Photovoltaic (PV) devices based on PbS quantum dot (QD) solids demonstrate high photon-to-electron conversion yields. However, record power conversion efficiencies remain limited mainly due to bulk and interfacial defects in the light absorbing material (QD solids). Interfacial defects can be formed when a semiconductor, such as QD solid, is contacted by another material and may predetermine the semiconductor/metal or semiconductor/metal-oxide junction properties. The objective of the work described in this dissertation was set to explore whether electrochemical contacting using liquid electrolytes can provide sufficient means of contacting the QD solids to investigate their PV performance without introducing the unwanted interfacial defects. I have initially focused on optimizing processing conditions for efficient QD solids deposition and studied their photovoltaic properties in a standardized solid-state, depleted heterojunction solar cell configuration. Further, a liquid contacting method was developed to study the relationship between photovoltages of QD solids and the energetics (e.g. reduction potentials) of the liquid contacting media. This electrochemical contacting of PbS QD solids was achieved by using anhydrous liquid electrolytes containing fast, non-coordinating, outer-sphere redox couples. Depending on the energetics of a redox couple, both rectifying and non-rectifying (Ohmic) PbS QD solid/electrolyte junctions were successfully formed with both p- and n-type QD solids. Furthermore, application of the liquid solution contacting method in studies of the PbS QD solids has unprecedentedly demonstrated that an ideal behavior of the photovoltage changes with respect to the changes in the energetics of the contacting media can be achieved. This fact supports the initially proposed hypothesis that such liquid contacting method will not introduce surface defects to the studied QD materials, allowing for their intrinsic properties to be better understood. The applicability of this method to both p- and n- type QD solids was demonstrated. Finally, a better understanding of the relationships between the surface and ligand chemistries of both p- and n-type QD solids and their photovoltaic properties was possible via applications of such method in conjunction with XPS and UPS studies.

Cette thèse, financée par le laboratoire SIMaP-EPM à Grenoble et l'INES à Chambéry, porte sur l'effet des contraintes thermomécaniques sur la qualité cristalline lors de l'élaboration de lingots de silicium pour applications photovoltaïques. Ainsi, ce travail commence par exposer comment l'industrie photovoltaïque construit les panneaux solaires et l'impact des dislocations, défauts issus des contraintes, sur leur rendement. Une revue bibliographique est également faite afin de présenter les modèles physiques et numériques traitant des dislocations dans le silicium, de leur mouvement et de leur multiplication. Différentes techniques de caractérisation de la densité de dislocations sont également décrites dans la première partie de ce travail.Dans le second chapitre du manuscrit, une étude comparative de différentes méthodes de caractérisation rapide est réalisée afin de montrer leurs forces et leurs faiblesses. Pour cela, un échantillon sert de référence pour la comparaison. Celui-ci a l'avantage d'être large, de ne pas être perturbé par des joints de grains et d'avoir des zones de forte et de faible densité de dislocations. La première technique de caractérisation étudiée dans ce manuscrit est la « méthode précise » consistant à dénombrer manuellement les dislocations à la surface de l'échantillon afin d'avoir une caractérisation fine de la densité de dislocations. Ensuite, la « méthode INES » utilise un traitement informatique d'images prise avec un microscope électronique à balayage afin de compter les dislocations. La « méthode Ganapati » relie les niveaux de gris d'une image de l'échantillon prise avec un scanner et la densité de dislocations. Enfin, la « méthode PVScan » utilisant l'appareil du même nom et qui se sert de la diffusion d'un faisceau laser à la surface de l'échantillon afin de réaliser la caractérisation. Cette étude comparative montre quelles sont les applications privilégiées pour chaque technique et surtout quels questions il est nécessaire de se poser avant de réaliser une caractérisation.La troisième partie de ce travail est dévolue à la mise en place de deux simulations numériques utilisant le code commercial Comsol afin de prédire la densité de dislocations dans un lingot de silicium à la fin de son élaboration. Pour cela, le modèle d'Alexander et Haasen, décrivant l'évolution de la densité de dislocations et de la relaxation plastique, est implémenté au logiciel et couplé avec le calcul des contraintes thermomécaniques. Le premier modèle, nommé « évolution continue », traite le lingot dans son ensemble, partie solide et liquide, et fait évoluer la température de façon continue durant la résolution. Dans le second modèle, nommée « pas à pas », seule la partie solide du lingot est prise en compte en modifiant la géométrie et la température à chaque pas de temps. Ces deux modèles sont comparés avec des simulations numériques réalisées par des équipes japonaise et norvégienne et le premier modèle est également comparé avec la caractérisation d'un échantillon. Ainsi, cette partie montre la pertinence de l'utilisation d'un code commercial pour l'estimation de la densité de dislocation dans un lingot à la fin de l'élaboration, de par sa simplicité d'utilisation et son adaptabilité à différentes géométries de fours.Dans un dernier chapitre, le problème de l'attachement entre le lingot et le creuset lors de la cristallisation est étudié car il est à l'origine de fortes contraintes et donc de dislocations dans le cristal. Ce problème est également traité par simulation numérique en utilisant le logiciel Comsol. Pour cela, un modèle physique est défini : l'intégrale J est utilisé pour évaluer l'énergie élastique exercée sur l'attachement et cette valeur est ensuite comparée à l'énergie d'adhésion entre le creuset et le lingot. Ce model est implémenté au logiciel et les résultats sont comparés avec une expérience réalisée au cours d'une thèse précédente (...) / SIMaP-EPM laboratory of Grenoble and INES institute of Chambery have both financed this thesis which investigates the effect of thermo-mechanical stresses on the crystal quality during production of silicon ingots for photovoltaic applications. This work begins by showing how photovoltaic industry makes solar panels and the influence of dislocations (defects induced by stresses) on the conversion efficiency. Bibliographic review is also performed in order to describe physical and numerical models of dislocation motion and their multiplication in silicon. Several characterization methods of the dislocation density at the surface of a sample are also presented in the first part of this work.In the second part of this manuscript, comparative study of different quick characterization methods is done in order to show their strength and weaknesses. Therefore, a sample, which is wide, not containing grain boundaries, and having areas of high and low dislocation density, is used as reference sample for the comparison. The first characterization technique studied in this work is the “accurate method” consisting in manually counting the dislocations at the surface of the sample in order to have a precise characterization of dislocation density. The “INES method” uses numerical treatment of SEM pictures to count dislocations. The “Ganapati method” links the grey scale of a sample picture taken with a scanner and the dislocation density. Finally, the “PVScan method”, using the eponymous device, uses diffusion of a laser beam on the surface of the sample for characterization. This comparative study underlines the best applications for each method and which questions should be thought about before performing dislocation characterization.The third part of this work is intended to build two numerical simulations using Comsol commercial software in order to predict dislocation density in silicon ingot at the end of its production. Therefore, Alexander and Haasen model, describing dislocation density and plastic relaxation rate, is implemented into the software and coupled with the thermo-mechanical stress calculation. In the first model, named “continuous evolution”, the entire ingot is taken into account (liquid and solid parts) and, during solving of this numerical simulation, temperature changes continuously. In the second model, named “step by step” only the solid part of the ingot is taken into account with new geometry and new temperature at each step. Both of these models are compared to numerical simulations performed by Japanese and Norwegian teams. Results of the first one are also compared to the experimental characterization of a sample. Thus, this part shows the pertinence of using commercial software for the prediction of dislocation density in a silicon ingot at the end of its production. Its use is simple and shows good adaptability to different furnace geometries and thermal fields.In the last part, ingot/crucible attachment is studied because it creates high stresses and then dislocations in the crystal. This problem is also solved by numerical simulation using Comsol software. Therefore, a physical model is created: the J-integral is used to estimate elastic energy at the attachment area and then this value is compared to ingot/crucible adhesion energy. This model is implemented into the software and the results are compared to an experiment realized during a previous thesis. This numerical simulation is also applied to two attachment configurations of a silicon ingot in order to study the attachment duration, the localization and the size of crystal area impacted by plasticity.

No description available.

Solar energy is the energy derived from the sun through the form of solar radiation. Solar powered electrical generation relies on photovoltaic (PV) systems and heat engines. These two technologies are widely used today to provide power to either standalone loads or for connection to the power system grid. Maximum power point tracking (MPPT) is an essential part of a PV system. This is needed in order to extract maximum power output from a PV array under varying atmospheric conditions to maximize the return on initial investments. As such, many MPPT methods have been developed and implemented including perturb and observe (P&O), incremental conductance (IC) and Neural Network (NN) based algorithms. Judging between these techniques is based on their speed of locating the maximum power point (MPP) of a PV array under given atmospheric conditions, besides the cost and complexity of implementing them. The P&O and IC algorithms have a low implementation complexity but their tracking speed is sluggish. NN based techniques are faster than P&O and IC. However, they may not provide the global optimal point since they are prone to multiple local minima. To overcome the demerits of the aforementioned methods, support vector regression (SVR) based strategies have been proposed for the estimation of solar irradiation (for MPPT). A significant advantage of SVR based strategies is that it can provide the global optimal point, unlike NN based methods. In the published literature of SVR based MPPT algorithms, however, researchers have assumed a constant temperature. The assumption is not plausible in practice as the temperature can vary significantly during the day. The temperature variation, in turn, can remarkably affect the effectiveness of the MPPT process; the inclusion of temperature measurements in the process will add to the cost and complexity of the overall PV system, and it will also reduce the reliability of the system. The main goal of this thesis is to present a novel sensorless SVR based multi-stage algorithm (MSA) for MPPT in PV systems. The proposed algorithm avoids outdoor irradiation and temperature sensors. The proposed MSA consists of three stages: The first stage estimates the initial values of irradiation and temperature; the second stage instantaneously estimates the irradiation with the assumption that the temperature is constant over one-hour time intervals; the third stage updates the estimated value of the temperature once every one hour. After estimating the irradiation and temperature, the voltage corresponding to the MPP is estimated, as well. Then, the reference PV voltage is given to the power electronics interface. The proposed strategy is robust to rapid changes in solar irradiation and load, and it is also insensitive to ambient temperature variations. Simulations studies in PSCAD/EMTDC and Matlab demonstrate the effectiveness of the proposed technique.

Back Amorphous-Crystalline silicon Heterojunction (BACH) solar cells which combine the benefits of back contact and heterojunction silicon solar cells have been fabricated at the University of Toronto. p- and n-type amorphous silicon deposited at low temperature (~<200 oC) by DC Saddle-Field PECVD system forms interdigitated hetero-emitter and base contacts on the rear-side. A photolithography approach using thermal oxide for electrical isolation demonstrates the proof-of-concept. Three methods for fabricating simplified and advanced BACH cells were explored. The best performing 1 cm2 cell showed an AM1.5G conversion efficiency of 8.11%, VOC = 0.536 V, JSC = 20.1 mA/cm2 and FF = 75.5%. The BACH cell performance is limited by poor surface passivation and un-optimized cell design. With completely low temperature processing, highly passivated front and rear surfaces, and independent optimization of front-side optical antireflective features and rear-side electrical junctions and contacts, the BACH cell has the potential of becoming highly cost competitive.

Back Amorphous-Crystalline silicon Heterojunction (BACH) solar cells which combine the benefits of back contact and heterojunction silicon solar cells have been fabricated at the University of Toronto. p- and n-type amorphous silicon deposited at low temperature (~<200 oC) by DC Saddle-Field PECVD system forms interdigitated hetero-emitter and base contacts on the rear-side. A photolithography approach using thermal oxide for electrical isolation demonstrates the proof-of-concept. Three methods for fabricating simplified and advanced BACH cells were explored. The best performing 1 cm2 cell showed an AM1.5G conversion efficiency of 8.11%, VOC = 0.536 V, JSC = 20.1 mA/cm2 and FF = 75.5%. The BACH cell performance is limited by poor surface passivation and un-optimized cell design. With completely low temperature processing, highly passivated front and rear surfaces, and independent optimization of front-side optical antireflective features and rear-side electrical junctions and contacts, the BACH cell has the potential of becoming highly cost competitive.

To address the commonly poor short wavelength response of the conventional solar cell structure which consists of a highly doped thin emitter layer on top of a thicker and less doped base, the novel concept of the Buried-Windowed-Emitter is introduced. This new solar cell structure makes use of a high quality semiconductor layer on top of the traditionally made highly doped emitter and greatly enhances the spectral response of the solar cell by giving the superficially generated carriers a higher chance of collection at the junction. In the proposed BWE structure the emitter is windowed in order to electrically connect the top layer to the base for current collection. The efficacy of the proposed novel device is proven by computer aided device simulations using the available device simulation tools such as MEDICI. The results of simulation show that the proposed novel Buried-Windowed-Emitter solar cell will not only improve the short wavelength spectral response of the overall cell as expected, but also will boost the spectral efficiency for all the wavelengths. Another exciting conclusion from the results of the computer simulation of the BWE solar cell is that the minority carrier lifetime in the top layer does not need to be very high for a superb performance and values as low as 1µs can still boost the short circuit current of the cell to values close to the theoretical limit of the photo-current collectable by a silicon solar cell. This is indeed a good news for manufacturability of this device as it should be practically feasible to achieve epitaxial films with minority carrier lifetime in this range. In order to increase the understanding about the rather complex structure of the proposed Buried-Windowed-Emitter solar cell, an analytical circuit level model, similar to the case of the standard solar cell, is developed for the proposed device. The developed analytical model helps to understand the importance of the main design parameters such as the dimensions of the pattern of the windowed emitter. On the path to fabricate the proposed BWE solar cell, great deal of work is done on the development of a low temperature (<300°C) epitaxial silicon technology using the benefits of Plasma Enhanced Chemical Vapor Deposition (PECVD). Highly doped epitaxial silicon layers of up to around 1µm thickness are achieved with sheet resistivity as low as 7Ω/sq which is much lower than what is reposted in the literature in similar deposition conditions. Intrinsic, phosphorous doped n-type and boron doped p-type epitaxial films have been developed on silicon substrates. Measurement of reflection spectra of the deposited epitaxial films is proposed as a fast, non destructive and process-integrate-able method to assess the crystalline quality of the epitaxial films. Effects of higher temperature post deposition annealing have been studied on the develop epitaxial films A full technology is developed for the fabrication of the proposed novel solar cells. Photo-masks are designed to create 10 different architectures for the design of the windowed emitter in the BWE cell. All the steps taken in the successful fabrication of the novel BWE cells are presented in detail and the relevant findings are discussed and proposed as future research topics. Three kinds of cells are fabricated using the developed technology to separately study the effects of partial coverage of the windowed emitter, the optical performance of the developed epitaxial silicon films and the performance and manufacturability of the novel BWE solar cell The results show that the concept of windowed-emitter by itself (even without the top layer) is capable of enhancing the performance of the solar cell when compared to a standard design. It also promises high conversion efficiency for the BWE solar cell in case a high quality top layer can be deposited on top of the windowed emitter. The results further reveal the lower than expected quality of the low temperature epitaxial films despite the indication of their full crystallinity through other analyses. Use of the epitaxial films as the emitter of the solar cell is proposed as a direct and effective method of studying the photovoltaic performance of the low temperature epitaxial films. Further development of the epitaxial technology will lead to feasibility of a BWE solar cell with very high photovoltaic performance.

This thesis explores the design of a smart photovoltaic (PV) module- a PV module in which PV cells in close proximity are electrically grouped to form a pixel and are connected to dc-dc converter blocks which reside embedded in the back pane of the module. An auto-connected flyback converter topology processing less than full power is used to provide high gain and perform maximum power point tracking (MPPT). These dc-dc converters interface with cascaded H-bridge inverter modules operating on feed forward control for dc-link voltage ripple rejection. By means of feed forward control, a significant reduction in dc link capacitance is achieved by enduring higher dc link ripple voltages. The dc link electrolytic capacitors are replaced with film capacitors thus offering an improvement in the reliability of the smart PV module. The proposed configuration is capable of producing 120V/ 240V AC voltage. The PV module now becomes a smart AC module by virtue of embedded intelligence to selectively actuate the individual dc-dc converters and control the output AC voltages directly, thus becoming a true plug and power energy system. Such a concept is ideal for curved surfaces such as building integrated PV (BIPV) system applications where gradients of insolation and temperature cause not only variations from PV module-to-PV module but from group-to-group of cells within the module itself. A detailed analysis along with simulation and experimental results confirm the feasibility of the proposed system.

This project employs the biological compounds hemin, melanin, and retinoic acid as photoactive dyes in dye-sensitized solar cells (DSSCs). These dyes are environmentally and economically superior to the standard ruthenium-based dyes currently used in DSSCs because they are nontoxic and widely available. Characterization by linear sweep voltammetry yielded averaged maximum overall conversion efficiency values of 0.059% for retinoic acid, 0.023% for melanin, and 0.015% for hemin. Absorption spectra of hemin and retinoic acid suggest that they would complement each other well when used in tandem in one cell because hemin has a secondary maximum absorption peak at 613nm and retinoic acid has maximum absorption at 352nm. Cells made with hemin or melanin performed better with the use of lower temperatures to seal the cells, and hemin cells performed exceptionally well with exclusion of the sealing procedure. These biologically-derived cells have the potential to advance the development of inexpensive and safer solar energy sources, which promise to serve as clean energy sources in the near future.

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