Tesis sobre el tema "Accumulation de charge photoinduite"

Inspirée de la nature, la conversion de l’énergie solaire par photosynthèse artificielle est l’une des solutions les plus prometteuses à la crise énergétique mondiale actuelle. Cependent, déployer des systèmes artificiels fonctionnels nécessite une compréhension approfondie des processus intégrés dans le fonctionnement des systèmes naturels car ils fournissent les directions pour réaliser des dispositifs de photosynthèse artificielle. Ces processus incluent l’absorption de la lumière, la séparation des charges, plusieurs étapes de transfert de charges menant à leur accumulation et, enfin, la catalyse. Dans ce travail, nous étudions toutes ces étapes élémentaires en utilisant des approches spectroscopiques résolues en temps, dans le but d’explorer la photophysique de différents systèmes moléculaires biomimétiques dédiés à la photoréduction du dioxyde de carbone (CO₂) pour produire des carburants solaires. Nous commençons par le développement d’un nouveau dispositif expérimental pompe-pompe-sonde, capable de déclencher et de détecter l’accumulation progressive de charges grâce à une sonde Raman résonante. Un système modèle contenant le dication méthylviologène (MV²⁺) comme double accepteur d’électrons, le complexe prototypique [Ru(bpy)₃]²⁺ comme photosensibilisateur, et l’ascorbate comme donneur réversible d’électron est utilisé pour une preuve de concept de la technique. En effet, avec la première pompe, MV•⁺ est formé et détecté grâce à son mode vibrationnel caractéristique à 1356 cm⁻¹. Lorsque la concentration transitoire de , MV•⁺ atteint son maximum, nous déclenchons la deuxième pompe laser pour montrer la possibilité de suivre la formation réversible du MV⁰ à travers d’un mode vibrationnel unique à 992 cm⁻¹. Nous passons ensuite à l’étude de systèmes catalytiquement actifs composés de dérivés de porphyrine de fer en tant que catalyseurs pour la réaction de réduction du CO₂. Ces porphyrines sont intégrées dans des systèmes biomimétiques multicomposants contenant du [Ru(bpy)₃]²⁺ et de l’ascorbate comme photosensibilisateur et donneur réversible d’électron, respectivement. Pour le dérivé fonctionnalisé avec des groupements urées (FeUr), un catalyseur contenant un réseau de liaisons hydrogène logé dans sa seconde sphère de coordination, nous fournissons une description mécanistique complète de tous les processus photoinduits conduisant à l’accumulation de charges et son activation vers le CO₂. En atmosphère inerte, en partant de l’état d’oxydation Feˡˡˡ, nous rapportons l’accumulation de deux électrons vers la formation de l’état Feˡ à la suite de la stratégie pompe-pompe-sonde. Sous conditions catalytiques en présence de CO₂, notre approche fournit des preuves convaincantes que l’état d’oxydation Feˡ, produit de deux étapes consécutives de transfert d’électron, est déjà catalytiquement actif, comme en témoigne l’accumulation de l’intermédiaire stable Feˡˡ‒CO caractéristique du cycle de réduction du CO₂. D’une façon générale, nous montrons également que Feˡ est catalytiquement actif indépendamment de la stratégie de fonctionnalisation du macrocycle de la porphyrine, remettant en cause l’interprétation classique de la catalyse de réduction du CO₂ promue par les porphyrines de fer. Enfin, nous nous éloignons du complexe prototypique Ru(bpy)₃]²⁺ pour étudier la photophysique de différents photosensibilisateurs basés sur des éléments abondants sur terre, y compris des complexes à base de cuivre(I), un dérivé porphyrine de zinc (ZnF₂₀), et un colorant carbocationique entièrement organique (TATA⁺). Notamment, nous montrons que le TATA⁺ est capable de photosensibiliser l’accumulation de charges sur le système actif à base de FeUr, activant ainsi la réaction de réduction du CO₂. La caractérisation de nouveaux photosensibilisateurs basés sur des éléments abondants sur terre est fondamentale pour le développement de photosystèmes artificiels avec des applications concrètes dans le monde réel
Inspired by nature’s masterpiece of evolution, the conversion of solar energy through artificial photosynthesis is one of the most promising solutions to the ongoing global energy crisis. Deploying functional artificial mimics of the photosynthetic apparatus, however, requires a deep understanding of the processes embedded in the functioning of naturally photosensitizing organisms as they provide the roadmap to realize artificial photosynthetic devices. These processes include light harvesting, charge separation, multiple charge transfer steps leading to effective charge accumulation and, finally, efficient catalysis. In this work, we investigate all of these elementary steps by employing state-of-the-art time-resolved spectroscopic approaches with the goal of exploring the photophysics of different biomimetic molecular systems devoted to the photoreduction of carbon dioxide (CO₂) to produce energy-rich solar fuels. We start with the development of a novel pump-pump-probe experimental setup that is capable of triggering and detecting the stepwise accumulation of charge through the powerful lens of a resonance-enhanced Raman scattering probe. A model system containing the methyl viologen dication (MV²⁺) as a dual electron acceptor, the prototypical [Ru(bpy)₃]²⁺ complex as a photosensitizer, and ascorbate as a reversible electron donor is used for a proof-of-concept of the technique. Indeed, with the first pump, MV•⁺ is formed and detected through its fingerprint vibrational mode at 1356 cm⁻¹. When the transient concentration of MV•⁺ peaks, we fire the second laser pump and show the possibility of tracking the reversible formation of the two-electron accumulated MV⁰ species through a unique vibrational mode at 992 cm⁻¹. We then move on to investigating catalytically active systems featuring iron porphyrin derivatives as CO₂ reduction catalysts. These porphyrins are integrated into multicomponent biomimetic systems that similarly contain [Ru(bpy)₃]²⁺ and ascorbate as photosensitizer and reversible electron donor, respectively. For the urea-functionalized derivative (FeUr), a catalyst with a hydrogen-bonding network lodged in its second coordination sphere, we provide a full mechanistic depiction of all photoinduced processes leading to charge accumulation and its activation towards CO₂. In inert atmosphere, starting from Feˡˡˡ, we report the stepwise formation of the formal Feˡ species as a result of the double pump excitation strategy. Remarkably, under catalytic conditions in the presence of CO₂, our spectroscopy-based approach provides compelling evidence that the Feˡ oxidation state of FeUr, product of two consecutive electron transfer steps, is already catalytically active, evidenced by the accumulation of the stable Feˡˡ‒CO intermediate of the CO₂ reduction cycle. Going beyond FeUr, we show that Feˡ is catalytically active irrespective of the design strategy used in the functionalization of the porphyrin macrocycle, challenging the classical picture of CO₂ reduction catalysis promoted by iron porphyrins. Finally, we move away from the prototypical [Ru(bpy)₃]²⁺ complex and dive into the photophysics of different photosensitizers based on earth-abundant elements, including copper(I)-based complexes, a perfluorinated zinc porphyrin derivative (ZnF₂₀), and a fully organic triazatriangulenium carbocationic dye (TATA⁺). Importantly, we show that the TATA⁺ dye is capable of photosensitizing charge accumulation on the active FeUr-based system, activating it towards the reduction of CO₂. The characterization of new photosensitizing units based on abundant elements is fundamental for the development of artificial photosystems with real-world applications

High voltage direct current (HVDC) based transmission of electric power, in comparison with high voltage alternating current (HVAC) transmission over a distance of over 800 km, has the advantages of higher transmission capability, lower line loss, more confined damage by fault, and that all AC lines connected to an HVDC system do not need to be synchronized. However, under DC conditions the electric field across the insulation space remains unidirectional and stable, which results in charge accumulation, leading to significant differences in the dielectric behavior and flashover characteristics of insulation materials in comparison with those under AC conditions. There exists limited amount of test data and research results on HVDC insulation behaviour. It is therefore necessary to carry out in-depth study of the discharge mechanisms and behavioral characteristics of the insulation materials required for HVDC transmission systems. The research in this thesis aims at the understanding of the charge transport and accumulation process inside and around insulators made of epoxy composite material. Firstly, the physical mechanisms are critically reviewed with appropriate experimental results selected for model verification. The model developed in the work encompasses all important mechanisms. Material properties obtained under different conditions are reviewed and values for the cases investigated in the present work determined. Charge transport is dominated by the drift of two types of oppositely charged particles in electric field with nonlinear generation source terms, giving rise to extreme difficulties for convergence of computation in strong electric field. As a result, special code is developed for a commercial software package (COMSOL based on finite element method) to implement the model and also the complicated boundary conditions. To gain confidence in the model and its implementation including the boundary conditions and material properties the model is verified in the work against experimental cases with different gas, geometry and applied voltage. An experimental study of the effect of surface charge accumulation on the potential distribution along the surface of an epoxy insulator under HVDC stress was carried out under non-ideal conditions.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2001.
Includes bibliographical references (p. 161-172).
We describe results obtained by a novel scanning technique, called Subsurface Charge Accumulation (SCA) Imaging, that enables the direct imaging of electronic systems buried inside semiconductor materials. Using SCA Imaging, we image and measure properties of a two-dimensional electron system (2DES) in a GaAs/AlGaAs heterostructure, in the regime of the integer quantum Hall effect. We observe general charging features in a plain 2DES near quantum Hall integer filling factors. We proceed by imaging low compressibility strips in the presence of an artificially created density gradient in the 2DES. We study them in detail at Landau level filling factors v = 2, 4. The strips appear significantly wider than predicted by theory and we account for the discrepancy by presenting a model that considers the disorder-induced nonzero density of states in the cyclotron gap. We also measure the charging properties of incompressible strips that form parallel to the edges of a metal gate deposited on the surface of our sample. An RC model considering charging of the 2DES across the strip, closely fits the data. This allows us to determine the longitudinal resistivity of the incompressible part of the edge state that runs parallel to the gate, for a range of filling factors. Surprisingly, the strip becomes more resistive in regions of high electronic density gradient, where its width is expected to decrease. By sensing charge from the motion of single electrons inside the 2DES we produce a topographic map of the random potential inside the integer quantum Hall liquid.
(cont.) We achieve this by creating a mobile quantum dot inside the 2DES. By scanning the dot, single electrons enter or leave it, in response to the local potential. Detection of this motion leads to the creation of a potential contour map. We find that the 2D electron screening of the random potential induced by external impurities, changes little between quantum Hall plateaus and within each plateau. We finally present preliminary results from a 2DES sample with a built-in backgate. The backgate enables us to deplete the 2DES and perform measurements in the regime of low electronic densities.
by Paul Ioannis Glicofridis.
Ph.D.

The focus of this work has been on hybrid insulation in inhomogeneous electric fields under lightning impulse voltage stress. The principal idea behind hybrid insulation is the intentional use of surface charges to re-distribute the electric field within an insulation system. This allows a significant part of the electric stress to be transferred from the dielectric weaker gas to the dielectric stronger solid insulation thus increasing the total electric strength of the insulation system.

The concept has been theoretically and experimentally addressed by means of a hemispheric rod covered with a layer of solid insulation. Discharge activity and surface charge accumulation have been studied in an air gap by measuring the voltage and discharge current and recording the discharge activity using a high-speed digital camera. New methods have been introduced and evaluated for the evaluation of surface charge measurements.

The experiments found that the increase in positive inception voltage was considerable compared to uncovered rods. This increase varied from 35% up to 100% depending on the electrode distance. The increase in breakdown strength is higher than the increase in inception voltage and dependent on the covered length of the rod. During the application of a lightning impulse, the discharge activity spreads upwards along the rod and out into the air gap. Positive discharges form numerous branches and bridge the air gap in most cases. Negative discharges are more diffuse, less light intensive and only form a few branches around the tip of the rod where the electric field is the strongest. Discharge activity along the insulating surface has been observed where the background field is lower than the critical electric field strength. Visible discharge activity is observed where the background field is higher than 2.3 kV/mm and 2.5 kV/mm for positive and negative impulses respectively.

During the application of lightning impulses, discharge activity starts in the air gap around the tip where the electric field is highest and spreads upwards along the rod. As expected, negative charges accumulate on the surface in the case of positive impulse voltage and vice versa. However, after more powerful discharges during negative impulse voltage application, surface charges of both polarities have been observed.

Accumulated surface charges decay exponentially with a time constant τ varying from micro-seconds to hours depending on the material properties of the solid insulation. The dominating relaxation mechanism is found to be conduction through the solid insulation.

Improved methods to calculate surface charges based on probe response for a 2D axial symmetric case have been developed and evaluated. The method that is best suited for this purpose is the λ-method with truncated singular value decomposition (TSVD) as regularization.

Surface charge calculations show that the accumulated surface charges for the used configuration typically have a maximum value of 0.6 to 1.5 µC/m² and 0.4 to 1 µC/m² after positive and negative impulses respectively. The surface charge density in the areas with the highest discharge activity is relatively uniform. Further upwards along the rod, the surface charge density is reduced relatively fast towards zero, and in some cases, it changes polarity before approaching zero.

Metal encapsulation with compressed sulphur hexafluoride gas insulation has reduced the size of power substations and improved their reliability. Due to the superior insulating properties of this gas, its application in Gas Insulated Transmission lines (GITL) and Gas Insulated Substations (GIS) is increasing. Such systems invariably require solid support spacers for supporting high voltage conductors and for compartmentalizing sections of the systems. It has been found that, although the dielectric strength of sulphur hexafluoride is high compared to other gases, in the presence of a solid spacer, the dielectric integrity of the system is impaired. For DC GIS and GITL, free of any particulate contamination, anomalous flashover during a polarity reversal of the applied voltage has been attributed to charge accumulation on the spacer surface. The present work examines the effect of switching impulse voltages on the charge accumulation on support spacers in sulphur hexafluoride gas and the effect of AC precharging on impulse flashover. The charge accumulation on the spacer has been measured using a capacitive probe. A three-dimensional surface charge simulation technique has been developed to convert the probe potential measurements to an equivalent charge distribution. Electric field variation on the spacer surface due to this computed charge can be then obtained with this program. The results indicate that spacers acquire charges even under impulse voltage application in non-uniform field gaps. These charges affect the prebreakdown phenomenon and the breakdown behavior of spacer gaps. Under AC voltages, spacers in uniform fields do not acquire charges as has been reported by previous investigators. Under non-uniform field conditions however, AC precharging of spacers does affect the impulse flashover. The variation however, seems to fall within the statistical scatter of pure impulse flashover.
Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate

Compressed gas insulated substation (GIS) and transmission line (GITL) equipment have been developed rapidly throughout the world during the past decades. Compactness is the main advantage of GIS and GITL over conventional air-insulated substations and transmission lines. Sulphur hexafluoride (SF₆) is used as an insulation gas in GIS and GITL because of its excellent insulating properties. Supporting spacers are identified as the most likely places for flashover to occur and they often determine the overall strength of a system. For energized system, surface charges have been observed on spacer surfaces and are considered to play an important role for anomalous flashover of a GIS or GITL system. The purpose of this research is to study the mechanisms and factors governing the magnitude and distribution of surface charges and their influence on flashover voltage. In this investigation, experiments to study surface charge accumulation under different experimental conditions have been conducted, with a rod-spacer-plane electrode system. The parameters varied are applied voltage levels, insulating gases, gas pressures, spacer materials, rod electrode diameters, and the duration of applied voltages. Experiments with impulse voltage pre-charging were also conducted. It was found that the mechanisms of surface charging are corona, gas conduction, and photoionization. Surface charge magnitude and distribution are strongly field dependent and are related to the duration of voltage application. Spacers in SF₆ gas accumulate less charges on the surface than in air and nitrogen , for a given geometry, spacer material, voltage, and gas pressure. In order to determine the effect of surface charges on overall electric field, a surface charge simulation program (the SSM program) was developed to calculate the overall electric field when there are surface charges on a spacer surface. The calculation results show that surface charges significantly distort the overall electric field magnitude, field direction, and distributions, which may explain the anomalous flashover in GIS/GITL systems. It was found that the flashover propagation field on a PTFE spacer surface may be between 6 kV/cm and 10 kV/cm. High speed photographic observations, with an image intensifier, are suggested to examine the propagation of a flashover. It may, therefore, be possible that, a reasonable model to predict flashover in SF₆ gas can be built by comparing the flashover propagation model with the electric field pattern on a spacer surface. The role of photoionization near the spacer surface in charge accumulation should also be studied in greater detail.
Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate

This thesis is part of the fields of Material Physics and Organic Electronics and aims to determine the charge carrier density and mobility in the hydrated conducting polymer–polyelectrolyte blend PEDOT:PSS. This kind of material combines electronic semiconductor functionality with selective ionic transport, biocompatibility and electrochemical stability in water. This advantageous material properties combination makes PEDOT:PSS a unique material to build organic electrochemical transistors (OECTs), which have relevant application as amplifying transducers for bioelectronic signals. In order to measure charge carrier density and mobility, an innovative 4-wire, contact independent characterization technique was introduced, the electrolyte-gated van der Pauw (EgVDP) method, which was combined with electrochemical impedance spectroscopy. The technique was applied to macroscopic thin film samples and micro-structured PEDOT:PSS thin film devices fabricated using photolithography. The EgVDP method revealed to be effective for the measurements of holes’ mobility in hydrated PEDOT:PSS thin films, which resulted to be <μ>=(0.67±0.02) cm^2/(V*s). By comparing this result with 2-point-probe measurements, we found that contact resistance effects led to a mobility overestimation in the latter. Ion accumulation at the drain contact creates a gate-dependent potential barrier and is discussed as a probable reason for the overestimation in 2-point-probe measurements. The measured charge transport properties of PEDOT:PSS were analyzed in the framework of an extended drift-diffusion model. The extended model fits well also to the non-linear response in the transport characterization and results suggest a Gaussian DOS for PEDOT:PSS. The PEDOT:PSS-electrolyte interface capacitance resulted to be voltage-independent, confirming the hypothesis of its morphological origin, related to the separation between the electronic (PEDOT) and ionic (PSS) phases in the blend.

Le transfert de charge des principaux aquahydroxocomplexes de Fer(III) a été étudié par spectroscopie électronique. Les résultats obtenus, confrontés à ceux de calculs semi-empiriques, ont permis d'assigner le type de transitions, ainsi que les orbitales moléculaires impliquées. La photolyse de l'ion hexaaquafer(III) a par la suite été explorée à l'aide d'alcools aliphatiques, employés comme pièges à radicaux. En absence totale d'oxygène, la dégradation photoinduite de ces substrats, a fourni des méthodes convergentes de détermination de rendements quantiques. L'impact des radicaux péroxyle sur les mécanismes réactionnels radicalaires a également été abordé, mettant en évidence de nouvelles réactivités, quantifiées par modélisation cinétique. Enfin les rendements quantiques de photoréduction des monomères Fe3+aq, Fe(OH)2+ et Fe(OH)2+, ainsi que du dimère Fe2(æ-OH)24+ ont été déterminés expérimentalement; des modèles de dépendance en longueur d'onde sont proposés
The charge transfer of main Iron(III) aquahydroxocomplexes has been studied using electronic spectrotroscopy. Confronting the results obtained with those of semi-empirical calculations, enabled to label which type of transitions, and molecular orbitals are involved. The photolysis of the hexaaquairon(III) ion was therefore explored, with aliphatic alcohols, utilised as radical cavengers. In absence of oxygen, the photoinduced degradation of these substrates provided consistent methods of quantum yield determination. The impact of peroxyl radicals on reaction pathways has also been investigated. Some new reactivities were highlighted, and assessed through kinetic modelling. Eventually, the photoreduction quantum yields of the Fe3+aq, Fe(OH)2+ and Fe(OH)2+ monomers, as well as of the Fe2(æ-OH)24+ dimer have been determined experimentally; wavelength dependence models are proposed

Les matériaux diélectriques sont présents dans de nombreux dispositifs en microélectronique. Ces derniers peuvent être soumis à de fortes contraintes électriques impactant leur durée de vie. Le stress électrique peut en effet provoquer le claquage du diélectrique ou la modification des performances des composants par accumulation de charges. Dans ces travaux de thèse, différentes méthodes de caractérisation et d'analyse physique ont été utilisées pour étudier la structure des échantillons et identifier les mécanismes en jeu dans le processus d'accumulation de charges dans des couches minces de nitrure de silicium. Puis un code de simulation modélisant les phénomènes de transport de charges dans les isolants a été développé. Le modèle prend en compte des phénomènes de transport par effet tunnel et par effet thermique, dans le volume du diélectrique et aux interfaces isolant-métal. Il permet d'étudier l'évolution de grandeurs physiques (courants, charge, champ électrique) en fonction du temps et de la profondeur dans la couche mince diélectrique. Des résultats de mesures sur des composants capacitifs ont pu être reproduits grâce aux simulations. Cet outil permet d'estimer l'intérêt d'un matériau diélectrique relativement à la fiabilité de composants capacitifs. Il peut également être utilisé en amont afin de définir un matériau aux propriétés idéales pour l'application visée ou aider au dimensionnement de dispositifs en microélectronique
Dielectric materials can be found in numerous devices in microelectronics. They can be subjected to significant electrical stress, which impacts their lifetime. Indeed, this electrical stress can lead to dielectric breakdown or modify the component performances by charge storage. In this work, several characterization methods and physical analysis have been used in order to study the samples and identify mechanisms involved in charge transport in silicon nitride thin films. Then a simulation code has been developed to model charge transport phenomena in insulators. This model takes into account tunnel and thermal effects in the dielectric and at the dielectric-metal interfaces. The temporal and spatial evolution of physical quantities (currents, charge, electric field) in the dielectric film are calculated. Measurement results on capacitive components can be obtained thanks to simulations. This simulation tool allows testing dielectric materials according to capacitive component reliability. It may be used to define optimal properties for materials depending on applications or to assist in device design in microelectronics

Ce mémoire porte sur l'étude des propriétés spectroscopiques, photo physiques et photochimiques ainsi que sur les calculs d'O. M. De complexes phényl-4'-terpyridine ( ptpy ), parasubstitués M(Xptpy)² ⁺/₂ (M =Fe(II), Ru(ll) ou Os (II) ;X =-OCH₃, CH₃, CI,-OH). La bande d'absorption visible MLCT des complexes Ru(Xptpy )² ⁺/₂ est plus intense et déplacée de 50 nm vers le rouge par rapport à celle de Ru(bpy)² ⁺/₃. Leur émission observée, à température ambiante, en solution et dans les membranes est très faible avec une durée de vie de quelques nanosecondes. Par spectroscopie éclair laser, nous avons déterminé les spectres d'absorption des états excités des complexes de Ru(II) et Os(II). La similarité de ces spectres avec ceux des anions des ligands XptpyϮ suggère fortement que la densité électronique est localisée sur un seul des deux ligands équivalents. Le complexe d'osmium dont l'état excité a une durée de vie relativement longue (Ϯ- 250 ns) est intéressant pour l'élaboration d'édifices moléculaires à deux composants PS-A (PS photosensibilisateur, A accepteur) dénommés "diades". Dans la diade Os(CH₃ptpy) (MV²+- -CH₂ptpy)⁴ + où l'accepteur est le méthylviologène MV² +, nous avons mis en évidence un phototransfert d'électron vectoriel intramoléculaire conduisant à l'état de sépa­ ration de charges PS+-A- de 720 ps de durée de vie. Ce résultat est une étape importante pour la réalisation de systèmes triades D-PS-A (D donneur) susceptibles de conduire à une séparation de charges de plus longue durée. Les calculs d'O. M. (EHM0) nous ont permis d'interpréter les spectres d'absorption de l'état fondamental et de l'état excité des complexes.

Cette These experimentale examine le transport par effet tunnel entre deux couches ferromagnetiques separees par une barriere isolante ultrafine. L'enjeu de ces travaux est de rapprocher la comprehension theorique, basee sur des systemes ideaux, de la realite experimentale domin´ee par des jonctions comprenant une barriere amorphe. Au moyen de jonctions partiellement ou entierement epitaxiees integrant le materiau La0:7Sr0:3MnO3 dont nous avons confirme la polarisation de spin tunnel quasi-totale, l'influence de la structure electronique de materiaux isolants tels que SrTiO3, Ce0:69La0:31O1:845, TiO2, MgO (epitaxies) et Al2O3 (amorphe) sur le magnetotransport tunnel est mise en evidence. La theorie soutendant ces resultats est testee au moyen de mesures XMCD effectuees sur des barrieres de Al2O3 et MgO. La demi-metallicite de La0:7Sr0:3MnO3 est ensuite utilisee dans des jonctions La0:7Sr0:3MnO3 /SrTiO3 /La0:7Sr0:3MnO3 et La0:7Sr0:3MnO3 /SrTiO3 /Co afin d'affirmer quantitativement le caractere spectroscopique du transport tunnel polarise en spin entre electrodes ferromagnetiques. Ces etudes en tension montrent l'influence de la generation d'ondes de spin lors du transport tunnel sur l'ordre ferromagnetique de l'interface manganate/ isolant proche de sa temperature de transition metal-isolant. Enfin, nous utilisons l'electromigration aux interfaces afin de modifier la densite d'etats et le profil de potentiel des interfaces. Nous montrons comment il est possible de r´ealiser un dispositif aux proprietes de magnetotransport bistables; et nous examinons dans le regime tunnel Fowler-Nordheim les repercussions de ces modifications sur la formation d'etats quantifies au sein de la barriere, et l'evolution du couplage d'echange indirect entre les electrodes ferromagnetiques.

This thesis outlines investigations of metal oxide/organic interfaces in photo-voltaic devices. It focuses on device instabilities originating from the metal oxide layer surface sensitivity and it presents suggested mechanisms behind these in- stabilities. A simple sol-gel solution deposition technique for the fabrication of stable and highly performing transparent conducting mixed metal oxides (ZnMO) is presented. It is demonstrated that the use of amorphous, mixed metal oxides allows improving the performance and stability of interfacial charge extraction layers for organic solar cells. Two novel ternary metal oxides, zinc-strontrium- oxide (ZnSrO) and zinc-barium-oxide (ZnBaO), were fabricated and their use as electron extraction layers in inverted organic photovoltaics is investigated. We show that using these ternary oxides can lead to superior devices by: prevent- ing a dipole forming between the oxide and the active organic layer in a model ZnMO/P3HT:PCBM OPV as well as lead to improved surface coverage by a self assembled monolayer and promote a significantly improved charge separation efficiency in a ZnMO/P3HT hybrid device. Additionally a spectroscopic technique allowing a versatility of characterisa- tion for long-term stability investigations of organic solar cells is reported. A device instability under broadband light exposure in vacuum conditions for an inverted ZnSrO/PTB7:PC71BM OPV is observed. Direct spectroscopic evidence and electrical characterisation indicate the formation of the PC71BM radical an- ion associated with a loss in device performance. A charge transfer mechanism between a heavily doped oxide layer and the organic layers is suggested and dis- cussed.

The task of the thesis was studding of tantalum capacitors with solid electrolytes properties. Ta – Ta2O5 – MnO2 capacitor by its construction represents MIS structure, where tantalum anode has metal conductivity and MnO2 cathode is semiconductor. Isolation layer consists of tantalum pentoxide Ta2O5 with relative permitivity r = 27. Dielectric thickness is typically in range from 30 to 150nm. The capacitor charge is not only stored and accumulated on electrodes but also in localised states (oxide vacancies) in isolation layer. The capacitor connected in normal mode represents MIS structure polarized in reveres direction when the applied voltage higher potential barrier between semiconductor - MnO2 cathode and isolation of Ta2O5. The transport of charge carriers via isolation layer is determined by Poole-Frenkel mechanisms and tunnelling. Poole-Frenkel mechanism of charge transport is dominant in low intensity of electric field. Tunnelling determines current at higher electric field intensity. During low intensity of electric field ohmic component is also presented which is determined by volume of resistance of impurities in isolation layer due to donor states of oxygen vacancies. Based on the modelling of measured VA characteristics is possible to estimate determine dielectric thickness of Ta2O5 and determine share of Poole-Frenkelov and tunnel current and charge transportation. The thesis is described charge transport and charge concentration on tantalum capacitor in low frequency area and analysis of capacitor behaviour at frequency band. The first impulse for the thesis was an effort to create equivalent circuit diagram of tantalum capacitor in respect of its physical and electrical behaviour. There is an opportunity to study and determine electric charge transport and its accumulation based on the equivalent circuit diagram structure. There is also a chance to define and trace potential barriers and charge distribution in the capacitor structure based on an measurement and carried out experiments. This methodology and analysis consists of electrical characteristic determination to create physical model of the capacitor describing it function, properties and behaviour.

碩士
國立成功大學
光電科學與工程學系
100
Pentacene and N,N’-dioctadecy1-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13H27) are used as active layers to fabricate organic non-volatile memory devices with various types of charge trapping layer. Several types of semiconductor film are used to determine the impact of surface morphology and electrical properties. In the first part, a single-layer structure that has a trapping layer, Polystyrene (Ps), and cross-linked poly (4-vinylphenol) (C-PVP) and a double-layer structure that has a modification layer and a trapping layer, PS + PS, C-PVP + Poly (a-methylstyrene) (PαMS), and PS + polymethylmethac- 0.rylate (PMMA) are used. The memory window of the double–layer structure is much larger than that of the single-layer structure. C-PVP + PαMS provides the largest memory window and the best stability. In the second part, three types of polyimide, namely DA7013, DA9000, and DA9000A, are used as a trapping layer to fabricate non-volatile memory devices. The main structures of DA9000 and DA9000A are the same, but DA9000A does not have a side chain and thus lacks the ability to capture carriers. DA9000A is thus unsuitable for a memory device. The opposite is true for DA9000. DA7013 exhibits an excellent n-type memory feature and durability.

This diploma thesis deals with photovoltaic rechargable regulators used in isolated, or off-grid photovoltaic systems. First, basic types of off-grid installations, including their functions and applications in practice are described. Then, possibilities of electric energy accumulation in photovoltaic systems are mentioned, considering actual, accesible technologies. Matters of electric energy accumulation in leaden accumulators are examined in detail. Main part of the diploma thesis is about electronic designs of photovoltaic, also called solar, rechargable regulators. These are the main connecting part between photovoltaic panels, accumulator, but also often connected charge, which is appliance. Individual electronic regulators concepts are described narrowly, then compared and evaluated in thesis´ conclusion.