1,720,962 research outputs found
Ultra‐Thin Metal Oxide Superstructure of Pd(001) as Passivation Interlayer at Organic/Metal Interface
Full text linkAbstract At organic molecule/metal interfaces for electronic applications, it is required of the metal surface to be passivated in view of preserving the molecular properties of the ordered organic layer. This can be achieved by screening the metal with a single atomic layer of O, namely, ultra‐thin metal oxide (UTMO) layers. Cobalt tetraphenylporphyrins (CoTPP) on oxygen passivated Fe(001), with 1 ML O coverage, have revealed a molecule/substrate decoupling effect due to the formation of an ultra‐thin Fe oxide layer at the interface. However, the threshold concentration of surface O required to observe the decoupling effect has not been assessed yet. In this work, the possibility of stabilizing different ultra‐thin Pd oxide superstructures, characterized by a different number of O atoms per unit cell, is exploited to investigate the O decoupling effect on CoTPP films. Two Pd oxide superstructures are considered: Pd(001)‐p(2 × 2)O and Pd(001)‐p(√5 × √5)R27°O, with 0.25 and 0.80 ML O coverages, respectively, which are characterized by low‐energy electron diffraction (LEED), X‐ray and ultra‐violet photoelectron spectroscopies (XPS/UPS) and inverse photoemission spectroscopy (IPES). The results suggest a lower limit of 0.80 ML O coverage as a passivation interlayer to obtain an ordered and decoupled CoTPP monolayer on Pd(001)
Probing the correlation between morphology and optical anisotropy in ZnTPP films grown at different temperatures
Full text linkFilms of tetraphenylporphyrins (TPP) have been recently investigated for their ability in protecting graphite electrode surfaces from degradation. The effectiveness of the protection depends on both the molecule–substrate and molecule–molecule interactions, which determine the type of film growth and its morphology. For instance, meso-tetraphenyl porphyrin-Zn(II) (ZnTPP) arranges differently depending on the substrate, conditions used for deposition and post-growth treatments. Since many parameters influence film morphology and strategies to reach an efficient protective coverage, here we investigate the role of the substrate temperature in determining morphological variations in thick ZnTPP films. ZnTPP molecules were sublimated by an organic molecular beam epitaxy (OMBE) system on a highly oriented pyrolytic graphite (HOPG) substrate, controlled in temperature by a variable temperature cryostat. The film morphology was characterized by atomic force microscopy (AFM), whereas the type of growth by reflectance anisotropy spectroscopy (RAS), which is highly sensitive to the orientation and type of arrangement of molecules on the substrate; changes in the film valence electronic structure were monitored in situ by UV photoemission spectroscopy (UPS). The comparison between AFM and RAS investigations clearly correlates a ZnTPP film morphology to the characteristic optical signal, both influenced by temperature conditions and diffusivity of molecules on the substrate
Photoelectron spectroscopic studies of alkali-treated Cu(In,Ga)Se2 absorbers and simulation studies of solar cell performances
Full text linkA rise in conversion efficiencies of Cu(In,Ga)Se2 (CIGSe)-based solar cells in recent years due to alkali treatment of CIGSe absorbers led to this thesis work. It presents a comprehensive study of the interaction of the alkali elements sodium (Na), potassium (K) and rubidium (Rb) with CIGSe absorbers. A combination of soft and hard X-ray photoelectron spectroscopy (XPS/UPS, HAXPES), X-ray absorption near-edge structure (XANES) spectroscopy, electrical measurements (J-V), numerical simulations (SCAPS) and chemical system modelling (DFT) has been used to gain a deeper insight into the chemical and electronic effects of the alkali treatment on absorber surfaces and at absorber/buffer interfaces, which are the key areas that determine the open-circuit voltage (Voc) and fill factor (FF) enhancement in devices. Starting with Na, an Auger parameter analysis led to the observation of different Na speciation at CIGSe surfaces with Na from post-deposition treatment (PDT) and Na from soda-lime glass (SLG) substrate. Na from PDT formed a Na-containing complex (NaxCu1-x)(InyOz) compound. Whereas, no interaction between Na from SLG and CIGSe-related elements was found. From device simulations it was found that Na PDT onto CIGSe might not always be beneficial for devices. It results in reduced electron affinity (χ) and increased acceptor density (NA) at the absorber/buffer interface, which lowers the FF. In contrast, CIGSe deposited on SLG substrate resulted in a type-inversion at the absorber/buffer interface. In a Na+K-treated CIGSe absorber, quantitative XPS analysis showed Cu-deficiency and Se-enrichment at the absorber surface, which could assist Na in diffusing away from the surface, leaving a higher K surface content. A surface band gap (Eg) of 2.46±0.31 eV was confirmed with UPS and XANES measurements. Device simulations indicated that the presence of such a surface layer on CIGSe might contribute to a Voc enhancement but not without a FF loss. K incorporation in CIGSe led to real devices with significantly improved Voc and FF. A 113 mV gain in Voc in the real devices could be reproduced with device simulations of a Cu(1-x)KxIn(1-y)GaySe2 (CKIGSe) absorber with a 0.12 eV higher Eg than that of CIGSe such that this difference in Eg of the absorbers alone manifested as the difference in Voc values of the respective devices. This shows the effect of bulk recombination on Voc gain. However, using the CKIGSe/CdS device model, the FF gain could not be reproduced. One of the factors that may influence the FF is type-inversion, which, in turn, is influenced by χ and defect-induced doping concentrations at absorber/buffer interfaces. Therefore, the alkali-induced chemical and electronic modifications observed at the KF- and RbF-treated CIGSe/CdS interfaces have been co-related to a theoretical model of the alkali metal induced point defects at a CuInSe2/CdS interface. Analysed with HAXPES bi-layer modelling, the near-interface regions showed Cu-poor, In-rich compositions at both the alkali-treated CIGSe/CdS interfaces. The KF-CIGSe/CdS interface showed low Cd-Cu, Cd-In intermixing and high S-Se intermixing, while the RbF-CIGSe/CdS interface showed little Cd-Cu intermixing. Cu vacancies (VCu defects) contribute to an increase in NA. Possible formation of KCu and RbCu defects could result in lower NA at the interfaces because of the Cu vacancies being filled up by K and Rb. NaCd and excess CdCu defects at the KF-CIGSe/CdS interface and only CdCu defects at the RbF-CIGSe/CdS interface might have formed that would result in higher donor densities (ND) at the interfaces. An increase in the valence band maxima w.r.t. the Fermi level in both alkali treatments indicated towards higher χ values. All these factors, which lead to enhanced type-inversion when applied in the device simulations, resulted in FF gain in addition to Voc gain. This shows the effect of interface recombination on FF gain.In den letzten Jahren konnten die Wirkungsgrade von Cu(In,Ga)Se2-Solarzellen (CIGSe) aufgrund einer neuartigen Alkalibehandlung deutlich gesteigert werden. Daher wurde in dieser Arbeit eine umfassende Studie über die Interaktion der Alkalielemente Natrium (Na), Kalium (K) und Rubidium (Rb) mit dem CIGSe-Absorber durchgeführt. Eine Kombination aus Weich- und Hartröntgen-Photoelektronenspektroskopie (XPS/UPS, HAXPES), Röntgenabsorptions-spektroskopie (XANES), elektrischen Messungen (J-V), numerischen Simulationen (SCAPS) und quantenmechanischen Materialmodellierungen (DFT) wurde verwendet, um einen tieferen Einblick in die chemischen und elektronischen Auswirkungen der Alkalibehandlung auf Absorberoberflächen und Absorber/Puffer-Grenzflächen zu erhalten. Diese Bereiche der Solarzelle sind es, die maßgeblich Leerlaufspannung (Voc) und Füllfaktor (FF) bestimmen. Ausgehend vom Na konnten durch Analyse der Augerparameter unterschiedliche Na-Verbindungen in Abhängigkeit davon nachgewiesen werden, ob eine Alkalinach-behandlung (PDT) durchgeführt wurde oder ob Na aus dem Substratglas stammte. Nach der PDT wurden komplexe (NaxCu1-x)(InyOz)-Verbindungen gefunden, während Na aus dem Glas keine nachweisbaren Verbindungen mit den Absorberelementen einging. Durch Simulationen wurde festgestellt, dass die Alkali-Nachbehandlung von CIGSe nicht immer vorteilhaft ist. Sie kann zu einer stark reduzierten Elektronenaffinität (χ) und erhöhter Akzeptordichte NA, an der Absorberoberfläche führen und dadurch den FF reduzieren. Im Gegensatz dazu führt das Vorhandensein von positiven Ladungen durch Na+ Ionen an der Oberfläche von CIGSe, welches auf einem Natron-Kalk-Glas-Substrat (SLG) abgeschieden wurde, zu einer vorteilhaften Typinversion. Durch den Einbau von Na und K in CIGSe wurde eine Cu-verarmte, jedoch K- und Se-angereicherte Oberflächenphase mit einer großen Bandlücke (Eg) von ca. 2,46±0,31 eV gebildet. Simulationen zeigen, dass das Vorhandensein einer solchen CIGSe-Oberflächenschicht zu einer Voc-Verbesserung beitragen kann, aber nicht ohne Verluste beim FF. Dagegen führte der K-Einbau in CIGSe-Absorbern zu deutlich verbesserten Werten in der Voc und dem FF bei realen Solarzellen. Ein Voc Zuwachs von ~113 mV der Solarzellen konnte durch Simulationen eines Cu(1-x)KxIn(1-y)GaySe2 (CKIGSe)-Absorbers mit einer 0,12 eV höheren Bandlücke als dem von CIGSe reproduziert werden, d.h. der Gewinn in der Voc geht lediglich auf die größere Bandlücke zurück. Um sowohl Voc als auch den FF modellieren zu können, wurden die Alkali-induzierten chemischen und elektronischen Veränderungen, die an den KF- und RbF-behandelten CIGSe/CdS-Grenzflächen beobachtet wurden, mit einem theoretischen Modell von alkalimetallinduzierten Punktdefekten an der CuInSe2/CdS-Grenzfläche in Beziehung gesetzt. Die Modellierung der HAXPES-Daten mit einem Zweischichtmodell offenbarte eine Cu-arme und In-reiche Zusammensetzung an beiden Alkali-behandelten CIGSe/CdS Grenzflächen. Die KF-CIGSe/CdS-Grenzfläche zeigte geringe Cd-Cu und Cd-In Interdiffusion und starke S-Se-Vermischung, während die RbF-CIGSe/CdS-Grenzfläche nur geringe Cd-Cu-Interdiffusion aufwies. Cu-Vakanzen (VCu) erhöhen die Akzeptordichte NA, während KCu- und RbCu-Defekte zu niedrigeren NA an der Grenzfläche führen können, weil die VCu durch K und Rb aufgefüllt werden. Die Bildung von NaCd- und CdCu-Defekten an der KF-CIGSe/CdS-Grenzfläche und CdCu-Defekten an der RbF-CIGSe/CdS-Grenzfläche können zu höheren Donatordichten ND führen. Eine Erhöhung des Valenzbandmaximums in Bezug zur Fermienergie nach den beiden Alkalibehandlungen deutete auf vergrößertes χ hin. Diese Faktoren, die bei der Anwendung in den Simulationen zu einer verstärkten Ladungsträgerinversion führen, zeigen neben dem Voc-Anstieg auch eine Verbesserung vom FF. Daher wurde der Schluss gezogen, dass der FF-Gewinn hauptsächlich durch die reduzierte Rekombination an den Grenzflächen beeinflusst wird
Non‐chemical route to PGM‐Free via N+ ion implantation in vertically aligned carbon nanotubes
No full textThe continuous research on electrocatalytic nanomaterials for critical raw materials replacement has returned the important class of Platinum-group-metal-free electrocatalysts (PGM-free), based on C, N, and non-noble transition metals. PGM-free are employed in Fuel Cells, Zinc-Air batteries, and Electrolyzers to catalyze important energy-related reactions. They are usually synthesized following a chemical route based on mixing, pyrolyzing, and postprocessing of specific N- and transition metal-containing compounds into conductive C structures, leading to a high content of byproducts. Herein, for the first time, a new approach for PGM-free synthesis is investigated, based on nitrogen implantation via ion beam produced with a Kaufman apparatus, and iron evaporation inside a clean chamber. The performed investigation, based on a model sp2 carbonaceous material (vertically aligned carbon nanotubes), showed similarities to the chemical route in terms of surface functionalization with the possibility to maximize the pyridinic N content, paving the way for the first time to the synthesis of nitrogen-compound-free nanoelectrocatalysts
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
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