Concentrated solar power (CSP) has the potential of fulfilling the world’s electricity needs. Parabolic-trough system using synthetic oil as the HTF with operating temperature between 300 and 400o C, is the most matured CSP technology. A thermal storage system is required for the stable and cost effective operation of CSP plants. The current storage technology is the indirect two-tank system which is expensive and has high energy consumption due to the need to prevent the storage material from freezing. Latent heat storage (LHS) systems offer higher storage density translating into smaller storage size and higher performance but suitable phase change materials (PCMs) have low thermal conductivity, thus hindering the realization of their potential. The low thermal conductivity can be solved by heat transfer enhancement in the PCM. There is also lack of suitable commercially-available PCMs to cover the operating temperature range. In this study, a hybrid cascaded storage system (HCSS) consisting of a cascaded finned LHS and a high temperature sensible or concrete tube register (CTR) stages was proposed and analysed via modelling and simulation. Fluent CFD code and the Dymola simulation environment were employed. A validated CFD phase change model was used in determining the heat transfer characteristics during charging and discharging of a finned and unfinned LHS shell-and-tube storage element. The effects of various fin configurations were investigated and heat transfer coefficients that can be used for predicting the performance of the system were obtained. A model of the HCSS was then developed in the Dymola simulation environment. Simulations were conducted considering the required boundary conditions of the system to develop the best design of a system having a capacity of 875 MWhth, equivalent to 6 hours of full load operation of a 50 MWe power plant. The cascaded finned LHS section provided ~46% of the entire HCSS capacity. The HCSS and cascaded finned LHS section have volumetric specific capacities 9.3% and 54% greater than that of the two-tank system, respectively. It has been estimated that the capital cost of the system is ~12% greater than that of the two-tank system. Considering that the passive HCSS has lower operational and maintenance costs it will be more cost effective than the twotank system considering the life cycle of the system. There is no requirement of keeping the storage material above its melting temperature always. The HCSS has also the potential of even lower capital cost at higher capacities (>6 hours of full load operation).

L’une des solutions proposées pour la diminution du coût par watt d'électricité produite par le photovoltaïque est de réduire la quantité des matériaux semiconducteurs entrants dans la fabrication de la cellule solaire. La 3ème génération des cellules solaires en couches minces nanostructurées vient pour répondre à cette exigence. Les matériaux CIGS sous leurs structures chalcopyrites, sont de nouveaux matériaux semiconducteurs fortement recommandés pour la fabrication des cellules solaires à base de couches minces. La synthèse par la méthode de pulvérisation cathodique et la caractérisation de ces derniers matériaux ont été l’objectif général de cette thèse. Toutes les couches ont été déposées grâce à une seule cible constituée par des grains nanométriques de CIGS, ces derniers ont été obtenus par la voie solvothermale. Dans la première partie de ce travail, nous avons étudié l’effet des différents paramètres de synthèse (température, durée de synthèse, le traitement thermique et l’effet du taux molaire de gallium et d’indium) sur les propriétés des nanoparticules CIGS, les mécanismes réactionnels mis en jeu ont été aussi étudié. Les conditions de synthèse optimales sont une température et une durée de synthèse égale à 220 °C et 24 heures. Après un traitement thermique, les nanoparticules de CIGS sont cristallisées suivant la structure chalcopyrite, avec l’absence des pics correspond aux phases secondaires, les diamètres des grains varie entre 15 et 30 nm. Dans la deuxième partie, Nous sommes intéressés à l’élaboration et la caractérisation des couches absorbantes ternaire et quaternaire de type CIS et CIGS (x = 0 et x = 0.3) obtenues par pulvérisation cathodique en variant la puissance de dépôt de 60 à 100 W. Toutes les couches élaborées présentent la phase chalcopyrite avec (112) comme axe d’orientation préférentiel de croissance. La taille moyenne des grains a le même ordre de grandeur que les poudres initiales. Les couches de CIGS sont généralement de type de conduction p avec des faibles valeurs de résistivités. Les caractérisations optiques des couches présentent une bonne absorption de l’ordre de 95 % dans la gamme de visible et le proche infra-rouge. La variation du coefficient d’absorption en fonction de l’énergie du photon, nous a permis de déterminer l’énergie du gap optique. Les valeurs obtenues pour les différentes couches sont cohérentes avec l’optimum pour la conversion photovoltaïque. / One of the proposed solutions for reducing the cost of electricity produced by the photovoltaic is to reduce the amount of incoming semiconductor materials in the manufacture of the solar cell. The 3rd generation solar cells based on nanostructured thin film come in response to this requirement. CIGS under their structures chalcopyrite are highly recommended for the manufacture of this solar cells type. The synthesis of these materials using sputtering method and their characterization were the overall goal of this thesis. All films were deposited onto glass substrates from single target composed trough nanoparticles of CIGS, which are obtained by the solvothermal route. In the first part of this work, we studied the effect of different synthesis parameters (temperature, synthesis time, the heat treatment and the effect of the molar ratio of gallium and indium) on the properties of CIGS nanoparticles. The reaction mechanisms were also studied. The optimum synthesis conditions are a temperature and a synthesis time equal to 220 ° C and 24 hours. After heat treatment, the nanoparticles are crystallized according CIGS chalcopyrite structure, with the absence of the peaks corresponding to the secondary phases, grain size between 15 and 30 nm. In the second part, we are interested in the deposition and characterization of ternary and quaternary absorbent thin film CIS and CIGS (x = 0 and x = 0.3) obtained by sputtering deposition by varying the power of pulverization from 60 to 100 W. All layers have crystallized in the chalcopyrite structure with the preferential orientation in the (112) plane were obtained. The average grain size has the same order of magnitude as the initial powders. All films are generally p-type conduction with low resistivity values. Optical characterizations of the layers exhibit a good absorption in the visible range and the near infrared. The variation of the absorption coefficient as a function of photon energy enabled us to determine the energy of the optical gap. The values obtained for the different layers are consistent with the optimum for the photovoltaic conversion.

Lanthanide (III) complexes are used extensively in solar conversion devices, such as Luminescent Solar Concentrators (LSCs) and Luminescent Down-Shifting (LDS) for their peculiar characteristics of narrow band emission, avoidance of re-absorption losses due to large Stokes shift and possibility of high photoluminescence quantum yield (PLQY). The study has looked into the synthesis of Ln (III) complexes of the general formula: [Ln(hfac)3DPEPO], where DPEPO = bis(2-(diphenylphosphino)phenyl)ether oxide, and hfac = hexafluoroacetylacetonate. The work presented in this thesis focuses on the synthesis, and subsequent photophysical characterisation of these Ln(III) complexes, plus characterisation and spectroscopic study of [Tb(pobz)3(hacim)2], (where Hpobz = phenoxybenzoic acid, and Hacim = acetylacetone imine), yielding results that open new design of functional Ln(III) systems. Spectroscopic study of Chromium dioxalate and analogous compounds has revealed that with the appropriate design, Cr(III)Ln(III) energy transfer can be achieved, while study of polyaromatic hydrocarbons (PAH) such as coronene, enable to explore a ligand with better absorption in the whole UV region. These results open attractive perspectives for light-conversion systems, such as LSC devices.

In screen-printed silicon-crystalline solar cells, the contact resistance of a thin interfacial glass layer between the silicon and the silver electrode plays a limiting role for electron transport. The motivation of this project is to gain increased understanding of the transport mechanisms of the electrons across this layer, which can be exploited to provide higher performance crystalline silicon solar cells. Our methodology throughout is to formulate and analyse mathematical models for the electron transport, based on the drift diffusion equations. In the first chapter we outline the problem and provide a summary of relevant theory. In Chapter 2 we formulate a one-dimensional model for electron transport across the glass layer, that we solve both numerically and by employing asymptotic techniques. Chapter 3 extends the model presented in Chapter 2 to two dimensions. To solve the two-dimensional model numerically we devise and validate a new spectral method. The short circuiting of current through thinner regions of the glass layer enables us to find limiting asymptotic expressions for the average current density for two different canonical glass layer profiles. In Chapter 4 we include quantum mechanical effects into the one-dimensional model outlined in Chapter 2 and find that they have a negligible effect on the contact resistance of the glass layer. We model the boundary effects present at the silicon emitter-glass interface in Chapter 5. Finally, in Chapter 6 we summarise our key results, suggest possible future work, and outline the implications of our work to crystalline silicon solar cell manufacturers.

Ce travail de thèse a pour sujet la conception des cellules solaires photovoltaïques à base de pérovskite hybride par le biais de la technologie d’impression jet d’encre. Les deux premiers chapitres font la présentation du contexte de la thèse, à savoir l’alimentation d’un réseau autonome de capteurs, et passent en revue les aspects scientifiques des technologies jet d’encre et photovoltaïque de nouvelle génération. Le troisième chapitre présente la mise au point d’une cellule photovoltaïque à l’état de l’art et son évolution vers une architecture imprimable à basse température de recuit. La problématique de la stabilité des cellules photovoltaïques à pérovskite est aussi abordée. La dernière partie présente les différents aspects et problématiques de l’impression par jet d’encre des trois couches internes d’une cellule solaire pérovskite. Au terme de ce travail la possibilité d’imprimer des cellules solaires pérovskites avec des rendements supérieurs à 10 % a été démontrée, le tout en condition ambiante et à basse température. / This thesis is about the design of photovoltaic solar cells based on hybrid perovskite using inkjet printing technology. The first two chapters present the context of the thesis, namely the powering of an autonomous sensor network, and review the scientific aspects of inkjet and photovoltaic technologies. The third chapter presents the development of a state-of-the-art photovoltaic cell and its evolution towards a printable architecture at low annealing temperatures. The problem of the stability of photovoltaic cells with perovskite is also discussed. The last part presents the different aspects and problems of the inkjet printing of the three inner layers of a perovskite solar cell. At the end of this work the possibility of printing perovskite solar cells with efficiencies higher than 10% has been demonstrated, all in ambient conditions and at low temperature.

Concentrating photovoltaic (CPV) systems are a key step in expanding the use of solar energy. Solar cells can operate at increased efficiencies under higher solar concentration and replacing solar cells with optical devices to capture light is an effective method of decreasing the cost of a system without compromising the amount of solar energy absorbed. CPV systems are however still in a stage of development where new designs, methods and materials are still being created in order to reach a low levelled cost of energy comparable to standard silicon based photovoltaic (PV) systems. This work outlines the different types of concentration photovoltaic systems, their various design advantages and limitations, and noticeable trends. Comparisons on materials, optical efficiency and optical tolerance (acceptance angle) are made in the literature review as well as during theoretical and experimental investigations. The subject of surface structure and its implications on concentrator optics has been discussed in detail while highlighting the need for enhanced considerations towards material and hence the surface quality of optics. All of the findings presented contribute to the development of higher performance CPV technologies. Specifically high and ultrahigh concentrator designs and the accompanied need for high accuracy high quality optics has been supported. A simulation method has been presented which gives attention to surface scattering which can decrease the optical efficiency by 10-40% (absolute value) depending on the material and manufacturing method. New plastic optics and support structures have been proposed and experimentally tested including the use of a conjugate refractive-reflective homogeniser (CRRH). The CRRH uses a reflective outer casing to capture any light rays which have failed total internal reflection (TIR) due to non-ideal surface topography. The CRRH was theoretically simulated and found to improve the optical efficiency of a cassegrain concentrator by a maximum of 7.75%. A prototype was built and tested where the power output increase when utilising the CRRH was a promising 4.5%. The 3D printed support structure incorporated for the CRRH however melted under focused light, which reached temperatures of 226.3°C, when tested at the Indian Institute of Technology Madras in Chennai India. The need for further research into prototyping methods and materials for novel optics was also demonstrated as well as the advantages of broadening CPV technology into the fields of biomimicry. The cabbage white butterfly was proven to concentrate light onto its thorax using its highly reflective and lightweight wings in a basking V-shape not unlike V-trough concentrators. These wings were measured to have a unique structure consisting of ellipsoidal pterin beads aligned in ladder like structures on each wing scale which itself is then tiled in a roof like pattern on the wing. Such structures of a reflective material may be the answer to lightweight materials capable of increasing the power to weight ratio of CPV technology greatly. Experimental testing of the large cabbage white wings with a silicon solar cell confirmed a 17x greater power to weight ratio in comparison to the same set up with reflective film instead of the wings. An ultrahigh design was proposed taking into account manufacturing considerations and material options. The geometrical design was of 5800x of which an optical efficiency of either ~75% with state of the art optics should produce and effective concentration of ~4300x. Relatively standard quality optics on the other hand should give an optical efficiency of ~55% and concentration ratio ~3000x. A prototype of the system is hypothesised to fall between these two predictions. Ultrahigh designs can be realised if the design process is as comprehensive as possible, considering materials, surface structure, component combinations, anti-reflective coatings, manufacturing processes and alignment methods. Most of which have been addressed in this work and the accompanied articles. Higher concentration designs have been shown to have greater advantages in terms of the environmental impact, efficiency and cost effectiveness. But these benefits can only be realised if designs take into account the aforementioned factors. Most importantly surface structure plays a big role in the performance of ultrahigh concentrator photovoltaics. One of the breakthroughs for solar concentrator technology was the discovery of PMMA and its application for Fresnel lenses. It is hence not an unusual notion that further breakthroughs in the optics for concentrator photovoltaic applications will be largely due to the development of new materials for its purpose. In order to make the necessary leaps in solar concentrator optics to efficient cost effective PV technologies, future novel designs should consider not only novel geometries but also the effect of different materials and surface structures. There is still a vast potential for what materials and hence surface structures could be utilised for solar concentrator designs especially if inspiration is taken from biological structures already proven to manipulate light.

Dans cette thèse nous évaluons la concentration optique sur cellules photovoltaïques micrométriques et nanométriques sans système de suivi de Soleil. Cette étude a deux objectifs principaux. La première partie est dédiée à l’évaluation de la faisabilité de la concentration optique sur des microcellules à base de Cu(In,Ga)Se2 via un concentrateur luminescent (LSC). Le LSC est bas-coût, concentre la lumière directe et diffuse, et est non imageant, ce qui très avantageux pour la concentration sur microcellules. Néanmoins, la sensibilité extrême aux non-idéalités explique la différence entre les performances théoriques et expérimentales. Un code de simulation est développé pour analyser ce système et ses mécanismes de perte. Un nouveau formalisme basé sur des données statistiques est proposé pour décrire les propriétés du LSC. Le couplage LSC/microcellules est effectué expérimentalement et des pistes d’amélioration explorées. La seconde partie tire profit de la fonction de conversion spectrale des LSC et développe un nouveau concept de nano-antenne photovoltaïque mono-résonant à base d’InP. Des simulations optiques montrent qu’un rendement de conversion de 10.7% peut être atteint avec une épaisseur moyenne d’absorbeur de moins de 20 nm. Les étapes technologiques de fabrication sont identifiées et réalisées en salle blanche. Le fort ratio surface/volume nous a amené à étudier la passivation de l’InP par du polyphosphazène. Des mesures de luminescence montrent que la surface est stabilisée durablement. Les résultats de cette thèse démontrent que le couplage nano-photonique / LSC est prometteur, alliant de très faibles volumes à d’excellentes efficacités optiques. / In this thesis we explore light concentration on nano and micro photovoltaic cells without Sun tracking. This study has two main aims. The first part is dedicated to the evaluation of light concentration feasibility on Cu(In,Ga)Se2-based microcells with luminescent solar concentrator (LSC). LSC is cheap, allows both direct and diffuse light concentration and is non imaging, making it advantageous for microcells concentration. Yet, the extremely high sensitivity to non ideality explains the gap between theoretical and real systems. A simulation code is developed to analyze the system and its loss mechanisms. A new formalism based on statistical data is proposed to describe LSC properties. LSC and microcells coupling is experimentally achieved and improving tracks investigated. The second part takes advantage of the LSC down-shifting effect to propose a new mono-resonant InP-based photovoltaic nano-antenna. Optical modeling on this new device shows that 10.7 % efficiency can be obtained with an absorber averaged thickness lower than 20 nm. Technological process steps to fabricate this device are identified and realized in a clean-room environment. The high ratio surface over volume leads us to study InP passivation through a mono-atomic polyphosphazen film. Luminescent measurements show that passivated InP surface is long-term stabilized. The results of this thesis demonstrate that nano-photonic / LSC coupling is promising, enabling high optical efficiency in extremely low volume.

The problems of lack of potable water, food and electrical power in and around the desert environment are analysed and their solutions based on the utilisation of indigenous renewable energy resources are evaluated in the current research. Self-contained, decentralised solar energy powered processes are devised in the research results as means to attain the objectives of supplying electrical power, potable water and food to the communities living in the deserts in environmentally sustainable manner. Needs analysis of desert community, a questionnaire survey, desert energy model utilisation scenarios, solar potential assessment and environmental emissions reduction strategy are used as means of analysis in the current research. A potential assessment of a desert site Islamkot, at Thar is carried out to demonstrate the capability of available solar potential to meet the energy needs of underground pumping, desalination of aquifer water potable water, cultivation of wheat, rice and pulses and domestic power consumption. The needs analysis estimates the amounts of electrical power needs of potable water desalination, agricultural commodities cultivation and electrical power needs per person per day, which can be scale up for any number of communities living in and around the deserts. The results reveals that indigenous solar potential capability can be used to produce the required amounts of electrical power to meet the water, wheat, rice, pulses, electrical power, drinking, non-drinking and cultivation water needs of the desert communities in environmentally sustainable manner. The research results are practicable and can be implemented to meet the energy needs of isolated communities living in and around the deserts in the long run. However, sustainable efforts would be required to encourage stakeholders to initiate a process of small, medium and large scale solar power utilization in and around the deserts.

Les perovskites organiques-inorganiques en halogénures de plomb sont des matériaux très prometteurs pour la prochaine génération de cellules solaires avec des avantages intrinsèques tels que leur faible coût de fabrication (grande disponibilité des matériaux de base et leur mise en œuvre à basse température) et leur bon rendement de conversion photovoltaïque. Cependant, les cellules solaires pérovskites sont encore instables et montrent des effets d'hystérésis courant-tension délétères. Dans cette thèse, des résultats de l’analyse physique de couches minces de pérovskite à base de CH3NH3PbI3-xClx et de cellules solaires ont été présentés. Les caractéristiques de transport électrique et les processus de vieillissement ont été étudiés avec différentes approches.Dans une première étape, la synthèse du matériau pérovskite a été optimisée en contrôlant les conditions de dépôt des films en une seule étape telles que la vitesse de rotation (6000 rpm) de la tournette et la température de recuit des films (80 °C). Dans un second temps, des cellules solaires perovskites à base de CH3NH3PbI3-xClx ont été fabriquées en utilisant la structure planaire inversée et caractérisées optiquement et électriquement.Grace à l’utilisation de la spectroscopie optique à décharge luminescente (GDOES), un déplacement des ions halogénures a été observé expérimentalement et de façon directe sous l’application d’une tension électrique. Une longueur de diffusion ionique de 140 nm et un rapport de 65% d'ions mobiles ont été déduits. Il est montré que l'hystérésis courant-tension dans l'obscurité est fortement affectée par la migration des ions halogénures provoquant un écrantage substantiel du champ électrique appliqué. Nous avons donc trouvé sous obscurité un décalage de la tension à courant nul jusque 0,25 V et un courant de fuite jusque 0,1 mA / cm2 en fonction des conditions de mesure. Grâce aux courbes courant-tension en fonction de la température, nous avons déterminé la température de transition de la conductivité ions/électrons à 260K et analysé les résultats expérimentaux en utilisant l'équation de Nernst- Einstein donnant une énergie d'activation de 0.253 eV pour les ions mobiles.Enfin, le processus de vieillissement de la cellule solaire a été étudié avec des mesures optiques et électriques. Nous avons déduit que le processus de vieillissement apparaît d'abord à la surface des cristaux de pérovskite ainsi qu’aux joints de grains. Les mesures GDOES nous indiquent que les caractéristiques électriques des cellules pérovskites sont perdues par une corrosion progressive de l'électrode supérieure en argent causée par la diffusion des ions iodures. / Organic-inorganic lead halide perovskites are very promising materials for the next generation of solar cells with intrinsic advantages such as a low-cost material due to the availability of source materials and low-temperature solution processing as well as a high power conversion efficiency of the sunlight. However, perovskite solar cells are still unstable and show deleterious current-voltage hysteresis effects. Inthis thesis, analyses of CH3NH3PbI3-xClx based perovskite thin films and solar cells are presented. The electrical transport characteristics and the ageing processes are investigated using different approaches.The synthesis of the halide perovskite materials is optimized in a first step by controlling the deposition conditions such as annealing temperature (80°C) and spinning rate (6000 rpm) in the one step-spin-casted process. CH3NH3PbI3-xClx based perovskite solar cells are then fabricated in the inverted planar structure and characterized optically and electrically in a second step.Direct experimental evidence of the motion of the halide ions under an applied voltage has been observed using glow discharge optical emission spectroscopy (GDOES). Ionic diffusion length of 140 nm and ratio of mobile iodide ions of 65 % have been deduced. It is shown that the current-voltage hysteresis in the dark is strongly affected by the halide migration which causes a substantial screening of the applied electric field. Thus we have found a shift of voltage at zero current (< 0.25 V) and a leakage current (< 0.1 mA/cm2) in the dark versus measurement condition. Through the current-voltage curves as a function of temperature we have identified the freezing temperature of the mobile iodides at 260K. Using the Nernst-Einstein equation we have deduced a value of 0.253 eV for the activation energy of the mobile ions.Finally, the ageing process of the solar cell has been investigated with optical and electrical measurements. We deduced that the ageing process appear at first at the perovskite grain surface and boundaries. The electrical characteristics are degraded through a deterioration of the silver top-electrode due to the diffusion of iodides toward the silver as shown by GDOES analysis.

This thesis reports the investigation of Bismuth oxybromide (BiOBr) semiconductor material as an efficient photocatalyst for the sunlight harvesting as well as environmental cleanup. I have utilised different synthetic methodologies to obtain BiOBr and its derivatives, such as co-precipitation, ultrasonification, and photo-deposition; and have studied their structural and optical properties by X-ray diffraction and surface analysis techniques. I report the synthesis and characterisation of two new p-n heterojunction systems, AgBr-BiOBr and BiOBr-ZnFe₂O₄, and have performed initial studies on photocatalytic reaction and their catalytic decomposition mechanisms. I have also reported the surface modification method including the deposition of noble metal on BiOBr to investigate the role played by the noble metal and the interactions between semiconductor and metal using various characterisation measurements. Furthermore, a continuous series of BiOBr-BiOI solid solutions were synthesised, characterised and the photocatalytic degradation was performed on the as-obtained semiconductors, to study the band structure properties of the solid solutions.