<p>This thesis is based on 9 papers dealing with the synthesis, physicochemical and biochemical properties of two types of chemically modified oligonucleotides which have the potential to down-regulate gene expression: (i) The first set is comprised of antisense oligonucleotides (AONs) conjugated with different chromophores of varying size, charge and π-electron density. Conjugation of the chromophores at the 3'- or 5'-end enhanced the target RNA binding affinity and RNase H recruitment capabilities compared to the native counterpart without changing the global helical conformation of their AON/RNA hybrid duplexes. The 3'-dipyridophenazine (DPPZ) has emerged as the most promising non-toxic chromophore in this series. (ii) The second set encompasses a new class of AONs containing <i>North</i>-<i>East</i> conformationally constrained 1',2'-oxetane-nucleosides. The introduction of oxetane-<b>T</b> and -<b>C</b> units imparts lowering of the T_m by ~ 6º and ~ 3 ºC/modification, respectively, of the AON/RNA hybrids, whereas the incorporation of the corresponding oxetane-<b>A</b> and-<b>G</b> units into AONs did not alter the thermostability in comparison with that of the native hybrid duplex. The oxetane-modified AONs have been found to possess enhanced serum stability compared to that of the native, whereas oxetane-<b>T</b> and -<b>C</b> containing AONs were more endonuclease-resistant than oxetane-<b>A</b> and-<b>G</b> modified AONs. All oxetane-modified mixmer AON/ RNA hybrid duplexes were, however, found to be excellent substrates for RNase H cleavage, which has been analyzed by Michaelis-Menten kinetics. The oxetane-modified mixmer AONs have shown effective down-regulation of the proto-oncogene c-myb mRNA in the K562 human leukemia cells, which was analyzed by QRT-PCR and Western Blot. Based on the amount of AON uptake after delivery, determined by slot blot, it was apparent that the oxetane-modified AONs are 5-6 times more effective antisense agents than the corresponding isosequential phosphorothioate analogues. The electrochemical assay based on sensitive nucleic acid mediated charge transport (CT) has revealed that the presence of oxetane-<b>T</b> unit causes more stacking perturbations in a DNA/DNA duplex than in a DNA/RNA duplex. </p>

Mit Seitenkettenpolystyrenen wurde ein neues Synthesekonzept für phosphoreszente polymere LED-Materialien aufgestellt und experimentell verifiziert. Zunächst erfolgten auf Grundlage strukturell einfacher Verbindungen Untersuchungen zum Einfluss von Spacern zwischen aktiven Seitengruppen und dem Polystyrenrückgrat. Es wurden Synthesemethoden für die Monomere etabliert, durch die aktive Elemente - Elektronen- und Lochleiter - mit und ohne diesen Spacer zugänglich sind. Durch Kombination dieser Monomere waren unter Hinzunahme von polymerisierbaren Iridium-Komplexen in unterschiedlicher Emissionswellenlänge statistische Terpolymere darstellbar. Es wurde gezeigt, dass die Realisierung bestimmter Verhältnisse zwischen Loch-, Elektronenleiter und Triplettemitter in ausreichender Molmasse möglich ist. Die Glasstufen der Polymere zeigten eine deutliche Strukturabhängigkeit. Auf die Lage der Grenzorbitale übten die Spacer nahezu keinen Einfluss aus. Die unterschiedlichen Makromoleküle kamen in polymeren Licht emittierenden Dioden (PLEDs) zum Einsatz, wobei ein deutlicher Einfluss der Spacereinheiten auf die Leistungscharakteristik der PLEDs festzustellen war: Sowohl Effizienz, Leuchtdichte wie auch Stromdichte waren durch den Einsatz der kompakten Makromoleküle ohne Spacer deutlich höher. Diese Beobachtungen begründeten sich hauptsächlich in der Verwendung der aliphatischen Spacer, die den Anteil im Polymer erhöhten, der keine Konjugation und damit elektrisch isolierende Eigenschaften besaß. Diese Schlussfolgerungen waren mit allen drei realisierten Emissionsfarben grün, rot und blau verifizierbar. Die besten Messergebnisse erzielte eine PLED aus einem grün emittierenden und spacerlosen Terpolymer mit einer Stromeffizienz von etwa 28 cd A-1 (bei 6 V) und einer Leuchtdichte von 3200 cd m-2 (bei 8 V). Ausgehend von obigen Ergebnissen konnten neue Matrixmaterialien aus dem Bereich verdampfbarer Moleküle geringer Molmasse in das Polystyrenseitenkettenkonzept integriert werden. Es wurden Strukturvariationen sowohl von loch- wie auch von elektronenleitenden Verbindungen als Homopolymere dargestellt und als molekular dotierte Systeme in PLEDs untersucht. Sieben verschiedene lochleitende Polymere mit Triarylamin-Grundkörper und drei elektronendefizitäre Polymere auf der Basis von Phenylbenzimidazol konnten erfolgreich in den Polymeransatz integriert werden. Spektroskopische und elektrochemische Untersuchungen zeigten kaum eine Veränderung der Charakteristika zwischen verdampfbaren Molekülen und den dargestellten Makromolekülen. Diese ladungstransportierenden Makro-moleküle wurden als polymere Matrizes molekular dotiert und lösungsbasiert zu Einschicht-PLEDs verarbeitet. Als aussichtsreichstes Lochleiterpolymer dieser Reihe, mit einer Strom-effizenz von etwa 33 cd A-1 (bei 8 V) und einer Leuchtdichte von 6700 cd m-2 (bei 10 V), stellte sich ein Triarylaminderivat mit Carbazolsubstituenten heraus. Als geeignetstes Matrixmaterial für die Elektronenleitung wurde ein meta-verknüpftes Di-Phenylbenzimidazol ausfindig gemacht, das in der PLED eine Stromeffizienz von etwa 20 cd A-1 (bei 8 V) und eine Leuchtdichte von 7100 cd m-2 (bei 10 V) erzielte. Anschließend wurden die geeignetsten Monomere zu Copolymeren kombiniert: Die lochleitende Einheit bildete ein carbazolylsubstituiertes Triarylamin und die elektronen-leitende Einheit war ein disubstituiertes Phenylbenzimidazol. Dieses Copolymer diente im Folgenden dazu, PLEDs zu realisieren und die Leistungsdaten mit denen eines Homopolymer-blends zu vergleichen, wobei der Blend die bessere Leistungscharakteristik zeigte. Mit dem Homopolymerblend waren Bauteileffizienzen von annähernd 30 cd A-1 (bei 10 V) und Leuchtdichten von 6800 cd m-2 neben einer Verringerung der Einsatzspannung realisierbar. Für die abschließende Darstellung bipolarer Blockcopolymere wurde auf die Nitroxid-vermittelte Polymerisation zurückgegriffen. Mit dieser Technik waren kontrollierte radikalische Polymersiationen mit ausgewählten Monomeren in unterschiedlichen Block-längen durchführbar. Diese Blockcopolymere kamen als molekular dotierte Matrizes in phosphoreszenten grün emittierenden PLEDs zum Einsatz. Die Bauteile wurden sowohl mit statistischen Copolymeren, wie auch mit Homopolymerblends in gleicher Zusammensetzung aber unterschiedlichem Polymerisationsgrad hinsichtlich der Leistungscharakteristik verglichen. Kernaussage dieser Untersuchungen ist, dass hochmolekulare Systeme eine bessere Leistungscharakteristik aufweisen als niedermolekulare Matrizes. Über Rasterkraft-mikroskopie konnte eine Phasenseparation in einem Größenbereich von etwa 10 nm für den hochmolekularen Homopolymerblend nachgewiesen werden. Für die Blockcopolymere war es nicht möglich eine Phasenseparation zu beobachten, was vorwiegend auf deren zu geringe Blocklänge zurückgeführt wurde. / A new synthetic approach for the synthesis of side chain polystyrenes was established and their use as phosphorescent polymers for polymer light emitting diodes (PLEDs) is shown by experiments. An assay was introduced to clarify influences on electroluminescent behavior for RGB-colored phosphorescent terpolymers with N,N-Di-p-tolyl-aniline as hole-transporting unit, 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tert-BuPBD) as electron-transporting unit, and different iridium complexes in RGB-colors as triplet emitting materials. All monomers were attached with spacer moieties to the “para” position of a polystyrene. PLEDs were built to study the electro-optical behavior of these materials. The gist was a remarkable influence of hexyl-spacer units to the PLED performance. For all three colors only very restricted PLED performances were found. In comparison RGB-terpolymers were synthesized with directly attached charge transport materials to the polymer backbone. For this directly linked systems efficiencies were 28 cd A−1 @ 6 V (green), 4.9 cd A−1 @ 5 V (red) and 4.3 cd A−1 @ 6 V (bluish). In summary it is assumed that an improved charge percolation pathways regarding to the higher content of semiconducting molecules and an improved charge transfer to the phosphorescent dopand in the case of the copolymers without spacers are responsible for the better device performance comparing the copolymers with hexyl spacers. It was found that the approach of the directly connected charge transport materials at the nonconjugated styrene polymer backbone is favored for further investigations as shown in the following. A series of styrene derived monomers with triphenylamine-based units, and their polymers have been synthesized and compared with the well-known structure of polymer of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine with respect to their hole-transporting behavior in PLEDs. A vinyltriphenylamine structure was selected as a basic unit, functionalized at the para positions with the following side groups: diphenylamine, 3-methylphenyl-aniline, 1- and 2-naphthylamine, carbazole, and phenothiazine. The polymers are used in PLEDs as host polymers for blend systems. It is demonstrated that two polymers are excellent hole-transporting matrix materials for these blend systems because of their good overall electroluminescent performances and their comparatively high glass transition temperatures. For the carbazole-substituted polymer (Tg = 246 °C), a luminous efficiency of 35 cd A−1 and a brightness of 6700 cd m−2 at 10 V is accessible. The phenothiazine-functionalized polymer (Tg = 220 °C) shows nearly the same outstanding PLED behavior. Hence, both these polymers outperform the well-known polymer of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine, showing only a luminous efficiency of 7.9 cd A−1 and a brightness of 2500 cd m−2 (10 V). Furthermore, novel styrene functionalized monomers with phenylbenzo[d]imidazole units and the corresponding homopolymers are prepared. The macromolecules are used as matrices for phosphorescent dopants to prepare PLEDs. The devices exhibit current efficiencies up to 38.5 cd A−1 at 100 cd m−2 and maximum luminances of 7400 cd m−2 at 10 V. Afterwards the most efficient monomers of this investigations were combined and statistical copolymers were synthesized. As hole-transporting monomer the carbazole substituted triarylamine and as electron-transporting monomer a disubstituted phenylbenzoimidazole was selected. This statistical copolymer was used in the following as matrix material for phosporescent PLEDs and the device performance was compared with a matrix system of a polymer blend matrix system of corresponding homopolymers. With this homopolymer blend efficiencies of about 30 cd A-1 at 10 V and luminances of 6800 cd m-2 beside a decreased onset voltage were realized. Finally bipolar blockcopolymers of structural basic monomers were synthesized via nitroxide mediated polymerization. With these technique and the chosen hole- and electron-transporting monomers a controlled radical polymerization was realized leading to blockcopolymers in different block lengths. These blockcopolymers were used as molecular doped matrix systems in green phosphoreszent PLEDs. The devices were compared in regard to their performances with PLEDs made of statistical copolymers and homopolymer blends. It was found that high molecular systems show a better device performance compared to low molecular polymer matrices. With atomic force microscopy it is shown that a phase separation takes place for the high molecular blend of homopolymers. For the synthesized blockcopolymers no phase separation could be verified, mainly because of the comparatively low molecular weight of these systems.

This thesis is based on 9 papers dealing with the synthesis, physicochemical and biochemical properties of two types of chemically modified oligonucleotides which have the potential to down-regulate gene expression: (i) The first set is comprised of antisense oligonucleotides (AONs) conjugated with different chromophores of varying size, charge and π-electron density. Conjugation of the chromophores at the 3'- or 5'-end enhanced the target RNA binding affinity and RNase H recruitment capabilities compared to the native counterpart without changing the global helical conformation of their AON/RNA hybrid duplexes. The 3'-dipyridophenazine (DPPZ) has emerged as the most promising non-toxic chromophore in this series. (ii) The second set encompasses a new class of AONs containing North-East conformationally constrained 1',2'-oxetane-nucleosides. The introduction of oxetane-<b>T</b> and -<b>C</b> units imparts lowering of the Tm by ~ 6º and ~ 3 ºC/modification, respectively, of the AON/RNA hybrids, whereas the incorporation of the corresponding oxetane-<b>A</b> and-<b>G</b> units into AONs did not alter the thermostability in comparison with that of the native hybrid duplex. The oxetane-modified AONs have been found to possess enhanced serum stability compared to that of the native, whereas oxetane-<b>T</b> and -<b>C</b> containing AONs were more endonuclease-resistant than oxetane-<b>A</b> and-<b>G</b> modified AONs. All oxetane-modified mixmer AON/ RNA hybrid duplexes were, however, found to be excellent substrates for RNase H cleavage, which has been analyzed by Michaelis-Menten kinetics. The oxetane-modified mixmer AONs have shown effective down-regulation of the proto-oncogene c-myb mRNA in the K562 human leukemia cells, which was analyzed by QRT-PCR and Western Blot. Based on the amount of AON uptake after delivery, determined by slot blot, it was apparent that the oxetane-modified AONs are 5-6 times more effective antisense agents than the corresponding isosequential phosphorothioate analogues. The electrochemical assay based on sensitive nucleic acid mediated charge transport (CT) has revealed that the presence of oxetane-<b>T</b> unit causes more stacking perturbations in a DNA/DNA duplex than in a DNA/RNA duplex.

π-Conjugated polymers have attracted considerable attention since they are potential candidates for various opto-electronic devices such as solar cells, light emitting iodes, photodiodes, and transistors. Electronic de vices based on conjugated polymers can be easily processed at low temperature using inexpensive technologies. This leads to cost reduction, a key-deriving factor for choosing conjugated polymers for various types of applications. In particular, polymer based solar cells are of special interest due to the fact that they can play a major role in generating clean and cheap energy in the future. The investigations described in thesis are aimed mainly at understanding charge transport and the role of energy le vels in solar cells based on polymer/acceptor bulk heterojunction (BHJ) active films. Best polymer based solar cells, with efficiency 4 to 5%, rely on polymer/fullerene BHJ active films. These solar cells are in an immature state to be used for energy conversion purposes. In order to enhance their performance, it is quite important to understand the efficiency-limiting factors. Solid films of conjugated polymers compose conjugation segments that are randomly distributed in space and energy. Such distributio n gives rise to the localization of charge carriers and hence broadening of electron density of states. Consequently, electronic wave functions have quite poor overlap resulting into absence of continuous band transport. Charge transport in polymers and organic materials, in general, takes place by hopping among the localized states. This makes a bottleneck to the performance of polymer-based solar cells. In this context, the knowledge of charge transport in the solar cell materials is quite important to develop materials and device architectures that boost the efficiency of such solar cells. Most of the transport studies are based on polyfluorene copolymers and fullerene electron acceptor molecules. Fullerenes are blended with polymers to enhance the dissociation of excited state into free carriers and transport free electrons to the respective electrode. The interaction within the polymer-fullerene complex, therefore, plays a major role in the generation and transport of both electrons and holes. In this thesis, we present and discuss the effect of various polymer/fullerene compositions on hole percolation paths. We mainly focus on hole transport since its mobility is quite small as compared to electron mobility in the fullerenes, leading to creation of spa ce charges within the bulk of the solar cell composite. Changing a polymer band gap may necessitate an appropriate acceptor type in order to fulfill the need for sufficient driving force for dissociation of photogenerated electron-hole pairs. We have observed that different acceptor types give rise to completely different hole mobility in BHJ films. The change of hole transport as a function of acceptor type and concentration is mainly attributed to morphological changes. The effect of the acceptors in connection to hole transport is also discussed. The later is supported by studies of bipolar transport in pure electron acceptor layers. Moreover, the link between charge carrier mobility and photovoltaic parameters has also been studied and presented in this thesis. The efficiency of polymer/fullerene-based solar cells is also significantly limited by its open-circuit voltage (Voc), a parameter that does not obey the metal-insulator-metal principle due to its complicated characteristics. In this thesis, we address the effect of varying polymer oxidation potential on Voc of the polymer/fullerene BHJ based solar cells. Systematic investigations have been performed on solar cells that comprise several polythiophene polymers blended with a fullerene derivative electron acceptor molecule. The Voc of such solar cells was found to have a strong correlation with the oxidation potential of the polymers. The upper limit to Voc of the aforementioned solar cells is thermodynamically limited by the net internal electric filed generated by the difference in energy levels of the two materials in the blend. The cost of polymer-based solar cells can be reduced to a great extent through realization of all-plastic and flexible solar cells. This demands the replacement of the metallic components (electrodes) by highly conducting polymer films. While hole conductor polymers are available, low work function polymer electron conductors are rare. In this thesis, prototype solar cells that utilizes a highly conducting polymer, which has a work function of ~ 4.3 eV, as a cathode are demonstrated. Development of this material may eventually lead to fabrication of large area, flexible and cheap solar cells. The transparent nature of the polymer cathode may also facilitate fabrication of multi-layer and tandem solar cells. In the last chapter of this thesis, we demonstrate generation of red and near infrared polarized light by employing thermally converted thin films of polyfluorene copolymers in light emitting diodes. This study, in particular, aims at fabricating polarized infrared light emitting devices. / On the day of the defence day the status of article III was In press and article VI was Manuscript.

In bulk-heterojunction solar cells, device performance is influenced by both the intrinsic properties of the individual components - typically conjugated polymers and fullerene derivatives - and how they assemble and interact at their interface. The ability of fullerene to intercalate within the side-chains of a conjugated polymer can significantly affect the microstructure and overall device performance. Here, a series of computational chemistry approaches are applied to investigate the relationships between structure and property in intercalated polymer:fullerene blend. Using a combination of molecular mechanics (MM) calculation and simulations of 2D grazing incidence X-ray diffraction (GIXD) patterns, we have determined the molecular packing configuration of poly (2,5-bis (3-tetradecyl thiophene-2-yl) thieno[3,2-b]thiophene) (PBTTT-C₁₄) and a blend of PBTTT-C₁₄ and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). Based on the confirmed packing structures, the electronic properties and morphological disorder were examined using density functional theory (DFT) and molecular dynamics (MD) calculations, respectively; we also investigated the intermolecular interaction energies behind the structure formation. Finally, we examined the vibrational, redox, and optical properties of the pristine polymer and a series of fullerene derivatives to understand the characteristic modes related to the various charged states of the systems.

Research efforts to determine the causes, effects and locations of mutations within the human genome have been widely pursued due to their role in the development of various diseases. The main cause of mutations in vivo is oxidative damage to DNA via oxidants and free radical species. Numerous studies have been performed in vitro to determine how oxidative damage is induced in DNA. Most of these in vitro studies require photosensitizers to initiate the oxidative damage through various mechanisms. For the purposes of this research, all the photosensitizers that were used initiated oxidative damage in DNA through the electron transfer mechanism. In the charge transport studies, an anthraquinone photosensitizer was covalently linked to the 5 end of DNA by a short carbon tether in order to determine the pattern of damage induced along the length of the DNA. Anthraquinone preferentially damages guanine bases. Our first work sought to determine the effects of charge transport through guanine rich quadruplex DNA dimers. The dimers were formed by the combination of two hairpins with duplex overhangs extending beyond the quadruplex region. This enabled the optimal comparison of the effects of charge transport between duplex and quadruplex DNA structures. Another area of research we pursued in this area was to determine the effects of charge transport in M-DNA (a novel DNA conformation that was reported to form in the presence of zinc ions at a pH above 8). Earlier work on M-DNA suggested that it behaved like a molecular wire. Our research attempted to determine the effects of charge transport on this structure in order to show the behavior of a DNA molecular wire as compared to the standard studies performed in this area on normal B-DNA structures. Lastly, in collaboration with Dr. Ramaiah and colleagues we designed some viologen linked acridine photosensitizers which were tested for any ability to cleave GGG bulges. In preliminary studies, these viologen linked acridine derivatives showed preferential cleavage for guanine bases. They were not covalently bound to DNA, although they could potentially form non covalent interactions with DNA such as intercalation and/or groove binding. Our overall research goal was to determine the extent and overall effect of oxidative damage (using different photosensitizers) on the various DNA structures mentioned above.

Nous étudions plusieurs problémes reliés aux propriétés de transport dans les systèmes corrélés. La thèse contient 3 parties distinctes, chacune d'entre elles décrivant un aspect particulier. Nous avons obtenu dans chacun des cas des résultats qui permettent une meilleure compréhension du transport. Nous étudions l'effet de la dissipation et d'une perturbation extérieure dépendant du temps sur le diagramme de phases d'un systèmes à N corps à température nulle et à température finie. En présence de perturbation dépendant du temps, la dissipation joue un rôle important dans l'évolution vers un état stable indépendant du temps. Nous utilisons le formalisme de Keldysh dans l'approximation adiabatique qui permet d'étudier le diagramme de phases du système en fonction de parameter et de la température. Dans la 2ième partie, nous étudions un concept important pour la physique des systèmes métalliques à plusieurs bandes, le concept d'hybridation, et la façon dont l'hybridation affecte la supraconductivité du métal. De façon générale, une hybridation dépendante ou non du vecteur d'onde k a tendance à détruire la supraconductivité. Nous montrons dans ce chapitre qu'une hybridation antisymétrique a l'effet inverse et renforce la supraconductivité. Nous montrons que si l'hybridation est antisymétrique, la supraconductivité a des propriétés non-triviales. Nous proposons que dans un tel système, il puisse exister des fermions de Majorana, même en l'absence de couplage spin-orbite. Le dernier chapitre de la thèse porte sur les effets du couplage spin-orbite sur le transport dans les nanostructures magnétiques. Dans les nanostructures, le couplage spin-orbite joue un rôle important en raison de la brisure de symmétrie à la surface ou aux interfaces. En particulier, nous étudions l'effet de l'interaction Dzyaloshinskii-Moriya (DM) sur le transport de spin dans un système tri-couche. Nous montrons qu'il existe une interaction DM entre les moments des couches et les électrons de conduction, et l'influence de cette interaction sur le transport est étudiée dans un modèle simplifié ou chaque couche est représentée par un point. / We study some important problems related to the transport properties of many body systems. It is divided in three parts, each one focusing in a specific topic. We obtain relevant results that improve our understanding of these systems. We investigate the effect of dissipation and time-dependent external sources, in the phase diagram of a many body system at zero and finite temperature. In the presence of time-dependent perturbations, dissipation is essential for the system to attain a steady, time independent state. In order to treat this time dependent problem, we use a Keldysh approach within an adiabatic approximation that allows us to study the phase diagram of this system as a function of the parameters of the system and temperature. We also discuss the nature of the quantum phase transitions of the system. Next, we study an important concept in the physics of metallic multi-band systems, that of hybridization, and how it affects the superconducting properties of a material. A constant or symmetric $k$-dependent hybridization in general act in detriment of superconductivity. We show here that when hybridization between orbitals in different sites assumes an anti-symmetric character having odd-parity it {it{enhances}} superconductivity. The antisymmetric hybridization in a problem study in this thesis (present in Chapter 3) allow us to propose a new system where it is possible to investigate Majorana fermions, even in absence of spin-orbit interactions. In the last part of this thesis we study the effect of spin-orbit coupling (SOC) on transport properties in magnetic nanostructures. In this system SOC plays an important role, because surfaces (or interfaces) introduce symmetry breaking which is a source of spin-orbit interaction. We study the role of Dzyaloshinshkii-Moriya (DM) interaction on spin-transport in a 3 layer system. We show that there is a DM interaction between magnetics ions in the layers and spin of conduction electrons. We study the influence of this DM interaction on transport within a simple model where each layer is represented by a point.

Oxygen gain is the difference in fuel cell performance operating on oxygen-depleted and oxygen-rich cathode fuel streams. Oxygen gain experiments provide insight into the degree of oxygen mass-transport resistance within a fuel cell. By taking these measurements under different operating conditions, or over time, one can determine how oxygen mass transport varies with operating modes and/or aging. This paper provides techniques to differentiate between mass-transport resistance within the catalyst layer and within the gas-diffusion medium for a polymer-electrolyte membrane fuel cell. Two extreme cases are treated in which all mass transfer limitations are located only (i) within the catalyst layer or (ii) outside the catalyst layer in the gas diffusion medium. These two limiting cases are treated using a relatively simple model of the cathode potential and common oxygen gain experimental techniques. This analysis demonstrates decisively different oxygen gain behavior for the two limiting cases. For catalyst layer mass transfer resistance alone, oxygen gain values are limited to a finite range of values. However, for gas diffusion layer mass transfer resistance alone, the oxygen gain is not confined to a finite range of values. This analysis is then extended to evaluate ionic effects within the catalyst layer. / text

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

La conversion photovoltaïque est un procédé très attractif pour la fourniture d’énergie propre et renouvelable. Cette filière est en plein essor grâce à une réduction constante des coûts de revient et des politiques incitatives de nombreux pays. Pourtant, l’ensemble des panneaux photovoltaïques installés ne produit qu’une faible part de la consommation mondiale en électricité. Les récents développements technologiques dans l’industrie photovoltaïque se sont surtout concentrés sur les cellules dites de seconde génération, à savoir les couches minces à base de CIGS, CdTe, a-Si, a-SiGe. Cette filière permet la fourniture d’électricité à coût inférieur à la technologie standard silicium, mais les rendements de conversion demeurent encore faibles, ce qui nécessite de larges surfaces disponibles. Il est à noter notamment que les cellules couches minces à base de matériaux semiconducteurs à gap direct comme le CIGS et le CdTe sont en plein essor puisqu’ils profitent en particulier d’une absorption accrue par rapport au silicium ; toutefois, ces matériaux sont présents en quantité limitée à la surface de la planète (In, Te). Dans ce contexte, les cellules à base de nanofils constituent une solution intéressante aux problèmes de l’absorption de la lumière, du transport et de la séparation des porteurs de charge photo-générés mais aussi de la quantité de matière utilisée. En effet, en utilisant une jonction radiale (i.e. entourant le nanofil), il est possible de séparer l’absorption de la lumière ( liée notamment à la longueur du nanofil) de la collecte des porteurs de charge (qui dépend du diamètre des nanofils). L’intérêt de ces structures réside également dans les propriétés de base des nanofils : la relaxation élastique favorable sur leur surface latérale ouvre le champ au dépôt de nanofils par hétéro-épitaxie sur tout type de substrat alors que la faible densité de défauts étendus en leur sein est propice à un transport efficace des porteurs de charges. Ainsi, la possibilité de réaliser des nanofils sur substrat souple en réduisant de manière importante la quantité de matière utilisée par rapport à une cellule en silicium cristallin massif peut être envisagée. Plusieurs laboratoires grenoblois ont déjà une expertise dans le domaine de la croissance des nanofils. Cette thèse a pour but de réaliser une analyse expérimentale approfondie des propriétés optoélectroniques des nanofils (par des mesures de réflectivité, de durée de vie des porteurs minoritaires et de recombinaisons en surface et aux interfaces) combinée à des simulations optiques (de type RCWA ou FDTD) et électriques (TCAD). L’objectif ultime étant de concevoir et de développer des cellules à base de nanofils de silicium et de ZnO/CdTe. Des démonstrateurs seront réalisés sur la base des simulations électro-optiques. Pour cela, les moyens d’élaboration, de caractérisation et de technologie des différents laboratoires et entités, ainsi que les compétences associées, seront mis en commun pour accompagner les travaux du doctorant. / Photovoltaic energy is a very attractive way to produce renewable energy. The current increase in the photovoltaic energy production mainly takes advantage of the continuous decrease in the solar cell cost as well as to incentive policy. However, installed photovoltaic panels only contribute to a very small part of the global electricity production. Therefore, important technological developments are dedicated to the second generation of solar cells (i.e. thin film solar cells) in order to reduce more their manufacturing cost despite the resulting lower conversion efficiency owing to a weaker structural and optical material quality. One alternative way to increase the solar cell efficiency is to fabricate nanowire-based solar cells since they may benefit from a higher light absorption and carrier collection efficiency. The light absorption is actually increased thanks to the high surface/volume ratio of nanowires but also to light trapping related to the nanowire length. Furthermore, the collection of minority charge carriers is more efficient in radial structures (i.e. core-shell structures) since the nanowire diameter is very small. This PhD thesis aims at investigating the optoelectronic properties of silicon and ZnO/CdTe nanowires (absorption, lifetime of minority charge carriers, bulk and surface recombination…) in order to design an optimised nanowire-based solar cell structure. Electromagnetic simulations will be first performed to define the best nanowire geometry for the absorbance, and then compared to experimental measurements of the absorption coefficient. Electrical characterisations (lifetime measurements, surface recombination…) will be also achieved to analyse the structural quality and to simulate the solar cell electrical properties. Some prototypes of optimised solar cells will eventually be fabricated.