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Introduction
No full textA brief overview of the book’s contents in relation to recent photovoltaic market developments is given. Major developments since the publication of Volume 1 are indicated.</p
Exploiting bi-facial deposition of ALD AlOx for PERC solar cells
No full textLAUREA MAGISTRALELe celle solari al silicio costituiscono attualmente lo standard nella produzione industriale. Tra le diverse tipologie realizzabili, quella denominata Aluminium Back Surface Field solar cell (Al-BSF solar cell) è la tecnologia con la più grande fetta di mercato, con circa l’80% dello share del mercato mondiale. Tuttavia, la maggior parte della nuova produzione aggiunta nell’ultimo anno è di un altro tipo: si tratta della tecnologia Passivated Emitter and Rear Contact solar cell (PERC solar cell), che è ampiamente riconosciuta essere la tecnologia che sorpasserà la Al-BSF come nuovo standard industriale, raggiungendo il monopolio del mercato entro i prossimi dieci anni.
Caratteristica fondamentale di quest’ultima tecnologia è, come si può intuire dal nome, la passivazione della superficie posteriore. Inoltre, per poter produrre questa struttura di cella solare è sufficiente aggiungere due step nella catena manifatturiera della già esistente Al-BSF. Per cui è lecito definire la PERC solar cell il diretto successore della presente cella solare standard. Uno dei due step sopracitati consiste nella deposizione di un film di materiale dielettrico sulla superficie posteriore della cella, realizzando cosi la passivazione della superficie stessa. Mentre per la superficie anteriore viene utilizzato un layer di SiNx che funge anche da anti-reflection coating (ARC) – utilizzato anche nella produzione della comune Al-BSF – per quanto riguarda la superficie posteriore della PERC solar cell, viene normalmente impiegato un layer di Al2O3 ricoperto ancora una volta dal SiNx che rende stabile la struttura.
Questo film di AlOx, normalmente di spessore tra i 7nm e i 10 nm, può essere depositato con varie tecniche ma, rispetto alle altre, Atomic Layer Deposition (ALD) offre un’eccezionale passivazione della superficie, insieme ad uniformità spaziale e ad un controllo ottimale della composizione. Tuttavia, la reazione di deposizione è molto lenta (un reattore ALD di laboratorio impiega infatti ore per depositare un film di AlOx) ed in fase di deposizione può verificarsi una non voluta deposizione di AlOx sulla faccia superiore della cella (il cosiddetto wrap-around effect). Lo sviluppo di nuovi reattori ALD per la produzione di massa come lo spatial ALD o il temporal ALD risolve il problema della velocità di deposizione, mentre il lavoro sperimentale svolto per questa tesi si indirizza a risolvere il problema della deposizione sulla faccia frontale. Infatti solamente pochi lavori fino ad oggi hanno trattato di questo wrap-around effect, tutti riportando un effetto negativo sulle performance della cella solare.
In questa tesi verrà dimostrato per la prima volta che un layer di ALD AlOx, depositato sulla faccia superiore della cella dopo lo standard ARC SiNx, non peggiora la performance della cella, bensì risulta in una migliore prestazione. In particolare verrà mostrato che un sottile layer di AlOx di 3 nm e 5 nm può diminuire le perdite associate con il contatto superiore della cella. In aggiunta sono state realizzate celle solari con 0, 3, 5, 7 e 10 nm di Al2O3 che ricopre entrambe le superfici della cella (in modo da simulare il wrap-around effect). I risultati mostrano che le celle solari realizzate con 3 nm di AlOx presentano un’efficienza maggiore rispetto ai provini standard che non presentano AlOx sulla faccia superiore.
Questi risultati possono essere di grande rilievo, soprattutto considerando che il design di reattori ALD dove il wrap-around effect viene evitato e/o limitato risulta in maggiore difficoltà di utilizzo e minore output, ed inoltre che recentemente è stato dimostrato che anche layer ultra-sottili di AlOx possono offrire buona passivazione superficiale.
Gli esperimenti sono stati realizzati tra agosto 2016 e marzo 2017 presso i laboratori della Scuola per le Energie Rinnovabili TYREE della University of New South Wales (UNSW) a Sydney, Australia. Il lavoro presentato in questa tesi verrà pubblicato come: “A. To, A. Garavaglia, S. Tahir, Wei Min Li, Xiang Li, B. Hoex, The Effect of Bi-facial AlOx deposition on silicon solar cells, in IEEE Journal of Photovoltaics”.P-type silicon aluminium back surface field (Al-BSF) solar cells are currently the dominant solar cell technology in high-volume manufacturing and as the industry moves to increase conversion efficiency, the Passivated Emitter and Rear Cell (PERC) cell structure is widely expected to take up increasing market share, becoming the dominant cell technology by 2026. The PERC cell structure may be distinguished from its predecessor, the full-area aluminium back surface field (Al-BSF) solar cell, for the passivated back surface and localised point contact scheme. Surface passivation is usually achieved with a plasma enhanced chemical vapour deposition (PECVD) of amorphous hydrogenated silicon nitride layer (simply often reffered to as SiNx) on the front side of the cell and an atomic layer deposited (ALD), or again PECVD, AlOx (capped with SiNx) film on the rear side.
ALD deposited films, compared to other kind of deposition systems, provide an excellent level of surface passivation, spatial uniformity and precise growth control. However, two main disadvantages may be identified for the ALD deposition: long processing time and, due to the intrinsic property of the process, unintended deposition of AlOx on the front side of the wafer. In the present work, this wrap-around effect will be addressed. Parasitic deposition of AlOx on the front side of the wafer penetrating up to 1 mm has been reported with detrimental consequences when the AlOx deposition precedes front side SiNx deposition. Consequently, much effort, particularly at the equipment design level, has been devoted to ensure monofacial thin film processing, often leading to more complex production equipment with lower throughput.
While new high-throughput deposition reactors, such as spatial ALD systems, solve the problem for the low deposition rates of lab-scale ALD reactors, the purpose of this work is to evaluate the effect of full-area AlOx wrap-around on the front side of the wafer when deposited on top of the SiNx film. It will be shown that a thin layer of AlOx of up to 5 nm on top of the standard SiNx ARC coating can actually reduce the contact resistance on the front metal grid. In particular, a 3 nm ALD AlOx layer resulted in even improved cell efficiency. Furthermore, the optimum firing temperature window appears to be wider for samples with a 3 or 5 nm layer in comparison to the control ones.
These results may be of fundamental importance, given that it allows for more freedom and potentially cheaper production equipment and that it has recently been demonstrared that even ultra-thin layers of AlOx give high-quality surface passivation on the rear surface of p-type PERC solar cells.
The work conducted for the purposes of this thesis is in preparation for publication as “A. To, A. Garavaglia, S. Tahir, Wei Min Li, Xiang Li, B. Hoex, The Effect of Bi-facial AlOx deposition on silicon solar cells, in IEEE Journal of Photovoltaics”
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
Colored PV Modules
No full textThis chapter covers the physics of colors in photovoltaics (PV) modules. It presents various options to realize colored silicon PV modules, as the largest market segment for PV modules. The chapter focuses on colored graphic designs on PV modules and the performance of these PV modules. It describes thin-film interference, which is a typical optical process related to colors in surfaces on top of PV modules. There are several options for coloring the different layers in a PV module, and the inks and pigments that can be used depend on which layer or material the color is applied. Coloring of glass for PV modules can be divided into bulk coloring and surface coloring. In case of bulk coloring, a metallic salt is added to the glass mold, giving the color to the final product. In this way, only homogeneous colored glass sheets can be obtained.</p
Deployment of Solar Photovoltaic Systems in Distribution Grids in Combination with Batteries
No full textIn addition to all the benefits that PV systems can bring, their integration into distribution grids is challenging as it will impact operation and control of power systems, mainly because PV systems have different characteristics from conventional fossil fuel-based power plants. Intermittency of PV-generated power causes strong fluctuations of DC power at micro-scales, on a daily basis, and over the seasons. This increases the possibility of mismatch between energy production and demand, leading to voltage and frequency fluctuations, and possibly curtailment of PV systems, i.e., their disconnection from electricity grids. Furthermore, evolving regulations and policies and energy economics can either enhance or deteriorate the integration of PV systems into distribution grids. Hence, in this chapter, various PV integration possibilities, technical challenges regarding integration of PV systems into the distribution grids, possible approaches such as integrating energy storage to tackle them, and solutions to deploy more solar energy efficiently in the grid will be discussed.</p
Environmental Impacts of Integrated Photovoltaic Modules in Light Utility Vehicles
No full textThis chapter allows to recognize the overall impact of Vehicle Integrated Photovoltaics (VIPV) on global warming. Life Cycle Assessment (LCA) is generally understood as a compilation and evaluation of a product or system’s inputs, outputs, and potential environmental impacts throughout its life cycle. The study reports a unique observation that integrating a photovoltaics system on a Light Utility Vehicle StreetScooter can improve the global warming potential of the vehicle when the reference case of an average shadowing factor of 30% and eight years of operation time are evaluated. Nevertheless, the results of the LCA show that viability is heavily dependent on the vehicle’s deployment region and usage scenario. One of the critical challenges of this LCA was finding an appropriate vehicle usage model to reproduce the ratio of using VIPV. Tests with radiation sensors investigating shading and reflection conditions would help cover missing data and provide irradiance profiles for the LCA.</p
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
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