This thesis explores a range of photoactive metal halide perovskite materials for use in photovoltaic applications. These materials are of huge interest due to their outstanding optoelectronic properties which result in high photovoltaic power conversion efficiencies. In particular, this thesis discusses perovskites with stoichiometry ABX₃ where A is a singly charged cation, for example methylammonium (MA), B is predominantly lead (Pb²⁺), and X is iodide (I-) and/or bromide (Br^-). At present the commercial applications of these materials are limited by the chemical instability of the A-site cation. In this thesis, the effect of chemical substitution of the A-site is investigated as a way to increase the stability of the perovskite material. Full replacement with the inorganic cation caesium (Cs⁺) is shown to significantly improve the chemical stability. However, the inorganic lead halide perovskites with ideal bandgaps for photovoltaic applications exhibit structural instability. Routes to achieve both chemical and structural stability for these perovskites are discussed. Consequently, this thesis represents pioneering work in the field of inorganic halide perovskites and will greatly assist the development of stable inorganic perovskite materials for optoelectronic applications such as tandem photovoltaics and LEDs. Chapters 1 and 2 of this thesis present the motivation for perovskite materials to be used in solar cells, along with relevant background information about these materials and solar cell operation in general. Chapter 3 details the methods utilised in the experimental results chapters which follow. The first experimental results chapter, Chapter 4, shows how incorporation of Br^- in place of I^- in CsPbI₃ leads to increased ambient stability of the perovskite structure, and the first solar cells with CsPbI₂Br as the absorbing photovoltaic material are reported. Chapter 5 remedies the deficit of information about the optoelectronic properties of the CsPbI_3-xBr_x (0 ≤ x ≤ 3) perovskites through magneto-optical measurements on thin-films. These measurements raise questions about the room temperature perovskite structure of the CsPbI_3-xBrx compositions with small x, previously thought to be cubic perovskite, which is shown in Chapter 6 to be an orthorhombic perovskite polymorph. This finding motivates preliminary work presented in Chapter 7 aimed at chemical stabilisation of this orthorhombic perovskite polymorph. Finally, Chapter 8 summarises the work presented in this thesis, and recommends further research for the development of stable perovskite materials for photovoltaics.

Residential Grid-Connected Photovoltaics (GPV) systems hold remarkable promise in their potential to reduce energy use, air pollution, greenhouse gas emissions, and energy costs to consumers, while also providing grid efficiency and demand-side management benefits to utilities. Broader adoption of customer-sited GPV also has the potential to transform the traditional model of electricity generation and delivery. Interest and activity has grown in recent years to promote GPV in north central Texas. This study employs a mixed methods design to better understand the status of residential GPV adoption in the DFW area, and those factors influencing a homeowner's decision of whether or not to install a system. Basic metrics are summarized, including installation numbers, distribution and socio-demographic information for the case study city of Plano, the DFW region, Texas, and the United States. Qualitative interview methods are used to gain an in-depth understanding of the factors influencing adoption for the Solarize Plano case study participants; to evaluate the effectiveness of the Solarize Plano program; and to identify concepts that may be regionally relevant. Recommendations are presented for additional research that may advance GPV adoption in north central Texas.

Renewable energy systems have received considerable attention as a sustainable technology in the building sector. Specifically, the use of ground-source heat pump (GSHP) and air-source heat pump (ASHP) for heating and cooling of buildings is increasing rapidly, and the combination with photovoltaic (PV) systems and heat pump systems provide energy savings and environmental benefits. This study investigates the feasibility of replacing conventional heating and cooling systems in a multifamily, residential building with GSHP and ASHP systems and their combination with PV. The integration of PV with GSHP and ASHP systems presents an opportunity for increased solar energy usage resulting in a reduction of electricity demanded and a reduction of emissions of greenhouse gases. To analyze different heat pumps systems with and without PV, system modeling and computer simulations are performed with RETScreen Expert software.<div><br></div><div>A multifaceted verification and validation study is conducted for the system model and computer simulation. The important objective of this part of the study is to understand and develop confidence for modelling individual studies in RETScreen Expert software. To accomplish this, RETScreen Expert is used for modeling and simulating the performance of PV systems in several geographical locations, including Fort Wayne, IN. A comparison is made to performance predictions from System Advisory Model (SAM) software. In addition, a study is done to compare predictions from both software to previously published data.<br></div><div><br></div><div>In the further phase of the study, eQUEST software, a tool for building energy simulation is used to predict outputs such as electricity consumption, heating loads, and cooling loads for the multifamily residential building considered in this study. These outputs, as well as, building parameters are used as inputs to RETScreen Expert. Since, this study focuses on modeling and simulating the heating and cooling systems coupled with PV for feasibility analysis, only a few minor modifications to the eQUEST default settings are made.<br></div><div><br></div><div>The outputs from eQUEST are used as inputs to RETScreen Expert and analysis of ASHP and GSHP systems, as well as their combination with a PV system are performed. The results include the technical performance and financial model of each system, which can be used to indicate feasibility. The results show that both GSHP and ASHP systems are environmentally friendly and reduce energy consumption. These systems are economically feasible, with payback periods of under 10 years, when electricity prices are high. When combined technology is preferred, PV-GSHP systems are more environmentally friendly and have fuel savings far better than any other proposed systems. But the feasibility of the both the GSHP and PV-GSHP systems strongly depends on loop installation cost.<br></div>

Cette thèse présente l’étude de nanocristaux semi-conducteurs pour leur intégration dans des dispositifs de conversion thermoélectrique. Ce phénomène permet de générer un courant à partir d’une différence de température entre deux faces, reliées par deux pieds conducteurs de charges. Les matériaux les plus efficaces à température ambiante sont basés sur le tellurure de bismuth Bi2Te3, qui est toxique et coûteux. Une étude théorique et bibliographique, portant sur les grandeurs caractéristiques de la conversion thermoélectrique, est réalisée. Elle permet de déterminer les matériaux d’intérêt en fonction de leur coût et de leur efficacité, que l’on peut optimiser à travers différents paramètres d’influence. La chalcopyrite, CuFeS2, présente des propriétés intéressantes en thermoélectricité, et offre une alternative intéressante aux matériaux classiques, car composée d’éléments abondants et non-toxiques. La synthèse par voie chimique choisie permet de contrôler la composition du matériau, et d’obtenir des nanocristaux de taille contrôlée entre 30 et 50 nm, pour diffuser les phonons dans le matériau et diminuer sa conductivité thermique. La thèse s’oriente autour de l’étude de ces nanocristaux semi-conducteurs de CuFeS2, organisée en deux parties principales.La première partie décrit la synthèse par voie chimique des nanocristaux et leur étude structurale. Deux méthodes de synthèse sont optimisées et permettent de contrôler finement la stœchiométrie du matériau, et d’accéder à des cristaux de différentes tailles et morphologies. Une étude complète de la composition des nanocristaux est réalisée par XPS, EDX et thermogravimétrie. L’étude du matériau par diffraction des rayons X met en évidence l’influence de la composition chimique des nanocristaux, et des conditions de température et de pression sur la phase cristalline du matériau. Une transition de phase de la wurtzite vers la chalcopyrite est décrite.Dans la seconde partie sont étudiées les propriétés thermoélectriques des nanocristaux synthétisés. Leur mise en forme en pieds thermoélectriques monolithiques est décrite, ainsi que l’optimisation de leurs propriétés thermoélectriques à travers trois stratégies. Le matériau obtenu est un conducteur de type n, qui permet la conduction des électrons. Sa conductivité thermique est réduite par nanostructuration. La première stratégie consiste à faire varier la composition des nanocristaux, et plus particulièrement le rapport entre charges cationiques et anioniques, pour modifier le taux de dopage du matériau, et ainsi modifier sa conductivité électrique et son coefficient Seebeck. La seconde voie d’amélioration consiste à remplacer les ligands isolants présents après la synthèse des nanocristaux par des ligands courts et conducteurs, pour augmenter la conductivité électrique du matériau. Enfin, des nanoparticules métalliques d’argent, d’étain et de cuivre sont introduites en mélange avec les nanocristaux afin d’augmenter la conductivité électrique du matériau nanocomposite ainsi créé.Cette thèse apporte des éléments de compréhension entre la structure et la composition de matériaux ternaires et leurs propriétés thermoélectriques, et permet d’envisager une amélioration de leurs performances. Les matériaux optimisés présentent des efficacités comparables aux résultats de la littérature pour cette famille de matériaux, notamment autour de la température ambiante. A travers une combinaison efficace des facteurs d’influence étudiés, ces efficacités pourront être dépassées lors de futurs travaux, et le matériau intégré à un dispositif de conversion thermoélectrique couplé à une cellule photovoltaïque, pour la conversion de l’énergie solaire par les deux phénomènes. / This thesis presents the studies made on semiconducting nanocrystals, to be integrated in thermoelectric generators. Thermoelectricity generates a current through a temperature difference between two faces, connected by thermoelectric legs which conduct the charges. Nowadays, the most efficient materials at room temperature contains tellurium, which is toxic and expansive due to its scarcity. A study on theory and literature is carried to understand the underlying phenomena which help us explain the thermoelectric conversion. The potentially interesting materials are selected for their cost and efficiency, tunable by varying different parameters. Chalcopyrite, of formula CuFeS2, presents promising properties for thermoelectricity, and offers an interesting way to replace classic materials as a non-toxic earth-abundant substitute. The chemical synthesis allows to control the composition of the material and to obtain 30 to 50 nm sized nanocrystals, able to scatter phonons and diminish the thermal conductivity of the material as a consequence. The thesis is describing the study of these semiconducting CuFeS2 nanocrystals, and is divided in two main parts.The first part describes the chemical synthesis of the nanocrystals and the characterization of their structure. Two ways of synthesis are developed and optimized, allowing to control the stoichiometry of the material, and to obtain crystals of different sizes and shapes. A complete study of the composition of the nanocrystals is made by XPS, EDX and thermogravimetric analysis. The study of the material by X-ray diffraction shows that the chemical composition of the nanocrystals, as well as the temperature and the pressure, have an influence on their crystalline phase. A phase transition from the wurtzite phase to the chalcopyrite phase is described.In the second part, are studied the thermoelectric properties of the nanocrystals. Their preparation as solid materials is described. The improvement made on their efficiency is following three main paths. The obtained material is a n type conductor, which means it carries electrons. Its thermal conductivity is reduced due to the nanostructuration. The first strategy consists in varying the composition of the nanocrystals, and especially the ratio between positive and negative charges, carried by ions, to modify the electrical conductivity and Seebeck coefficient of the material through doping. The second way of improvement is by replacing the native insulating ligands of the nanocrystals by short inorganic conducting ones, to increase the electrical properties of the material. Finally, metallic nanoparticles, of silver, tin and copper, are blended with the nanocrystals to improve the electrical conductivity of the resulting nanocomposite material.This thesis helps one to understand the relation between structure, composition and thermoelectrical properties of ternary semiconducting materials. It is possible to think of ways of improvement for the studied materials. Our best results are state of the art for this family of materials, especially around room temperature. There is room for improvement, with a proper combination of the studied parameters. During a future work, the optimized material could be integrated to a thermoelectric - photovoltaic device, for conversion of the solar energy through the two phenomena.

Nanowire solar cells have great potential as candidates for high efficiency, next-generation solar cell devices. To realize their potential, accurate and efficient modeling techniques en- compassing both optical and electrical phenomena must be developed. In this work, a coupled optical and electronic model of GaAs nanowire solar cells was developed, with the goal of building a platform for automated, algorithmic device optimization. Significant work was done on the optical portion of model, with the goal of reducing run- times and improving the level of automation. Enhancements were made to an open-source implementation of the Rigorous Coupled Wave Analysis method for solving Maxwell’s equations, to make it more accurate for modeling nanowire solar cells. Its accuracy and efficiency were thoroughly investigated, and with the enhancements presented here it was shown to be an effective technique for rapid optical modeling of nanowire devices. Purely optical optimizations of a sample AlInP-passivated GaAs nanowire on a GaAs substrate were performed to demonstrate the efficacy of the technique using a Nelder-Mead simplex optimization of device geometry. The optical model was then coupled into a finite volume method based electrical model implemented in TCAD Sentaurus, to compute device efficiencies and ultimately optimize electrical device performance. As a first step, an algorithmic optimization of a p-i-n nanowire solar cell consisting of an AlInP-passivated GaAs nanowire on a Si substrate was performed using the generation rates computed by the enhanced RCWA implementation. The overall geometry was fixed to the result of the optical optimization, and only internal electrical parameters were optimized. The results showed that significant performance improvements can be obtained with the right choice of doping levels and doping region configurations, even without optimizing the global device geometry.

High performance multijunction solar cells based on polycrystalline thin films will require a wide bandgap top cell with at least 15% efficiency. With the bottom cell being CIGS which have already demonstrated the required efficiencies, this work aims to study the complete fabrication and performance of Cd1-xZnxTe solar cells with a bandgap of 1.7eV on glass and flexible polyimide substrates. Cd1-xZnxTe films were deposited by RF magnetron co-sputtering with CdTe and ZnTe sources. By varying the composition of Cd1-xZnxTe being deposited the required bandgap of 1.7eV was achieved. The optical and structural properties of the films were studied with optical transmission, SEM and XRD measurements. The films exhibited high optical transmission and pinhole free grain structure. CZT solar cells were fabricated on glass and flexible polyimide substrate and were analyzed by J-V and spectral response measurements. The effect of post deposition treatments and the effect of N2 during sputtering on CZT device performance were studied.

Copper Indium Gallium DiSelenide absorber layers are fabricated using a two step manufacturing-friendly process. The first step involves the sequential deposition of Copper and Gallium and codeposition of Indium and Selenium, not necessarily in that order, at 275o C. This is followed by the second stage, where the substrate is annealed in the presence of Selenium and a thin layer of Copper is deposited to neutralize the excess Indium and Gallium on the surface to form the Copper Indium Gallium diSelenide absorber layer. Elimination of the need for high degree of control and elimination of toxic gases like hydrogen selenide aid in the easy scalability of this process to industry. The performance of CuInGaSe2/CdS/ZnO solar cells thus fabricated was characterized using techniques such as I-V, C-V, Spectral Response and EDS/SEM. Cells with open circuit voltages of 450-475 mV, short circuit current densities of 30-40 mA/cm², fill factors of 60-68% and efficiencies of 8-12% were routinely fabricated. Gallium in small amounts seems to improve the open circuit voltages by 50-100 mV without significantly affecting the short circuit currents and the band gap in Type I precursors. Gallium also improves the adhesion of the CIS layer to the molybdenum back contact. Efforts are also being aimed at improving the short circuit current densities in our high bandgap devices. It is believed that improperly bonded Ga is hurting the electronic properties of the CIGS films. A part of this work involves the reduction of the detrimental effect of Ga on the Jsc's by modifying the base process, so as to improve the homogeneity of the film. The modifications include lowering the Ga level as well as fine-tuning the annealing step. Ar annealing of the samples has also been incorporated. The short circuit current densities have been improved significantly by the above mentioned modifications. At present, the best Jsc's are in the 33-35 mA/cm² range. The Voc's have also been improved by splitting the Ga into two layers and replacing the top Cu layer by a Ga layer. Light soaking studies of the absorber have also been carried out. The baseline Type I process has also been adapted to a new load-locked in-line evaporator system. Device performance dependence on Ga and In thickness as well as the top selenization temperature has been determined in this research. The effect of moisture on the quality of the films has been studied. Bandgap variations due to the presence/absence of Se during the Cu deposition has been investigated. The impact of substrate cleaning/Moly deposition conditions on the device performance has been explored. Insitu Ar annealing studies of CIGS absorbers have been carried out. Alternate buffer layers have been pursued. Devices with Voc's as high as 480 mV, Jsc's as high as 40.7 mA/cm² and fill factors of 66% have been fabricated.

The goal of this research project was to contribute to the understanding of CuGaSe2/CdS photovoltaic devices, and to improve the performance of these devices. The initial part of the research dealt with the optimization of a Sequential Deposition process for CuIn(Ga)Se2 absorber formation. As an extension of this, a recipe (Type I Process) for CuGaSe2 absorber layer fabrication was developed, and the deposition parameters were optimized. Electrical characterization of the thin films and completed devices was carried out using techniques such as Two-Probe and Three-Probe Current-Voltage, Capacitance-Frequency, Capacitance-Voltage, and Spectral Response measurements. Structural/chemical characterization was done using XRD and EDS analysis. Current densities of up to 15.2 mA/cm2, and Fill Factors of up to 58% were obtained using the Type I CuGaSe2 Process. VOC's, however, were limited to less than 700 mV. Several process variations, such as changes in the rate/order/temperature of depositions and changes in the thickness of layers, resulted in little improvement. With the aim of breaking through this VOC performance ceiling, a new absorber recipe (Type II Process) was developed. VOC's of up to 735 mV without annealing, and those of up to 775 mV after annealing, were observed. Fill Factors were comparable to those obtained with Type I Process, whereas the Current Densities were found to be reduced (typically, 10-12 mA/cm2, with the best value of 12.6 mA/cm2). This performance of Type II devices was correlated to a better intermixing of the elements during the absorber formation. To gain an understanding of the performance limitations, two simulation techniques, viz. SCAPS and AMPS, were used to model our devices. Several processing experiments and SCAPS modeling indicate that a defective interface between CuGaSe2 and CdS, and perhaps a defective absorber layer, are the cause of the VOC limitation. AMPS simulation studies, on the other hand, suggest that the back contact is limiting the performance. Attempts to change the physical back contact, by changes in the absorber processing, were unsuccessful. Processing experiments and simulations also suggest that the CuGaSe2/CdS solar cell involves a true heterojunction between these two layers.

In the first part of the study, power conversion efficiency enhancement of organic solar cells by addition of gold nanorods and gold nanostars into PEDOT: PSS (Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) layer was investigated. Efficiency of each sample set has been characterized by measuring current density-voltage characteristics. The best efficiencies obtained during this study are 2.88 % and 2.54 % by addition of gold nanostars and nanorods, respectively. The increase in PCEs is notable when these values are compared with the ones (1.67 %) obtained with a reference device which is prepared without adding any gold nanoparticles under the same conditions. In the second part of the study, branched gold nanoparticles were succesfully grown directly on different types of surfaces such as glass, silicon wafer, and indium-tin-oxide (ITO) coated glass with a simple solution-based method in order to utilize them for further applications.

With an increasing interest for renewable power, photovoltaics (PV) have becomemore and more common in the distribution network. If a customer wants to install aPV system, or another type of distributed generation (DG), the distribution systemoperators (DSO) needs a good way to determine if it the grid can handle it or not. InSweden, a guideline to aid the DSO was published in 2011. However, this guidelineonly considers one connection without considering other DG units. This project isabout developing new guidelines for DG connections in grids with a large number ofDG units. Based on a literature study it has been concluded that one of the mostcritical issue is over-voltage, which is the main focus of this project. Two new methods have been developed; the first proposed method is based onneglecting reactance and losses in the grid, a simple linear relationship between thevoltage level, the resistance in the lines, and the installed power is obtained. Thisrelationship is then used to calculate the voltage level at critical points in the grid. Thesecond method is to find the weakest bus, with a connected DG unit. By assumingthat all power is installed at that point we get a very simple guideline; it is veryconservative but can be used before the first method. A simulation tool has been developed in order to analyze the voltage level in grids forvarious cases with connected DG units. The simulated results have proven that theproposed guidelines are, when considering voltage issues, very reliable and can beuseful. However, further work needs to be done to ensure that other problems donot occur.