1,720,953 research outputs found
Photosynthesis-inspired device architectures for organic photovoltaics
Full text linkThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 153-166).Organic semiconductor photovoltaics offer a promising route to low-cost, scalable, emissions-free electricity generation. However, achieving higher power conversion efficiencies is critical before these devices can play a larger role in our future energy generation landscape. Organic photovoltaic devices are currently limited by two primary challenges: (1) a trade-off between light absorption and exciton diffusion and (2) low open-circuit voltage due to charge recombination at the donor-acceptor interface. In this work, we demonstrate two new device architectures inspired by photosynthesis that aim to overcome these two challenges. First, we overcome the trade-off between light absorption and exciton diffusion by introducing an external light absorbing antenna layer. We model energy transfer from the antenna to the charge generating layers via surface plasmon polariton modes in the interfacial thin silver contact and via radiation into waveguide modes. We experimentally demonstrate devices with both single layer antennas and strongly absorbing resonant cavity antennas. We measure energy transfer efficiency from the antenna layer to the PV active layers as high as 51±10%. We discuss structural design criteria and describe ideal antenna material characteristics. Second, we reduce charge transfer state recombination in organic photovoltaics by inserting a thin interfacial layer at the donor-acceptor interface. The thin interfacial layer creates a cascade energy structure that destabilizes the Coulombically bound charge transfer state formed immediately following exciton dissociation. We nd the optimal interfacial layer thickness to be approximately 1.5 nm. In CuPc/C₆₀ devices, under simulated solar illumination the short-circuit current increased 34%, the open-circuit voltage increased 33%, and the power conversion eciency increased 49%. Thin interfacial layers can also be used to study the physics of exciton separation.by Timothy David Heidel.Ph.D
Tradeoffs between revenue enhancements and emissions reductions with energy storage-coupled photovoltaics
Full text linkThesis (S.M. in Technology and Policy)--Massachusetts Institute of Technology, Engineering Systems Division, Technology and Policy Program, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 111-114).Energy storage has the potential to dramatically change the operation of photovoltaics by allowing for a delay between generation and use. This flexibility has the potential to impact both the revenue from generating electricity using photovoltaics and the associated emissions reductions. This thesis attempts to quantify the impacts of adding energy storage to photovoltaics. The thesis formulates an optimization problem to solve for the optimal use of photovoltaics with energy storage from 2000 to 2005 in New England. The optimization is first solved using perfect information about historical solar generation, energy prices, and marginal emissions rates. Then, the model is solved using forecasted energy prices and emissions rates. The analysis finds that adding energy storage to photovoltaics can increase annual revenues by over 30%. With energy storage capacity and power equal to solar capacity, annual revenues were found to increase between 19.3% and 31.1% with an energy storage efficiency of 100%. Unfortuneately, the potential revenue increases were found to fall to between 9.1% and 21.3% with 80% efficient storage and between 3% and 14.5% with 60% efficient storage. However, when owners utilize energy storage to maximize revenue, the changes in avoided emissions with energy storage are found to be negligible. Alternatively, it is possible to achieve significant increases in the emissions offset by photovoltaics with energy storage. However, when energy storage is utilized to maximize emissions reductions, revenue decreases.(cont.) This tradeoff between the economic and environmental benefits that can be achieved when energy storage is added to photovoltaics means it is unlikely to be possible, without policy, to simultaneously achieve large increases in both revenue and avoided emissions. Policy mechanisms could be used to enable energy storage to enhance both the revenue from photovoltaics and avoided emissions.by Timothy David Heidel.S.M.in Technology and Polic
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
Surface plasmon polariton mediated energy transfer from external antennas into organic photovoltaic cells
Full text linkThesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.Includes bibliographical references (leaves 47-52).Despite significant improvements in the performance of organic photovoltaic devices in recent years, the tradeoff between light absorption and charge separation efficiency remains pervasive; increasing light absorption by increasing the device thickness leads to a decrease in exciton diffusion efficiency and vice versa. In this thesis, I demonstrate organic solar cells with an external light absorbing antenna. Light is absorbed by the external antenna and subsequently transferred into the photovoltaic cell via surface plasmon polariton modes in an interfacial thin silver contact. By decoupling the optical and electrical functions of the cell, this new architecture has the potential to circumvent the tradeoff between light absorption and charge separation efficiency. Non-radiative energy transfer is discussed and modeling finds that efficient energy transfer is mediated by surface plasmon polaritons. Devices with two very different antenna systems are demonstrated experimentally. Antennas with high photoluminescence efficiency are found to exhibit energy transfer efficiencies of approximately 50% while strongly absorbing antennas exhibit increases in photocurrent as high as 700% when compared to devices with non-functioning antennas even with very low photoluminescence efficiencies near 4%. These results suggest that this new device architecture could lead to significantly higher power conversion efficiencies by allowing the independent optimization of the optical and electrical components of organic photovoltaic cells.by Timothy David Heidel.M.Eng
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