This work is aimed to design power supply for nodes of wireless sensors networks using solar energy in indoor spaces. The proposal is made for the particular layout, but the process described can be applied generally. The result is the proposed involvement of the operating power circuit for node of wireless sensor network used indoors.

This thesis is focused on the area of analysis and diagnostic of monocrystalline solar cells, using local light radiation. Main goals of this thesis are focused on explanation of generation and behavior microplasma in solar cell, in dependence of temperature and reversed bias voltage. As a next focus of this thesis is strong regions of microplasma sources, analyzed by scanning electron microscope and a detailed analysis of edge and surface structure is made. The influence of the environment (air pressure, level of vacuum and nitrogen gas) on the microplasma generation and behavior is observed in the vacuum chamber of electron microscope. The results from microplasma method are correlated with noise diagnostic method and on the base of these results are set a new thesis and mathematical equations for the defects behavior in different conditions. The outcomes of the research are shared with the manufacturer od silicon solar cells, Solartec s.r.o. company. It is the advanced diagnostic methods that allow improving the quality of the production process, through early detection of individual groups of defects.

This thesis describes the deposition of nanoparticles into nanoporous alumina templates. Nanoparticles of cadmium sulphide were chosen thanks to wide band gap of 2,45 eV. CdS is desired semiconductor promising better efficiency when comes to solar radiation conversion to useful energy. Theoretical part consists of explanation of photovoltaic principles, band-gap theory of materials, currently available technologies to manufacture solar cells and their efficiencies. Next part introduces the approaches of manufacturing nanoporous templates from aluminium (Anodized Aluminium Oxide, AAO) in details together with options to deposit cadmium sulphide into these structures. There is also brief description of deposition based on vacuum filtration. Experiments were made in Laboratory of Microsensors and Nanotechnologies at Faculty of Electrical Engineering and Communication in Brno University of. The experimental chapter describes manufacturing process of AAO and deposition via vacuum filtration. Samples were evaluated by Scanning Electron Microscopy.

Investigations have been done as of why dust particles appear in a circular pattern on the backside of solar cells produced in sputtering chambers at Midsummer AB. An experimental approach was conducted, where solar cells were produced at standard conditions and their backside studied by material analytical methods. The solar cells dust particles were analyzed by energy-dispersive x-ray spectroscopy and x-ray diffraction, deducing that they consisted of iron selenide (Fe0.89Se). Furthermore, the dust particles appear due to formation of a thin iron selenide film that cracks and delaminate upon cooling from process temperature to room temperature. Iron selenide film thickness was found by energy-dispersive x-ray spectroscopy to occur in a pattern with radial symmetry with respect to the cell center, correlating with the film delamination pattern. The reason to the film formation was due to selenium reacting with the substrate steel at high temperatures (>400 ◦C) in deposition chambers having a selenium environment. The film delamination occurs at a critical film thickness at which stresses in the film is high enough for the film to yield and fracture. It was concluded that iron selenide film formation or delamination must be minimized in order to control dust particle formation. These two phenomena can be mitigated by protective substrate films, change of substrate material, selenium environment optimization or temperature profile optimization and should be researched further to find the most effective and viable solution.

Ce travail a pour but d'implémenter des procédés d’implantation ionique pour des cellules solaires à hétérojonctions de silicium (SHJ) afin d'en simplifier le procédé de fabrication ou d’en augmenter les performances.Nous avons d'abord étudié le procédé pour réaliser le dopage des couches de silicium amorphe hydrogéné (a-Si:H). Par ce nouveau procédé, il est possible de réaliser des dopages localisés de manière simple, à travers des masques, ce qui peut permettre une diminution des coûts de fabrication de certains types de cellules SHJ comme les cellules à contacts interdigités à l'arrière. Les implantations de phosphore et de bore ont été étudiées, pour la réalisation de dopage respectivement de type n et p. Les comportements et les conclusions sont très différents pour ces deux types de dopage. Le phosphore étant plus lourd que le bore, il est possible de l'implanter dans des couches très minces sans endommager fortement l'interface avec le silicium cristallin, mais la création très importante de défauts dans le a-Si:H, résistant à des recuits post-implantation, conduit à de fortes dégradations des propriétés électriques du a-Si:H, et il n'a pas été possible d'atteindre des niveaux de conductivité suffisants. Au contraire, pour le bore, conformément à des résultats de la littérature, les atomes sont activés plus facilement par un recuit post-implantation grâce à la forte diminution de la concentration de défauts localisés. Cependant, le bore, implanté plus profondément, atteint e plus facilement l'interface, ce qui nécessite des recuits à plus haute température pour guérir les défauts d'interface. Néanmoins, pour des couches de a-Si:H de l'ordre de 25 nm, nous avons pu trouver des conditions technologiques permettant d'obtenir des propriétés comparables à celles obtenues par le procédé classique de dépôt de (p) a Si:H assisté par plasma, à savoir des valeurs élevées de conductivités du a-Si:H (10-4 Ω-1cm-1) et de passivation d’interface (i VOC > 700 mV).Une deuxième partie de ce travail est consacrée à l’étude d’une nouvelle cellule, dite à homo hétérojonction de silicium (HHJ) comprenant un homo-émetteur additionnel (p+) c-Si à l’hétéro-interface côté émetteur. Le but est d’améliorer la passivation de l’interface afin d’augmenter le rendement de la cellule. Des simulations numériques ont mis en évidence une augmentation de FF de la cellule HHJ, que nous avons pu attribuer à une meilleure passivation par effet de champ et à une diminution de la résistance globale du a-Si:H due à des modifications des courbures de bandes. Elles ont aussi montré la nécessité d’un homo-émetteur suffisamment mince et fortement dopé (5×1018 cm-3). De ce fait, nous avons utilisé le procédé d’implantation ionique pour développer des profils de bore adéquats et avons pu vérifier expérimentalement que l'incorporation de la couche de (p+) c-Si permet la diminution de la résistance de contact et l'amélioration de la passivation de l'interface (i) a-Si:H/(p+) c-Si par effet de champ lorsque la concentration de bore en surface n'est pas trop importante. Ces deux améliorations ont pu être concrétisées dans la réalisation de cellules présentant une amélioration du facteur de forme et de meilleurs rendements de conversion par rapport à des cellules SHJ de référence. Cette réalisation constitue la première preuve de concept pour les cellules de type HHJ. / This work aims at investigating the use of ion implantation to process silicon heterojunction solar cells (SHJ) in order to improve the ratio of cost to produced power (€/Wp) of the cells either by cost reduction due to manufacturing simplification or by increase of the cell performance.A first part of the work consists in doping hydrogenated amorphous silicon (a-Si:H) layers by ion implantation. Using hard masks, doping of localized regions required in cell architectures like interdigitated back contact cells can thus be easily achieved at lower cost. Both boron and phosphorus implantation have been studied for p- and n-type doping, respectively. These two types behave very differently. Phosphorous being heavier than boron, very shallow implantation can be achieved on thin a-Si:H layers onto crystalline wafers without damaging the interface. However very high defect densities are created in a-Si:H which cannot be annealed out by post-implantation annealing treatments. Therefore it was not possible to reach conductivity values suitable for solar cell applications. For B implantation, consistently with previous work, the activation of B atoms has been achieved upon annealing thanks to a decrease of localized bandgap states. Also, boron can penetrate deeper and reach high concentration at the a-Si:H/c-Si interface, which requires higher temperature annealing compared to P implantation to recover a good interface passivation quality. Nevertheless, for a-Si:H layers of about 25 nm process conditions allowing similar properties to PECVD-doped (p) a-Si:H deposition (i.e. conductivity of 10-4 Ω-1cm-1 and interface passivation allowing i-VOC > 700 mV) have been obtained.A second study is dedicated to the study of a new cell concept, named silicon homo-heterojunction (HHJ) which comprise an additional homo-emitter (p+) c-Si at the emitter interface. The goal is to improve the interface passivation in order to increase the cell efficiency. Numerical simulations have evidenced an improved fill factor in this cell that is attributed to a field effect passivation improvement and a decrease in series resistance related to band bending changes in the a-Si:H layers. The need of sufficiently shallow and strongly doped (> 5×1018 cm-3) emitter has also been evidenced. Therefore, ion implantation has been used to develop suitable boron profiles and both the increase in fill factor and the decrease in contact resistances have been obtained when the boron surface concentration is not too high. These improvements have been validated by processing HHJ solar cells that exhibit a fill factor improvement and an improved efficiency compared to SHJ cells. This achievement is a first proof of concept of the HHJ architecture.

abstract: Interconnection methods for IBC photovoltaic (PV) module integration have widely been explored yet a concrete and cost-effective solution has yet to be found. Traditional methods of tabbing and stringing which are still being used today impart increased stress on the cells, not to mention the high temperatures induced during the soldering process as well. In this work and effective and economical interconnection method is demonstrated, by laser welding an embossed aluminum (Al) electrode layer to screen-printed silver (Ag) on the solar cell. Contact resistivity below 1mΩ.cm2 is measured with the proposed design. Cross-sectional analysis of interfaces is conducted via Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDXS) methods. Typical laser weld phenomenon observed involves Al ejection at the entrance of the weld, followed by Al and Ag fusing together mid-way through the weld spot, as revealed by cross-sectional depth analysis. The effects of voltage and lamp intensity are also tested on the welding process. With the range of voltages tested, 240V seems to show the least process variability and the most uniform contact between Al and Ag layers, upon using an Ethylene-Vinyl Acetate (EVA) encapsulant. Two lamp intensities were also explored with a Polyolefin (POE) encapsulant with Al and Ag layers seen welded together as well. Smaller effect sizes at lamp 2 intensity showed better contact. A process variability analysis was conducted to understand the effects of the two different lamps on welds being formed. Lamp 2 showed a bi-modal size distribution with a higher peak intensity, with more pulses coupling into the sample, as compared to lamp 1. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2019

abstract: Semiconductor devices often face reliability issues due to their operational con- ditions causing performance degradation over time. One of the root causes of such degradation is due to point defect dynamics and time dependent changes in their chemical nature. Previously developed Unified Solver was successful in explaining the copper (Cu) metastability issues in cadmium telluride (CdTe) solar cells. The point defect formalism employed there could not be extended to chlorine or arsenic due to numerical instabilities with the dopant chemical reactions. To overcome these shortcomings, an advanced version of the Unified Solver called PVRD-FASP tool was developed. This dissertation presents details about PVRD-FASP tool, the theoretical framework for point defect chemical formalism, challenges faced with numerical al- gorithms, improvements for the user interface, application and/or validation of the tool with carefully chosen simulations, and open source availability of the tool for the scientific community. Treating point defects and charge carriers on an equal footing in the new formalism allows to incorporate chemical reaction rate term as generation-recombination(G-R) term in continuity equation. Due to the stiff differential equations involved, a reaction solver based on forward Euler method with Newton step is proposed in this work. The Jacobian required for Newton step is analytically calculated in an elegant way improving speed, stability and accuracy of the tool. A novel non-linear correction scheme is proposed and implemented to resolve charge conservation issue. The proposed formalism is validated in 0-D with time evolution of free carriers simulation and with doping limits of Cu in CdTe simulation. Excellent agreement of light JV curves calculated with PVRD-FASP and Silvaco Atlas tool for a 1-D CdTe solar cell validates reaction formalism and tool accuracy. A closer match with the Cu SIMS profiles of Cu activated CdTe samples at four different anneal recipes to the simulation results show practical applicability. A 1D simulation of full stack CdTe device with Cu activation at 350C 3min anneal recipe and light JV curve simulation demonstrates the tool capabilities in performing process and device simulations. CdTe device simulation for understanding differences between traps and recombination centers in grain boundaries demonstrate 2D capabilities. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

The evolution of the internet of things (IoT) opens the market opportunity for organic photovoltaic cells, especially for indoor applications where the lifetime of the organic cells is longer than outdoor. For example, IoT requires off-grid energy sources for many devices with low power consumption. In this work, new materials were tested as candidate components in the active layer of printed organic photovoltaics by fabrication of devices. The initial electrical performance of these devices and their stability over time were investigated by measurements of the current-voltage characteristics. Three selected active layers were further investigated with atomic force microscopy (AFM) measurements. The current-voltage measurements showed that the addition of a solvent additive to the active layer ink affects the initial electrical performance as well as the stability of the devices. The AFM measurements showed that the surface topography of the active layer was affected by the sort of solvent additive that was used. Three new electron acceptor material and two solvent additives showed promising electrical performance and stability.

Photovoltaics (PV) are a renewable energy technology experiencing rapidly increasing commercial adoption today. Nonetheless, many proposed PV applications still require higher efficiencies, lower costs and comparable reliability to currently available in commercial devices (typically made from silicon). To enable the rigorous study of a much wider range of materials and novel design concepts, particularly those based on compound thin films, Concentrated Photovoltaics (CPV), cells with bifaciality, a comprehensive modeling framework is developed to couple photon absorption, carrier transport, photon recycling, and thermal transport in PV devices. The universality of this framework manifest itself in approaching various PV related problems as follows: 1) exploring the novel design of wide-Eg GaInP solar cells as an intermediate step to enhance the efficiency of multijunction PV devices; 2) characterizing the open-circuit voltage (VOC) degradation in thin-film vapor liquid solid (TF-VLS) grown InP solar cell through combined device and circuit model for interpreting photoluminescence (PL) image; 3) establishing optic-electric-thermal coupled framework to assess and compare the passive cooling effect for Silicon CPV devices that employ porous soda-lime glass radiative cooler and conventional copper cooler respectively; 4) Investigating and formulating the analytic solution of the optimal design that minimizes combined optical shadowing loss and electrical resistive loss for two types of bifacial PV devices: a) interdigitated back contact (IBC) Silicon heterojunction (SHJ) solar cells and b) Copper Indium Gallium DiSelenide (CIGSe) solar cell with Al2O3 passivation; and 5) Constructing an Neural Network Autoen- coder (NNA) that compresses and reconstructs the J-V characteristics obtained from TCAD simulation and literature for rapid screening and automated classification.

CdTe an II-VI semiconductor has been a leading thin film photovoltaic material due to its near ideal bandgap and high absorption coefficient [1]. The typical thin film CdTe solar cells have been of the superstrate configuration with CdS (Eg-2.42eV) as the n-type heterojunction partner. Due to the relatively narrow bandgap of CdS, a wider bandgap n-type window layer has recently emerged as a promising substitute: alloys of MgyZn1-yO have been successfully used as the emitter or window layer. The benefits in the usage of MgyZn1-yO (MZO) are its tunable bandgap and wide optical spectrum on optoelectronic devices. Due to an increasing bandgap of the window layer, the carrier collection can be improved in the short wavelength range (<500 nm). In addition alloys of CdSexTe1-x (CST) have also been used in the absorber layer (i.e., CST/CdTe) for the fabrication of CdTe devices to improve the carrier collection and lifetime [2]. The lower bandgap of the CST alloy can lead to higher short-circuit current (JSC), but it can also result in lower open circuit voltage (VOC). Another critical aspect of the CdTe solar cell is the use of copper as a p-type dopant, which is typically incorporated in the cell during the fabrication of the back contact. The most challenging issue related to further advancing the CdTe solar cell efficiency is the relatively low level of p-type doping, which limits the VOC. Efforts to dope CdTe with group V dopants are yet to produce the desired results. ZnO has been used as an effective high resistivity transparent. When CdTe is deposited directly on sputtered ZnO, VOC of typically 500-600 mV is produced. Band alignment measurements indicate that a negative conduction band offset with CdS exists; alloying with MgO to produce MgyZn1-yO with a composition of y = 0.15 can produce a flat conduction band alignment with CdS. This material has an additional benefit for improving the energy bandgap of the MZO for better UV light transmission in the short wavelengths. By changing the magnesium content from y = 0 to 0.30 allowed researchers to make the tunable conduction band offset from a “cliff” to a “spike,” with both increased open-circuit voltage and fill factor as increasing magnesium compositions [3] — the bandgap gains as expected with increased magnesium composition. The large compositions (y > 0.30) of MgyZn1-yO cause the enormous spike result in S-kink in the IV measurement so that the FF decreases. Besides, due to the instability of MZO material, the fabrication process has to proceed carefully. The properties of CST films and cells were investigated as a function of Se composition (x), substrate temperature (TSUB), and ambient used during the CSS deposition. The higher ratio of Se in CST alloy causes the smaller grain structures and lower bandgap, which profoundly detrimental to the device performance (VOC). However, the CST can be deposited in various substrate temperatures and different inert ambient gas to improve the grain structure by utilizing the especial Close Space Sublimation (CSS) deposition system. Therefore, despite the fact that the CST (25% Se) has the optical bandgap (1.37eV), the improvement of grain structure can slightly increase the doping concentration and decrease the grain boundary (GBs) due to increased alloys grain size 3X larger, which is contributed to improving the VOC [4]. The study of higher ratio Se of CST alloy is significant to achieve the high efficiency polycrystalline CST/CdTe photovoltaic devices. The effect of Cu doping back contact in CdSexTe1-x (CST)/CdTe solar cells with varying amounts of Se (x) has been investigated. The Cu-based back contact was annealed at different thermal temperatures in order to vary the amount of Cu in-diffusion. Net p-type doping was found to increase as the back-contact annealing temperature increased. All cells exhibited a decrease in VOC with increased annealing temperature (i.e., higher Cu concertation), presumably due to a degradation of the lifetime with increased amounts of Cu [5]. However, cells with the highest Se composition appeared to exhibit a higher degree of tolerance to the amount of Cu – i.e., they exhibited a smaller loss in VOC with the increased amount of Cu. Extrinsic p-type doping of CdSeTe can be fabricated using two different experimental processes. Firstly, by using group I elements such as, Cu to substitute Cd, which is promising during the back contact process. Secondly, using group V (P, As, Sb) elements to substitute Te, and this is suitable for Cd-rich of intrinsic CdTe. Intrinsic CST alloy has lower hole density concentration as higher Se composition with limitation of the VOC. Thus, in order to increase the p-type net doping up to 1016 cm-3 the extrinsic P or As doping have been widely investigated recently. The research studies show the CST/CdTe devices lead to improve VOC up to 850 mV with higher hole density in higher Se compositions of As doped CST alloys. Nevertheless, the group V doped CdTe still cause the formation of compensating defects limits the upper boundary of dupability on the CdTe thin film solar cells. Even if a high hole density concentration is achieved for intrinsically-doped p-type CST/CdTe, it is believed the poor carrier lifetime in the CdTe side would still limit the VOC.