Academic literature on the topic 'Meyer-Neldel'

Abstract Meyer-Neldel behaviour of the conductivity of phase separated La1−xCaxMnO3 manganite system in the low Ca-doping range has been investigated. Evolution of the isokinetic temperature of the conductivity, modified by Ca-doping, hydrostatic pressure and current bias has been determined. In addition, the evolution of the isokinetic temperature with ageing has also been studied. It is found that the Meyer-Neldel behaviour of the manganite system stems from multi-excitation entropy mechanism. The isokinetic temperatures estimated from pressure and doping effects coincide but differ from those determined using current and ageing controlled conductivity changes. It is concluded that in the presence of a detailed theoretical model of the excitations coupling in manganites, the investigations of the Meyer-Neldel effect may became a powerful tool for characterization and investigation of transport mechanisms in phase separated manganites.

2009 - 2010
Organic Thin-Film Transistor (OTFT) can be considered one of the building blocks of Organic Large-Area Electronics. The role of this kind of switching device is crucial in the organic information displays field but also in a wide range of possible applications which take advantage of such switching devices. For these reasons, major technology investments have been made to optimize the characteristics related to switching the power status of the pixel in order to obtain sufficient dynamics for a proper representation of moving pictures, movies, etc. In addition to technological and industrial fallouts of OTFT utilization, it should be noted that materials science in Organic Electronics often employs these transistors as an investigation method - an experiment – in order to characterize the physical properties of semiconductors, insulators and interfaces by leveraging device’ principles of operation and physics. The motivation of the present investigation is related to the evidence that gate dielectrics properties and dielectric-semiconductor interface physics are known to govern growth of partiallyordered channel films with a tremendous impact on the morphology of their polycrystalline phase and then to electric performances. Such relationships are still not clearly understood nor fully exploited in a wide spectrum of cases. Keeping in mind that gate insulators can be considered as a key-factor in OTFT device modeling and optimization, the purpose of this thesis work has been the analysis and the interpretation of the role played by such dielectrics and their interface in the organic thin-film transistors performance. The key aspects which have been investigated about dielectrics are the gate leakages and the models to extract the channel current, the relationship between wettability of dielectric surfaces and the growth of pentacene, the channel morphology, charge transport and its thermal activation. In particular, device’ operation regimes and performance parameters have been studied taking into account non-ideal behaviours which can hardly affect physical interpretations of charge transport mechanisms in organic semiconducting films and bring to misleading considerations. In such analysis, the parasitic gate dielectric conduction has been emphasized because it appears appealing both from a scientific point and from an industrial perspective. In fact, gate leakages often appear as a hidden problem in many literature reports and nevertheless they become dominant in technological considerations because they have a relevant impact when working on very thin insulating films or leaky dielectrics like polymers or solution-processed materials because they are responsible of static dissipation in OFET-based circuitry. In order to obtain improved devices, we studied the OTFT performances when varying the dielectric material. We considered the surface wettability as a key factor to be decreased in order to obtain performing channels. Thus, after taking into account standard gate dielectrics at different film thicknesses, and studying mobility in a gate-leakage-aware modeling framework, we acted on the nature and interface of insulators to increase the hydrophobicity and obtain a large-grain growth of pentacene channel semiconductor. In the experimental, among other things, we compared the utilization of highly hydrophobic compounds in gate dielectric layer fabrication to surface conditioning treatments of usual insulating polymers and to the deposition of buffer layers. In the aim to prepare an improved device, we introduced in device’ processing a novel insulating material, an organic-inorganic hybrid material based on a Tetraethyl Orthosilicate / 1H,1H,2H,2H-Perfluorodecyl triethoxysilane solution commonly named “PFTEOS:TEOS”. The abovementioned layer is characterized by perfluoroalkyl units which are responsible of the desired highly hydrophobic properties. It has been solutionprocessed and finally deposited by a spin-coating-based sol-gel technique on the metallic gate layer. Following an optimization path, a thin film (<10nm) of Poly(methyl methacrylate) has been employed to bufferize the PFTEOS:TEOS surface to reduce gate currents. A mobility-morphology trend for analyzed dielectrics in OTFTs has been extrapolated and analyzed denoting PFTEOS:TEOS as an exception to a well-assessed empirical rule. Escaping from obvious considerations about the effect of grain boundaries in channel performances, the singularity of PFTEOS:TEOS has lead to the adoption of thermal activation of charge carriers as an instrument to open to a deeper interpretation of channel defects. Thermal analyses of charge transport activation for the considered samples have been performed showing a general validity of the Meyer-Neldel rule also for hybrid dielectrics. Furthermore, the extraction of energetic parameters in Arrhenius plots applied to static electrical characterizations has revealed differences of maximum mobility trends versus the Meyer-Neldel characteristic Energy (EMN) when comparing polymer dielectric-based OTFTs to PFTEOS:TEOSbased OTFTs. The differences in dielectric/OSC interface appared to be correlated to the isokinetic temperature and activation energy and then to the disorder parameter “:” of the Density Of States in the valence band of the organic channel. Then, instead of considering the contribution of inband density of states of the channel material, the amplitude of the distribution of energetic states has been exploited in the investigation of surface properties and dielectric-specific features remarked. The activation energy analysis showed a trend inversion in the Meyer-Neldel Temperature (TMN)/mobility relationship between PMMA samples and PFTEOS:TEOS samples revealing an effect induced by the very nature of insulator rather than the OSC/dielectric interface on thermally activated processes. The dielectric is then acknowledged to be responsible of a wide range of thermally-activated behaviours in the response of disordered OSC used in OTFTs. Then, thermal analyses have proven to be a key discriminant factor to address non-conventional dielectrics surface-features characterizations in electronic devices able to quantify nanoscopic disorder in polycristalline mediums. In conclusion the behavior of a novel sol-gel gate insulator has been characterized and analyzed comparing it to plain cases and finding an original behavior of mobility/Activation energy which exhibits an inverse (decreasing) trend between energetic disorder and charge transport. This has been completely opposite to trends found for PMMA devices encouraging studying, exploiting and characterizing more in depth PFTEOS:TEOS material for OTFT fabrication purposes. Organic Electronics has still to face some key challenges to assert itself and become competitive in market sectors left still partially unexplored by the inorganic electronic technology. From this point of view, the possibility to exploit dielectric materials singularities to break technological performance trends, united to the availability of second-order modelling techniques both in insulator non-idealities and in charge transport activation can be a non-trivial starting point for further investigations.[edited by author]
IX n.s.

Šiame darbe nagrinėjamos galimybės patobulinti plačiajuostės dielektrinės spektroskopijos metodus naudojant skaitmeninius ir analitinius daugelio modų dielektrinės skvarbos skaičiavimo metodus, tiriami (1-x)(Na1/2 Bi1/2)TiO3 - xLa(Mg1/2 Ti1/2)O3 (NBT-LMT) keramikų grupės laidumo ypatumai. Nagrinėjama galimybė pritaikyti HFSS skaitmeninio modeliavimo programą dielektrinės spektroskopijos tikslams. Naudojant šią programinę įrangą, apskaičiuojama dielektrinė ir magnetinė skvarbos komplikuotiems mikrojuostelinės linijos ir dalinai užpildyto bangolaidžio matavimo grandinių atvejams. Pateikiami patobulinti kondensatoriaus koaksialinėje linijoje ir ribotų matmenų atviro galo koaksialinės linijos matematiniai modeliai. Šie modeliai patikrinami skaitmeniniu metodu. Naudojant daugiamodį kondensatoriaus modelį, atsižvelgus į magnetinio lauko pasiskirstymą koaksialinėje matavimo grandinėje, pasiūlomas būdas pamatuoti mažai bandinio magnetinei skvarbai, kai dielektrinė skvarba didelė (dešimtim ar šimtais kartų didesnė už magnetinę skvarbą). Atviro galo koaksialinei linijai siūloma keletas kalibravimo būdų, įrenginys bandiniui prispausti prie linijos. Bešvinė NBT-LMT keramika buvo tyrinėjama dielektrinės spektroskopijos metodais. Gauti rezultatai rodo, kad NBT-LMT keramikų grupėje elektriniam laidumui galioja Maerio-Neldelio taisyklė.
One of the basic problems in measurements of the electrical properties of materials is finding relations between measured electrical values and characteristics of the material, especially when the field distribution in the device under test with a sample inside is complex. Commercial electromagnetic simulation software was used in unconventional way to calculate materials' electrical properties. Because the software is not adopted for this task, built-in optimisation option was used. In this way dielectric and magnetic properties of materials in very complex shape were calculated. The method was tested by calculation of complex dielectric permittivity of Bi1.5ZnNb1.5O7-xF2 pyrochlore ceramics as well as by calculation of both complex dielectric permittivity and complex dielectric permeability of carbon-coated capsules of Ni embedded into polyurethane matrix from experimental results. Due to the fact that using numerical methods is time consuming, a mathematical model on mode matching approach of rectangular rod in a waveguide and a new model of multimode capacitor were developed. Models were checked by numerical methods. On a high frequency, when electric field in the sample is inhomogeneous, the magnetic field in the sample is much stronger than in the transmission line. Thus, the magnetic permeability of the sample will affect scattering parameters. Based on this circumstance, the method to measure small magnetic permeability using capacitor like in coaxial line when... [to full text]

Šiame darbe nagrinėjamos galimybės patobulinti plačiajuostės dielektrinės spektroskopijos metodus naudojant skaitmeninius ir analitinius daugelio modų dielektrinės skvarbos skaičiavimo metodus, tiriami (1-x)(Na1/2 Bi1/2)TiO3 - xLa(Mg1/2 Ti1/2)O3 (NBT-LMT) keramikų grupės laidumo ypatumai. Nagrinėjama galimybė pritaikyti HFSS skaitmeninio modeliavimo programą dielektrinės spektroskopijos tikslams. Naudojant šią programinę įrangą, apskaičiuojama dielektrinė ir magnetinė skvarbos komplikuotiems mikrojuostelinės linijos ir dalinai užpildyto bangolaidžio matavimo grandinių atvejams. Pateikiami patobulinti kondensatoriaus koaksialinėje linijoje ir ribotų matmenų atviro galo koaksialinės linijos matematiniai modeliai. Šie modeliai patikrinami skaitmeniniu metodu. Naudojant daugiamodį kondensatoriaus modelį, atsižvelgus į magnetinio lauko pasiskirstymą koaksialinėje matavimo grandinėje, pasiūlomas būdas pamatuoti mažai bandinio magnetinei skvarbai, kai dielektrinė skvarba didelė (dešimtim ar šimtais kartų didesnė už magnetinę skvarbą). Atviro galo koaksialinei linijai siūloma keletas kalibravimo būdų, įrenginys bandiniui prispausti prie linijos. Bešvinė NBT-LMT keramika buvo tyrinėjama dielektrinės spektroskopijos metodais. Gauti rezultatai rodo, kad NBT-LMT keramikų grupėje elektriniam laidumui galioja Maerio-Neldelio taisyklė.
Šiame darbe nagrinėjamos galimybės patobulinti plačiajuostės dielektrinės spektroskopijos metodus naudojant skaitmeninius ir analitinius daugelio modų dielektrinės skvarbos skaičiavimo metodus, tiriami (1-x)(Na1/2 Bi1/2)TiO3 - xLa(Mg1/2 Ti1/2)O3 (NBT-LMT) keramikų grupės laidumo ypatumai. Nagrinėjama galimybė pritaikyti HFSS skaitmeninio modeliavimo programą dielektrinės spektroskopijos tikslams. Naudojant šią programinę įrangą, apskaičiuojama dielektrinė ir magnetinė skvarbos komplikuotiems mikrojuostelinės linijos ir dalinai užpildyto bangolaidžio matavimo grandinių atvejams. Pateikiami patobulinti kondensatoriaus koaksialinėje linijoje ir ribotų matmenų atviro galo koaksialinės linijos matematiniai modeliai. Šie modeliai patikrinami skaitmeniniu metodu. Naudojant daugiamodį kondensatoriaus modelį, atsižvelgus į magnetinio lauko pasiskirstymą koaksialinėje matavimo grandinėje, pasiūlomas būdas pamatuoti mažai bandinio magnetinei skvarbai, kai dielektrinė skvarba didelė (dešimtim ar šimtais kartų didesnė už magnetinę skvarbą). Atviro galo koaksialinei linijai siūloma keletas kalibravimo būdų, įrenginys bandiniui prispausti prie linijos. Bešvinė NBT-LMT keramika buvo tyrinėjama dielektrinės spektroskopijos metodais. Gauti rezultatai rodo, kad NBT-LMT keramikų grupėje elektriniam laidumui galioja Maerio-Neldelio taisyklė.

Solarzellen aus organisch-anorganischen hybriden Perowskithalbleitern gelten als mögliche Schlüsseltechnologie zur Erzeugung günstiger und umweltfreundlicher elektrischer Energie und somit als Meilenstein für die Energiewende. Um die weltweit stetig wachsende Nachfrage an elektrischer Energie zu decken, bedarf es Solarzellentechnologien, welche gleichzeitig eine hohe Effizienz nahe dem Shockley-Queisser-Limit als auch eine hinreichend gute Stabilität aufweisen. Während die Effizienz von Solarzellen auf Basis von Perowskithalbleitern in dem letzten Jahrzehnt eine bemerkenswerte Entwicklung erfahren hat, lassen sich die wesentlichen physikalischen Mechanismen dieser Technologie noch nicht vollständig erklären. Die elektronisch-ionische Mischleitfähigkeit ist eine dieser Eigenschaften, welche die Effizienz und besonders die Stabilität der Perowskit-Solarzelle beeinflusst. Zentrales Thema dieser Arbeit ist daher die Untersuchung von mobilen ionischen Defekten und deren Einfluss auf Solarzellenparametern. Es wird gezeigt, dass die Migrationsraten ionischer Defekte in Perowskit breiten Verteilungen unterliegen. Durch die Anwendung eines neu entwickelten Regularisationsalgorithmus für inverse Laplace-Transformationen und verschiedener Messmoden für transiente Störstellenspektroskopie kann somit geklärt werden, warum sich berichtete ionische Defektparameter aus der Literatur für gleiche Defekte stark unterscheiden können. Dieses grundlegende Verständnis kann angewendet werden, um den Einfluss von kleinen stöchiometrischen Variationen auf die Defektlandschaft zu untersuchen und das Zusammenspiel zwischen elektronischen und ionischen Eigenschaften besser zu verstehen. Der Einsatz der Meyer-Neldel Regel ermöglicht ferner eine Kategorisierung ionischer Defekte in Perowskithalbleitern. Im letzten Teil dieser Arbeit wird gezeigt, dass elektrische und optische Methoden wie intensitätsmodulierte Spektroskopie geeignet sind, um Informationen über mobile Ionen in hybriden Perowskiten zu erhalten. Zusätzlich wird das elektronische Rekombinationsverhalten näher untersucht.
Solar cells made of organic–inorganic hybrid perovskite semiconductors are considered as a possible key technology for the conversion of cheap and environmentally friendly electrical energy and thus as a milestone for the turnaround in energy policy. In order to meet the steadily growing global demand for electrical energy, solar cell tech- nologies are required that are both highly efficient, i.e. close to the Shockley–Queisser limit, and sufficiently stable. While the efficiency of solar cells based on perovskite semi- conductors has undergone a remarkable development in the last decade, the essential physical mechanisms of this technology cannot yet be fully explained. The electronic- ionic mixed conductivity is one of these properties, which influences the efficiency and especially the stability of the perovskite solar cell. The central topic of this thesis is therefore the investigation of mobile ionic defects and their influence on solar cell parameters. It is shown that the migration rates of ionic defects in perovskites are attributed to wide distributions. By application of a newly developed regularisation algorithm for inverse Laplace transform and different measurement modes for deep-level transient spectroscopy, it can thus be clarified why reported ionic defect parameters from the literature for the same defects can differ significantly. This basic understanding can be used to study the influence of small stoichiometric variations on the defect landscape and to better understand the interaction between electronic and ionic properties. Us- ing the Meyer–Neldel rule also allows the characterisation of ionic defects in perovskite semiconductors. The last part of this thesis shows that electrical and optical methods such as intensity-modulated spectroscopy are suitable for obtaining information about mobile ions in hybrid perovskites. In addition, the electronic recombination behaviour is examined more closely.