1,721,133 research outputs found
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
Characterization and simulation of three-dimensional solid-state solar cells
Get PDFIntroduction : The newest PV technology that uses less material and cheaper materials and deposition methods (in order to lower the cost even more) resulted in extremely thin absorber solar cells, or ETA-cells. Such ETA-layers need to be applied to a nano-structured substrate. Dye-sensitized solar cells (DSSC’s) already fully use this potential but encounter a lot of long-term stability problems. In the cells studied in this thesis, both the organic absorber dye and the liquid electrolyte of the DSSC are replaced by a solid-state semiconductor and are called 3D-cells. The nanostructured substrate is nanoporous titanium dioxide (TiO2).
Objective : Charge carrier generation and their transport to the contacts is separated in 3D-cells. This seems to be an advantage with respect to the one-dimensional standard and thin film technology, where charge generation and transport take place in the same materials. Our and other’s nanostructured solid-state cells however, do not seem to benefit from this three-dimensional feature like DSSC’s do. The question is why?
Interface recombination : Different kind of nano- and microstructured 3D-cells are characterized and analyzed. In particular a dynamic model is presented which enables us to quantify the influence of recombination in these cells. Interface recombination turns out to be a crucial element in the determination of the final performance of a 3D-cell.
Diffusion models : Detailed effective medium models are set-up which describe both steady-state and dynamic response of the cells. In one of the models, the cell is treated as an infinite network of flat band unit cells (i.e.
there is no electric field on a microscopic level). In the other, an analytic diffusion model is set up with boundary conditions specific for the treated cell. Both models assume diffusion as the main driving force for electron transport; charge separation occurs due to the difference in electron negativity of the materials.
Results : The diffusion coefficients that are found are in the same range as those found for DSSC’s. The main difference is in the lifetime of the electrons which is 104 times smaller for 3D-cells than for DSSC’s. Both analytic approximations of and full simulations with the analytic diffusion model revealed that the diffusion length is much smaller than the cell thickness, and also than the absorption depth of the light. In that case, the characteristic time constant associated with the break frequency of the optoelectronic photocurrent transfer function is equal to half the electron life time, the characteristic time constant associated with the break frequency of the optoelectronic photovoltage transfer function is equal to the
electron life time. The quantum response is equal to the product of the electron diffusion length and the absorption coefficient, which in our case indeed yields a quantum response that is much smaller than one. At last, the analytic diffusion model turned out to represent the nanostructured solar cell in the same way as the proposed infinite network of flat-band unit cells
Characterization and modelling with SCAPS of the performance of solar cells
No full textResearch in the field of solar cells is continuously increasing in order to fabricate high efficiency cells and using inexpensive materials and technologies. The guideline of this thesis is the use of numerical simulation and electrical characterization to obtain a better understanding and insight in various types of solar cells. In the third generation solar cells, impurity photovoltaic effect (IPV) was suggested some time ago as a new approach to improve solar cell performance, especially the short-circuit current, by using sub-band gap photons and extending the infrared response. In our modeling of this effect using Scaps software (Gent, Belgium) which was extended for this purpose, we had the unique ability to thoroughly examine the influence of the solar cell’s material, the doping and impurity concentration, the position and characteristics of the optically active impurities, the internal reflection in the cell, in Si and GaAs based solar cells. We also investigated a number of niche applications of GaAs solar cells: micro solar cells, the temperature effects under low sun concentration, and the role of an intrinsic layer in a pin structure. Finally we could perform measurements and numerical analysis to a highly advanced cell type: thin films CIGSe solar cells grown on different flexible metallic foils with and without barrier layer. Flexible solar cells offer distinct advantages over cells on rigid glass substrate, in terms of manufacturability and application possibilities, and this has motivated quite a lot of studies of these cells. Increasing activities to develop and to fabricate flexible CIGS-based devices have been observed over the past years. Measurements and simulation have been carried out on these cells in order to find a correlation between the properties of the CIGS devices and the substrate treatment. Besides the electrical characterization, additional measurements have been performed on these cells such as XPS and DLTS characterization to investigate the nature of defects existing in these cells, and limiting or not their performance
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
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