1,721,034 research outputs found
Transparent selective contacts for silicon-based heterojunction solar cells
Full text linkRenewable energies are nowadays recognised to be the protagonists of a necessary sustainable transition in the energy production process. Among the existing technologies, photovoltaic (PV) stands out for the infinite availability of the source, the direct efficient energy transformation and for low global energy costs. These reasons drive the global research toward materials and methods to obtain higher and higher efficiencies.
Beyond the well-assessed silicon-based homojunction technology, that covers the 95% of the total PV production, the amorphous/crystalline-silicon heterojunction concept is gaining increasing market share. Heterojunctions (HJs) are able to overcome some shortcomings of homojunctions, such as the high thermal budget connected to the diffusion process to achieve the doping and to the recombination losses that limit the VOC. However HJs still require complex architectures and the use of toxic and dangerous gases, which demand costly measures for a safe industrial manufacturing. A new approach to circumvent some of the above-mentioned issues is currently explored with increasing interest, and involves the use of non-doped, non-silicon films to replace the amorphous-silicon ones. A great variety of materials has already been proposed and tested for the purpose, but some challenges still have to be completely solved, first of all the resistance to thermal annealing at 200-250°C, that is a necessary step in module assembling. Moreover, in view of an industrial application, the manufacture of these novel materials needs to satisfy several constrains and then guarantee an easy and high-throughput process.
In this thesis the attention is focused on some of these novel materials for both emitter and base contacts of a heterostructure solar cell based on n-type doped crystalline silicon. In particular nickel oxide and titanium oxide or zinc sulphide are exploited as selective emitter and base contact respectively. These materials have to be more transparent than the doped amorphous layer counterpart to enhance the quantum yield of the solar cell in the blue region of the solar spectrum where the amorphous films introduce parasitic absorptions. Moreover the substitute layers must be deposited on the well-assessed thin buffer layer of amorphous silicon suboxide used in our laboratory to passivate the surfaces of the silicon wafer, therefore the most relevant constrain to fulfil is to avoid any contamination and any detrimental thermal stress of the surface passivation. On the basis of these requirements, particular care will be given to the choice of the deposition procedures as well as to the choice of the doping elements to be introduced into the selective contacts. The measurement of silicon surface passivation will be adopted as the most relevant method to evaluate the suitability of materials and manufacturing methods. Numerical simulations will also be used to evaluate the ideal band diagram of the proposed heterostructures and to verify how differences in the experimental properties of the materials with respect to ideality could affect the heterostructure device performances.
In analogy with the doped amorphous layers used in conventional HJs, also the proposed novel materials for selective contacts need to be coupled to a transparent conductive oxide to make up for the poor lateral conductivity that still remains a basic issue of any selective layer proposed in the literature up to now. In this thesis another promising material is tested as an alternative to indium tin oxide, namely tungsten-doped indium oxide. It is found that non-reactive radio-frequency sputtering is suited to grow transparent films with very high mobility and low resistivity due a low carrier concentration that reduces parasitic absorptions. Most importantly, the sputtering damage to surface passivation of the crystalline silicon substrates can be completely recovered after a suitable thermal annealing.
All the discussed results show that ideal materials or totally qualified deposition methods for obtaining alternative selective contacts have yet to be found, mostly from the point of view of the resistance to thermal treatments. This thesis adds some tiles to the road to high-efficiency, low-thermal budget, low-toxicity, high-throughput silicon-based heterojunctions
Routing Strategies in WDM Networks: the Impact of Transmissions Issues and Network Topology
No full textLuminescence from Erbium-Doped Silicon Epi Layers Grown by Liquid-Phase Epitaxy
No full textDislocation-related photoluminescence at 0.806 and 0.873 e V is observed in erbium-doped silicon epi layers grown by liquid-phase epitaxy on (100) Si wafers. These signals are detected at T = 2 K only on epi layers deposited on Czochralski grown silicon substrates. No luminescence is observed when float zone-grown substrates are used. The peak intensity shows temperature quenching, but the signal remains detectable up to 195 K. The luminescence apparently is due to dislocations in silicon in the simultaneous presence of high oxygen concentration and erbium impurities. A comparison with the typical infrared emission fro erbium-implanted silicon samples is presente
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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
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