nonetheless, the efficiency is improved from eta(max)=2.1% for n-C60 and n-HATNA-Cl6 solar cells to eta(max)=2.3, and 2.4% for n-Bis-HFl-NTCDI and n-NTCDA devices, respectively. This development is supported by generally higher Voc and FF in solar cells with transparent ETM. Finally, p-i-n type solar cells with varying ETM are aged at a temperature of 50°C and an illumination intensity of approximately 2 suns. Having extrapolated lifetimes t(80) of 36, 500, and 14000h and nearly unchanged jV-characteristics after 2000h, n-C60 and n-Bis-HFl-NTCDI devices exhibit the best stability. In contrast, n-NTCDA devices suffer from a constant decrease in Isc while n-HATNA-Cl6 solar cells show a rapid dscegradation of both Isc and FF associated with a decomposition of the material or a complete de-doping of the ETM. Here, lifetimes of only 4500h and 445hare achieved.

Die zunehmenden Leistungsanforderungen an Energiespeichersysteme, insbesondere die Energiedichte betreffend, führen dazu, dass der Bedarf durch die am häufigsten verwendete Lithium-Ionen-Technologie bald nicht mehr bedient werden kann. Unter den Batterietechnologien der nächsten Generation zeigt die Lithium-Schwefel-(Li-S)-Batterie ein großes Potenzial. Jedoch verhindern die technologischen Herausforderungen der Li-S-Zellchemie (z. B Anodenkorrosion, Elektrolytzersetzung oder Polysulfid-Shuttle) eine breite Kommerzialisierung. Zusätzlich liegen die praktisch erreichbaren gravimetrischen Energiedichten noch weit unter den theoretischen Werten. Im Rahmen dieser Arbeit wurden verschiedene poröse Kohlenstoffe und kovalente Triazin Netzwerke (covalent triazine framework - CTF) zu Schwefelkathoden verarbeitet und in unterschiedlichen Elektrolytsystemen elektrochemisch analysiert. Ein Ziel dieser Arbeit war es, ein skalierbares N-dotiertes Kathodenmaterial für den Einsatz in Prototypzellen zu entwickeln. Weiterhin wurde der Einfluss der Kohlenstoffporosität in unterschiedlichen Elektrolytsystemen diskutiert. Es wurde erfolgreich eine skalierbare, post-synthetische Imprägnierungsroute entwickelt, um N-dotierte Kohlenstoffe aus kommerziellen Kohlenstoffmaterialien und Melamin herzustellen. Mit Hilfe des Veredelungsprozesses wurde erstmals ein N-dotierter Kohlenstoff aus einem porösen Ruß (Ketjenblack) im größeren Maßstab (~100 g) hergestellt und erfolgreich zu doppelseitigen Li-S-Kathoden verarbeitet. Sowohl die galvanostatischen Ergebnisse (vs. Li/Li+) als auch die Analyse im symmetrischen Zellaufbau (S8 vs. Li2S) zeigten den positiven Einfluss der N-Dotierung auf die Lebensdauer der Knopfzellen. Trotz des geringen Stickstoffgehalts von 1,08 Gew.% im N-Dotierten Kathodenmaterial erzielte die fünflagige Demonstratorzelle eine gravimetrische Energiedichte von 238 Wh kg-1 (~1,30 Ah) und eine erhöhte Zyklenstabilität gegenüber dem undotierten Referenzmaterial. Somit wurde zum ersten Mal der positive Effekt der N-Dotierung erfolgreich von der Knopfzelle auf die Pouchzelle übertragen. Als alternatives N-dotiertes Kathodenmaterial erlangten kovalente Triazin Netzwerke große Aufmerksamkeit. Allerdings hat die Charakterisierung von CTF-Kathoden mit kovalent gebundenem Schwefel gezeigt, dass zuvor berichtete Vorteile dieser Gerüststrukturen überdacht werden müssen. Anhand der elektrochemischen Ergebnisse im carbonatbasierten LP30-Elektrolyten wurde geschlussfolgert, dass CTF-Materialien im Gegensatz zum literaturbekannten Referenzsystem S-PAN einen anderen Umwandlungsmechanismus durchlaufen. Die Charakterisierung im DME/DOL-Elektrolyten zeigte, dass die Synthesetemperatur und damit die Leitfähigkeit der CTF-Materialien einen deutlich größeren Einfluss auf die Zellperformance hat als bisher in der Literatur angenommen. Vielmehr haben die Ergebnisse darauf hingedeutet, dass der Umwandlungsmechanismus wahrscheinlich über das leitfähige Kohlenstoffadditiv (Ketjenblack) abläuft und das CTF-Material eher als Wirtsstruktur für Polysulfide bzw. den Elektrolyten fungiert. Ein anderer Forschungszweig beschäftigt sich mit dem Einschluss von Schwefel in Poren, da die eingeschlossenen Schwefelspezies im carbonatbasierten Elektrolyten betrieben werden können. Die Ausbildung einer schützenden Passivierungsschicht auf der Kathodenoberfläche (cathodic electrolyte interface – CEI) realisiert die Quasi-Festkörperumwandlung des Schwefels. Es wurden verschiedene mikroporöse Modellkohlenstoffe und ein mesoporöser Ruß als Schwefelkathoden im carbonatbasierten LP30-Elektrolyten untersucht. Obwohl die Komposite nach einem Verdampfungsschritt bei 300 °C geringe Schwefelgehalte (22 Gew.% und 30 Gew.%) aufwiesen, wurden stabile Zellperformances bis zu 200 Zyklen sowie der Quasi-Festkörperkonversionsmechanismus beobachtet. Außerdem zeigten die elektrochemischen Analysen die Ausbildung einer schützenden CEI-Schicht auf der Kathodenoberfläche. Für das schmelzinfiltrierte mesoporöse KB/S-Komposit wurde nahezu keine Schwefelausnutzung beobachtet, da wahrscheinlich keine geschlossene CEI-Schutzschicht ausgebildet wurde. Dagegen zeigten die schmelzinfiltrierten mikroporösen C/S-Proben nach einem höheren initialen Kapazitätsverlust eine moderate Schwefelausnutzung. Weiterhin wurde mit Hilfe von Röntgenabsorptionsspektroskopie (X-ray absorption spectroscopy – XAS) die Natur der adsorbierten Schwefelmoleküle analysiert. Die XAS-Ergebnisse deuteten darauf hin, dass, wenn überhaupt, nur ein kleiner Anteil des Aktivmaterials als kurzkettige Schwefelspezies (S2, S4) in den Mikroporen vorliegt. Dennoch wurden erstmals Aussagen zur chemischen Umgebung von eingeschlossenen Schwefelspezies in Mikroporen gemacht. Somit konnte geschlussfolgert werden, dass die Hauptrolle der mikroporösen Kohlenstoffmatrix (dp < 2 nm) darin besteht, die Ausbildung einer geschlossenen schützenden CEI-Schicht auf den Kohlenstoffpartikeln zu ermöglichen, anstatt kurzkettige Schwefelspezies im Inneren als notwendige Voraussetzung für den reversiblen Betrieb bereitzustellen. Die Entwicklung von innovativen Elektrolyten mit reduzierter Polysulfid-Löslichkeit ist ein weiterer Lösungsansatz, um höhere Energiedichten in Prototypzellen zu realisieren. Um den Einfluss der Gerüstporosität (Mikroporen, Mesoporen, hierarchische Poren) auf den modifizierten Konversionsmechanismus zu untersuchen, wurden verschiedene poröse Kohlenstoffmaterialien mit variierendem Porenvolumen als Schwefelkathoden in zwei Elektrolyten mit geringer Polysulfid-Löslichkeit (TMS/TTE & HME/DOL) evaluiert. Die mikroporösen Elektroden zeigten im TMS/TTE-Elektrolyten ein zusätzliches drittes Entladungsplateau, welches durch den Quasi-Festkörpermechanismus hervorgerufen wird. Dieses Phänomen wurde zuvor noch nicht für den TMS/TTE-Elektrolyten beschrieben. In den elektrochemischen Ergebnissen wurde die Bildung einer CEI-Schicht beobachtet, die aus der Zersetzung des TTE-Lösungsmittels an der Kohlenstoffoberfläche resultiert. Für die weiteren untersuchten Kohlenstoffe konnte ein kombinierter Reaktionsmechanismus aus fest-flüssig-fest sowie quasi-fest-zu-fest Umwandlung nicht ausgeschlossen werden, wobei vorwiegend die gewöhnliche fest-flüssig-fest Schwefelkonversion beobachtet wurde. Für den HME/DOL-Elektrolyten konnte aufgrund der eingeschränkten Polysulfid-Löslichkeit ebenfalls eine Kombination aus der fest-flüssig-fest Schwefelkonversion und dem Quasi-Festkörpermechanismus angenommen werden. Im Vergleich zum TMS/TTE-System scheint jedoch eine abgewandelte Form der quasi-fest-zu-fest Umwandlung stattzufinden, da sich die Spannungsprofile deutlich unterscheiden und keine CEI-Ausbildung festgestellt wurde. Vermutlich beeinflusst die Kohlenstoffporosität den gehinderten Massentransport während der Schwefelumsetzung im HME/DOL-Elektrolyten. Zusammenfassend wurden neue mechanistische Einblicke für den Betrieb von Li-S-Batterien gewonnen, bei denen Elektrolyte mit geringer Polysulfid-Löslichkeit angewendet werden.:Abkürzungsverzeichnis I 1 Einleitung und Motivation 1 2 Theorie und Stand der Forschung 6 2.1 Thermodynamische Grundlagen 6 2.2 Definitionen wichtiger Batteriekenngrößen 8 2.2.1 Einfluss von Überspannungseffekten 8 2.2.2 Einführung der spezifischen Kapazität und Energiedichte 11 2.2.3 Einführung der C-Rate und der Coulomb-Effizienz 12 2.2.4 Einführung des Spannungsprofils und der Zyklenstabilität 12 2.3 Die Lithium-Schwefel-Batterie 13 2.3.1 Der grundsätzliche Reaktionsmechanismus der Li-S-Batterie 13 2.3.2 Herausforderungen der Li-S-Technologie 19 2.3.3 Elektrolyte für die Li-S-Batterie 22 2.3.4 Überblick über die Anoden- und Separator-Entwicklung im Li-S-System 31 2.3.5 Überblick über die Kathodenentwicklung in Li-S-Batterien 37 2.4 Charakterisierungsmethoden 47 2.4.1 Stickstoff-Physisorption 47 2.4.2 Elektrochemische Charakterisierung 52 3 Experimenteller Teil 56 3.1 Verwendete Chemikalien 56 3.2 Kohlenstoffsynthesen 58 3.2.1 Synthese von hierarchisch porösen Kohlenstoffen (HPC) 58 3.2.2 Synthese von TiC-CDC 59 3.2.3 Stickstoffdotierung von Kohlenstoffen 59 3.3 Synthese der Kohlenstoff/Schwefel-Komposite 61 3.3.1 Verdampfungsprozess bei 300 °C 61 3.4 Elektrodenherstellung 62 3.4.1 Kohlenstoff/Schwefel-Kathoden 62 3.4.2 Kovalent-Triazin-Netzwerk-Kathoden 62 3.5 Elektrolytherstellung 63 3.6 Einlegetest im Elektrolyten 64 3.7 Strukturelle Charakterisierungsmethoden 64 3.7.1 Stickstoff-Physisorption 64 3.7.2 Wasser-Physisorption 65 3.7.3 Thermogravimetrische Analysen 65 3.7.4 Elementaranalysen 65 3.7.5 Raman Spektroskopie 65 3.7.6 Pulver-Widerstandsmessung 65 3.7.7 Rasterelektronenmikroskopie 66 3.7.8 Transmissionselektronenmikroskopie 66 3.7.9 Röntgenphotoelektronenspektroskopie 66 3.7.10 Röntgenabsorptionspektroskopie 67 3.8 Elektrochemische Charakterisierung 68 3.8.1 Elektrochemische Analyse in Knopfzellen 68 3.8.2 Elektrochemische Analyse in Pouchzellen 72 4 Auswertung und Diskussion 73 4.1 Thermische Veredelung von porösen Kohlenstoffmaterialien 73 4.1.1 Funktionalisierung des porösen Rußes Ketjenblack mit Melamin 73 4.1.2 Funktionalisierung von weiteren porösen Kohlenstoffmaterialien 94 4.2 Kovalente Triazin-Netzwerke als Li-S-Kathoden 102 4.2.1 Diskussion der mechanistischen Rolle von CTF-Materialien in Li-S-Kathoden 103 4.2.2 Elektrochemische Charakterisierung der S@CTF-Kathoden 105 4.3 Die Rolle der Kohlenstoffporosität bei der Ausbildung einer kathodischen Passivierungsschicht in Li-S Zellen 112 4.3.1 Materialcharakterisierung der mikroporösen Kohlenstoffe 114 4.3.2 Elektrochemische Charakterisierung in Knopfzellen vs. Li/Li+ 120 4.4 Der Einfluss der Kohlenstoffporosität auf die Schwefelumsetzung bei Anwendung von Elektrolyten mit geringer Polysulfid-Löslichkeit 130 4.4.1 Materialcharakterisierung der verschiedenen porösen Kohlenstoffe 131 4.4.2 Stabilitätstest der Li-S-Kathoden in verschiedenen Elektrolyten 136 4.4.3 Voruntersuchungen der Kathodenmaterialien im DME/DOL-Standardelektrolyten (vs. Li/Li+) 140 4.4.4 Elektrochemische Evaluierung im TMS/TTE Elektrolyten (vs. Li/Li+) 146 4.4.5 Elektrochemische Evaluierung im HME/DOL Elektrolyten (vs. Li/Li+) 157 5 Zusammenfassung und Ausblick 166 6 Anhang 171 7 Literaturverzeichnis 184 8 Abbildungsverzeichnis 194 9 Tabellenverzeichnis 200 10 Wissenschaftliche Beiträge 201 11 Eidesstattliche Erklärung 203

L'équipe micro-capteurs de l'IM2NP développe des capteursde gaz dont le principe de détection est basé sur la mesure de la variationde la conductance en présence de gaz. Le matériau utilisé comme élémentsensible est l'oxyde de tungstène (WO3) en couches minces. L'objet de cettethèse est donc d'étudier la surface de WO3 dans sa reconstruction c(2x2),obtenue par clivage selon la direction [001]. Cette étude a été également suivied'une étude des lacunes par des calculs ab initio basés sur la DFT, dans lesdeux approximations LDA et GGA. Ensuite, l'dsorption de molécules de gazsimples (O3, COx, NOx) sur des surfaces plus ou moins riches en oxygènea été effectuée. Pour simuler ces systèmes, nous avons fait le choix du codeSIESTA basé sur la DFT et qui présente l'avantage de pouvoir travailler. / The team of micro sensors at IM2NP mainly focuses onthe development of gas sensors based on measurement in conductancevariation in presence of gas. The material used as sensitive element istungsten oxide (WO3) thin film. The objective of present thesis is to studythe surface properties of WO3 in its reconstruction c(2x2), obtained bycleavage along the [001] direction. This study is also followed by a gapanalysis using ab initio calculations based on DFT in both LDA andGGA approximations. Then, the adsorption of molecules of simple gases((O3, COx NOx) for these surfaces (more or less rich in oxygen), is performed.To simulate these systems, we have chosen the SIESTA code based onDFT which is used for the larger number of atoms as compared to other codes.