1,720,961 research outputs found
Photophysics of polymer : fullerene blends and organic photovoltaic devices
No full textSCGCGR-53 978; EPFL - SB - SCGC. - Consultable sur demande à la Bibliothèque de l'EPFL / Offered in consultation at the EPFL library
A Time-Resolved Photophysical Study of Hybrid Organic-Inorganic Perovskite Photovoltaic Materials
Full text linkThe search for new photovoltaic materials has been driven by the combined need to exploit sources of energy that are clean and sustainable, while simultaneously doing it in a cost effective manner. In this light, hybrid organic-inorganic perovskites have recently emerged as an extremely promising photonic material, and their application as a functional photovoltaic layer has resulted in device efficiencies that rival long established silicon based technologies. Rapid progress in device efficiencies have occurred over the last years (22.1% being the current record). However, the simultaneous growth in basic studies of the material have not resulted in a conclusive understanding of the fundamental process that occur subsequent to photoexcitation. Hence, there is a pressing need to identify the pathways that currently limit device performance and provide direction for future work in materials and device engineering. Towards this goal, we investigate two perovskite compositions (CH3NH3PbI3 and (FAPbI3)0.85(MAPbBr3)0.15) using time-resolved (THz and electroabsorption) spectroscopic techniques. Chapter 1 and 2 provide a general introduction into the investigated system and the experimental techniques that have been used. In chapter 3, we detail time-resolved THz measurements and report on experimental evidence for carrier recombination through an indirect transition, as well as a direct recombination pathway that is present at higher carrier densities. We calculate temperature dependent carrier mobilities (at THz frequencies) and bimolecular recombination constants. Through which we identify phonon scattering as the primary limiting mechanism for carrier transport, and temperature dependent bimolecular recombination that is mediated by the relative mobility of the charge carriers. Analysis of the complex photoconductivity spectra using the Drude-Smith model revealed a large difference in carrier scattering between the two perovskite films that could be attributed to the significantly different morphologies. In chapter 4 we apply time-resolved electroabsorption spectroscopy (TREAS) to insulated CH3NH3PbI3 layers and investigate the macroscopic carrier transport dynamics under applied electric fields. Transport within the 40nm perovskite grain was discovered to be diminished by a factor ¿ 2 relative to the high frequency mobility obtained through THz spectroscopy. The averaged carrier mobility across the 280nm film length was reduced by a factor ¿ 4, due to the presence of grain boundaries and defects. Preliminary investigations also identified spectral signatures associated with carrier accumulation at the perovskite interface and delayed extraction at the contacts. Chapter 5 deals with complete solar cells formed using (FAPbI3)0.85(MAPbBr3)0.15 as the active layer and we report the first application of the TREAS technique to complete perovskite devices. Our results reveal that the improved morphology of the film results in film averaged mobilities that are near the intrinsic values obtained using THz spectroscopy. Analysis of transient absorption spectra revealed an electroabsorption signature, its dynamics can be correlated with the disassociation of a transient excitonic species to form free charge carriers.GR-M
Charge Separation Pathways in a Highly Efficient Polymer:Fullerene Solar Cell Material
Full text linkPBDTTPD is one of the best conjugated polymers for solar cell applications (up to 8.5% efficiency). We have investigated the dynamics of charge generation in the blend with fullerene (PCBM) and addressed highly relevant topics such as the role of bulk heterojunction structure, fullerene excitation, and excess energy. We show that there are multiple charge separation pathways. These include electron transfer from photoexcited polymer, hole transfer from photoexcited PCBM, prompt (<100 fs) charge generation in intimately mixed polymer:fullerene regions (which can occur from hot states), as well as slower electron and hole transfer from excitons formed in pure PBDTTPD or PCBM domains (diffusion to an interface is necessary): Very interestingly, all the charge separation pathways are highly efficient. For example, the yield of long-lived carriers is not significantly affected by the excitation wavelength, although this changes the fraction of photons absorbed by PCBM and the amount of excess energy brought to the system. Overall, the favorable properties of the PBDTTPD:PCBM blend in terms of morphology and exciton delocalization allow excellent charge generation in all circumstances and strongly contribute to the high photovoltaic performance of the blend.GR-M
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
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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