1,721,273 research outputs found
Electron Transfer Dynamics in Dye-Sensitized Solar Cells
No full textIn this review, we address the materials design parameters that control the processes of charge separation, and thereby device efficiency, in dye-sensitized photoelectrochemical solar cells. The review starts with an overview of the structure, energetics and kinetics of dye-sensitized solar cells. It then goes on to consider in more detail the parameters determining the efficiency of the two primary charge separation steps in these devices: electron injection from the dye excited state into the metal oxide electrode, and regeneration of the dye ground state by the redox electrolyte. We consider the kinetic competition between these desired charge separation steps and the undesired loss pathways of excited state decay to ground and electron recombination with dye cations. The review avoids detailed mathematical and spectroscopic discussion, but rather tries to summarize the key conclusions relevant to materials design. A recurring theme of the review is the energy cost of achieving charge separation, and how this limits device performance. A further factor addressed in this review is real as opposed to ideal materials behavior, including, for example, consideration of the implications of empirical observations of an exponential density of acceptor states in the metal oxide, as well as identification of unresolved issues in our current understanding
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
Charge separation, triplets and photochemical stability in polymer / fullerene solar cells
Full text linkCharge generation and stability are key issues that have great impact upon the commercial viability of organic solar cells. In this thesis, a range of donor polymers, mainly of donor-acceptor class, were employed. Various materials characterisation, photophysical and photostability studies were performed on neat polymer films and polymer/fullerene blend films with the aim of establishing relationships between material structure and device function/stability.
Transient absorption spectroscopy (TAS) was employed for the photophysical studies on neat films. The photophysics of triplet excitons are found to strongly correlate with relative polymer crystallinity as determined from wide-angled x-ray diffraction (WAXD), with the more amorphous polymers exhibiting longer triplet lifetimes. The rate constant and yield of oxygen quenching of these triplet states also showed clear correlations with material crystallinity.
Charge generation in polymer/fullerene blend films was investigated using TAS and steady state optical spectroscopies. Compositional dependence studies with varying fullerene loadings were conducted on two polymers of different crystallinity, with a stronger dependence being observed in the more amorphous blend films. A comparison of charge generation pathways via electron or hole transfer suggests that the energetics between donor and acceptor can affect the efficiency of these pathways. This is consistent with the observation of a correlation between polaron yield and the energy offset driving charge separation for a series of blend films. Furthermore, the polaron yield estimated from TAS was correlated with device photocurrent.
Photochemical stability is of significant concern in organic solar cells, as organic materials are susceptible towards photo-oxidation. Accelerated photodegradation in neat and blend films was monitored using steady state absorption spectroscopy under oxygen atmosphere. More crystalline polymers with shorter triplet lifetimes are found to be more stable. The mechanism of photodegradation involving triplet-mediated singlet oxygen generation was investigated with a molecular fluorescent probe, and found to be a significant photodegradation pathway.Open Acces
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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