1,721,140 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
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
Growth and Characterization of Thin Film Nanocrystalline Silicon Materials and Solar Cells
No full textThe need for electrical energy is growing fast as a result of the expanding world population and economic activities. On top of this the energy need of each individual is also growing. At present the growth in energy demand is not matched by the growth in energy generation because of insufficient energy production. This energy gap therefore needs to be bridged. In addition, most conventional means of energy generation are not environmentally-friendly and in turn affect human lives. Solar energy is one of the alternatives for renewable energy generation. It can be extracted directly in electrical form from solar radiation using photovoltaic (PV) solar cells or solar concentrators. The PV market is dominated by crystalline-silicon based solar cells. However, thin-film silicon solar cells are becoming increasingly important, because they are deposited at relatively low temperatures and as such offer the possibility to produce flexible and light-weight solar panels. The latter can be applied on for instance the roofs of factory buildings. Thin film nanocrystalline silicon (nc-Si:H) is an important material for application in thin-film silicon solar cells. It has been mainly developed because unlike amorphous silicon (a-Si:H) it is stable against light-induced degradation, and because with this material the solar cells have an extended spectral response up to the infrared wavelength region. Because of this extended response nc-Si:H films are used in combination with a-Si:H to form multi-junction solar cells, leading to an increased solar-cell efficiency. The focus of this thesis is on the development and characterization of nc-Si:H layers and solar cells. In chapter 3 we present results of research on how the deposition parameters during the growth affect nc-Si:H material properties and device performance when using radio frequency plasma enhanced chemical vapor deposition. Particular attention is paid to p-type and intrinsic films and their application in nc-Si:H solar cells. For the p-layer development, the effects of deposition power, the substrate roughness, and doping are investigated. Intrinsic layer investigation focused on the effect of substrate temperature, deposition power, deposition pressure, and the gas-flow ratio. Intrinsic layers deposited at the amorphous-to-nanocrystalline transition during growth are investigated in detail. At this transition nc-Si:H films with favorable properties for application in thin film solar cells can be made. Within our growth regime, this transition shows high sensitivity to the deposition parameters hence narrowing the process window. We show that materials deposited at the amorphous-to-nanocrystalline transition, but at different deposition settings, can have similar crystalline mass fraction while showing different electrical properties. In chapter 4 we present how seed layers are used to enhance crystal nucleation at the onset of nc-Si:H growth. By this approach, uniform and rapid evolution of the crystalline mass fraction as a function of thickness is obtained. Our results show a possibility to grow thin-film nc-Si:H without the usual amorphous incubation layer. A depth profile Raman technique that enables the ex-situ investigation of the crystalline mass fraction depth profile in nc-Si:H films is presented. This approach reveals different growth phases in the development of nc-Si:H. From transmission electron microscopy (TEM) analyses, we observe that crystal sizes are not affected by seed layers. However, horizontal cracks are observed to dominate the early growth of nc-Si:H in p-i-n solar cells and this effect is reduced upon seeding. For the n-i-p cells the appearance of these cracks is not affected by seeding. X-ray diffraction (XRD) results indicate that the use of seed layers does not affect the crystal size, but affects the preferential orientation of the crystals. Solar-cell external parameters show that seeding of p-i-n solar cells leads to an increase in solar cell efficiency, mainly due to increase in the short-circuit current density. The investigation of seeding on the crystallinity development is further extended in chapter 5. Here, we show that different substrates have different profile for the development of the crystalline mass fraction. For the three substrates investigated, we found a gradual development of the crystalline mass fraction, starting from the amorphous incubation layer. By means of seeding, rapid nucleation is observed as indicated by the high value of the crystalline mass fraction from the onset of growth. The effect of the substrate is reduced as all three substrates show a similar development profile of the crystalline mass fraction upon seeding. In the last chapter of this thesis, the possibility to use Raman spectroscopy to determine the preferred crystal orientation in nc-Si:H is demonstrated. The preferred orientation of crystals in nc-Si:H can give insight into the film growth mechanism and is often determined from transmission electron microscopy and x-ray diffraction. The method presented in this thesis is based on the fact that molecular vibrations in films under polarized light give rise to polarization-dependent Raman scattering intensity, depending on the grain crystal orientation of the irradiated material. This approach has been tested on a series of nc-Si:H samples and the results comparable with x-ray diffraction results.Electrical sustainable energyElectrical Engineering, Mathematics and Computer Scienc
Degradation of CIGS solar cells
No full textThin film CIGS solar cells and individual layers within these solar cells have been tested in order to assess their long term stability. Alongside with the execution of standard tests, in which elevated temperatures and humidity levels are used, the solar cells have also been exposed to a combination of elevated temperature and humidity and illumination, which also allowed in-situ analysis of the changes in the electrical parameters. Additionally, the samples have been tested in the presence of water and various atmospheric species, like CO2, nitrogen and oxygen, in order to assess the impact of these species. Based on these experiments, it was concluded that CIGS solar cells can rapidly lose efficiency due to the migration of sodium, which occurs when exposed to illumination and water vapor. It was also observed that the transparent top electrode, consisting of ZnO:Al degraded rapidly in the presence of a combination of water and CO2, while it is stable in the presence of water combined with e.g. N2 and O2. The thesis also contains an extensive literature study on the stability of CIGS solar cells and a study on the temperature dependency of these solar cells.PhotoVoltaic Devices and MaterialsElectrical Engineering, Mathematics and Computer Scienc
Improvement of a-Si:H devices by analysis, simulations and experiment
No full textDue increasing energy use per capita and increasing world population, the world faces an energy-crisis. However, the energy required to solve it is abundantly available: solar energy. The technology to convert this energy to electricity, solar cell technology, is well-developed. The technical issue is how to manufacture solar cells at such a large scale that they can produce a significant part of the global energy demand. For that purpose the cost of solar electricity must be reduced. Solar cells based on hydrogenated amorphous silicon (a-Si:H) offer the potential of low-cost solar electricity, mainly because of two reasons. Firstly, a-Si:H solar cells are manufactured at low temperatures, thus saving energy. Secondly, the costs for raw materials are significantly reduced as the solar cells are very thin (Ç 300 nm). Cost can further be reduced by enhancing the conversion efficiency. That is the topic of this thesis. Before we can study how to improve the performance, first we require a good understanding of the loss factors in a-Si:H solar cells. In chapter 2 we perform a systematic study of the electrical losses in the a-Si:H material itself, i.e. the recombination of charge carriers, and investigate the effect of different recombination paths on the theoretical maximal performance of a solar cell based on a-Si:H. We find that recombination processes limit the open circuit voltage of these solar cells. In actual a-Si:H solar cells, the recombination at the p-i interface is a significant loss-factor. Simulations in chapter 3 show that the charge carrier density at the pi interface is very high. By inserting a thin, high band-gap material with suitable electrical properties at this interface we show that the charge carrier density and consequently the losses at this interface can be reduced, leading to a higher open circuit voltage and efficiency of the solar cell. In the second part of chapter 3 we search for a material that has these properties by experiments on proto-crystalline silicon (pc-Si:H), hydrogenated amorphous silicon carbide (a-SiC:H) and hydrogenated amorphous silicon nitride (a-SiN:H). Consecutively, these materials are tested in solar cells. We find that a-SiN:H is unsuitable and that a-SiC:H gives better performance than pc-Si:H. In the final chapter we optimize the deposition process of a-Si:H material for use in an industrial flexible solar cell product. For that reason a material study is performed. The relations between the measurements of the hydrogen content, band gap, Urbach energy and defect density are explained in terms of a novel model for a-Si:H, the disordered network with hydrogenated vacancies (DNHV). Next lab-scale solar cells on glass substrate have been optimized. The results are transferred to the industrial proces on flexible substrate.PVMDElectrical Engineering, Mathematics and Computer Scienc
Doped nanocrystalline silicon oxide for use as (intermediate) reflecting layers in thin-film silicon solar cells
No full textIn summary, this thesis shows the development and nanostructure analysis of doped silicon oxide layers. These layers are applied in thin-film silicon single and double junction solar cells. Concepts of intermediate reflectors (IR), consisting of silicon and/or zinc oxide, are applied in tandem cells. Multi-stack Bragg reflector IRs are tested in tandem cells, increasing the top cell current output. Finally, mechanical polishing is applied on intermediate reflectors, creating asymmetrically textured IRs. Doped silicon oxide layers have proven their versatility as multipurpose layers in thin-film silicon solar cells. In chapter 3, the search for device grade n- and p-doped silicon oxide material is described. The nanostructure of silicon oxide films with a wide array of optical and electrical properties is studied in detail by TEM, Raman, FTIR and XPS. Silicon oxide is found to be a very heterogeneous material with complex nanostructure. Both the amorphous and crystalline phases of silicon oxide are studied in detail. Differences are found between the p- and n-doped materials. It is found that the n-doped material has a nanostructure of silicon crystal grains embedded in an amorphous silicon oxide matrix. The p-doped material, however, contains silicon filaments in an amorphous silicon oxide matrix. These filaments are of intrinsic amorphous silicon with crystalline silicon grains. Intrinsic amorphous silicon is mainly responsible for good conductivity in both n-doped and p-doped silicon oxide, however, minimum crystalline content is also required. Finally, the relations between each phase and element content is related to optical and electrical properties. N-doped silicon oxide used as a back reflector in single junction solar cells reflects unabsorbed light back into the absorber layer, increasing its current output. The blue part of the spectrum is absorbed in one pass, therefore the response in the red part of the spectrum is expected to increase. However, an increase in the blue part of the spectrum is observed as well and is the topic of chapter 4. This increase is attributed to a combination of factors, but mostly to the prevention of a native oxide formation on the standard a-Si:H n-layer. The standard n-layer is covered with the n-doped silicon oxide layer which prevents the standard layer from oxidizing in ambient air. The silicon oxide also provides a better contact interface with silver. Other factors increasing the blue response include: 1. The lower activation energy of n-doped silicon oxide in comparison with the standard a-Si:H n-layer. 2. The changing of the band states due to the larger bandgap of n-doped silicon oxide in reference to n-doped a-Si:H. 3. The thinner a-Si:H n-layer as the one in the reference cell is twice as thick. 4. The lower parasitic plasmonic absorption in the silver back contact due to the common interface with silicon oxide. P-doped silicon oxide exhibits anti-reflective properties, increasing cell current output in the blue part of the spectrum as well. An initial efficiency of 11.4% is achieved with the application of both p- and n-doped silicon oxide layers in a single junction a-Si:H solar cell. Intermediate reflector concepts are explored in chapter 5. Distributed Bragg Reflectors (DBR) have tunable reflective properties and are an interesting candidate for intermediate reflectors in tandem cells. They exhibit nearly the same reflectance range under various angles of incidence. DBRs can be easily designed with the help of optical simulation software such as ASA. The design sequence is as follows: 1. The desired reflectance range inside a solar cell is simulated by varying the thickness of each material. 2. This stack is then simulated in a glass – air environment. 3. The stack is deposited on a glass substrate. 4. The measured reflectance is compared with the air – glass simulation. If a good fit is achieved, the DBR will give the desired simulated reflectance inside the cell. DBRs greatly enhance the top cell current in a tandem cell, reaching up to 13,5 mA/cm2 in a 175 nm-thick a-Si:H layer. Current matching and lowering of Voc remain issues. Texture control in the IR is important in order to provide good light scattering for both the top and bottom cells of a tandem and to provide a good substrate for the growth of a defect-free nanocrystalline absorber layer. An approach to modify the texture of ZnO serving as an asymmetric IR in a tandem cell is developed. Because of the excellent performance of the top amorphous silicon cell deposited on an Asahi VU substrate, it is beneficial to keep this substrate texture for the top cell and integrate different textures (with larger surface features) in the layers processed after the top cell. Two approaches to create an asymmetrically-textured IR are chosen: wet etching and mechanical polishing. The wet etching approach is done with two dilution levels of HCl. Then the IR interface facing the top cell has a typical Asahi VU texture while the IR interface facing the bottom cell has larger surface features beneficial for long-wavelength scattering. The second approach is about applying mechanical polishing to silicon oxide and ZnO IRs. This approach successfully flattened the Asahi-induced texture, leaving it in the IR interface facing the top cell and on the other flat side allowing higher-quality nc-Si:H growth.Electric Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc
Materials and Light Management for High-Efficiency Thin-Film Silicon Solar Cells
No full textDirect conversion of sunlight into electricity is one of the most promising approaches to provide sufficient renewable energy for humankind. Solar cells are such devices which can efficiently generate electricity from sunlight through the photovoltaic effect. Thin-film silicon solar cells, a type of photovoltaic (PV) devices which deploy the chemical-vapor-deposited hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) and their alloys as the absorber layers and doped layers, are one of the promising PV technologies. Compared to other PV technologies, thin-film silicon solar cells have several important advantages such as the use of abundant and non-toxic materials, low processing temperature, short energy payback time and mature large-area manufacturing techniques. Despite the many advantages, thin-film silicon (TF-Si) technology is suffering from the drop in the PV market share due to the relatively low efficiency compared to c-Si, CIGS, and CdTe solar cells. This thesis is devoted to the development of advanced materials and novel light-trapping structures to increase the power conversion efficiency of thin-film silicon solar cells. To achieve maximal light absorption in the absorber layers, implementation of light-trapping structures is crucial for thin-film silicon solar cells. The status of light-trapping techniques is briefly summarized in chapter 1, together with the background knowledge for thin-film silicon solar cells. To design an effective light-trapping scheme for solar cells, both the optical performance and the influence on the electrical performance of solar cells have to be considered. The light-trapping structure itself should not give additional parasitic absorption losses, or this loss should be minimized. The morphology of the light-trapping substrate should be suitable for the growth of high-quality materials. Meanwhile, absorption losses in the supporting layers such as front electrodes, doped layers, and back reflectors should be minimized. In chapter 2, the fabrication of plasmonic back reflectors (BRs) based on self-assembled Ag nanoparticles and their application in a-Si:H solar cells are presented. It has been experimentally demonstrated that the optimized plasmonic back reflector can provide light trapping performance comparable to state-of-the-art random textures, without obvious deterioration of open-circuit voltage (Voc) and/or fill factor (FF). This conclusion is based on the fair comparison with high performance n-i-p solar cells and state-of-the-art p-i-n solar cells deposited on Asahi-VU substrates. The combined optical and electrical design of plasmonic back reflectors follows in chapter 3. The design rules of plasmonic back reflectors based on self-assembled Ag nanoparticles are discussed in detail. The shape of Ag NPs, the thickness of ZnO:Al spacer layers, materials on top of Ag NPs, and nanoparticle size are crucial for the performance of plasmonic BRs. By following the design ruless, an 8.4% efficiency plasmonic a-Si:H solar cell has been achieved. The application of the plasmonic back reflector in low bandgap nc-Si:H solar cells is discussed in chapter 4. The light trapping performance in nc-Si:H solar cells is improved by using the plasmonic BRs with a broad angular scattering and low parasitic absorption loss through tuning the size of silver nanoparticles. The nc-Si:H solar cells deposited on the improved plasmonic BRs demonstrate a high photocurrent comparable to the one achieved by the state-of-the-art textured Ag/ZnO BR. The commonly observed deterioration of fill factor is avoided by using nc-SiOx:H as the n-layer for solar cells deposited on plasmonic BRs. In chapter 5, micro-textures on glass with large opening angles and smooth U-shape morphology are proposed and applied to nc-Si:H solar cells for the first time. The micro-textures can provide both efficient light trapping and suitable morphology for the growth of high-quality nc-Si:H materials under a high deposition rate. A higher Voc and FF can be achieved in reference to the cells using nano-textured substrates. For thick solar cells (i-layer thicker than 2 µm), high Voc and FF values are maintained. Particularly, the Voc only drops from 564 to 541 mV as solar cell thickness increases from 1 to 5 ?m. The use of micro-textures paves the road to develop multijunction solar cells with a higher efficiency as will be shown in chapter 7. High-efficiency multijunction thin-film silicon solar cells require both high Voc and high blue spectral response in the top a-Si:H cell. In chapter 6, the mixed-phase p-SiOx films are investigated and used as window layer in high Voc a-Si:H p-i-n solar cells. The use of p-SiOx as window layer results in a higher Voc and a better spectral response than the standard p-SiC based window layer. Consequently, a-Si:H solar cells with Voc >1 V and FF >70% have been obtained. A high initial efficiency of 14.4% has been achieved in a-Si:H/nc-Si:H tandem solar cells deposited on the Asahi-VU substrates. Chapter 7 presents the implementation of highly transparent modulated-surface-textured (MST) front electrodes as light-trapping structures in multijunction TF-Si solar cells. The MST substrates comprise a micro-textured glass as developed in chapter 5, a thin layer of hydrogenated indium oxide (IOH), and a sub-micron nano-textured ZnO layer grown by low-pressure chemical vapor deposition (LPCVD ZnO). The MST front electrode has a good transparency and conductance, can provide efficient light trapping for each subcell and a suitable morphology for the growth of high-quality silicon layers. Efficiencies of 14.8% (initial) and 12.5% (stable) have been achieved for a-Si:H/nc-Si:H tandem solar cells with the MST front electrode and the high-performance a-Si:H top cells as developed in chapter 6, surpassing efficiencies obtained on state-of-the-art LPCVD ZnO. A short summary of this thesis is given in chapter 8. Perspectives to further improve the performance of thin-film silicon solar cells are suggested and discussed. The light-trapping performance of modulated-surface-textured front electrodes can be further improved by replacing the wet-etched glass with honeycomb textures, without sacrifice in electrical performance of solar cells. The honeycomb textures can be easily applied to superstrate configuration by mature UV-NIL technique. In the end, the hybrid a-Si:H/organic multijunction device configuration is proposed to avoid the use of thick nc-Si:H solar cells. A high efficiency of 11.6% has been achieved in the hybrid tandem configuration with a total absorber layer thickness less than 500 nm. By deploying the triple-junction structure, a high efficiency of 13.2% has been obtained while the thickness of absorber layers stack is below 1µm. With further efforts on this concept, performance comparable to the traditional devices based on a-Si:H and nc-Si:H can be expected while the total processing time is much shorter and the cost for manufacturing and materials is lower.Electrical Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc
On the Scalar Scattering Theory for Thin-Film Solar Cells
No full textNano-textured interfaces between two media of different refractive indices scatter light. The angular distribution and the intensity of the scattered light are deter- mined by the geometry of the nano-textures and the difference of the refractive indices of the two media. Thin-film silicon solar cells (TFSSC), which convert sunlight directly into electricity, have nano-textured interfaces. These interfaces scatter the light incident on the solar cell. The scattering leads to a longer average path length of the photons in the absorber layer of the solar cell. Therefore more light can be absorbed and thus converted to electricity. To introduce nano-textured interfaces into the solar cells, usually transparent conductive oxide (TCO) layers are used. Some TCO materials obtain nano-textured surfaces during the production process, while others are made rough by post processing, e.g. by etching. Nano-textures have been successfully implemented in TFSSC for almost 30 years by academia and industry; however, theoretical investigations on the relation between the nano-textures and the scattered electromagnetic fields have only been performed for about ten years. It is very important to investigate how the nano-textures can be optimized for scattering. In this thesis a scattering model is developed to tackle this important problem. The scattering model is based on the scalar scattering theory, i.e. it neglects the vector- character of the electromagnetic field and thus the light. Despite this strong assumption we have demonstrated that the model is suitable for simulating descriptive parameters of the scattered field in both reflection and transmission. The model is based on the fact that the transmitted field behind the nano-texture and the scattered field are related via Fourier transforms. By making simple assumptions for the transmitted field the model can be implemented using Fast Fourier trans- form algorithms, i.e. the model is very fast. The scattering model is formulated such that in principle it works for rough interfaces between arbitrary materials. We successfully evaluated it for several of these interfaces. We further showed that the model is also able to produce first predictions for the scattering parameters at oblique incidence. However, in this case the deviations between measured and simulated values are larger. Combining the scattering model with the ASA opto-electrical device simulator allows predicting how the nano-textures affect the performance of solar-cells. This combination can also be used to perform the major motivation for the development of scattering models: To investigate how the morphology of the nano- textures can be optimised. For this optimisation we use the “simulated annealing” optimisation algorithm. The optimisation and a subsequent evaluation reveal that the lateral feature size of the nano-textures is crucial for scattering into large angles: The smaller the lateral feature size, the more light is scattered into large angles. If, however, the lateral feature size becomes too small, less light is scattered since the nano texture then appears as effective medium. The vertical feature size hardly influences the shape of the scattered field. Nonetheless, it determines the fraction of the total light that is scattered away from the specular direction. If the rms-roughness, a measure for the vertical modulation of the texture, is kept constant, a nano- texture with the optimal lateral feature size is preferable to a texture that consists of a superposition of textures with different lateral feature sizes. However, due to the effect of the nano-textures on the electrical properties of the solar cells, a superposition of a texture consisting of large lateral and vertical features with another texture with small lateral and vertical features is preferable to a texture consisting of small lateral but large vertical features, i.e. sharp spikes. The results of our work give the direction to push absorption in solar cells towards the theoretical limits.Electrical Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc
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