Passivating-contact technologies fabricated from polycrystalline-silicon (poly-Si) are increasingly considered by the crystalline silicon (c-S) PV industry to be key enablers towards record performance. This is largely thanks to their ability to provide excellent carrier collection and surface passivation, while being compatible with industrial scale production. Poly-Si based passivating contacts consist of a stack of an ultrathin silicon oxide (SiOx) film on the surface of crystalline silicon (c-Si), covered by a doped silicon film. Thin films of SiOx can be grown by several different methods: chemically, thermally, or via UV-ozone exposure. However, each of these methods presents challenges towards industrial implementation. Here, we report an alternative method to grow SiOx films using an in-situ plasma process, where we subsequently deposit the doped poly-Si layer in the same process chamber by plasma enhanced chemical vapor deposition (PECVD). This process presents several advantages, such as ease of fabrication, inherently single-side oxide growth and poly-Si deposition, and the combined deposition in one chamber, lowering capital expenditure. Subsequently, we studied the structure of the SiOx films and the doped poly-Si(p+) capping layers using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) in order to determine the films’ elemental composition, and the band alignment at the semiconductor/oxide interfaces. A less p-type polysilicon was observed grown on top of a wet SiOx/c-Si with the origin tentatively attributed to depletion of the boron dopant via pin holes evidenced by AFM. A surface photo-voltage (SPV) was observed by XPS under in-situ light bias (AM 1.5) and a representation of the band alignment of the c-Si/SiOx/p-polysilicon under illumination is derived. The SPV was attributed to the photo accumulation of holes at the p-polysilicon and a splitting of quasi-fermi levels with its magnitude correlated to the device measured iVoc . Finally, a valuable application for this contact technology is the integration of silicon with perovskite solar cells, in the so-called monolithic tandem configuration. This approach is very promising to develop a new generation of PV with unmatched performances. Here, poly-Si contacts offer a variety of advantages, thanks to their broader material selection and to the stability at high processing temperature.

No description available.

In-vessel tungsten components of a future demonstration fusion reactor are likely to be operated in the material's non-ductile regime. Assessment of the components' reliability is not possible with current ductile design codes or through experimental qualification. There is therefore an urgent need for non-ductile assessment procedures. One such approach currently considered is Weibull's weakest link theory, which is based on linear-elastic fracture mechanics and has its origins in ceramics. A full assessment of its validity has been performed, and the challenge of obtaining irradiated material data addressed. Bend tests at the macroscopic scale confirm previous findings that the scatter in strength of pure tungsten follows a two-parameter Weibull distribution, provided the material fractures within its elastic regime. However, tests conducted over a range of specimen sizes reveal the technique's shortcomings in accurately predicting the material's size effect in fracture, questioning its applicability to pure tungsten and also other brittle metallic materials. Fracture strength tests conducted at the micrometre scale through cantilever bending have addressed the challenge of obtaining irradiated material data. An ultra-fine grained self-passivating tungsten alloy, considered as an alternative contender to tungsten for in-vessel components, is shown to fracture within its linear-elastic regime at the microscopic scale. A reliable and repeatable measurement of its strength of approximately 5.9 GPa is obtained. The scatter in measurements is shown to be greater than random errors, and to be described well by a two-parameter Weibull distribution. Cantilever tests conducted over a range of specimen sizes reveal a strong size effect (4.3 - 9.0 GPa), which is accurately predicted by Weibull's weakest link theory. Ion implantations, conducted in the tungsten alloy to mimic neutron induced elastic collision damage, result in a statistically confirmed drop (6 %) in cantilever measured fracture strength at low doses (0.7 dpa), and an increase (9-16 %) at higher doses (7 dpa).The cantilever test technique is therefore suitable for the measurement of ion and neutron irradiation effects on the material's fracture strength. Provided a full validation of Weibull's weakest link theory strength extrapolation from the micro- to macroscopic scale is realised on a future heterogeneity free material batch, irradiated material data obtained from cantilever tests could be used to assess the reliability of in-vessel components fabricated from a self-passivating tungsten alloy, and fill the current gap in non-ductile design assessment procedures.

Submitted by Rafael Santana (rafael.silvasantana@ufpe.br) on 2017-08-01T17:34:40Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) LEITE, S, S - Dissertação de Mestrado - 2013.pdf: 1926842 bytes, checksum: 88836c854c84c6018d77c2cccbdfd82f (MD5) / Made available in DSpace on 2017-08-01T17:34:40Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) LEITE, S, S - Dissertação de Mestrado - 2013.pdf: 1926842 bytes, checksum: 88836c854c84c6018d77c2cccbdfd82f (MD5) Previous issue date: 2013-08-30 / CAPES / Esta dissertação tem por objeto a assistência social na conjuntura brasileira recente. Tratase de um trabalho que pretende avançar no descortinamento dos determinantes e determinações que envolvem a expansão da assistência social na atualidade do Brasil. Objetiva portanto analisar as particularidades e tendências dessa política social no âmbito dos governos Lula. Para tanto, o processo investigativo privilegiou a realização de revisão bibliográfica acerca da temática da assistência, bem como a utilização de dados secundários apreendidos nos relatórios resultantes das conferências nacionais de assistência social realizadas no período referido. Destes últimos documentos foram analisados tanto os discursos governamentais proferidos no espaço das conferências, quanto as deliberações daí advindas para a área da assistência social. Também outros documentos, entre os quais, alguns oficiais dessa política social construídos nas gestões do governo Lula, compuseram o material investigativo dessa pesquisa (destaca-se aí a Política Nacional de Assistência Social e a Norma Operacional Básica do Sistema Único de Assistência Social). Os dados da investigação, avaliados a luz da perspectiva teórico-metodológica que concebe a realidade como lócus central de desvendamento do real, sendo este uma totalidade complexa, possibilitaram chegar aos seguintes resultados: a assistência social na conjuntura analisada manteve um substrato fortemente atrelado à perspectiva burguesa, presente tanto em algumas diretrizes teóricas, como na lógica das ações dessa política social. Contracena com essa constatação a recorrência da afirmação da assistência como direito social, sendo esta concepção defendida tanto a título de deliberação das conferências nacionais de assistência realizadas no governo Lula, como nos pronunciamentos governamentais analisados e, constantemente vinculada ao debate do enfrentamento da pobreza e da conformação de um novo “Estado social” no Brasil, embora tais discursos sejam também repletos de contradições por vezes reforçadoras de um viés conservador para a referida política social. Tais resultados, considerados a partir da sua inserção numa totalidade social, acenaram para a conclusão de que a assistência social assumiu nos governos Lula um papel de grande relevância na conformação da dominância burguesa, atrelada a uma ideologia de desenvolvimento que compatibiliza crescimento econômico e desenvolvimento social, estando implicados nesse processo tanto a utilização do potencial dessa política (principalmente dos seus programas de transferência de renda) para a consagração de um mercado consumidor interno, quanto para a passivização dos segmentos mais pauperizados da classe trabalhadora brasileira. / This dissertation has by object the social assistance in the conjuncture brazilian recent. It treats of a work that pretends to advance in the unveiling of the determinants and determinations that wrap the expansion of the social assistance in the actuality of Brazil. Objective therefore analyse the peculiarities and tendencies of this social politics in the field of the governments Lula. For so much, the process investigativo privileged the achievement of review bibliographic about the thematic of the assistance, as well as the utilisation of secondary data confiscated in the resultant reports of the national conferences of social assistance realised in the period referred. Of these last documents were analysed so much the governmental speeches proferidos in the space of the conferences, what the deliberations from here advindas for the area of the social assistance. Also other documents, between which, some officials of this social politics built in the managements of the government Lula, They composed the material investigative of this investigation (stands out here the National Politics of Social Assistance and the Basic Operational Norm of the Only System of Social Assistance. The data of the investigation, evaluated the light of the theoretical perspective-methodological that conceives the reality like lócus centrical of unveiling of the real, Being this a complex whole, made possible to arrive to the following results: The social assistance in the conjuncture analysed kept a substrate strongly atrelado to the perspective bourgeois, present so much in some theoretical guidelines, as in the logic of the actions of this social politics. contrascene With this finding the recorrência of the affirmation of the assistance like social, Being this conception defended so much to title of deliberations of the national conferences of assistance performed in the government Lula, as in the pronouncements governmental analysed an Constantly linked to the debate of the clash of the poverty and of the forming of a new “social State” in Brazil, although such speeches are also full of contradictions by times reinforcing of a viés conservative for the referred social politics. Such results, considered from his insertion in a social whole, waved for the conclusion that the social assistance it assumed in the governments Lula a paper of big importance in the forming of the dominance bourgeois, linked to an ideology of development that makes compatible economic growth and social development, being involved in this process so much the use of the potential of this politics (mainly of his programs of transfer of income) for the consecration of a market internal consume how much for the passivating of the segments more pauperized of the Brazilian hard-working class.

Silicon photovoltaic (PV) is a promising solution for energy shortage and environmental pollution. We are experiencing an era when PV is exponentially increasing. Global cumulative installation had reached 380 GW in 2017. Among which, silicon-based PV productions share more than 90% market. Performance of the first two-generation commercial popular silicon solar cells - Al-BSF and PERC - are limited by metal/Si contacts, where interface defects significantly reduce the open-circuit voltage. In this context, full-area passivation concepts are proposed for c-Si solar cells, with expectation to enhance the efficiency via reducing carrier recombination loss at the contact regions. In this thesis, poly silicon on oxide (POLO) passivating contact is developed for high efficiency c-Si solar cells. We unveiled the working mechanisms of POLO cells and then optimized the device performance based on our conclusion. We use boiling nitric acid to oxidize c-Si surface, which is of significance to determine the POLO working mechanisms. Phosphorus and boron doped silicon films are deposited by plasma enhanced vapor deposition (PECVD) or low-pressure vapor deposition (LPCVD) followed by high temperature (>800°C) annealing. SiOx structural evolution process under different annealing temperature was observed and the corresponding effects on passivation have been elucidated. The carrier transport mechanisms in the POLO contact annealed at high temperature, e.g. 800°C  900°C, were explored. We unveil that carrier transport in POLO structure is a combination of tunneling and pinhole transport, but dominant at varied temperature regions. Phosphorus-doped n-type POLO contact is optimized by several parameters, such as doping concentration, film thickness, annealing temperature, film deposition temperature, film relaxation time during annealing process, etc. We successfully obtained minority carrier lifetime over 10ms and contact resistivity lower than 30 mΩ·cm2. Boron-doped p-type POLO contact is also optimized by changing the doping concentration and annealing temperature. Finally, further hydrogen passivation is applied to enhance p-type POLO contact passivation, achieving an iVoc>690 mV, J0 <5 fA/cm2 and contact resistivity 1.3 mΩ·cm2. With the optimized n-type and p-type POLO contacts, an efficiency over 18% is achieved on n-type c-Si solar cells with a flat front surface.

Renewable and clean energy is urgently needed to cope with the climate crisis. Photovoltaics (PV) has been the fastest growing technology in the clean energy market due to its low cost, and the abundance of solar energy. The capacity of silicon-based PV is rapidly expanding with evolving technologies. Passivating the solar cell’s electrical contacts is a widely accepted strategy for the PV industry to improve device power conversion efficiency (PCE). Polycrystalline silicon (Poly-Si) passivating contacts are one of the promising concepts in the emerging class of passivating contacts. In this dissertation, the passivation mechanism of Poly-Si passivating contacts is investigated. Moreover, the influence of dopant diffusion on the passivation quality is revealed. To address the side-effects of dopant diffusion, a thin buffer layer is inserted between the Poly-Si(p) layer and the $SiO_x$ layer. With such a buffer layer, the passivation of the Poly-Si passivating contact is improved, which in turn, enhances the device PCE. In addition to passivating contacts, this dissertation also explores carrier-selective contact of crystalline silicon (c-Si) and low work function metal – Li. Li is a very reactive metal which makes the fabrication process a challenge. To overcome such a challenge, the c-Si/ Li contact is fabricated by thermally decomposing stable $Li_3N$ powder instead of metal evaporation. The c-Si/Li contact shows an excellent electron-selective transport performance with a 0.39 eV energy barrier. Full-area Si/Li rear contact devices are fabricated, and >19% PCE and >80% fill factor are achieved. To accelerate the device optimization, a physical model embedded machine-learning approach is applied to transparent conductive oxide (TCO) materials optimization. In this work, empirical correlations between sputtering parameters and the deposited TCOs’ electrical properties are established. Then a Bayesian Parameter Estimation (BPE) algorithm is applied to learn the empirical model. With this BPE network, the TCOs’ electrical properties are successfully predicted with limited material characterizations. Thanks to the combination of BPE and a physical model network, the material optimization process is significantly accelerated. In summary, this dissertation explores different aspects to develop novel passivating and carrier-selective contacts for c-Si solar cells, and introduces an approach to accelerate the development processes.

Engineering : Chemical / M. Tech. (Chemical Engineering)

The electrochemical corrosion behavior of mild steel in apple, grape, mango, orange and the mixture of these juices was investigated in this study. Open circuit potential and weight loss analytical techniques were employed to establish the comparative corrosion rates of this material in the agro media over the interval of five days for a sixty-day immersion period at an ambient temperature. The chemical compositions of both mild steel and the agro media were determined to ascertain corrosion mechanism for the reaction. Polarization behavior of mild steel in the agro media were determined by Tafel extrapolation curves. The analysis showed that cathodic polarization curves were almost identical irrespective of the concentration of the various media while the anodic polarization curves exhibited varying active and passive corrosion behaviour due to passivating oxide films. The analysis of the results further showed that the corrosion rate of the metallic sample decreased with longer immersion periods which could be attributed to a gradual decline in acidity of these media as revealed by the pH results. Hence, the evolution of hydrogen gas and reduction of dissolved oxygen molecules from the reacting system were presumed to be major factors retarding corrosion of the solution involved. Similarly, the presence of suspended particles on the surface of the test steel sample could have also led to the impediment to corrosion rates from the surrounding atmosphere. Microscopic analysis of the corroded mild steel specimens revealed uniform and localized corrosion with Fe2O3 spotted as the main corrosion product. The result obtained from the electrochemical study showed that the corrosion rate of the mild steel sample was highest in the orange medium (1.53mm/yr), followed by grape medium (1.40mm/yr), mixture of these media (0.67mm/yr), mango medium (0.40mm/yr) while the metal corroded least in the apple medium (0.30mm/yr) over the duration of immersion. / Engineering : Chemical / M. Tech. (Chemical Engineering)

Aufkonversions-Nanokristalle (UCNC) zeichnen sich als einzigartige Lumineszenzreporter aus, die Nah-infrarotes Anregungslicht in Photonen höherer Energie umwandeln. Für die gezielte Anpassung von Eigenschaften, bedarf es ein tiefes Verständnis der Prozesse der Aufwärtskonversionslumineszenz (UCL) und deren Abhängigkeit von Material und Partikeldesign. Diese Doktorarbeit untersucht die UCL-Prozesse von Yb3+,Er3+ dotierten SrF2-UCNC und zielt darauf ab, die UCL-Eigenschaften der bisher unterschätzten kubischen Wirtsgitter zu verstehen und zu steigern. Hierbei wird die fluorolytische Sol-Gel-Synthese als neuartige Syntheseroute für UCNC vorgestellt. Vorteile wie ausgezeichnete Reproduzierbarkeit, viele Freiheitsgrade bei der Temperaturbehandlung und Partikelgestaltung werden anhand von SrF2 UCNC demonstriert. Die UCNC wurden mittels UCL-Spektren, UCL-Quantenausbeuten, leistungsdichte-abhängiger relativer spektraler Verteilung sowie der Lumineszenzabklingkinetiken unter Einbeziehung kristalliner Eigenschaften wie der Kristallphase, der Kristallitgröße, der Gitterparameter und der Teilchengröße untersucht. Die Abhängigkeit der UCL-Eigenschaften von der Dotierungsmenge wurde mit einer umfassenden Dotierungsreihe beschrieben und der optimale Dotierungsbereich (Yb3+,Er3+) von kleinen, ungeschalten SrF2-UCNC eingegrenzt. Bei der Studie dotierter Kerne mit passivierenden Schalen wurde der Einfluss von Temperaturbehandlung auf die UCL-Mechanismen und die Kern-Schale-Vermischung untersucht. Anhand von unterschiedlich kalzinierten UCNC Pulvern wurde die Empfindlichkeit der UCL gegenüber der Änderung kristalliner Eigenschaften, wie Kristallphase, Kristallinität, und Kristallitgröße betrachtet. Zusammen liefern die Dotierungs-, die Kern-Schale- und die Kalzinierungsstudie wertvolle Einblicke in das gitterspezifische Verhalten der UCL-Eigenschaften als Funktion der Energiemigration und der Kristalleigenschaften. / Upconversion nanocrystals (UCNC) represent a unique type of luminescence reporters that convert near-infrared excitation light into higher energy photons. Tailoring UCNC with specific luminescence properties requires an in-depth understanding of upconversion luminescence (UCL) processes and their dependence on material and particle design. This Ph.D. thesis focuses on the UCL processes of Yb3+,Er3+ doped SrF2-UCNC and aims to understand and enhance the UCL properties of the previously underestimated cubic host lattices. Herein, fluorolytic sol-gel synthesis is introduced as a novel synthetic route for UCNC. Advantages such as excellent reproducibility, high flexibility in temperature treatment and particle design are demonstrated using SrF2 UCNC. The UCNC were characterized by UCL spectra, UCL quantum yields, excitation power density-dependent relative spectral distribution, and luminescence decay kinetics involving crystalline properties such as crystal phase, crystallite size, lattice parameters, and particle size. The dependence of UCL properties on doping amount was described in a comprehensive doping study, and the optimal doping range (Yb3+,Er3+) of small, unshelled SrF2-UCNC was identified. In a core-shell study of doped core UCNC with passivating shells, the influence of temperature treatment on UCL mechanisms and core-shell mixing was investigated. Further, using different calcined UCNC powders, the sensitivity of UCL to the change of crystalline properties, such as crystal phase, crystallinity, and crystallite size, was assessed. Together, the doping, core-shell, and calcination studies provide valuable insight into the lattice-specific behavior of UCL properties as a function of energy migration and crystal properties.