1,721,071 research outputs found
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
Molybdenum oxide as a case study for the diagnosis of carrier selective contacts in silicon heterojunction solar cells
No full textEnvironmental challenges impose a rapid energy transition. The renewable power capacity is expected to increase by 50% in the next 5 years, and recent scenarios plan that photovoltaics (PV) will be leading the new energy sources. Combining high efficiencies and simple fabrication process, silicon heterojunction (SHJ) solar cells have the potential to be the main technology for PV world wide deployment. Challenges yet remain, notably due to the use of a-Si:H as contact layers. MoOx appears to be one of the most promising alternative to tackle some of these challenges, owing its transparency and excellent electronic properties. This thesis focuses on the analysis and development of MoOx-based hole contact for SHJ solar cells. Contact passivation, selectivity and transport are assessed by varying the contact parameters, using standard (p)a-Si:H as a benchmark, and using characterization adapted for contact-limited SHJ.
By thinning down theMoOx layer from the standard 10 nm thickness to 4 nm, excellent passivation and selectivity were maintained. A current gain of 1.3mAcm-2 was obtained as compared with the standard a-Si:H contact. A 2x2cm2 screenprinted certified 23.5% world record device was achieved through this analysis, outperforming the reference (p)a-Si:H-based device. Further thinning the MoOx layer provoked several losses. Using many characterization methods combined to fittings using an innovative analytical description of contact-limited solar cells proposed by Roe et al., we could conclude that the losses stem from a passivation decrease combined with a selectivity reduction inducing a drop of the majority hole and an increase of parasitic electron currents in the contact, due to the reduction of the contact's work function.
Doping and deposition power of the ITO top layer was then modified and tested over several MoOx layer thicknesses, to estimate MoOx screening length towards ITO work function and investigate the influence of ITO on transport. 4-nm-thickMoOx showed to efficiently screen the ITO work function on the tested range while the 2-nm-thick-MoOx-based contact was affected. The ITO deposited at low power drastically enhanced the FF (» 1%absolute). In situ work function estimation of the ITOs (4.6 to 4.8 eV) and hole contact (» 5.0 eV) was conducted using fittings according to Roe's formalism. PECVD-deposited-a-SiOx:H, commonly used on top of fully-processed SHJ to boost the JSC, degraded theMoO3-x-based devices FF, ascribed to the presence of H in the plasma reducing MoOx.
MoOx-based devices showed to be remarkably stable over time. An annealing step performed on solar-cell samples prior toMoOx deposition was shown to slightly damaged the passivation but protected MoOx selectivity degradation for processes temperature up to 170 °C. Similarly to standard SHJ devices, current injection in dark in forward bias significantly improved the FF of the devices by 1.3%absolute. The cells were encapsulated and degraded by UV radiations, which showed to strongly affect selectivity.
A review of main transport mechanisms was gathered in the last chapter to parallelize with temperature-dependent J-V measurements (-160 to 70 °C) performed for MoO3x- and (p)a-Si:H-based SHJ. Voc(T) and FF(T) evolution showed similar trends in both cases, confirming the expected transport behavior from comparing band-diagrams. An extension of the method based on Roe et al. model is proposed to rapidly identify selectivity loss from S-shaped J-V curves.PV-LA
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
Characterisation of Silicon Heterojunction Solar Cells Beyond Standard Test Conditions
No full textThe global photovoltaic market is mostly dominated by solar cells based on crystalline silicon (c-Si), which are covering 95% of the market. This thesis concerns silicon heterojunction (SHJ), a high-efficiency technology with a 2% market share in 2018, but showing a steady global production capacity growth and potential for industry-compatible modules with a conversion efficiency above 21%. This thesis focuses on improving the device efficiency in real operating conditions. More specifically, different effects affecting a cell used in non-standard conditions are assessed, including different temperatures and illuminations, during prolonged exposure to light and the effect of surface inhomogeneities.
The main SHJ technology specificity consists of the use of thin hydrogenated amorphous silicon (a-Si) layers that provide excellent c-Si surface passivation. This enables very high operating voltages compared to other c-Si-based technologies, which directly reduces the loss of efficiency induced by higher operating temperatures. The cell's properties are measured at very low temperatures are studied, which amplifies charges transport phenomena. The voltage shows a deviation from its theoretical curve, which we explain to be related to a decrease of the positive contact holes selectivity due to the influence of the transparent conductive oxide layer. An increase in the resistive losses at low temperature is measured. These losses are caused by a resistivity increase in the different a-Si layers and interfaces because of a thermal energy decrease at low temperatures. These effects may also affect cell performances at room temperature and under operating conditions. This implies that optimisation at standard test conditions (STC), generally used to assess the performance of PV modules, does not necessarily result in the same optimum as in real climatic conditions, which generally correspond to a higher temperature and/or a lower illumination. We show that by adding carbon in the front doped layer, a transparency increase, and thus a current gain is observed at the expense of larger ohmic losses, which overall reduces the efficiency at STC. Using simulations and real climate data, we show that using such a layer leads to a relative energy production gain of 0.5 to 0.8%, both in a moderate and arid climate.
Another specificity of SHJ cells is a slight efficiency increase when exposed to light for a few days. We show that in case of a too-small positive doped layer charges reserve, cell performance degradation is observed instead of an improvement. In order to avoid such degradation, the positively doped layer should be sufficiently thick and doped to screen the influence of the subsequent transparent conductive oxide layer. A treatment in which a forward bias voltage is applied to the cell allows triggering the gain in efficiency without causing the detrimental effects of light exposure.
Finally, the influence of a non-homogeneous surface is studied. Parasitic electric currents are induced between the well-passivated area and poor passivation area through the metallisation. Using dedicated samples, correspondence between these currents, and an often measured drop of the low injection lifetime is described and linked to the detrimental effect of either the sample edges or by surface defects. Concretely, these effects can affect the performances of the cell at low illumination, reducing the performances in real outdoor conditions.PV-LA
Contact Design for Silicon Heterojunction Solar Cells
No full textToday more than ever the world needs clean energy sources and thus a fast deployment and scaling up of the photovoltaic industry. In this context improving solar cell efficiency plays a major role. In order to achieve the maximum single junction efficiency for this material, technologies featuring carrier selective passivating contacts are foreseen to be the next successors of actual industrial cells. Among them, the silicon heterojunction (SHJ) solar cell is a promising technology which demonstrated efficiencies up to 26.7%. Its main specificity consists in the use of intrinsic and doped amorphous silicon (a-Si:H) to provide surface passivation and carrier selectivity to the c-Si, which allows to reach very high operating voltages in comparison to other c-Si based technology.
The major drawbacks of SHJ solar cell is the large parasitic absorption occurring in those layers. To address this challenge, a strong focus on alternative non-silicon based thin films with wider band gap has built up in the literature. A first part of this thesis focuses on reviewing the fundamental process behind carrier selectivity and discuss the ability of different models from the literature to draw informations directly from the peculiar shapes of IV curves commonly observed in the solar cells featuring newly developed materials. We first discuss the simple four ideal diodes model of Roe et al to explain the occurrence of S-shapes depending on the contact quality. We discuss from a theoretical point of view how to improve the model by modifying the ideal diodes by other circuit elements as well as adding bulk recombinations. We test the theory experimentally and identified modifications that should be incorporated in the model.
In a second part of the thesis, we explore the development of doped nano-crystalline silicon (nc-Si) layers to replace classical amorphous silicon layers. We investigate the influence of TMB and BF3 as dopant sources on the transparency and contact properties of nc-Si:H(p) layers as well as their integration in solar cells. We expose the roles of both gas to modify the crystallinity of the layer and find different optimum for both of them to lead high efficiencies, illustrated by a certified 23.9%-efficient solar cell. We report also on further optimization steps of this front junction architecture resulting in certified efficiency of 24.44%.
We explore next the n-type nc-Si layers for application as window layer. In particular, we perform thickness and doping series to unravel the layer properties along its growth direction. In order to improve the low doping of the nucleation zone of nc-Si:H(n) at the i/n interface, a thin a-Si:H(n) buffer layer is introduced and shown to improve the passivation, selectivity and contact resistivity. Finally, we also report on the beneficial effect of an additional SiOx capping layer which improves both the reflection properties as well as all contact properties, broadening the optimal parameter window. The absorption loss in the front silicon layers remains however the highest share of losses, and therefore we explore theoretically the opportunity of using thinner front side layers or using thin TCO / silicon nitride bilayers to reduce the absorption losses. We finish the discussion by presenting an optimization roadmap, showing a possibility for double-side contacted SHJ cells to reach efficiencies above 26% without requiring any patterning or localization step.PV-LA
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
Plasma-initiated rehydrogenation of amorphous silicon to increase the temperature processing window of silicon heterojunction solar cells
Full text linkabstract: The dehydrogenation of intrinsic hydrogenated amorphous silicon (a-Si:H) at temperatures above approximately 300 °C degrades its ability to passivate silicon wafer surfaces. This limits the temperature of post-passivation processing steps during the fabrication of advanced silicon heterojunction or silicon-based tandem solar cells. We demonstrate that a hydrogen plasma can rehydrogenate intrinsic a-Si:H passivation layers that have been dehydrogenated by annealing. The hydrogen plasma treatment fully restores the effective carrier lifetime to several milliseconds in textured crystalline silicon wafers coated with 8-nm-thick intrinsic a-Si:H layers after annealing at temperatures of up to 450 °C. Plasma-initiated rehydrogenation also translates to complete solar cells: A silicon heterojunction solar cell subjected to annealing at 450 °C (following intrinsic a-Si:H deposition) had an open-circuit voltage of less than 600 mV, but an identical cell that received hydrogen plasma treatment reached a voltage of over 710 mV and an efficiency of over 19%.This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Applied Physics Letters and may be found at http://aip.scitation.org/doi/10.1063/1.4958831
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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