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Langzeit Stabilisation der Organischen Solarzellen mittels Additiven
No full textOrganische Solarzellen (OPV) bestechen durch hohes wirtschaftliches
Potenzial. Große Anstrengungen wurden bereits in Hinblick auf die Effizienz
unternommen. Aktuelle Spitzenwirkungsgrade liegen bei 12%, vergleichbar mit
anorganischen Dünnschichtsolarzellen. Allerdings bleibt die Stabilität eine
Schwachstelle auf dem Weg zur Marktreife. Aufgrund ihrer organischen Natur,
ist OPV sehr empfindlich gegenüber Licht, Wärme, Sauerstoff und
Feuchtigkeit, wobei alle unter Standard-Arbeitsbedingungen gleichzeitig
vorliegen. In meiner Arbeit untersuche ich die Stabilisierung gegen
photooxidative Degradation, unter Verwendung zusätzlicher stabilisierender
Verbindungen, welche direkt in die Aktivschicht eingebracht werden. Hierzu
wurde eine Vielzahl an Verbindungen mit unterschiedlichen
Stabilisierungsmechanismen im Modell-System
Poly(3-Hexylthiophen-2,5-diyl):Phenyl-C61-Buttersäure Methylester
(P3HT:[60]PCBM) getestet. Die hergestellten Solarzellen wurden unter
Beleuchtung, bei mäßiger Temperatur und in Gegenwart von Luft gealtert und
ihre Langzeitstabilität evaluiert. Eine enorme Zunahme der akkumulierten
Leistungserzeugung um einen Faktor größer 3, unter Verwendung von
Wasserstoffdonatoren und UV-Absorbern, konnte mittels spektroskopischer und
mikroskopischer Untersuchungen auf eine verringerte Bildung freier Radikale
zurückgeführt werden. Meine Arbeit unterstreicht das Potenzial von
Additiven zur Stabilisierung von OPV. Es muss allerdings beachtet werden,
dass das Additiv die Schichtmorphologie und photophysikalischen Prozesse
innerhalb der photoaktiven Schicht nicht negativ beeinflussen. Anhand des
Modellsystems P3HT:[60]PCBM, konnte gezeigt werden, dass die photoaktive
Schichten bestehend aus einer ternären Mischung von Polymer, Fulleren und
Stabilisator angewendet werden können, um die Lebensdauer von OPV
wesentlich zu verlängern. Dieser Ansatz kann prinzipiell auf andere OPV
Systeme übertragen werden, sofern das Additiv die geschilderten
Anforderungen erfüllt. Durch Zugabe kleinster Mengen kostengünstiger
Stabilisatoren kann, unabhängig von der intrinsischen photochemischen
Stabilität der eingesetzten Materialien, eine signifikante Verbesserung der
Lebensdauer und somit gesteigertem Wert des Gesamtsystems organische
Solarzelle erreicht werden. Daher haben diese Erkenntnisse eine große
Bedeutung für die Vermarktung von OPV.Organic solar cells (OSC) have a high commercial potential, although
certain issues still have to be resolved. Big efforts have already been
invested into perfecting the constituent materials with regard to
efficiency, and the current peak efficiency lies at 12 %, as reported by
Heliatek on vacuum deposited oligomer tandem solar cells. However, the
stability of devices remains their weak point. Due to their organic nature,
OSC are extremely sensitive to light, heat, oxygen and humidity, all of
which are commonly present in standard working conditions. In my thesis I
examine photooxidative stabilization by introducing stabilizing compounds
into the active layer. A variety of compounds with different stabilizing
mechanisms, including radical scavengers, hydrogen donors, UV absorbers and
hydroperoxide decomposers, were tested in the model active layer system
consisting of poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid
methyl ester (P3HT:[60]PCBM). Such devices were aged in lifetime setup at 1
Sun illumination at moderate temperature in air. A tremendous increase in
accumulated power generation, by over a factor of 3, was achieved with
stabilizers of hydrogen donor and UV absorber classes, which was
attributed, via spectroscopic and microscopic studies, to a reduction in
formation of free radicals. These results underscore the potential of
compounds known to successfully stabilize insulating plastic for
stabilization of OSC. However, unlike in the case of insulating plastics,
additional care has to be given that the energy levels of the additive are
not overlapping with the effective band gap of the photoactive layer and
that the resulting blend morphology does not disrupt the photoinduced
charge transfer. Using P3HT:[60]PCBM as the model system, the proof of
principle could be demonstrated that ternary blending OSC photoactive
layers of with antioxidants and UV absorbers can be applied to
substantially prolong the lifetime of devices. This approach could easily
be extended to other polymer:fullerene systems, under the condition that
its presence does not influence the morphology. These findings hold a great
importance for the commercialization of OSC, as they offer a route to
achieving highly reliable devices by adding small portions of cheap
stabilizer compounds, regardless of the intrinsic photochemical stability
of the polymer and the fulleren
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
Inhibiting Photo-oxidative Degradation in Organic Solar Cells using Stabilizing Additives
No full textIn the last decades, organic electronics has become an important field due to their numerous advantages, among others low-cost processability, high scalability, flexibility, light weight and short energy payback time. However, due to their organic nature, the functioning of the devices is impaired in the presence of oxygen, light, heat and humidity — factors found in a normal working environment. Several directions can be followed in order to minimize these losses. Main strategies include development of efficient encapsulation layers, implementation of additional barrier and UV blocking layers, synthesis of intrinsically more photochemically stable active layer materials, and stabilization of active layers by blending of stabilizing compounds such as antioxidants, radical scavengers and UV absorbers. This chapter places the focus on additive-assisted stabilization of organic devices, specifically organic photovoltaics. In this context, we are going to give an overview of the main classes of compounds that are being used for this purpose, and discuss the role of additives to promote both photochemical and morphological stabilization. The chapter will give an overview of the existing results in the field, along with an extensive collection of additives known to have been tested in different organic devices
Organic light-emitting transistors
No full textOrganic light-emitting transistors (OLETs) combine the switching properties of a transistor with the emission properties of a light-emitting diode in a single device. Such multifunctional devices can potentially simplify the circuitry for displays and can provide very high electroluminescence quantum efficiencies. Also, the planar geometry allows integration of various optical components such as waveguides and resonators with the transistor platform for more complex devices. Finally, OLETs are suitable model systems for studies of various physical processes in organic devices including charge injection and transport as well as emission processes. In this chapter, we review some of the latest developments in the field of OLETs. The first part describes the device geometry, fabrication processes and typically used materials, while the second section discusses the light-generation processes. The third section will provide examples of OLET devices with integrated photonic elements to spectrally tune the emission, while the fourth and final section will provide a summary and outlook
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
Degradation Mechanisms in Organic Photovoltaic Devices
No full textPhotovoltaic devices are subjected to many stresses during their lifetimes: thermal stress, continuous illumination, and mechanical deformation. Depending on the quality of the encapsulation barrier used, contaminants such as water and oxygen can also diffuse into the devices. These different factors can lead to intrinsic degradation (e.g. morphological changes) as well as extrinsic degradation (e.g. photo-oxidation). Numerous degradation pathways can be exhibited by a device under operating conditions, and in order to understand the different mechanisms present, it is important to study ageing systematically. This includes studying single components of devices, such as the donor organic semi-conductor, or complete devices - including, in particular the organic-organic or organic-inorganic interfaces.</p
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Get PDFWe 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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