Os eletrodos de filme de SnO2 suportado sobre sílica ou titânio apresentam propriedades eletroquímicas e morfológico-estruturais bastante distintas e são empregados em diferentes aplicações. Estes dois tipos de materiais foram preparados e investigaram-se suas propriedades através de diversas técnicas eletroquímicas e de análise de superfície. Os eletrodos de Ti/SnO2(Sb) apresentaram a superfície porosa e rugosidade de 500 nm. Para os filmes finos (~40 nm) depositados sobre sílica, porém, observou-se alta transparência, superfície uniforme e rugosidade de 1 nm. Mesmo entre os eletrodos transparentes, dependendo do dopante (Cl, F ou Sb), verificaram-se diferenças quanto às orientações cristalográficas predominantes, espessura, resistividade e tamanho das partículas. As investigações eletroquímicas também revelaram diferenças consideráveis entre ambos os tipos de eletrodos. Para as reações de óxido-redução do K4Fe(CN)6, p.ex., a transferência de elétrons foi muito mais rápida sobre o eletrodo de Ti/Sn)2(Sb), possivelmente porque este eletrodo apresentava maior número de portadores de carga e menor resistência ôhmica que os eletrodos transparentes. Investigou-se ainda o comportamento eletroquímico de eletrodos modificados com um filme de octadeciltriclorosilano (OTS). Os estudos revelaram que a presença do OTS não alterou o mecanismo para a reação de geração de oxigênio, porém, alterou significativamente o comportamento do eletrodo de Ti/SnO2(Sb) na eletro-oxidação do metanol. Aparentemente, o OTS induziu a formação de espécies poliméricas que bloquearam o eletrodo, indicando, portanto, que estes eletrodos modificados não são indicados para atuar como ânodos na eletro-oxidação de compostos orgânicos. Os processos de corrosão induzidos por polarização anódica ou catódica foram investigados por microscopia de forças atômicas e de efeito túnel para os eletrodos transparentes modificados ou não com OTS. As análises revelaram que os processos de corrosão iniciaram-se nos contornos dos grãos e atuaram de maneira diferenciada sobre as partículas que constituem o filme. A corrosão foi muito menos pronunciada para os eletrodos modificados com OTS, indicando que este organosilano poderia proteger a superficie do SnO2. Porém, a aplicabilidade do eletrodo Ti/SnO2(Sb)-OTS seria muito limitada, considerando que o OTS poderia promover a polimerização de espécies orgânicas na superficie do eletrodo. / The electrochemical and morphological properties of tin oxide electrodes deposited on silica or titanium substrates were investigated by electrochemical and surface analysis techniques. The Sb doped SnO2 film deposited on Ti was prepared by thermal decomposition of stannic chloride solution and investigated by SEM and interferometric microscopy. This Ti/SnO2(Sb) exhibited a porous surface, with a rms roughness of 500 run. The transparent film of SnO2 doped with Cl, F or Sb, which was deposited on silica by Spray Pyrolysis, were analyzed by XPS, XRD, SEM, AFM and STM techniques. These films were polycrystalline and had an uniform and smooth surface with a rms roughness ranging from 1 to 2 nm. Their microstructure was constituted of coalesced particles with an average diameter and height of 30 nm and 6 nm, respectively. Thickness, conductivity, grain size, and surface composition of transparent films depended on the dopant. The electrochemical behavior of both types of SnO2 electrodes were investigated by Cyclic Voltammetry and Electrochemical lmpedance Spectroscopy (EIS). For the reversibility of K4Fe(CN)O6 redox reaction, while the Ti/SnO2(Sb) electrode supported high rate for electron transfer, the transparent electrodes exhibited a poor electrochemical response, which was attributed to the high ohmic resistance. The effect of electrochemical perturbation in 0.1 M NaClO4, pH 2, on the microstructure oftransparent electrodes was investigated by in-situ STM and ex-situ AFM, SEM and other techniques. Anodic polarization at potentials near the OER region and potential cycling induces little alterations on the microstructure of the electrode, observable only in sub-micron range by AFM analysis. Cathodic polarization at potentials in the region of the HER, however, strongly damages the filmo AFM and STM examination revealed that the grain boundaries are attacked, and both vertical and lateral dimensions of the particles decrease. From the observations, it can be inferred that SnO is formed during the cathodic polarization and, destruction of the film occurs by dissolution of that more soluble oxide. In-situ STM experiments showed that, at the initial stages, the particles of the damaged film can acquire dimensions even larger than the initial size by anodic polarization. Finally, it can be concluded that a smooth and compact film, with little defects is less susceptible to corrosion.

Avec la demande toujours croissante d'énergie à laquelle l’homme fait face, le photovoltaïque (PV), qui convertit le rayonnement solaire en électricité, a connu ces dernières décennies un développement important. Bien que le marché PV actuel soit principalement dominé par les technologies à base de Si cristallin, la technologie PV à base de couches minces porte toujours l'espoir de contribuer efficacement à l'avenir vis-à-vis de la crise énergétique en raison de son coût beaucoup plus faible et d'une efficacité raisonnable.Les matériaux transparents conducteurs (TCM), principalement des oxydes conducteurs transparents (TCO), sont une composante essentielle dans la plupart des types de cellules solaires à couches minces car ils servent d'électrode de collecte des porteurs photo-générés sur la face avant de la cellule, c’est-à-dire celle face au soleil. Afin d'améliorer l'absorption optique (limitée par l'épaisseur de l'absorbeur) dans des cellules solaires à couches minces, on souhaite souvent que les TCO soient texturés (avec une rugosité de surface significative) de manière à bien diffuser la lumière, et ainsi de présenter des valeurs élevées de facteur de diffusion de la lumière. Ce dernier, que l’on peut appeler facteur de haze, est défini comme le rapport entre la transmittance (respectivement réflectance) diffuse et la transmittance (respectivement réflectance) totale. Plus ce facteur est élevé plus le TCO diffuse la lumière. Par voie de conséquence, la longueur du trajet optique est augmentée et ainsi le piégeage de la lumière dans la cellule solaire est amélioré, donnant lieu à une absorption de lumière plus importante dans les couches actives et augmentant potentiellement le rendement de conversion photovoltaïque des cellules solaires.Dans ce travail, des nano-composites innovants à base de SnO2 dopé au fluor (FTO) en combinaison avec les nanoparticules ZnO, S:TiO2 et Al2O3 ont été développés en utilisant un processus économique et facile constitué de deux étapes. Ces nano-composites à base de FTO présentent une transmittance totale de 70-80% et une résistance de 10-15 Ohm/sq, satisfaisant ainsi aux exigences requises pour des oxydes transparents conducteurs utilisés au sein de dispositifs photovoltaïques. En modifiant la concentration de la suspension de nanoparticules le facteur de haze de ces nano-composites peut être varié, de manière contrôlée, de presque 0% à 80%. Les propriétés morphologiques, structurales, électriques et optiques de ces nano-composites à base de FTO sont étudiées en détail et elles apparaissent étroitement dépendantes des nanoparticules sous-jacentes. Avant de discuter de l'intégration des nano-composites FTO au sein de cellules solaires, des efforts ont également été consacrés à une bonne compréhension de l'interface FTO/TiO2 qui est couramment présente au sein de divers types de cellules solaires à couches minces émergentes telles que les cellules solaires sensibilisées au colorant (DSSCs). Enfin, les nano-composites FTO diffusants ont été intégrés comme électrodes transparentes au sein de différents types de cellules solaires à couches minces et l'effet du facteur de haze sur la performance du dispositif a été étudié.En ajustant correctement le type et la concentration des nanoparticules sous-jacentes, les propriétés des nano-composites à base de FTO peuvent être variées de manière à répondre aux exigences d'électrodes pour une technologie photovoltaïque spécifique. Notre concept de préparation du TCO nano-composite en combinant les TCO et les nanoparticules propose une ligne directrice générale qui conduit à l’élaboration d’électrodes à caractère diffusant variable; permettant ainsi une bonne absorption des photons pour le photovoltaïque en couches minces. / With the increasing demand for energy that human beings are faced with, the photovoltaics (PV) technology which converts solar radiation into electricity has undergone increasingly development. Although the current PV market is mainly dominated by the crystalline Si based technologies, thin film PV still bears the hope to become the solution to the energy crisis in the future due to its much lower cost and reasonable efficiency.Transparent conductive materials (TCMs), mostly transparent conductive oxides (TCOs), are an essential component in most types of thin film solar cells as the current-collecting electrode on the sun-facing side of the cell. In order to improve the optical absorption (which is restricted by the limited absorber thickness) in thin film solar cells, the TCOs are often desired to be textured (with significant surface roughness) to show high values of haze factor. Haze factor is defined as the ratio of the diffuse transmittance/reflectance to the total transmittance/reflectance. The hazier a TCO is (i.e. with higher haze factor), the more light it scatters. As a consequence, the optical path length is increased and thus the light trapping in the solar cell is improved, giving rise to higher light absorption in the active layers and photon-to-current conversion efficiency of the solar cells.In this work, innovative nanocomposites of fluorine doped SnO2 (FTO) in combination with ZnO, S:TiO2 and Al2O3 nanoparticles have been developed using an economic and facile 2-step process. These FTO nanocomposites exhibit 70-80% total transmittance and 10-15 Ohm/sq sheet resistance, satisfying the basic requirements as transparent conductive oxides used in photovoltaics devices. By changing the nanoparticle suspension concentration, the haze factor of these nanocomposites can be varied, in a controlled way, from almost 0% up to 80%. The morphological, structural, electrical, and optical properties of these FTO nanocomposites are investigated in great details and are found to be closely related to the underlying nanoparticles. Before discussing the integration of the FTO nanocomposites into real solar cell devices, efforts have also been made to shed some light on the understanding of FTO/TiO2 interface commonly adopted in various types of emerging thin film solar cells such as dye sensitized solar cells (DSSCs). Finally, the hazy FTO nanocomposites have been used as transparent electrodes in different types of thin film solar cells and the effect of haze factor on the device performance has been examined.By properly tuning the type and concentration of the underlying nanoparticles, the properties of the FTO nanocomposites can be tuned to meet the electrode requirement for specific photovoltaic technology. Our concept of preparing TCO nanocomposite by combining TCOs and nanoparticles provides a general guideline to design hazy electrodes as light management structures in thin film photovoltaics.

Solid state lighting is a new environmentally friendly light source. So far, light emitting diodes (LEDs) and organic LEDs (OLEDs) have been presented as candidates with potentially high efficiency. Recent advances of OLEDs in device architecture, light-out coupling, and materials have ensured high efficiency, exceeding that of incandescent light bulbs. In contrast to conventional point source LEDs, OLEDs distribute light throughout the surface area and are not restricted by their size. Additionally, OLEDs are expected to reach sufficient stability in the near future. The remaining challenge for OLEDs is their cost. New OLED technologies provide cost effective manufacturing methods which could be presented for transparent electrode materials because indium tin oxide (ITO), a widely used material as a transparent electrode for OLEDs, is less than optimal due to its high element price. In this work, alternative transparent electrodes for OLEDs as a replacement of ITO were studied. First, Al doped ZnO (ZnO:Al) which is composed of abundant materials was investigated with DC magnetron sputtering under a wide range of experimental conditions. The optimised ZnO:Al received comparable performance with conventional ITO films, low sheet resistance of 22.8 Ω/sq as well as a high transparency of 93.1 % (average value in the visible range). Various type of p-i-n OLEDs were employed on the structured ZnO:Al using photolithography. Green OLEDs with double emission layers have been archived stable efficiencies even at higher luminance. Also, OLEDs using two fluorescent colour system on ZnO:Al anode showed a purely white emission. It has been found that the OLEDs on ZnO:Al anode has comparable or better device efficiencies and operational lifetime compared to OLEDs on conventional ITO anode. As another alternative electrode, the conductive polymer Baytron®PH510 (PEDOT:PSS) was investigated. Due to a relatively high sheet resistance of PEDOT:PSS, metal grid was designed for large size OLEDs. White OLEDs on PEDOT anode with a size of 5 × 5 cm2 have achieved more than 10 lm/W of power efficiency using a scattering foil. Furthermore, up-scaled devices on 10 × 10 cm2 were also demonstrated. These results showed ZnO:Al and PEDOT are suitable for OLEDs as anode and have high potential as alternative transparent electrode materials.

Organic solar cells offer attractive properties for novel applications and continuous advances in material and concept development have led to significant improvements in device performance. To exploit their full potential (roll-to-roll production of flexible and top-illuminated devices, using e.g. opaque metal foil or textile as substrate), highly transparent, conductive, mechanically flexible, and cost-efficient top electrodes are of great importance. The current standard material indium tin oxide (ITO) is rigid, expensive and requires a high energy / high temperature deposition process, limiting ITO (and other transparent conductive oxides) to bottom electrode applications. This work presents fundamental investigations to understand and control the properties of transparent conductors and documents four different approaches to prepare transparent electrodes on top of efficient small molecule organic solar cells, with the aim to replace ITO. Fullerene C60 layers are investigated as completely carbon-based electrodes. For an optimized doping concentration, sheet resistance and transmittance are improved and efficient solar cells are realized. Since the lateral charge transport is still limited, a combination with a microstructured conductor is suggested. Pulsed laser deposition allows for the first time a damage-free preparation of gallium doped zinc oxide (ZnO:Ga) layers on top of organic devices by careful optimization of the deposition atmosphere. ZnO:Ga electrodes with a transmittance of Tvis = 82.7 % and sheet resistance Rs = 83 Ohm/sq are obtained. The formation of local shunts due to ZnO:Ga droplets is identified and then prevented by a shadow mask between the target and the sample, enabling solar cells with similar efficiency (2.9 %) compared to a reference device using a state-of-the-art metal top contact. Another very promising alternative are intrinsically flexible, ultra-thin silver layers. By introducing an oxide interlayer, the adverse interpenetration of silver and organic materials is prevented and the charge extraction from the solar cells is improved. With a second oxide layer on top, the silver electrode is significantly stabilized, leading to an increased solar cell lifetime of 4500 h (factor of 107). Scanning electron micrographs of Ag thin films reveal a poor wetting on organic and oxide substrates, which strongly limits the electrode performance. However, it is significantly improved by a 1 nm thin seed layer. An optimized Au/Ag film reaches Tvis = 78.1 % and Rs = 19 Ohm/sq, superior to ITO. Finally, silver electrodes blended with calcium show a unique microstructure which enables unusually high transmittance (84.3 % at 27.3 Ohm/sq) even above the expectations from bulk material properties and thin film optics. Such values have not been reached for transparent electrodes on top of organic material so far. Solar cells with a Ca:Ag top electrode achieve an efficiency of 7.2 %, which exceeds the 6.9 % of bottom-illuminated reference cells with conventional ITO electrodes and defines a new world record for top-illuminated organic solar cells. With these electrodes, semi-transparent and large-area devices, as well as devices on opaque and flexible substrates are successfully prepared. In summary, it is shown that ZnO:Ga and thin metal electrodes can replace ITO and fill the lack of high performance top electrodes. Moreover, the introduced concepts are not restricted to specific solar cell architectures or organic compounds but are widely applicable for a variety of organic devices.

Les électrodes transparentes sont les composants indispensables de nombreux dispositifsoptoélectroniques commerciaux (cellules solaires, écrans plats, écrans tactiles ou encorediodes électroluminescentes). Elles sont constituées le plus souvent d’oxyde d’indium etd'étain (ITO). Les électrodes à base d'ITO sont produites par un procédé relativementcoûteux et sont très fragiles à la contrainte mécanique, ce qui limite leur utilisation au seinde dispositifs optoélectroniques flexibles. Des matériaux alternatifs, sans indium, à base deréseaux de nano-fils d’argent, font actuellement l'objet d'un grand nombre de recherches.Ces réseaux à base de nanostructures métalliques ont des propriétés opto-électroniquescomparables voire supérieures à celles de l’ITO. Ils sont adaptables à des substrats flexibleset sont compatibles avec les procédés de dépôt « roll to roll ». L'objectif de cette thèse estd'explorer de nouvelles voies de synthèse et de modification de surface de nanofils d'argentpour développer des électrodes transparentes plus performantes. De nouvelles nanostructuresmétalliques, différentes de celles commercialisées, ont été élaborées : (i) des fils d’argentultra-longs (ii) des fils d’argent présentant une architecture inhabituelle i.e avec desramifications. Des paramètres clés du procédé polyol ont été modifiés pour élaborer les filsà facteur de forme très élevé. Ils ont permis d'accroître les performancesrésistance/transparence des dispositifs conventionnels. Des nano-fils d’argent de forme « Y» ou « V » ont également été synthétisés en soumettant le milieu de croissance à des ultrasons.Ces nanostructures devraient permettre de limiter les problèmes de conduction quiapparaissent, à l'heure actuelle, au niveau des contacts entre les fils dans les dispositifsnanostructurés. Par ailleurs, des réseaux de fils d'argent modifiés en surface avec de l'acide11-mercaptoundecanoïque (MuA) ont été élaborés. Ils constituent une solution trèsintéressante pour améliorer la stabilité chimique des réseaux métalliques. Le MuA limite l'oxydation de surface du métal et permet aux électrodes de conserver leurs transparence etconductivité initiales. / Transparent electrodes are a necessary component in a number of devices such as solar cells,flat panel displays, touch screens and light emitting diodes. The most commonly usedtransparent conductor, indium tin oxide (ITO), is expensive and brittle, the latter propertymaking it inappropriate for up-and-coming flexible devices. Films consisting of randomnetworks of solution-synthesized silver nanowires have emerged as a promising alternative toITO. They have transparency and conductivity values better than competing new technologies(e.g. carbon nanotubes films, graphene, conductive polymers, etc.) and comparable to ITO.Furthermore, these silver nanowire films are cheap, flexible, and compatible with roll-to-rolldeposition techniques. The main objectives of this PhD work are to improve the properties ofsilver nanowire electrodes and to study and solve issues that are currently hindering their usein commercial devices. Specifically, I studied the important areas of electrode conductivity andstability. To increase the conductivity of nanowire electrodes, two silver nanostructuresdifferent from what is commercially available were synthesized i) ultra-long nanowires and (ii)branched nanowires. Regarding (i), by using 1.2-propanediol as the medium rather than thetypical ethylene glycol in the polyol synthesis process, as well as the molecular weight of PVP,the temperature of the process, or the concentration of silver nitrate, we obtained silvernanowires with an aspect ratio between their lengths and diameters of 1050. Among all theultra-long silver nanowires elaborated in polyol process reported in the literature, they have themaximum length. The synthesis developed is also cheap and the reaction time takes less than2h. Moreover, they have a high yield of 2 mg/ml. Electrodes with a sheet resistance of 5 Ω/Sqfor a transparency of 94% were obtained (with post thermal treatment applied). However, thispost-deposition anneal is shown to have a small influence on the decrease of the sheetresistance. It is thus not required to elaborate electrodes with good performance, which is veryadvantageous for the elaboration of electrodes on plastic substrates. Regarding (ii), “V-like shape” or “Y-like branched” nanowires were elaborated thanks to the input of ultrasonicirradiation during the polyol process. Unfortunately, their length being short (6 μm), theirinterest is limited to enhance the performance of transparent electrodes. In addition, structuralanalyses of both branched and unbranched nanowires revealed the nanostructures notmonocrystalline. Concerning the stabilities issues, the thermal stability of silver nanowireelectrodes coated with graphene was investigated. This coating allows a better homogeneity ofthe heat through the network, decreasing the number of hot spots and thus increasing thelifetime of the electrodes. The corrosion of silver nanowire and the resulting electrode resistanceincrease over time is a severe problem hindering their use in commercial devices. 11-mercaptoundecanoic acid (MuA) was identified as a promising passivation agent of silvernanowires. Lifetime testing showed that the electrode resistance increased more slowly (12%)than any other passivated electrodes reported in the literature. Furthermore, unlike many otherpassivation methods, the MuA molecule itself does not negatively affect the conductivity ortransparency of the electrode and is very inexpensive, all contributing to the commercialviability of the passivation method.

L’intérêt pour les électrodes transparentes (TEs) concerne un large spectre de domaines technologiques, tels que les dispositifs optoélectroniques (cellules solaires, LEDs, écrans tactiles), les films chauffants transparents, ou les applications électromagnétiques. Les TEs de nouvelle génération auront à combiner à la fois un très haut niveau de conduction électrique, de transparence optique, mais aussi de flexibilité mécanique. L’oxyde d’Indium dopé Etain (ITO) domine actuellement le marché des matériaux transparents conducteurs (TCMs). Cependant, la rareté de l’Indium, combinée à ses faibles performances en flexion mécaniques et ses coûts de fabrication élevés ont orienté les recherches vers des TCMs alternatifs. Les réseaux percolants de nanofils métalliques, en particulier les nanofils d’argent (AgNWs), se sont imposés comme l’une des alternatives les plus sérieuses à l’ITO, en raison de leurs propriétés physiques très attractives. Ces réseaux interconnectés offrent également la possibilité d’utiliser des méthodes de synthèse en voie chimique et d’impression à bas coût, sur de grandes surfaces. De manière générale, les premières estimations concernant les coûts de fabrication sont inférieures à celles de l’ITO. De plus, grâce au très haut facteur de forme des nanofils et à la nature percolante des réseaux, les besoins en quantité de matières premières nécessaires pour atteindre un haut niveau de performance sont moindres.Ce travail de thèse s’intéresse à l’étude des propriétés physiques fondamentales – inexplorées ou non encore suffisamment étudiées – des réseaux d’AgNWs, afin d’améliorer la robustesse, la fiabilité et la compatibilité de ce type d’électrodes avec les critères de performance industriels. La première partie est consacrée à l’étude des méthodes d’optimisation utilisées pour diminuer au mieux la résistance électrique des électrodes. Les mesures électriques in situ effectuées au court d’un recuit thermique et/ou après traitement chimique fournissent de précieuses informations concernant les mécanismes d’activation au niveau des jonctions entre nanofils. A l’échelle du réseau, notre capacité à distinguer les zones qui participent efficacement à la conduction électrique de celles qui seraient potentiellement inactives est un défi majeur. Pour les réseaux dont la densité en nanofils est proche du seuil de percolation, un processus d’activation discontinu de chemins efficaces de percolation à travers le réseau a pu être mis en évidence. De manière générale, l’influence de plusieurs paramètres (densité du réseau, tension, niveau d’optimisation) sur l’homogénéité et la stabilité électrique et thermique des électrodes a été étudiée, à l’aide de techniques de cartographie électrique et de simulations. A tension élevée, un processus d’emballement thermique conduisant à la formation d’une fissure physique à travers un réseau d’AgNWs soumis à des plateaux de tension croissants a pu être détecté visuellement. Des modèles de simulation via les logiciels Matlab et Comsol ont aussi été construits afin de confirmer, voire anticiper, les phénomènes observés expérimentalement. Par ailleurs, encouragés par la demande croissante pour les dispositifs électroniques portatifs en toute circonstance, des tests préliminaires ont été conduit pour évaluer le comportement des réseaux d’AgNWs sous contrainte d’étirement mécanique lorsqu’ils sont transférés sur des substrats élastiques. Ce travail de thèse a également donné lieu à l’intégration de réseaux d’AgNWs dans des dispositifs. Des études ont été menées afin d’améliorer la stabilité des films chauffants transparents à base d’AgNWs et de mieux appréhender les mécanismes favorisant l’émergence de défauts. L’utilisation des réseaux d’AgNWs pour des applications électromagnétiques (antennes, blindage) a également fait l’objet de tentatives préliminaires dont les résultats sont commentés à la fin du manuscrit. / Transparent electrodes attract intense attention in many technological fields, including optoelectronic devices (solar cells, LEDs, touch screens), transparent film heaters (TFHs) and electromagnetic (EM) applications. New generation transparent electrodes are expected to have three main physical properties: high electrical conductivity, high transparency and mechanical flexibility. The most efficient and widely-used transparent conducting material is currently indium tin oxide (ITO). However the scarcity of indium associated with ITO’s lack of flexibility and the relatively high manufacturing costs have prompted search into alternative materials. With their outstanding physical properties, silver nanowire (AgNW)-based percolating networks appear to be one of the most promising alternatives to ITO. They also have several other advantages, such as solution-based processing, and compatibility with large area deposition techniques. First cost estimates are lower for AgNW based technology compared to current ITO fabrication processes. Unlike ITO, AgNW are indeed directly compatible with solution processes, never requiring vacuum conditions. Moreover, due to very large aspect ratio of the NWs, smaller quantities of raw materials are needed to reach industrial performance criteria.The present thesis aims at investigating important physical assets of AgNW networks – unexplored (or not explored enough) so far – in order to increase the robustness, reliability, and industrial compatibility of such technology. This thesis work investigates first optimization methods to decrease the electrical resistance of AgNW networks. In situ electrical measurements performed during thermal ramp annealing and/or chemical treatments provided useful information regarding the activation process at the NW-NW junctions. At the scale of the entire network, our ability to distinguish NW areas taking part in the electrical conduction from inactive areas is a critical issue. In the case where the network density is close to the percolation threshold, a discontinuous activation process of efficient percolating pathways through the network was evidenced, giving rise to a geometrical quantized percolation phenomenon. More generally, the influence of several parameters (networks density, applied voltage, optimization level) on the electrical and thermal homogeneity and stability of AgNW networks is investigated via a dual approach combining electrical mapping techniques and simulations. A thermal runaway process leading to a vertical crack and associated to electrical failure at high voltage could be visually evidenced via in situ electrical mapping of AgNW networks during voltage plateaus. Moreover many efforts using Matlab and Comsol softwares were devoted to construct reliable models able to fit with experimental results. Due to the increasing demand for portable and wearable electronics, preliminary tests were also conducted to investigate the stretching capability of AgNW networks when transferred to elastomeric substrates. Finally, integrations of AgNW networks in several devices were performed. Specifically, studies were conducted to understand the mechanisms leading to failure in AgNW-based transparent film heaters, and to improve their overall stability. Preliminary investigations of the benefits of incorporating of AgNW networks into electromagnetic devices such as antennas and EM shielding devices are also discussed at the end of the manuscript.

Os eletrodos de filme de SnO2 suportado sobre sílica ou titânio apresentam propriedades eletroquímicas e morfológico-estruturais bastante distintas e são empregados em diferentes aplicações. Estes dois tipos de materiais foram preparados e investigaram-se suas propriedades através de diversas técnicas eletroquímicas e de análise de superfície. Os eletrodos de Ti/SnO2(Sb) apresentaram a superfície porosa e rugosidade de 500 nm. Para os filmes finos (~40 nm) depositados sobre sílica, porém, observou-se alta transparência, superfície uniforme e rugosidade de 1 nm. Mesmo entre os eletrodos transparentes, dependendo do dopante (Cl, F ou Sb), verificaram-se diferenças quanto às orientações cristalográficas predominantes, espessura, resistividade e tamanho das partículas. As investigações eletroquímicas também revelaram diferenças consideráveis entre ambos os tipos de eletrodos. Para as reações de óxido-redução do K4Fe(CN)6, p.ex., a transferência de elétrons foi muito mais rápida sobre o eletrodo de Ti/Sn)2(Sb), possivelmente porque este eletrodo apresentava maior número de portadores de carga e menor resistência ôhmica que os eletrodos transparentes. Investigou-se ainda o comportamento eletroquímico de eletrodos modificados com um filme de octadeciltriclorosilano (OTS). Os estudos revelaram que a presença do OTS não alterou o mecanismo para a reação de geração de oxigênio, porém, alterou significativamente o comportamento do eletrodo de Ti/SnO2(Sb) na eletro-oxidação do metanol. Aparentemente, o OTS induziu a formação de espécies poliméricas que bloquearam o eletrodo, indicando, portanto, que estes eletrodos modificados não são indicados para atuar como ânodos na eletro-oxidação de compostos orgânicos. Os processos de corrosão induzidos por polarização anódica ou catódica foram investigados por microscopia de forças atômicas e de efeito túnel para os eletrodos transparentes modificados ou não com OTS. As análises revelaram que os processos de corrosão iniciaram-se nos contornos dos grãos e atuaram de maneira diferenciada sobre as partículas que constituem o filme. A corrosão foi muito menos pronunciada para os eletrodos modificados com OTS, indicando que este organosilano poderia proteger a superficie do SnO2. Porém, a aplicabilidade do eletrodo Ti/SnO2(Sb)-OTS seria muito limitada, considerando que o OTS poderia promover a polimerização de espécies orgânicas na superficie do eletrodo. / The electrochemical and morphological properties of tin oxide electrodes deposited on silica or titanium substrates were investigated by electrochemical and surface analysis techniques. The Sb doped SnO2 film deposited on Ti was prepared by thermal decomposition of stannic chloride solution and investigated by SEM and interferometric microscopy. This Ti/SnO2(Sb) exhibited a porous surface, with a rms roughness of 500 run. The transparent film of SnO2 doped with Cl, F or Sb, which was deposited on silica by Spray Pyrolysis, were analyzed by XPS, XRD, SEM, AFM and STM techniques. These films were polycrystalline and had an uniform and smooth surface with a rms roughness ranging from 1 to 2 nm. Their microstructure was constituted of coalesced particles with an average diameter and height of 30 nm and 6 nm, respectively. Thickness, conductivity, grain size, and surface composition of transparent films depended on the dopant. The electrochemical behavior of both types of SnO2 electrodes were investigated by Cyclic Voltammetry and Electrochemical lmpedance Spectroscopy (EIS). For the reversibility of K4Fe(CN)O6 redox reaction, while the Ti/SnO2(Sb) electrode supported high rate for electron transfer, the transparent electrodes exhibited a poor electrochemical response, which was attributed to the high ohmic resistance. The effect of electrochemical perturbation in 0.1 M NaClO4, pH 2, on the microstructure oftransparent electrodes was investigated by in-situ STM and ex-situ AFM, SEM and other techniques. Anodic polarization at potentials near the OER region and potential cycling induces little alterations on the microstructure of the electrode, observable only in sub-micron range by AFM analysis. Cathodic polarization at potentials in the region of the HER, however, strongly damages the filmo AFM and STM examination revealed that the grain boundaries are attacked, and both vertical and lateral dimensions of the particles decrease. From the observations, it can be inferred that SnO is formed during the cathodic polarization and, destruction of the film occurs by dissolution of that more soluble oxide. In-situ STM experiments showed that, at the initial stages, the particles of the damaged film can acquire dimensions even larger than the initial size by anodic polarization. Finally, it can be concluded that a smooth and compact film, with little defects is less susceptible to corrosion.

Organic solar cells offer attractive properties for novel applications and continuous advances in material and concept development have led to significant improvements in device performance. To exploit their full potential (roll-to-roll production of flexible and top-illuminated devices, using e.g. opaque metal foil or textile as substrate), highly transparent, conductive, mechanically flexible, and cost-efficient top electrodes are of great importance. The current standard material indium tin oxide (ITO) is rigid, expensive and requires a high energy / high temperature deposition process, limiting ITO (and other transparent conductive oxides) to bottom electrode applications. This work presents fundamental investigations to understand and control the properties of transparent conductors and documents four different approaches to prepare transparent electrodes on top of efficient small molecule organic solar cells, with the aim to replace ITO. Fullerene C60 layers are investigated as completely carbon-based electrodes. For an optimized doping concentration, sheet resistance and transmittance are improved and efficient solar cells are realized. Since the lateral charge transport is still limited, a combination with a microstructured conductor is suggested. Pulsed laser deposition allows for the first time a damage-free preparation of gallium doped zinc oxide (ZnO:Ga) layers on top of organic devices by careful optimization of the deposition atmosphere. ZnO:Ga electrodes with a transmittance of Tvis = 82.7 % and sheet resistance Rs = 83 Ohm/sq are obtained. The formation of local shunts due to ZnO:Ga droplets is identified and then prevented by a shadow mask between the target and the sample, enabling solar cells with similar efficiency (2.9 %) compared to a reference device using a state-of-the-art metal top contact. Another very promising alternative are intrinsically flexible, ultra-thin silver layers. By introducing an oxide interlayer, the adverse interpenetration of silver and organic materials is prevented and the charge extraction from the solar cells is improved. With a second oxide layer on top, the silver electrode is significantly stabilized, leading to an increased solar cell lifetime of 4500 h (factor of 107). Scanning electron micrographs of Ag thin films reveal a poor wetting on organic and oxide substrates, which strongly limits the electrode performance. However, it is significantly improved by a 1 nm thin seed layer. An optimized Au/Ag film reaches Tvis = 78.1 % and Rs = 19 Ohm/sq, superior to ITO. Finally, silver electrodes blended with calcium show a unique microstructure which enables unusually high transmittance (84.3 % at 27.3 Ohm/sq) even above the expectations from bulk material properties and thin film optics. Such values have not been reached for transparent electrodes on top of organic material so far. Solar cells with a Ca:Ag top electrode achieve an efficiency of 7.2 %, which exceeds the 6.9 % of bottom-illuminated reference cells with conventional ITO electrodes and defines a new world record for top-illuminated organic solar cells. With these electrodes, semi-transparent and large-area devices, as well as devices on opaque and flexible substrates are successfully prepared. In summary, it is shown that ZnO:Ga and thin metal electrodes can replace ITO and fill the lack of high performance top electrodes. Moreover, the introduced concepts are not restricted to specific solar cell architectures or organic compounds but are widely applicable for a variety of organic devices.

The aim of this work was to investigate silver nanowire as well as carbon nanotube networks as transparent conducting electrodes for small molecule organic solar cells. In the framework of the nanowire investigations, a low-temperature method at less than 80 °C is developed to obtain highly conductive networks directly after the deposition and without post-processing. In detail, specific non-conductive organic materials act as a matrix where the nanowires are embedded in such that a mutual attraction based on capillary forces and hydrophobic interaction is created. This process is mediated by the ethanol contained in the nanowire dispersion and works only for sublayer materials which exhibit hydrophobic and hydrophilic groups at the same time. In contrast to high-temperature processed reference electrodes (210 °C for 90 min) without matrix, a slightly lower sheet resistance of 10.8 Ohm/sq at a transparency of 80.4 % (including substrate) is obtained by using polyvinylpyrrolidone as the sublayer material. In comparison to annealed silver nanowire networks, the novel approach yields a performance enhancement in corresponding organic solar cells which can compete with ITO-based devices. Furthermore, a novel approach for scalable, highly conductive, and transparent silver nanowire top-electrodes for organic optoelectronic devices is introduced. By utilizing a perfluorinated methacrylate as stabilizer, silver nanowires with high aspect ratio can be transferred into inert solvents which do not dissolve most organic compounds making this modified dispersion compatible with small molecule and polymer-based organic optoelectronic devices. The inert silver nanowire dispersion yields highly performing top-electrodes with a sheet resistance of 10.0 Ohm/sq at 80.0 % transparency (including substrate) directly after low-temperature deposition at 30 °C and without further post-processing. In comparison to similarly prepared reference devices comprising a thin-metal film as transparent top-electrode, reasonable power conversion efficiencies are demonstrated by spray-coating this dispersion directly on simple, air-exposed small molecule-based organic solar cells. Moreover, a deeper understanding of the percolation behavior of silver nanowire networks has been achieved. Herein, direct measurements of the basic network parameters, including the wire-to-wire junction resistance and the resistance of a single nanowire of pristine and annealed networks have been carried out for the first time. By putting the values into a simulation routine, a good accordance between measurement and simulation is achieved. Thus, an examination of the electrical limit of the nanowire system used in this work can be realized by extrapolating the junction resistance down to zero. The annealed silver nanowires are fairly close to the limit with a theoretical enhancement range of only 20 % (common absolute sheet resistance of approximately 10 Ohm/sq) such that a significant performance improvement is only expected by an enlargement of the nanowire length or by the implementation of new network geometries. In addition, carbon nanotube networks are investigated as alternative network-type, transparent bottom-electrode for organic small molecule solar cells. For that purpose, cleaning and structuring as well as planarization procedures are developed and optimized which maintain the optoelectronic performance of the carbon nanotube electrodes. Furthermore, a hybrid electrode consisting of silver nanowires covered with carbon nanotubes is fabricated yielding organic solar cells with only 0.47 % power conversion efficiency. In contrast, optimized electrodes comprising only carbon nanotubes show significantly higher efficiency. In comparison to identically prepared ITO devices, comparable or lower power conversion efficiencies of 3.96 % (in p-i-n stack), 4.83 % (in cascade cell) as well as 4.81 % (in p-n-i-p architecture) are demonstrated. For an inverted n-i-p stack design, the highest power conversion efficiency of 5.42 % is achieved.

Solid state lighting is a new environmentally friendly light source. So far, light emitting diodes (LEDs) and organic LEDs (OLEDs) have been presented as candidates with potentially high efficiency. Recent advances of OLEDs in device architecture, light-out coupling, and materials have ensured high efficiency, exceeding that of incandescent light bulbs. In contrast to conventional point source LEDs, OLEDs distribute light throughout the surface area and are not restricted by their size. Additionally, OLEDs are expected to reach sufficient stability in the near future. The remaining challenge for OLEDs is their cost. New OLED technologies provide cost effective manufacturing methods which could be presented for transparent electrode materials because indium tin oxide (ITO), a widely used material as a transparent electrode for OLEDs, is less than optimal due to its high element price. In this work, alternative transparent electrodes for OLEDs as a replacement of ITO were studied. First, Al doped ZnO (ZnO:Al) which is composed of abundant materials was investigated with DC magnetron sputtering under a wide range of experimental conditions. The optimised ZnO:Al received comparable performance with conventional ITO films, low sheet resistance of 22.8 Ω/sq as well as a high transparency of 93.1 % (average value in the visible range). Various type of p-i-n OLEDs were employed on the structured ZnO:Al using photolithography. Green OLEDs with double emission layers have been archived stable efficiencies even at higher luminance. Also, OLEDs using two fluorescent colour system on ZnO:Al anode showed a purely white emission. It has been found that the OLEDs on ZnO:Al anode has comparable or better device efficiencies and operational lifetime compared to OLEDs on conventional ITO anode. As another alternative electrode, the conductive polymer Baytron®PH510 (PEDOT:PSS) was investigated. Due to a relatively high sheet resistance of PEDOT:PSS, metal grid was designed for large size OLEDs. White OLEDs on PEDOT anode with a size of 5 × 5 cm2 have achieved more than 10 lm/W of power efficiency using a scattering foil. Furthermore, up-scaled devices on 10 × 10 cm2 were also demonstrated. These results showed ZnO:Al and PEDOT are suitable for OLEDs as anode and have high potential as alternative transparent electrode materials.