Dissertations / Theses on the topic 'Thin set'

The main topic of the Thesis is the optimization of the compliance of thin elastic structures.The problem consists in finding the most robust configurations, when an infinitesimal amount of elastic material is subjected to a fixed force, and contained within a region having infinitesimal volume.The resistance to a load can be measured by computing a shape functional, the compliance, in which the shape represents the volume occupied by the elastic material. Thus we are led to study a minimization problem of a shape functional, under suitable constraints.In particular, we treat the case in which the design region is a thin rod, represented by a cylinder with infinitesimal cross section. The study finds its motivation in engineering problems: thin structures are very convenient to be used in practical applications.The approach we adopt draws inspiration from some recent works by I. Fragalà, G. Bouchitté and P. Seppecher, in which the authors deal with the case of thin elastic plates [G. Bouchitté, I. Fragalà, P. Seppecher: Structural optimization of thin plates: the three dimensional approach., Arch. Rat. Mech. Anal. (2011)]. We point out that these two problems are not merely technical variants one of the other, due to the substantial difference between the limit passages 3d-1d and 3d-2d, namely from 3 to 1 and from 3 to 2 dimensions.The study of optimal configurations led us to face another interesting variational problem: actually to establish whether homogenization phenomena occur in bars in pure torsion regime turns out to be equivalent to solve a nonstandard free boundary problem in the plane. This new problem is very challenging and, besides the link with torsion rods, it has mathematical interest in itself. One of the tools which can be employed to attack the problem is shape derivative for minima of integral functionals. The theory of shape derivatives is a widely studied topic (see e.g. the monograph by A. Henrot and M.Pierre Variations et Optimisation de Formes. Une Analyse Géométrique, Springer Berlin (2005), and the references therein), but the approach we propose in new and relies on assumptions which are weaker that the classical ones.

In many applications, one would like to perform calculations on smooth manifolds of dimension d embedded in a high-dimensional space of dimension D. Often, a continuous description of such manifold is not known, and instead it is sampled by a set of scattered points in high dimensions. This poses a serious challenge. In this thesis, we approximate the point-set manifold as an overlapping set of smooth parametric descriptions, whose geometric structure is revealed by statistical learning methods, and then parametrized by meshfree methods. This approach avoids any global parameterization, and hence is applicable to manifolds of any genus and complex geometry. It combines four ingredients: (1) partitioning of the point set into subregions of trivial topology, (2) the automatic detection of the local geometric structure of the manifold by nonlinear dimensionality reduction techniques, (3) the local parameterization of the manifold using smooth meshfree (here local maximum-entropy) approximants, and (4) patching together the local representations by means of a partition of unity. In this thesis we show the generality, flexibility, and accuracy of the method in four different problems. First, we exercise it in the context of Kirchhoff-Love thin shells, (d=2, D=3). We test our methodology against classical linear and non linear benchmarks in thin-shell analysis, and highlight its ability to handle point-set surfaces of complex topology and geometry. We then tackle problems of much higher dimensionality. We perform reduced order modeling in the context of finite deformation elastodynamics, considering a nonlinear reduced configuration space, in contrast with classical linear approaches based on Principal Component Analysis (d=2, D=10000's). We further quantitatively unveil the geometric structure of the motility strategy of a family of micro-organisms called Euglenids from experimental videos (d=1, D~30000's). Finally, in the context of enhanced sampling in molecular dynamics, we automatically construct collective variables for the molecular conformational dynamics (d=1...6, D~30,1000's).

A numerical treatment of non-linear higher-order geometric evolution equations with the level set and the finite element method is presented. The isotropic, weak anisotropic and strong anisotropic situation is discussed. Most of the equations considered in this work arise from the field of thin film growth. A short introduction to the subject is given. Four different models are discussed: mean curvature flow, surface diffusion, a kinetic model, which combines the effects of mean curvature flow and surface diffusion and includes a further kinetic component, and an adatom model, which incorporates in addition free adatoms. As an introduction to the numerical schemes, first the isotropic and weak anisotropic situation is considered. Then strong anisotropies (non-convex anisotropies) are used to simulate the phenomena of faceting and coarsening. The experimentally observed effect of corner and edge roundings is reached in the simulation through the regularization of the strong anisotropy with a higher-order curvature term. The curvature regularization leads to an increase by two in the order of the equations, which results in highly non-linear equations of up to 6th order. For the numerical solution, the equations are transformed into systems of second order equations, which are solved with a Schur complement approach. The adatom model constitutes a diffusion equation on a moving surface. An operator splitting approach is used for the numerical solution. In difference to other works, which restrict to the isotropic situation, also the anisotropic situation is discussed and solved numerically. Furthermore, a treatment of geometric evolution equations on implicitly given curved surfaces with the level set method is given. In particular, the numerical solution of surface diffusion on curved surfaces is presented. The equations are discretized in space by standard linear finite elements. For the time discretization a semi-implicit discretization scheme is employed. The derivation of the numerical schemes is presented in detail, and numerous computational results are given for the 2D and 3D situation. To keep computational costs low, the finite element grid is adaptively refined near the moving curves and surfaces resp. A redistancing algorithm based on a local Hopf-Lax formula is used. The algorithm has been extended by the authors to the 3D case. A detailed description of the algorithm in 3D is presented in this work
In der Arbeit geht es um die numerische Behandlung nicht-linearer geometrischer Evolutionsgleichungen höherer Ordnung mit Levelset- und Finite-Elemente-Verfahren. Der isotrope, schwach anisotrope und stark anisotrope Fall wird diskutiert. Die meisten in dieser Arbeit betrachteten Gleichungen entstammen dem Gebiet des Dünnschicht-Wachstums. Eine kurze Einführung in dieses Gebiet wird gegeben. Es werden vier verschiedene Modelle diskutiert: mittlerer Krümmungsfluss, Oberflächendiffusion, ein kinetisches Modell, welches die Effekte des mittleren Krümmungsflusses und der Oberflächendiffusion kombiniert und zusätzlich eine kinetische Komponente beinhaltet, und ein Adatom-Modell, welches außerdem freie Adatome berücksichtigt. Als Einführung in die numerischen Schemata, wird zuerst der isotrope und schwach anisotrope Fall betrachtet. Anschließend werden starke Anisotropien (nicht-konvexe Anisotropien) benutzt, um Facettierungs- und Vergröberungsphänomene zu simulieren. Der in Experimenten beobachtete Effekt der Ecken- und Kanten-Abrundung wird in der Simulation durch die Regularisierung der starken Anisotropie durch einen Krümmungsterm höherer Ordnung erreicht. Die Krümmungsregularisierung führt zu einer Erhöhung der Ordnung der Gleichung um zwei, was hochgradig nicht-lineare Gleichungen von bis zu sechster Ordnung ergibt. Für die numerische Lösung werden die Gleichungen auf Systeme zweiter Ordnungsgleichungen transformiert, welche mit einem Schurkomplement-Ansatz gelöst werden. Das Adatom-Modell bildet eine Diffusionsgleichung auf einer bewegten Fläche. Zur numerischen Lösung wird ein Operatorsplitting-Ansatz verwendet. Im Unterschied zu anderen Arbeiten, die sich auf den isotropen Fall beschränken, wird auch der anisotrope Fall diskutiert und numerisch gelöst. Außerdem werden geometrische Evolutionsgleichungen auf implizit gegebenen gekrümmten Flächen mit Levelset-Verfahren behandelt. Insbesondere wird die numerische Lösung von Oberflächendiffusion auf gekrümmten Flächen dargestellt. Die Gleichungen werden im Ort mit linearen Standard-Finiten-Elementen diskretisiert. Als Zeitdiskretisierung wird ein semi-implizites Diskretisierungsschema verwendet. Die Herleitung der numerischen Schemata wird detailliert dargestellt, und zahlreiche numerische Ergebnisse für den 2D und 3D Fall sind gegeben. Um den Rechenaufwand gering zu halten, wird das Finite-Elemente-Gitter adaptiv an den bewegten Kurven bzw. den bewegten Flächen verfeinert. Es wird ein Redistancing-Algorithmus basierend auf einer lokalen Hopf-Lax Formel benutzt. Der Algorithmus wurde von den Autoren auf den 3D Fall erweitert. In dieser Arbeit wird der Algorithmus für den 3D Fall detailliert beschrieben

A numerical treatment of non-linear higher-order geometric evolution equations with the level set and the finite element method is presented. The isotropic, weak anisotropic and strong anisotropic situation is discussed. Most of the equations considered in this work arise from the field of thin film growth. A short introduction to the subject is given. Four different models are discussed: mean curvature flow, surface diffusion, a kinetic model, which combines the effects of mean curvature flow and surface diffusion and includes a further kinetic component, and an adatom model, which incorporates in addition free adatoms. As an introduction to the numerical schemes, first the isotropic and weak anisotropic situation is considered. Then strong anisotropies (non-convex anisotropies) are used to simulate the phenomena of faceting and coarsening. The experimentally observed effect of corner and edge roundings is reached in the simulation through the regularization of the strong anisotropy with a higher-order curvature term. The curvature regularization leads to an increase by two in the order of the equations, which results in highly non-linear equations of up to 6th order. For the numerical solution, the equations are transformed into systems of second order equations, which are solved with a Schur complement approach. The adatom model constitutes a diffusion equation on a moving surface. An operator splitting approach is used for the numerical solution. In difference to other works, which restrict to the isotropic situation, also the anisotropic situation is discussed and solved numerically. Furthermore, a treatment of geometric evolution equations on implicitly given curved surfaces with the level set method is given. In particular, the numerical solution of surface diffusion on curved surfaces is presented. The equations are discretized in space by standard linear finite elements. For the time discretization a semi-implicit discretization scheme is employed. The derivation of the numerical schemes is presented in detail, and numerous computational results are given for the 2D and 3D situation. To keep computational costs low, the finite element grid is adaptively refined near the moving curves and surfaces resp. A redistancing algorithm based on a local Hopf-Lax formula is used. The algorithm has been extended by the authors to the 3D case. A detailed description of the algorithm in 3D is presented in this work.
In der Arbeit geht es um die numerische Behandlung nicht-linearer geometrischer Evolutionsgleichungen höherer Ordnung mit Levelset- und Finite-Elemente-Verfahren. Der isotrope, schwach anisotrope und stark anisotrope Fall wird diskutiert. Die meisten in dieser Arbeit betrachteten Gleichungen entstammen dem Gebiet des Dünnschicht-Wachstums. Eine kurze Einführung in dieses Gebiet wird gegeben. Es werden vier verschiedene Modelle diskutiert: mittlerer Krümmungsfluss, Oberflächendiffusion, ein kinetisches Modell, welches die Effekte des mittleren Krümmungsflusses und der Oberflächendiffusion kombiniert und zusätzlich eine kinetische Komponente beinhaltet, und ein Adatom-Modell, welches außerdem freie Adatome berücksichtigt. Als Einführung in die numerischen Schemata, wird zuerst der isotrope und schwach anisotrope Fall betrachtet. Anschließend werden starke Anisotropien (nicht-konvexe Anisotropien) benutzt, um Facettierungs- und Vergröberungsphänomene zu simulieren. Der in Experimenten beobachtete Effekt der Ecken- und Kanten-Abrundung wird in der Simulation durch die Regularisierung der starken Anisotropie durch einen Krümmungsterm höherer Ordnung erreicht. Die Krümmungsregularisierung führt zu einer Erhöhung der Ordnung der Gleichung um zwei, was hochgradig nicht-lineare Gleichungen von bis zu sechster Ordnung ergibt. Für die numerische Lösung werden die Gleichungen auf Systeme zweiter Ordnungsgleichungen transformiert, welche mit einem Schurkomplement-Ansatz gelöst werden. Das Adatom-Modell bildet eine Diffusionsgleichung auf einer bewegten Fläche. Zur numerischen Lösung wird ein Operatorsplitting-Ansatz verwendet. Im Unterschied zu anderen Arbeiten, die sich auf den isotropen Fall beschränken, wird auch der anisotrope Fall diskutiert und numerisch gelöst. Außerdem werden geometrische Evolutionsgleichungen auf implizit gegebenen gekrümmten Flächen mit Levelset-Verfahren behandelt. Insbesondere wird die numerische Lösung von Oberflächendiffusion auf gekrümmten Flächen dargestellt. Die Gleichungen werden im Ort mit linearen Standard-Finiten-Elementen diskretisiert. Als Zeitdiskretisierung wird ein semi-implizites Diskretisierungsschema verwendet. Die Herleitung der numerischen Schemata wird detailliert dargestellt, und zahlreiche numerische Ergebnisse für den 2D und 3D Fall sind gegeben. Um den Rechenaufwand gering zu halten, wird das Finite-Elemente-Gitter adaptiv an den bewegten Kurven bzw. den bewegten Flächen verfeinert. Es wird ein Redistancing-Algorithmus basierend auf einer lokalen Hopf-Lax Formel benutzt. Der Algorithmus wurde von den Autoren auf den 3D Fall erweitert. In dieser Arbeit wird der Algorithmus für den 3D Fall detailliert beschrieben.

This paper examines the extent to which thin markets in conjunction with tax induced kinks in the budget constraint cause consumer demand to be skewed. To illustrate the principles I focus on the demand for owner-occupied housing. Housing units are indivisible and heterogeneous while tastes for housing are at least partly idiosyncratic, causing housing markets to be thin. In addition, prior to 1998, capital gains tax provisions introduced a sharp kink in the budget constraint of existing owner-occupiers in search of a new home: previous homeowners under age 55 paid no capital gains tax if they bought up, but were subject to capital gains tax if they bought down. I first characterize the economic conditions under which households err on the up or down side when choosing a home in the presence of a thin market and a kinked budget constraint. I then specify an empirical model that takes such effects into account. Results based on Monte Carlo experiments indicate that failing to allow for skewness in the demand for housing leads to biased estimates of the elasticities of demand when such skewness is actually present. In addition, estimates based on American Housing Survey data suggest that such bias is substantial: controlling for skewness reduces the price elasticity of demand among previous owner-occupiers from 1.6 to 0.3. Moreover, 58% of previous homeowners err on the up while only 42% err on the down side. Thus, housing demand is skewed.
Ph. D.

Theater
M.F.A.
Urinetown: The Musical is a meta-theatrical musical that addresses issues such as sustainability, corruption, and human intentions. For my design of Urinetown: The Musical, I sought to poetically represent the world of the play without interfering with the story telling. From start to end, this is my process.
Temple University--Theses

The consequence of many psychiatric and neurodegenerative disorders, such as Parkinson's disease and schizophrenia, is an impairment in ‘executive functioning'; an umbrella term for several cognitive processes, including the focussing and shifting of attention and the inhibition of responding. The ability to form an ‘attentional set' involves learning to discriminate qualities of a multidimensional cue, and to subsequently learn which quality is relevant, and therefore predictive of reward. According to recent research, the subthalamic nucleus (STN) and possibly the adjacent zona incerta (ZI) may mediate the formation of attentional set. Dysregulation of the STN as a result of Parkinson's disease contributes to characteristic motor symptoms, and whilst deep-brain stimulation of this region may treat gross motor impairments, it may also impair cognition. The work in this thesis aimed to expand our understanding of the mechanisms of attentional set-formation, and the role of the STN in this process. This thesis evaluates new methods for examining set-formation in the attentional set-shifting task; rather than inferring this behaviour solely from the cost of shifting set, modifications to the task design in Chapters 3 & 4 explored several hypotheses designed to exploit a deficit in this behaviour. Chapter 6 revealed that inhibition of this region with designer receptors leads to a disruption in attentional selectivity, which compromises the ability to form an attentional set. This manifested as an inability to parse relevant information from irrelevant, and instead, animals learned the stimuli holistically. The findings in this thesis also suggested that reversal and attentional shifting processes do not operate independently, but rather in a hierarchy, and that consequently, the STN is a region that may be crucial in selecting appropriate responses during associative learning that leads to the formation of an attentional set.

Thesis (M.S. in Information Systems and Operations)--Naval Postgraduate School, June 2006.
Thesis Advisor(s): Douglas Brinkley. "June 2006." Includes bibliographical references (p. 85). Also available in print.

The massive mortality rates of the American Civil War challenged mid-nineteenth century Americans' understandings and relationship with death. Faced with inadequate methods of individual identification and record-keeping that were unable to keep up with the overwhelming mobilization of both men and resources that the war demanded, many soldiers simply disappeared or were buried under a stone marked "unknown." Even soldiers who kept their names died far from home, away from family, in a manner that challenged nineteenth century traditions of death. These factors caused many soldiers to seek some manner of permanence to ensure that their name would not be forgotten following death. This project examines the ways in which soldiers used visual culture, particularly graffiti, drawings, and studio photographs, to find permanence amidst the destruction and death of war. By looking at the subjects of and the ways that soldiers used the visual culture they created, this thesis seeks to understand the value of visual culture as both an outlet for soldiers of the Civil War and as an invaluable source for historical research today. This project first explores the role of religion as both a subject of and an influence on visual culture. It then moves on to examine how soldiers used visual culture as a means finding permanence, including as a means of claiming a place in and piece of the war and as a form of memorialization. By examining the power of visual culture for finding permanence, this project provides insight into the ways in which soldiers sought to remember each other and their own experiences while also adding to the human conversation on mortality.
Master of Arts

EDS is a tool for quantitative and qualitative analysis of the materials. In electron microscopy, the energy of the electrons determines the depth of the region where the X-rays come from. By varying the energy of the electrons, the depth of the region where the X-rays come from can be changed. If a thin film is used as a specimen, different quantitative ratios of the elements for different electron energies can be obtained. Unique thickness of a specific film on a specific substrate gives unique energy-ratio diagram so the thickness of a thin film can be calculated by analyzing the fingerprints of the energy-ratio diagram of the EDS data obtained from the film.

Cu(In,Ga)Se₂ thin film solar cells have attracted a lot of interest because they have shown the highest achieved efficiency (21%) among thin film photovoltaic materials, long-term stability, and straightforward optical bandgap engineering by changing relative amounts of present elements in the alloy. Still, there are several opportunities to further improve the performance of the Cu(In,Ga)Se₂ devices. The interfaces between layers significantly affect the device performance, and knowledge of their chemical and electronic structures is essential in identifying performance limiting factors. The main goal of this research is to understand the characteristics of the Cu(In,Ga)Se₂-back contact interface in order to design ohmic back contacts for Cu(In,Ga)Se₂-based solar cells with a range of band gaps and device configurations. The focus is on developing either an opaque or transparent ohmic back contact via surface modification or introduction of buffer layers in the back surface.

In this project, candidate back contact materials have been identified based on modeling of band alignments and surface chemical properties of the absorber layer and back contact. For the first time, MoO₃ and WO ₃ transparent back contacts were successfully developed for Cu(In,Ga)Se ₂ solar cells. The structural, optical, and surface properties of MoO ₃ and WO₃ were optimized by controlling the oxygen partial pressure during reactive sputtering and post-deposition annealing. Valence band edge energies were also obtained by analysis of the XPS spectra and used to characterize the interface band offsets.

As a result, it became possible to illuminate of the device from the back, resulting in a recently developed "backwall superstrate" device structure that outperforms conventional substrate Cu(In,Ga)Se₂ devices in the absorber thickness range 0.1-0.5 µm. Further enhancements were achieved by introducing moderate amounts of Ag into the Cu(In,Ga)Se₂ lattice during the co-evaporation method resulting in a 9.7% cell (with 0.3 µm thickness) which has the highest efficiency reported for ultrathin CIGS solar cells to date.

In addition, sulfized back contacts including ITO-S and MoS ₂ are compared. Interface properties of different contact layers with (Ag,Cu)(In,Ga)Se₂ absorber layers with various Ga/(Ga+In) and Ag/(Ag+Cu) ratios are discussed based on the XPS analysis and thermodynamics of reactions.

The remarkable spread of the Internet has generated new opportunities for companies to promote themselves and communicate with their customers. The most used Internet service of finding information today is search engines (e.g. Google, Bing, Yahoo) were almost three million searches are preformed every minute worldwide. This trend has led to an importance for companies having their websites visible on these search engines. In addition, people tend to be more and more on the go and are searching with their smartphones instead of traditional personal computer (PC). Meaning that companies’ websites can be available for customers 24 hour a day, 7 days a week, independent on customers location. Consequently, companies have the possibilities to reach the attention of customers when they actually search for a product/service and are therefore most likely to purchase.   In this thesis a marketing strategy has been developed for managers regarding how to master this new communication tool, in order to reach success and competitive advantage. This strategy was developed by investigate in what characteristics and functions users desire when searching with their smartphone. Together with three hypotheses tested if the perceived user-friendliness, relative advantage and compatibility within mobile search have a positive affect on the intention of use this new technology.   In this bachelor thesis a survey was conducted among people in generation C living in Sweden, which resulted in 397 completed responses. The hypotheses were tested with use of a regression analysis and the findings were that the relative advantage and compatibility in mobile search has a positive affect on the intention of use. Therefore, this thesis suggests companies to develop their mobile search engine marketing strategy with the aim of delivering value and satisfying the customer. Moreover, this thesis recommend managers to keep their mobile website quick, relevant and making sure having valuable and informative information about their business in the description text visible to users in the search engine hit list.   Finally, the majority of the population does already act on this new market and it is therefore of great importance for companies satisfying these users in order to stay competitive.

North Sea Archaeologies traces the way people engaged with the North Sea from the end of the last ice age, around 10,000 BC, to the close of the Middle Ages, about AD 1500, drawing upon archaeological research from many countries, including the UK, Netherlands, Germany, Denmark, Sweden, Norway, Belgium, and France. It addresses topics which include the first interactions of people with the emerging North Sea, the origin and development of fishing, the creation of coastal landscapes, the importance of islands and archipelagos, the development of seafaring ships and their use by early seafarers and pirates, and the treatment of boats and ships at the end of their useful lives. The study offers a ‘maritime turn’ in Archaeology through the investigation of aspects of human behaviour that have been, to various extents, disregarded, overlooked, or ignored in archaeological studies of the land. The study concludes that the relationship between humans and the sea challenges the frequently invoked dichotomy between pre-modernity and modernity, since many ancient beliefs, superstitions, and practices linked to seafaring and engagement with the sea are still widespread in the modern era.

In this thesis CIGS absorber layers have been deposited by gas flow sputtering with an extra supply of selenium, a method that displays promise for large scale production because of its one-step sputtering route which deposits low energy particles in a high deposition rate. In this thesis a method was developed where selenium was added to the sputtering process inside the sputter chamber in a controllable manner and in larger amount than done in previous projects. A total of five samples were manufactured with altered evaporation temperatures and an extra supply of selenium which then were finalized to solar cells using the standard baseline process of the Ångström solar center. The characteristics of the CIGS layer and solar cells were analyzed by XRF, IV- and QE measurements. A cell with a conversion efficiency of 11.6 %, Jsc of 27.9 mA/cm2, Voc of 0.63 V and fill factor of 66.2 % was obtained on a 0.5 cm2 cell area without an antireflective coating. All samples contained cells with obtained efficiencies above 10 %, but over the whole samples the efficiencies varied considerably. The samples that were deposited with moderately large selenium evaporation provided the highest efficiencies with a relatively good homogeneity over the substrate. Results show a deficiency of copper in the CIGS films compared to the target composition. The copper content was lower than 70 % expressed in Cu/(Ga+In), which probably resulted in a low diffusion length for electrons, leading to limited cell efficiencies.  Through the duration of the thesis issues that concerned the power supply- and the controls to the substrate heaters as well as the control of the evaporation temperature during the depositions arose that required problem solving and needs to be resolved for the future progression of this work. The conclusions drawn from this thesis are that it is possible to vary the temperature of the selenium source and thereby control the amount of selenium that evaporates during the deposition process even though there is a lot of additional heating in the sputter chamber from both the substrate heaters and the sputter source which could affect the ability to control the amount of selenium being evaporated. That the most likely reason for the limited efficiencies is due to the low copper content in the CIGS films and that a larger amount of evaporated selenium compared to previous work did not result in higher obtained efficiencies.

Thesis (M.A. in Security Studies (Middle East, South Asia, Sub-Saharan Africa))--Naval Postgraduate School, September 2008.
Thesis Advisor(s): Russell, James. "September 2008." Description based on title screen as viewed on November 5, 2008. Includes bibliographical references (p. 65-71). Also available in print.

The solar cell technology based on Cu(In,Ga)Se2 (CIGS) thin-films provides a promising route to cost competitive solar electricity. The standard device structure is ZnO:Al/ZnO/CdS/CIGS/Mo films on a glass substrate, where the first three layers are n-type semiconductors with wide bandgaps, forming a pn-junction with the p-type CIGS absorber layer; the Mo layer serves as a back contact. This thesis deals with analysis of the generation and recombination of electron-hole pairs throughout the device. These processes determine the maximum output power: generation limits the current; recombination limits the voltage. The generation is calculated with an optical model based on complex refractive indices determined for the individual layers. The main features of the optical response of the solar cell can be reproduced with a specular model neglecting scattering. A model including ideally Lambertian scattering at the front and back surface of the CIGS absorber layer is introduced to investigate the possibility to maintain a high current generation with thin absorber layers. The result highlights the relatively poor optical performance of the Mo back contact. TiN and ZrN are explored as alternatives, and improved optical performance is experimentally demonstrated for both materials. The recombination analysis emphasizes that, in general, more than one recombination path of comparable magnitude are operative in parallel. For cells with absorber bandgap increasing from 1.0 eV (CuInSe2) to 1.7 eV (CuGaSe2), a relative increase of interface recombination is found. When these cells are subject to accelerated ageing, degradation is smallest for intermediate bandgaps; an explanation involving different sensitivity to decreased absorber band bending and activation of grain boundaries is suggested. The optical gain with ZrN back contacts is counteracted by increased back contact recombination and contact resistance, but an intermediate layer of MoSe2 is shown to alleviate these problems, allowing for an overall improved efficiency.

In this thesis, research on synthesis, structure and characterization of amorphous carbide-based thin films is presented. Crystalline and nanocomposite carbide films can exhibit properties such as high electrical conductivity, high hardness and low friction and wear. These properties are in many cases structure-related, and thus, within this thesis a special focus is put on how the amorphous structure influences the material properties. Thin films within the Zr-Si-C and Cr-C-based systems have been synthesized by magnetron sputtering from elemental targets. For the Zr-Si-C system, completely amorphous films were obtained for silicon contents of 20 at.% or higher. Modeling of these films, as well as experimental results suggest that the films exhibit a network-type structure where the bond types influence the material properties. Higher hardness and resistivity were observed with high amounts of covalent Si-C bonds. Several studies were performed in the Cr-C-based systems. Cr-C films deposited in a wide composition range and with substrate temperatures of up to 500 °C were found to be amorphous nanocomposites, consisting of amorphous chromium carbide (a-CrCx) and amorphous carbon (a-C) phases. The carbon content in the carbidic phase was determined to about 30-35 at.% for most films. The properties of the Cr-C films were very dependent of the amount of a-C phase, and both hardness and electrical resistivity decreased with increasing a-C contents. However, electrochemical analysis showed that Cr-C films deposited at higher substrate temperature and with high carbon content exhibited very high oxidation resistance. In addition, nanocomposite films containing Ag nanoparticles within an amorphous Cr-C matrix were studied in an attempt to improve the tribological properties. No such improvements were observed but the films exhibited a better contact resistance than the corresponding binary Cr-C films. Furthermore, electrochemical analyses showed that Ag nanoparticles on the surface affected the formation of a stable passive film, which would make the Cr-C/Ag films less resilient to oxidation than the pure Cr-C films.

The development of low cost, efficient photovoltaic devices is a major technological challenge which demands suitable materials and fabrication processes. Thin film polycrystalline heterojunction solar cells appear to be most appropriate with respect to cost and ease of manufacture, and it is anticipated that the next generation of photovoltaic devices will be based entirely on thin film technologies. Copper based ternary and multinary compounds are well established as exceptional semiconductors with potential applications in the fields of solar cells for both terrestrial and space applications, infra-red detectors, light emitting diodes etc. The chalcopyrite forms of these compounds have large absorption coefficients and exhibit superior radiation resistance. Among these compounds, CuInSe2 (CIS) and CuIn1-xGaSe2 (CIGS) have raised the most interest and recent thin film heterojunction photovoltaic devices based on these materials have achieved efficiencies of the order 15.5% and 16.9% respectively. The higher efficiencies realised in CIGS based devices is due to the fact that the band gap of the material can be adjusted to wards the optimum value (1.45eV) by the partial substitution of gallium for indium. In this work, thin films of both CIS and CIGS were deposited onto glass substrates by flash evaporation of the respective pre-reacted source materials. The substrate temperature was varied between room temperature and 200 degrees C. Two types of evaporation sources, a flat tungsten strip and a molybdenum twin chimney were used. The effect of the growth conditions on the film properties was observed. The structural, compositional and electro-optical properties were studied using a variety of analytical techniques including x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis with x-ray (EDAX), x-ray fluorescence (XRF), Rutherford backscattering spectroscopy (RBS), four point and thennal probe techniques, photoconductivity (PC) and photoacoustic spectroscopy (PAS). The as-grown films were found to have a columnar structure and a strong preferred orientation with the <112> plane parallel to the substrate. Results from EDAX, XRF and RBS indicated that the as-grown films were slightly deficient in selenium, otherwise the composition was comparable with that of the starting polycrystalline material. Electrical measurements revealed both n- and p-type conductivities with resistivity values in the range 10-2 to 106 gcm. The as-grown films were subsequently processed under several sets of conditions including vacuum, selenium, inert and forming gas ambients at different temperature and times. A two stage post-deposition heat treatment of the films was developed to improve the composition and crystal structure and to optimise the electro-optical properties. It was observed that the first annealing stage (in a selenium ambient) produced an excellent improvement in the composition of the film. An increase in the film grain size (to > 2pm) was observed when the films were subsequently annealed in a forming gas ambient. Significant improvements were also observed in the optical properties. The as-grown and annealed films were analysed using the PAS technique which revealed the existence of several donor and acceptor states originating from intrinsic defect levels. The results were compared with those obtained from single crystals. Photoconductivity measurements were also performed on the as-grown thin films.

Since the publication of Laura Mulvey's influential article 'Visual Pleasure and Narrative Cinema,' in which she identifies the pervasive presence of the male gaze in Hollywood cinema, scholars have sought to account for the female spectator in her paradigm of gendered vision. This thesis suggests that women writers have long debated the problem of the female spectator through literary depictions of the female artist. Women writers of the nineteenth and twentieth centuries'including Elizabeth Stuart Phelps, Edith Johnstone, and Virginia Woolf'recognized the power of the woman artist to undermine the trope of the male gazing subject and a passive female object. Examining Phelps's The Story of Avis (1877), Johnstone's A Sunless Heart (1894), and Woolf's To the Lighthouse (1927) illustrates how the woman artist's active vision disrupts Mulvey's 'active/male and passive/female' binary of vision. Phelps's painter-heroine Avis destabilizes the power of the male gaze not only by exerting her own vision, but also by acting as an active object to manipulate the way she is seen. Johnstone uses artist Gasparine to demonstrate the dangers of vision shaped by either aesthetic or political conventions, suggesting that even feminist idealism can promote the objectification of its heroines. Finally, Woolf redefines the terms of objectification through painter Lily Briscoe, whose vision imbues material objects with subjectivity, thereby going beyond the boundaries between male and female to blur the distinction between subject and object. Through their novels, Phelps, Johnstone, and Woolf suggest that depictions of human experience need to be radically re-thought in order to adequately represent the complexity of subjectivity.
M.A.
Department of English
Arts and Humanities
English MA

Theater
M.F.A.
This thesis documents a production of the French playwright Moliere's comedy Tartuffe at Temple Repertory Theater in Philadelphia. Set in Philadelphia's Main Line, this production provides an updated version of the play that comments on religion in contemporary America. Tartuffe is one of the best comedies written by Moliere. Banned for many years, it deranged people and touched a very tender spot in 17th century French society. America has a very different approach to religion than France. The many churches in existence make this country a deist democracy as opposed to the French secular state. The use of religion in politics, the "in God we trust" on the dollar bill, and seeing people pray in restaurants before their dinner have dictated this production be set in contemporary America. The use of clown techniques helped in finding American archetypes that would fit the characters. Directing choices, designers and actors created a world that tells the story of a rich American family threatened by the extremist religious discourse of a hypocrite. All these choices try to honor the heightened style of Moliere's language and, here, of Ranjit Bolt's very modern and fluid translation.
Temple University--Theses

En quelques années, l'efficacité des cellules solaires à base de Cu(In,Ga)Se2 (CIGS) est passée de 20% à 22.6%. La rapidité de ce développement montre que le CIGS est un matériaux idéal pour les technologies solaires en couches minces. Pourtant, le coût de production cette technologie doit encore être abaissé pour une meilleure compétitivité. La fabrication d'un module avec une couche CIGS plus fine permettrait d'augmenter la production d'une usine et de réduire sa consommation en métaux. Ce travail de thèse vise à réduire l'épaisseur du CIGS d'un standard de 2.0-2.5 µm à une épaisseur inférieure à 500 nm sans altérer les performances des cellules. Cependant, comme rapporté dans la littérature, nous avons observé une diminution des rendements, ce que nous avons analysé en détail en comparant simulations et caractérisations d'échantillons. Celle-ci est causée à la fois par une faible absorption de la lumière dans la couche de CIGS et par un impact important du contact arrière (fortes recombinaisons et faible réflectivité). Pour dépasser ces limites, nous démontrons à la fois théoriquement et expérimentalement que le contact arrière en molybdène peut être remplacé par un oxyde transparent conducteur couplé à un miroir métallique. Nous obtenons de cette manière de meilleurs rendements de cellules. Pour atteindre ce résultat, une optimisation du dépôt de CIGS a été nécessaire. De plus, nous prouvons qu'une couche d'oxyde perforée, insérée entre le CIGS et le contact arrière, limite les recombinaisons des porteurs de charges et réduit l'influence des courants parallèles. Au final, nous avons fabriqué une cellule avec un rendement de 10.7% sur SnO2:F passivé par Al2O3
In the past three years, record efficiency of Cu(In,Ga)Se2 (CIGS) based solar cells has improved from 20% up to 22.6%. These results show that CIGS absorber is ideal for thin-film solar cells, even if this technology could be more competitive with a lower manufacture cost. The fabrication of devices with thinner CIGS absorbers is a way to increase the throughput of a factory and to reduce material consumption. This PhD thesis aims to develop cells with a CIGS thickness below 500 nm instead of the conventional 2.0-2.5 µm. However, as reported in the literature, we observed a decrease in cell performance. We carefully analyzed this effect by the comparison between simulations and sample characterizations: it is attributed, on one hand, to a lack of light absorption in the CIGS layer and, on the other hand, to an increased impact of the back-contact (high recombination and low reflectivity). To resolve these problems, we demonstrated theoretically and experimentally that the use of an alternative back-contact, other than molybdenum, such as a transparent conducting oxide coupled with a light reflector, improves the cell efficiency. To achieve this result, an optimization of the CIGS deposition was necessary. Moreover, we proved that a porous oxide layer inserted between the CIGS and the back-contact limits the charge-carrier recombination and removes some parasitic resistance. Finally, an efficiency of 10.7% was achieved for a 480-nm-thick CIGS solar cell with a SnO2:F back-contact passivated with a porous Al2O3 layer

Vanadium dioxide (VO_2) is a material of particular interest due to its exhibited metal to insulator phase transition at 68°C that is accompanied by an abrupt and significant change in its electronic and optical properties. Since this material can exhibit a reversible drop in resistivity of up to five orders of magnitude and a reversible drop in infrared optical transmission of up to 80%, this material holds promise in several technological applications. Solid phase crystallization of VO_2 thin films was obtained by a post-deposition annealing process of a VO_{x,x approx 2} amorphous film sputtered on an amorphous silicon dioxide (SiO_2) layer. Scanning electron microscopy (SEM) and electron-backscattered diffraction (EBSD) were utilized to study the morphology of the solid phase crystallization that resulted from this post-deposition annealing process. The annealing parameters ranged in temperature from 300°C up to 1000°C and in time from 5 minutes up to 12 hours. Depending on the annealing parameters, EBSD showed that this process yielded polycrystalline vanadium dioxide thin films, semi-continuous thin films, and films of isolated single-crystal particles. In addition to these films on SiO_2, other VO_2 thin films were deposited onto a-, c-, and r-cuts of sapphire and on TiO_2(001) heated single-crystal substrates by pulsed-laser deposition (PLD). The temperature of the substrates was kept at ~500°C during deposition. EBSD maps and orientation imaging microscopy were used to study the epitaxy and orientation of the VO_2 grains deposited on the single crystal substrates, as well as on the amorphous SiO_2 layer. The EBSD/OIM results showed that: 1) For all the sapphire substrates analyzed, there is a predominant family of crystallographic relationships wherein the rutile VO_2{001} planes tend to lie parallel to the sapphire's {10-10} and the rutile VO_2{100} planes lie parallel to the sapphire's {1-210} and {0001}. Furthermore, while this family of relationships accounts for the majority of the VO_2 grains observed, due to the sapphire substrate's geometry there were variations within these rules that changed the orientation of VO_2 grains with respect to the substrate's normal direction. 2) For the TiO_2, a substrate with a lower lattice mismatch, we observe the expected relationship where the rutile VO_2 [100], [110], and [001] crystal directions lie parallel to the TiO_2 substrate's [100], [110], and [001] crystal directions respectively. 3) For the amorphous SiO_2 layer, all VO_2 crystals that were measurable (those that grew to the thickness of the deposited film) had a preferred orientation with the the rutile VO_2[001] crystal direction tending to lie parallel to the plane of the specimen. The use of transmission electron microscopy (TEM) is presented as a tool for further characterization studies of this material and its applications. In this work TEM diffraction patterns taken from cross-sections of particles of the a- and r-cut sapphire substrates not only solidified the predominant family mentioned, but also helped lift the ambiguity present in the rutile VO_2{100} axes. Finally, a focused-ion beam technique for preparation of cross-sectional TEM samples of metallic thin films deposited on polymer substrates is demonstrated.

Polycrystalline Cu(In,Ga)Se2 (CIGS) based thin-film solar cells achieve power-conversion efficiencies of almost 23% on the laboratory scale, one of the highest among thin-film solar cells. The aim of further CIGS research and development is to reach conversion efficiencies of 25%, which is currently the efficiency of the best single-crystalline Si based solar cells. To reach this goal, the factors limiting efficiency, e.g. non-radiative recombination of charge carriers, should be minimized. Such recombination processes may occur at line or planar defects present in the CIGS absorbers (among other interfaces, such as absorber and buffer layer). In the present study, the structure and composition of several defects as well as their evolution during the growth were investigated for an enhanced understanding. Highest efficiencies in CIGS solar cells are achieved, when the absorber is fabricated with a three-stage co-evaporation process. During the second stage of this process, Cu and Se are evaporated on the initially formed (In,Ga)2Se3 layer. The composition of the absorber becomes Cu-rich ([Cu]/([In] + [Ga]) > 1) during this stage. The change in composition leads to recrystallization, i.e. grain growth and defect annihilation, thus enabling higher conversion efficiencies. Therefore, it is crucial to investigate the recrystallization and the evolution of the microstructure at the second-stage of the CIGS growth. In the literature, two methods were suggested for this purpose: i) investigating the microstructural evolution of diffusion couples during a heating study; ii) ex-situ comparison of a growth-interrupted and a growth-finished sample. In the first part of this study, a Cu-poor ([Cu]/([In] < 1) CuInSe2 (CIS) precursor layer with a Cu2-xSe capping layer was prepared and heated in a scanning transmission electron microscope (STEM) to mimic the recrystallization. During the Cu diffusion from the Cu-rich Cu2-xSe phase into the Cu-poor CIS phase, the growth of defect-free grains towards the grains with closely-spaced planar defects (PDs) was monitored by low-angle annular dark-field (LAADF) imaging, whereas elemental depth profiles were analyzed by energy-dispersive X-ray spectroscopy (EDXS) before and after heating. The substantial impact of the Cu excess on the recrystallization was also indicated by an in-situ heating experiment of a Cu-poor CIS film without a Cu2-xSe layer on top, in which neither grain growth nor defect annihilation was detected. Monitoring of the recrystallization within the CIS absorber layers was performed for the first time by means of STEM and provided direct evidence for the currently accepted theory of the grain growth mechanism. In the second part, a CIGS absorber grown via co-evaporation was analyzed. During the growth, one piece of the sample was removed before the recrystallization at the second stage. For the remaining piece, the three-stage process was completed. The defect concentrations as well as the in-depth elemental analysis were performed by STEM-LAADF imaging and EDXS, respectively. Similar to the in-situ heating results, much larger grains with reduced linear/planar defect concentrations were detected in the absorber layer for which the growth had been completed. Although most of the structural defects were annihilated after the recrystallization, few structural defects were detected by LAADF imaging after the recrystallization, and even after the completion of the three-stage growth process. Further analyses were performed via aberration-corrected, high-resolution STEM (HR-STEM) in combination with electron energy-loss spectroscopy (EELS) to elucidate the nature of individual microstructural defects from various stages of the growth. HR-STEM and EELS results revealed the structure and chemistry of defects that were present in both growth-interrupted and growth-finished samples: Σ3-twin boundaries and stacking faults with stoichiometric elemental distribution; grain boundaries, tilt boundaries and dislocations with cation redistribution, i.e. Cu enrichment and In depletion. Stoichiometric inversion boundaries, Cu enriched ‘complex’ PDs and an extrinsic Frank partial dislocation were detected only in the growth-interrupted Cu-poor samples, whereas a ‘Cu2-xSe secondary phase’ was detected only in the growth-finished absorber layer. The present work provided direct insight into the recrystallization of CIGS absorbers and evolution of structural defects, as well as a thorough investigation of individual defects in CIGS absorbers.

This thesis is dealing with application of Correlative Probe and Electron Microscopy. All measurements were carried out by atomic force microscope LiteScope which is designed especially to be combined with electron microscopes. Advantages of Correlative AFM/SEM Microscopy are demonstrated on selected samples from field of nanotechnology and material science. Application of the correlative imaging was proposed and then realized particularly in case of low-dimensional structures and thin films. Further, this thesis deals with the possibility of combining Correlative AFM/SEM Microscopy with other integrated techniques of an electron microscope such as Focused Ion Beam and Energy Dispersive X-rays Spectroscopy.

Electrodeposition of diamond-like carbon (DLC) films was studied on different substrates using two different electrochemical methods. The first electrochemical method using a three-electrode system was studied to successfully deposit hydrogenated DLC films on Nickel, Copper and Brass substrates. The as-deposited films were characterized by scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR) and cyclic voltammetry (CV). A variety of experimental parameters were shown to affect the deposition process. The second electrochemical method was developed for the first time to deposit hydrogen free DLC films on Ni substrates through a two-electrode system. The as-deposited films were characterized by Raman spectroscopy and FTIR. According to Raman spectra, a high fraction of diamond nanocrystals were found to form in the films. Several possible mechanisms were discussed for each deposition method. An electrochemical method was proposed to deposit boron-doped diamond films for future work.

Cu(In,Ga)Se2 (CIGS) is one of the most promising semiconductor compounds for large-scale production of efficient, low-cost thin film solar cells, and several research institutes have announced their plans for CIGS production lines. But for the CIGS technology to become a commercial success, a number of issues concerning manufacturability, product definition, and long-term stability require further attention. Several studies indicate that CIGS-based modules are stable over many years in field operation. At the same time, it is shown in the present work that they may have difficulties in passing standard accelerated lifetime test procedures like the IEC 1646 damp heat test. In particular, CIGS modules are sensitive to humidity penetrating through the module encapsulation, which will increase the resistive losses in the front contact and cause severe corrosion of the back contact. It is also shown that cells experience degradation in both voltage and fill factor, and the causes of these effects are addressed. By concentrating the light falling onto a solar cell, the device will deliver a higher power output per illuminated absorber area, which can lower the electricity production costs. For CIGS-based solar cells, low-concentrated illumination could be an economically viable approach. In this work it is shown that the yearly performance of a photovoltaic system with CIGS modules can be significantly improved at a moderate cost by using parabolic aluminum mirrors as concentrating elements. However, in order to avoid detrimental power losses due to high temperatures and current densities, the modules need to be designed for the higher light intensity and to be sufficiently cooled during operation. A design where the front contact of the module is assisted by a metal grid has shown promising results, not only for concentrated illumination but also for normal operation. The benefits are enhanced window processing tolerance and throughput, as well as improved degrees of freedom of the module geometry.

The Cu(In,Ga)Se2-based thin film solar cell is a promising candidate for becoming one of the more important solar cell technologies in the near future. In order to realize such a development a significant reduced production cost of the Cu(In,Ga)Se2 (CIGS) layer is needed. This work shows a possible way towards such a reduction by increasing the deposition rate and decreasing the CIGS thickness with almost maintained device efficiency. Obtaining an improved device performance in CIGS-based solar cells by using an in-depth variation of the band gap has earlier been investigated without any clear conclusions. In this work an extensive experimental study of the beneficial effect of band gap profiling has been performed and firmly based conclusions have been made. For standard CIGS devices the band gap profiling can result in an improved efficiency of around 0.4 % units. This gain is related to improved field-assisted carrier collection. For reduced CIGS thicknesses the importance of a band gap profiling is enhanced, and at a CIGS thickness of 0.5 μm an efficiency gain of 2.5 % units is obtained, resulting in a 13.4 % efficient device. The main reason for the gain is passivation of the back contact, which becomes increasingly detrimental for the device performance as the CIGS thickness is reduced. With an optimized band gap profile the CIGS thickness can be reduced 3-4 times, with almost solely absorption related losses. The potential for increasing the deposition rate of co-evaporated CIGS layers is shown to be large. An increase of up to 10 times compared to commonly used deposition rates is possible with only minor losses in efficiency. By using band gap profiled thin CIGS layers deposited at high rates, the production from a single evaporation system can be increased up 30 times. Such an increase will lead to the needed reduction of the production cost of the complete solar cell module.

Thin-film solar cells based on Cu(In,Ga)Se2 contain a thin buffer layer of CdS in their standard configuration. In order to avoid cadmium in the device for environmental reasons, Cd-free alternatives are investigated. In this thesis, ZnO-based films, containing Mg or S, grown by atomic layer deposition (ALD), are shown to be viable alternatives to CdS. The CdS is an n-type semiconductor, which together with the n-type ZnO top-contact layers form the pn-junction with the p-type Cu(In,Ga)Se2. From device modeling it is known that a buffer layer conduction band (CB) position of 0-0.4 eV above that of the Cu(In,Ga)Se2 layer is consistent with high photovoltaic performance. For the Cu(In,Ga)Se2/ZnO interface this position is measured by photoelectron spectroscopy and optical methods to –0.2 eV, resulting in increased interface recombination. By including sulfur into ZnO, a favorable CB position to Cu(In,Ga)Se2 can be obtained for appropriate sulfur contents, and device efficiencies of up to 16.4% are demonstrated in this work. From theoretical calculations and photoelectron spectroscopy measurements, the shift in the valence and conduction bands of Zn(O,S) are shown to be non-linear with respect to the sulfur content, resulting in a large band gap bowing. ALD is a suitable technique for buffer layer deposition since conformal coverage can be obtained even for very thin films and at low deposition temperatures. However, deposition of Zn(O,S) is shown to deviate from an ideal ALD process with much larger sulfur content in the films than expected from the precursor pulsing ratios and with a clear increase of sulfur towards the Cu(In,Ga)Se2 layer. For (Zn,Mg)O, single-phase ZnO-type films are obtained for Mg/(Zn+Mg) < 0.2. In this region, the band gap increases almost linearly with the Mg content resulting in an improved CB alignment at the heterojunction interface with Cu(In,Ga)Se2 and high device efficiencies of up to 14.1%.

Chalcopyrite copper indium gallium diselenide (Cu(In,Ga)Se2 or CIGS) solar cells have achieved the highest laboratory power conversion efficiency among thin film PV technologies, and have the potential to be manufactured cost-effectively at large scale. In this thesis, detailed studies on the growth of CIGS solar cells by means of a pilot scale inline co-evaporation system have been performed to explore the beneficial deposition techniques to successfully transfer the research achievements into cost-effective industrial production. The work involved the development and optimisation of several new processes along with the design of the inline pilot scale system. Effects of the thin absorber layers (with thickness of ~1 μm) on material quality and the device performance have been investigated using (i) a two-step process with room temperature evaporated metal precursors, (ii) a three-stage process at constant substrate temperature and (iii) a three-stage process at varied substrate temperatures. Changing the copper and gallium compositions of the CIGS layers was observed to have a strong impact on the structural and electronic properties and hence on the performance of the solar cells. In one study, simple mechanical compression was employed on a batch of low quality porous CIGS films to significantly improve the surface morphological and the optoelectronic properties. Photoluminescence measurements revealed the band-to-tail defect-related recombination that was detrimental to the quality of as-grown and compressed films. In another study, rapid thermal processing (RTP) was applied to the low temperature Cu-In-Ga-Se precursor layers in an attempt to reduce the cost and optimise the reaction mechanism of the chalcopyrite material. After fine tuning of the process conditions, the CIGS layers with larger grain size exhibited power conversion efficiencies up to 10.8%. While these results are promising, the device performance was mainly limited by the low open circuit voltage and the low fill factor. It has been found that the Cu-In-Ga-Se precursors doped with low concentrations of sodium were beneficial for re-crystallisation of the CIGS films due to the reduced grain size and crystallinity of the precursors. An in-situ X-ray diffraction technique was used to investigate the phase evaluation in both Na-free and Na-doped Cu-In-Ga-Se precursor layers as a function of temperature. The results confirmed that the formation of both CIGS as well as secondary phases in the layer with the Na-doped precursors started at higher temperatures compared to Na-free precursors. It is therefore expected that further improvements in the solar cell efficiency might be achieved following this RTP and low-temperature precursor process.

Although the passive films that form on stainless steels have been extensively studied, the concentration depth profiles are not fully understood. Their thinness makes passive films hard to study, but angle-resolved X-ray photoelectron spectroscopy (ARXPS) is a non-destructive technique that can be used to obtain depth information. An iterative approach to deconvolute ARXPS measurements into depth profiles is discussed, and the chemistry of passive films on a molybdenum-containing 316L stainless steel is investigated. Bipolar electrochemistry, in which the sample is placed along an electric field created by two driving electrodes in an electrolyte, is investigated as a screening tool. It is shown that the method is useful to create corrosion gradients on 304 stainless steel, both under pitting and non-pitting conditions. Chromium carbide thin films were deposited by magnetron sputtering with a variety of deposition parameters on stainless steel, and subsequently analyzed. It is shown that these films present a promising material system for protective coatings to improve the corrosion resistance of stainless steels while also maintaining other useful properties, such as low interfacial contact resistance. Particular attention is given to the electrochemical evaluation of the films, whose high carbon concentrations necessitates different interpretations of the electrochemical results compared to for stainless steels.

The use of solar cells for energy production has indeed a bright future. Reduction of cost for fabrication along with increased efficiency are key features for a market boom, both achieved as a result of increased knowledge of the technology. Especially the thin film solar cell technology with absorbers made of Cu(In,Ga)Se2 (CIGS) is promising since it has proven high power conversion efficiency in combination with a true potential for low cost fabrication. In this thesis different recipes for fabrication of the Cu(In,Ga)Se2 absorber layer have been studied. The deposition technique used has been co-evaporation from elemental sources. For all depositions the substrate has been heated to a constant temperature of 500 ºC in order for the growing absorber to form a chalcopyrite phase, necessary for the photovoltaic functionality. The selenium has been evaporated such to always be in excess during depositions whereas the metal ratio Cu/(In+Ga) has been varied according to different recipes but always to be less than one at the end of the process. In the work emphasis has been on the radiative properties of the CIGS film during growth. The substrate heater has been temperature controlled to maintain the constant set temperature of the substrate, regardless of varying emitted power caused by changing surface emissivity. Depending on the growth conditions the emissivity of the growing film is changing, leading to a readable variation in the electrical power to the substrate heater. Since the thermal radiation from the substrate during growth has been of central focus, this has been studied in detail. For this reason the substrate has been treated as an optical stack composed of glass/Mo/Cu(In,Ga)Se2/CuxSe which determine the thermally radiated power by its emissivity. An optical model has been adopted to simulate the emissivity of the stack. In order to use the model, the optical constants for Cu(In,Ga)Se2 and CuxSe have been derived for the wavelength interval 2 μm to 20 μm. The simulation of the emissivity of the stack during CIGS growth agreed well with what has been seen for actual growth. Features of the OP-signal could hereby be explained as a result of film thickness of Cu(In,Ga)Se2 and CuxSe respectively. This is an important knowledge for an efficient fabrication in large scale.

El objetivo de esta tesis es la síntesis de películas delgadas de SnS utilizando técnicas de bajo coste con el fin de fabricar células solares. Nuestra contribución radica en estudiar nuevos materiales susceptibles de ser utilizados para aplicaciones fotovoltaicas, y que puedan ser preparados con técnicas de bajo coste como la técnica de Pyrolysis de Spray Químico (CSP) y caracterizar algunos materiales elegidos para este fin como el Sulfuro de Estaño (SnS). Se han fabricado células solares a partir de la disposición de capas: Mo / SnS / Tampón / i-ZnO / ZnO: Al / Al / Metal. Las capas de buffer serían: In2S3 o CdS. En la primera etapa hemos procedido a la optimización de los parámetros de deposición de películas delgadas de SnS usando la técnica de la CSP, -Variación de la relación [S] / [Sn]. -Variación de la temperatura del substrato. -Variación de la naturaleza del sustrato utilizando sustrato como vidrio, óxido de estaño de indio (ITO) y vidrio recubierto de molibdeno. Las fuentes de productos químicos y disolventes utilizados son: - Cloruro de dihidrato dihidratado para Tin (Sn), Thiourea, Agua destilada como disolvente de la solución, Ethanol (10% de 50mL) con el fin de reducir la tensión superficial del agua que es 72 Nm-1, para permitir la dispersión de la solución depositada sobre el sustrato fácilmente. En una segunda etapa se han dopado pel¿culas delgadas de SnS con algún elemento en la tabla de Mendeleiev para modificar las propiedades f¿sicas y qu¿micas de las pel¿culas. Los elementos químicos utilizados fueron: Plata, Aluminio y Hierro. Se han utilizado varias técnicas de caracterización: - Difracción de rayos X (XRD) para la estructura cristalina de las películas - Espectroscopía Raman para la calidad de las películas - Microscopía electrónica de barrido (SEM) para morfología superficial - Microscopía de Fuerza Atómica (AFM) para topografía de superficie - Análisis dispersivo de energía de rayos X (EDAX) adjunto a SEM para la composición de la película - Espectrofotometría óptica para la transmisión y la determinación del gap - Método de 4 puntas para medición de resistividad del SnS dopado -Mott-Schottky para determinar el tipo de semiconductor y la concentración de portadores Los principales resultados obtenidos en esta tesis pueden resumirse como sigue: -Las películas delgadas mono-sulfuro (SnS) deben depositarse sobre un sustrato de vidrio con [S] / [Sn] igual a una (1) y la temperatura del sustrato igual a 350 ° C para obtener películas densas, bien cubiertas y homogéneas sin agujeros Y grietas. Distancia entre la boquilla al sustrato 25 cm, volumen pulverizado 5 ml, presión de aire 0,7 bar y velocidad de pulverización de 1,5 ml / min. - Para películas dopadas por Plata y Aluminio, todas las películas son estructura ortorrómbica con (111) como pico principal. La intensidad del pico principal aumenta cuando el porcentaje de elemento dopante aumenta en la solución inicial sin ninguna fase secundaria para el dopaje con Al y con Ag8SnS6 y Ag para el dopaje Ag. - El análisis de SEM y AFM demuestra que el elemento dopante Ag no tiene efecto en la morfología y ni en la topografía mientras que el dopaje Al actúa sobre la morfología superficial produciendo una morfología que presenta muchos agujeros para muestras dopadas de 3% a 7%. - EDAX destaca un aumento de Ag en películas cuando la cantidad de Ag aumenta en la solución con S/ Sn¿0,98 cerca de 1 al 5% de porcentaje de dopado de Ag donde como para el dopaje EDAX destaca la mejora de la estequiometría con un aumento del porcentaje de Al Atómica en películas cuando la concentración de Al aumenta en la solución inicial con S / Sn = 0, 99 al 10%. - La resistividad de las muestras dopadas con Ag y Al aumenta con la concentración de dopado y se observa un aumento del gap óptico de 1.66eV a 1.70eV para SnS dopado por Ag y SnS dopado por Al, respectivamente.
ß-In2S3 thin films deposited by Chemical Spray Pyrolysis technique at different substrate temperatures (250 °C-300 °C-350 °C) showed well crystallized thin films with (0 0 12) as preferred direction perpendicular to the plane containing the surface of glass substrate. SEM images showed dense, uniform, well-covered layers that adhere well to substrates and no crack and void space were noted for all substrate temperatures. Microanalysis X confirms the presence of In and S elements with good stoichiometry after vacuum annealing for 30 minutes. Raman spectroscopy analysis confirms ß-In2S3 phase with more prominent modes after vacuum annealing. We also noted a reduction in the gap energies after annealing for films prepared at 250 °C and 350 °C substrate temperatures while for those prepared at 300 °C, the energy of the gap remains stable. Tin mono-sulfide (SnS) thin films must be deposited onto glass substrate with [S]/[Sn] ratio equal to one (1) and substrate temperature equal to 350 °C to obtained dense, well-covered, and homogeneous films without pinholes and cracks. Distance between nozzle to substrate is kept to 25cm, sprayed volume 5mL, air pressure 0.7bar and spray rate 1.5 mL/min. Films doped with Silver (Ag) and Aluminum (Al) were all orthorhombic structure with (111) as main peak. The intensity of main peak increased when the percentage of dopant element increased in the initial solution without any secondary phase for Al-doping films and with Ag8SnS6 and Ag for Ag-doping ones. SEM and AFM analysis showed that Ag-doping element had no effect in the morphology and the topography while Al-doping affected the surface morphology with "fishing net" like morphology with lots of holes for samples doped from 3% to 7%. EDS highlighted an increase of Ag in films when its amount increased in the solution with S/Sn¿0.98 near to 1 at 5% of Ag-doping percentage where as for Al-doping EDS highlighted improvement of stoichiometry with an increase of Al percentage atomic in films when Al concentration increased in the initial solution with S/Sn¿0.99 at 10%. Electrical and energy band gap measurement showed a decrease of resistivity when Ag and Al percentages increased in the solution to reach relatively low resistivity of 108¿.cm and 170¿.cm at 10% for both, and an increased of energy band gap when the Ag and Al-doping elements increased in the solution with 1.66eV and 1.70eV for SnS doped with Ag and SnS doped with Al, respectively. Spray pyrolyzed SnS thin films doped with indium were studied using various optical and electrical techniques. Structural analysis shows that all films crystallize in orthorhombic structure with (111) as a preferential direction without secondary phases. Doping of SnS layers with indium results in better morphology with increased grain size. Absorption measurements indicate dominant direct transition with energy decreasing from around 1.7 eV to 1.5 eV with increased indium supply. Apart from direct transition, an indirect one, of energy of around 1.05 eV, independent on indium doping was identified. The photoluminescence study revealed two donors to acceptor transitions between two deep defect levels and one shallower with energy of around 90 meV. The observed transitions did not depend significantly on In concentration. The conductivity measurements reveal thermal activation of conductivity with energy decreasing from around 165 meV to 145 meV with increased In content. Finally, we were investigated the J-V characteristics of FTO/CdS/SnS,FTO/ZnO/CdS/SnS, FTO/ZnO:Al/CdS/SnS, FTO/ZnO:Al/SnS and FTO/In2S3/SnS solar cells and we found that efficiencies are very low due probably to the recombination at the junction, grain boundaries, etc.
L'objectiu d'aquesta tesi és la síntesi de pel·lícules primes de SnS utilitzant tècniques de baix cost per tal de fabricar cèl·lules solars amb alta eficiència. La nostra contribució rau en estudiar nous materials susceptibles de ser utilitzats per a aplicacions fotovoltaiques, i que puguin ser preparats amb tècniques de baix cost com la tècnica de Spray Piròlisis Químic (CSP) i caracteritzar alguns materials triats per a aquest fi, com ara el Sulfur de estany (SnS). S'han fabricat cèl·lules solars a partir de la disposició de capes: Mo/SnS /Tampó/i-ZnO/ZnO: Al/ Metall. Les capes de per al bufer intermèdi has sigut de In2S3 i CdS. En la primera etapa hem procedit a l'optimització dels paràmetres de deposició de pel·lícules primes de SnS usant la tècnica CSP. -Variació de la relació [S] / [Sn]. -Variació de la temperatura Ts del substrat. -Variació de la naturalesa del substrat utilitzant substrat com: vidre simple, òxid d'estany d'indi (ITO) i vidre recobert de molibdè. Les fonts de productes químics i dissolvents utilitzats han sigut; Clorur d'estany per a l'estany (Sn), thiourea per sofre (S). Aigua destil·lada com a dissolvent de la solució. Ethanol (10% de 50ml) per tal de reduir la tensió superficial de l'aigua que és 72 Nm-1, per a permetre la dispersió de la solució dipositada fàcilment sobre el substrat. En una segona etapa s'han dopat pel.lícules primes de SnS amb algun element en la taula de Mendeleiev per modificar les propietats físiques i químiques de les pel.l¿cules. Els elements químics utilitzats són: Plata (Ag+), alumini (Al3+), Ferro (Fe2+), Coure (Cu2+) i Antimoni (SB3+) com a font de nitrat de plata (AgNO3), Clorur d'alumini (AlCl3) (FeCl2·4H2O ), Clorur de Coure (CuCl2 i Clorur de Antimoni (SbCl3). S'han utilitzat diverses tècniques de caracterització: - Difracció de raigs X (XRD) per a l'estructura de les pel·lícules i cristal - Raman Spectroscopy per a la qualitat de les pel·lícules - Microscòpia electrònica de rastreig (SEM) per morfologia superficial - Microscòpia de Força Atòmica (AFM) per topografia de superfície - Anàlisi dispersiu d'energia de raigs X (EDAX) adjunt a SEM per a la composició de la pel·lícula -Espectrofotometría per a la transmissió i el mesurament de la banda d'energia utilitzant la trama de Tauc - Tècnica de punta-sonda per a mesurament de resistivitat amb dopat SnS -Mott-Schottky per determinar el tipus de semiconductor i la concentració de portadors Els principals resultats obtinguts en aquesta tesi poden resumir així: -Les pel·lícules primes mico-sulfur (SnS) han de dipositar-sobre un substrat de vidre amb [S]/[Sn] igual a una (1) i la temperatura del substrat igual a 350 °C per obtenir pel·lícules denses, ben cobertes i homogènies sense forats I esquerdes. Distància entre el filtre al substrat 25 cm, volum polvoritzat 5 ml, pressió d'aire 0,7 bar i velocitat de polvorització de 1,5 ml / min. Per pel·lícules dopades per Plata i alumini, totes les pel·lícules són estructura ortorrómbica amb (111) com pic principal. La intensitat del pic principal augmenta quan el percentatge d'element dopant augmenta en la solució inicial sense cap fase secundària per al dopatge amb Al i amb Ag8SnS6 i Ag per al dopatge Ag. L'anàlisi de SEM i AFM demostra que l'element dopant Ag no té efecte en la morfologia i la topografia mentre que el dopatge en actua sobre la morfologia superficial produint una morfologia que presenta molts forats per a mostres dopades de 3% a 7%. EDAX destaca un augment de Ag en pel·lícules quan la quantitat d'Ag augmenta en la solució amb S / Sn¿0,98 prop d'1 a 5% de percentatge de dopatge d'Ag on com per al dopatge EDAX destaca la millora de l'estequiometria amb un augment del percentatge d'al Atòmica en pel·lícules quan la concentració d'al augmenta en la solució inicial amb S / Sn = 0,99 al 10%.
Sall, T. (2017). Preparation and Characterization of SnS thin films by Chemical Spray Pyrolysis for fabrication of solar cells [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/95412
TESIS

The aim of this thesis is to provide an in-depth investigation of zinc tin oxide, Zn1-xSnxOy or ZTO, grown by atomic layer deposition (ALD) as a buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells. The thesis analyzes how changes in the ALD process influence the material properties of ZTO, and how these in turn affect the performance of CIGS solar cells. It is shown that ZTO grows uniformly and conformably on CIGS and that the interface between ZTO and CIGS is sharp with little or no interdiffusion between the layers. The band gap and conduction band energy level of ZTO are dependent both on the [Sn]/([Zn]+[Sn]) composition and on the deposition temperature. The influence by changes in composition is non-trivial, and the highest band gap and conduction band energy level are obtained at a [Sn]/([Zn]+[Sn]) composition of 0.2 at 120  °C. An increase in optical band gap is observed at decreasing deposition temperatures and is associated with quantum confinement effects caused by a decrease in crystallite size. The ability to change the conduction band energy level of ZTO enables the formation of suitable conduction band offsets between ZTO and CIGS with varying Ga-content. It is found that 15 nm thin ZTO buffer layers are sufficient to fabricate CIGS solar cells with conversion efficiencies up to 18.2 %. The JSC is in general 2 mA/cm2 higher, and the VOC 30 mV lower, for cells with the ZTO buffer layer as compared to cells with the traditional CdS buffer layer. In the end comparable efficiencies are obtained for the two different buffer layers. The gain in JSC for the ZTO buffer layer is associated with lower parasitic absorption in the UV-blue region of the solar spectrum and it is shown that the JSC can be increased further by making changes to the other layers in the traditional CdS/i-ZnO/ZnO:Al window layer structure. The ZTO is highly resistive, and it is found that the shunt preventing i-ZnO layer can be omitted, which further increases the JSC. Moreover, an additional increase in JSC is obtained by replacing the sputtered ZnO:Al front contact with In2O3 deposited by ALD. The large gain in JSC for the ZTO/In2O3 window layer stack compensates for the lower VOC related to the ZTO buffer layer, and it is demonstrated that the ZTO/In2O3 window layer structure yields 0.6 % (absolute) higher conversion efficiency than the CdS/i-ZnO/ZnO:Al window layer structure.