1,720,958 research outputs found
Manipulating the Hydrogenated Amorphous Silicon Growing Surface
No full textOur modern lifestyle is currently fueled by two billion years of accumulated energy reserves. For several years now there has been a strong rise in research interest and more recently also implementation of renewable energy sources in the European Union. Driving factors for these developments are the increasing awareness of global warming in our society, the limited nature of current fossil fuel sources, the need for energy security & independence and the opportunity to develop new technologies and business opportunities. Obtaining energy from solar radiation via solar cells based on cyrstalline silicon has seen continuous development since the demonstration of the first photovoltaic devices in the 1950’s. These types of solar cells are still the dominant technology today, but other solar cell technologies are about to make the transition from research labs into significant production volumes. One of these technologies is the hydrogenated amorphous silicon (a-Si:H) thin film solar cell, which promises to be a cheap alternative to the crystalline silicon solar cell, albeit at a reduced energy conversion efficiency. The deposition technique used to grow a-Si:H thin films in this work is the expanding thermal plasma chemical vapor deposition (ETP-CVD). Main benefit of ETP-CVD are the high growth rates of more than 1 nm/s that can be achieved, which is very beneficial for the growth of the thick intrinsic absorber layer in a solar cell. Substrate temperatures of >300ºC are required to obtain dense a-Si:H films suitable for solar cell application – temperatures that leads to degradation of previously deposited p-doped layers in a typical p-i-n solar cell. In order to reduce the temperature load during deposition, ion bombardment was employed with the goal to provide the growing film surface with kinetic energy from the bombarding ions, thereby allowing to deposit dense a-Si:H films at substrate temperatures around 200ºC. The results discussed in chapter 4 and 5 show that significant progress towards this goal has been achieved, as demonstrated by solar cells with > 7% initial efficiency at a growth rate of 1 nm/s and substrate temperatures between 200 – 180ºC for the intrinsic layer. To achieve this we have utilised two different types of substrate biasing, sinusoidal RF biasing and pulse-shaped biasing, with focus on the latter. Both biasing techniques result in ion bombardment of the film surface during deposition. Both biasing techniques enable the deposition of solar-grade intrinsic a-Si:H at substrate temperatures below the 350ºC required for unbiased deposition. In chapter 3 we demonstrate how RF substrate biasing leads to the growth of vacancy-dominated material up to 50ºC below the temperature required for unbiased deposition. This conclusion is based on FTIR analysis where a transition form vacancy- to void-dominated material is observed. We investigated the dependence of this transition on the reactor pressure. Ion bombardment at a DC bias voltage VDC of 14 V has hardly any effect on the transition, yet an increase in void fraction was observed for all temperatures and increasing pressures. Ion bombardment at VDC = 20 V for the high pressure series resulted in an increase of the transition from 9% total hydrogen concentration unbiased to 13% total hydrogen concentration biased. This is attributed to a reduced incorporation of ionic clusters and polymers into the film. The second type of substrate biasing utilized in this work is the pulseshaped biasing, PSB. As shown in chapter 4, with PSB we are able to obtain accurate control over the ion energy distribution in a range of 0 - 200 eV without the formation of a strong secondary plasma typically present for RF substrate biasing. Control over the ion energy was confirmed by retarding field energy analyzer measurements for conductive substrates, and non-conductive substrates covered with a conductive surface layer which is connected to the sample holder. For intrinsic a-Si:H deposited with PSB at a growth rate of 1 nm/s and substrate temperatures in the range of 180 - 200ºC we can distinguish roughly between two regions: region I 4.8 eV/Si atom. In region I we observe an increase in material density due to a decrease in nanovoid concentration as deduced from FTIR analysis. At the transition between region I and II around 4.8 eV/Si atom the densest material with low nanovoid concentration is obtained. The increase in material density and the reduction in surface roughness in region I are attributed to an increase in surface mobility of mobile species as well as surface atom displacement. Above 4.8 eV/Si atom we see an increase in Urbach energy which is related to bulk atom displacement in subsurface layers at higher ion energies. We report unique experimental evidence which indicates that the band gap is not correlated to the total hydrogen concentration, cH, as usually reported in literature, but instead to the presence of nanovoids in the film, asdetermined from the cHSM mode. Intrinsic a-Si:H deposited with PSB under the same conditions as the films from chapter 4 have also been implemented in p-i-n solar cells. These cells are discussed in chapter 5 and a reproducible record initial energy conversion efficiencies of 7.4% was obtained for cells grown by ETP-CVD at such high growth rates of 1 nm/s and low substrate temperatures of 200ºC. This efficiency was obtained for cells grown around deposited energies of around 1 eV/Si atom. The open-circuit voltage has a maximum of 0.82 V around 1 eV/Si atom and decreases at higher deposited energies per Si atom, which is attributed to the low band gap at higher deposited energy. The short-circuit current density reaches a maximum around 4.8 eV/Si atom and decreases at higher deposited energies, which is attributed to a reduced hole collection determined from external quantum efficiency measurements. The fill factor decreases above 1 eV/Si atom which we attribute to a lower mobility-lifetime product due to an increase in charge carrier recombination. This indicates defect formation at deposited energies above 1.7 eV/Si atom, significantly below the reported increase in Urbach energy around 4.8 eV/Si atom reported in chapter 4. Chapter 6 discusses the development of the a-Si:H surface roughness as function of several parameters like substrate temperature, RF substrate biasing and hydrogen dilution at growth rates of 0.1 nm/s. Important for this analysis was the spectroscopic ellipsometry technique, which allows to monitor the surface roughness development in-situ during film deposition. In the first section, from depositions with and without RF substrate biasing at different substrate temperatures the presence of a hydrogen-rich layer was suggested, which is removed/densified upon externally induced ion bombardment. In the second section of chapter 6, the surface roughness development was investigated as function of hydrogen dilution. We observe a discrepancy in the surface roughness development between spectroscopic ellipsometry and AFM measurements. We interpret this as another indication of the presence of a hydrogen rich/low density overlayer. At higher hydrogen dilutions, we obtain a thicker overlayer dominated by lower hydrides. From additional PSB experiments at two different dilutions we conclude that the hydrogen rich/low density layer is densified by the induced ion bombardment and/or excess hydrogen is removed.Electrical Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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