Дисертації з теми "Solid-state electronics devices"

The demands for advanced power sources with high energy efficiency, minimum environmental impact, and low cost have been the impetus for the development of a new generation of batteries and fuel cells. One of the key challenges in this effort is to develop and fabricate effective electrodes with desirable composition, microstructure and performance. This work focused on the design, fabrication, and characterization of nanostructured electrodes in an effort to minimize electrode polarization losses. Solid-state diffusion often limits the utilization and rate capability of electrode materials in a lithium-ion battery, especially at high charge/discharge rates. When the fluxes of Li+ insertion or extraction exceed the diffusion-limited rate of Li+ transport within the bulk phase of an electrode, concentration polarization occurs. Further, large volume changes associated with Li+ insertion or extraction could induce stresses in bulk electrodes, potentially leading to mechanical failure. Interconnected porous materials with high surface-to-volume ratio were designed to suppress the stress and promote mass transport. In this work, electrodes with these unique architectures for lithium ion batteries have been fabricated to improve the cycleability, rate capability and capacity retention. Cathodic interfacial polarization represents the predominant voltage loss in a low-temperature SOFC. For the first time, regular, homogeneous and bimodal porous MIEC electrodes were successfully fabricated using breath figure templating, which is self-assembly of the water droplets in polymer solution. The homogeneous macropores promoted rapid mass transport by decreasing the tortuosity. And mesoporous microstructure provided more surface areas for gas adsorption and more TPBs for the electrochemical reactions. Moreover, composite electrodes were developed with a modified sol-gel process for honeycomb SOFCs. The sol gel derived cathodes with fine grain size and large specific surface area, showed much lower interfacial polarization resistances than those prepared by other existing processing methods. Nanopetals of cerium hydroxycarbonate have been synthesized via a controlled hydrothermal process in a mixed water-ethanol medium. The formation of the cerium compound depends strongly on the composition of the precursors, and is attributed to the favored ethanol oxidation by Ce(IV) ions over Ce(IV) hydrolysis process. Raman studies showed that microflower CeO2 preferentially stabilizes O2 as a peroxide species on its surface for CO oxidation.

The thesis deals with the application of compensators andswitches based on power electronics in AC transmission anddistribution systems. The objective of the studieddevices/equipment is the power flow and voltage control intransmission systems and the mitigation of voltage sags andmomentary interruptions to critical loads in distributionsystems.

For validating the power electronics based devices/equipmentdescribed in the thesis, scaled models at a real-time simulatorhave been built. Simulation results of these models arepresented and discussed in the thesis.

The equipment studied in the thesis exploit the fast controlactions that can be taken by power electronics devices, whichare much faster than the speed of conventional equipment andprotection systems, based on electromechanical devices. In thisway, the power quality of distribution systems is improved,regarding duration and magnitude of voltage sags (dips) andmomentary interruptions, which are the most relevant types ofdisturbances in distribution systems.

The thesis presents some compensators based onforced-commutation voltage-source converters for correctingvoltage sags and swells to critical loads. The seriesconverter, usually denoted Dynamic Voltage Restorer (DVR), hasbeen proved suitable for the task of compensating voltage sagsin the supply network. The use of solid-state devices ascircuit breakers in distribution systems has also been studiedwith the objective of achieving fast interruption or limitationof fault currents. The location and practical aspects for theinstallation of these solid-state breakers are presented. Ithas beenshown that a configuration based on shunt and seriesconnected solid-state devices with controllable turn-offcapability can also provide voltage sag mitigation, without theneed of transformers and large energy storage elements.

The operation and control of two Flexible AC TransmissionSystem (FACTS) devices for voltage and power flow control intransmission systems, namely the Static Synchronous Compensator(STATCOM) and the Unified Power Flow Controller (UPFC),respectively, are also studied. A faster response compared totraditional equipment consisting of mechanically based/switchedelements is then achieved. This allows a more flexible controlof power flow and a secure loading of transmission lines tolevels nearer to their thermal limits. The behaviour of thesedevices during faults in the transmission system is alsopresented. Keywords: power electronics, power quality, voltagesags, voltage-source converters, Custom Power, FACTS, real-timesimulations, solid-state devices.

In the world around us, it is easy to think in different situations in which there are temperature gradients available. These could be converted into a great source of energy if using the proper technology. Thermoelectric devices are environmentally friendly solid-state harvesters able to play this role by converting temperature differences into an electric voltage and vice-versa. These devices, besides being highly reliable since they have no moving parts, if engineered and fabricated in a shape-adaptable manner, are able to fit in countless industrial or domestic applications to improve their efficiency or power low-consumption devices like sensors. If, on top of it, the whole fabrication process is cost-effective and easily scalable, the outcoming thermoelectric devices could potentially reach numerous markets banned to date due to a mix of low efficiencies and high prices of the currently existing solutions. The first milestone towards cost-effective thermoelectric devices relies on the improvement of the thermoelectric conversion efficiency of the constituents materials. However, such improvement cannot be at all costs. New materials with significant improved performance need to be designed and engineered with relatively low production cost. In this framework, solution-processed techniques are an outstanding alternative for the production of thermoelectric materials and devices. In particular, the bottom-up assembly of colloidal nanoparticles, with controlled size, shape, crystal phase and composition, has no competing technology to precisely design functional metamaterials without the need of a high capital equipment or complex procedures, not only for thermoelectrics, but also for a wide range of applications. Nevertheless, some limitations still need to be overcome to exploit the full potential of solution-processed assembly technologies, and two different challenges should be addressed. The first one is regarding materials efficiency enhancement, and the second one to the device development itself. In this work, we undertake a journey from the material development to the engineering of the final device. The thesis is structured in 5 chapters, starting from the Chapter 0 or General Introduction that intends to situate the reader into the broader context of the technology and present the main objectives of the work. Chapter 1 presents a general view of the solution-processed route for the development of bottom-up engineered nanoparticle-based thermoelectric nanomaterials and devices. It is an extended and comprehensive text where main concepts, challenges, advantages and opportunities that the technology offers are exposed. Chapter 2 is built around 3 publications that cover the three different steps of the solution-processed nanomaterials preparation, and how the efficiency can be enhanced within each one. First article is focused on the synthesis stage, and presents the production of core-shell nanoparticles as a way to design nanocomposites. The second one is related to the purification step, showing how, taking advantage of the nanoparticle surface, an HCl-mediated ligand displacement is able to introduce controlled amounts of dopants in the nanoparticle. Last one, is focused on the final assembly phase, in which by properly assembling two different kinds of nanoparticles, a semiconductor and a metal, the efficiency could be greatly enhanced. In Chapter 3 a step is made towards the production of a ring-shape device, taking PbSe as a model material. These results have been submitted for their publication. Chapter 4, presents the integration of a thermoelectric device together with a nanoparticle-based temperature sensor. This integrated assembly, including an ultra-low-power electronic management, was implemented as an autonomous soil moisture sensor, and shows the great opportunity that both solution-processed techniques and thermoelectrics technology offer for the development of new applications. Finally, some conclusions over the presented project and future work are listed.
Al món que ens envolta és fàcil pensar en situacions en què hi ha gradients de temperatura disponibles. Aquests, es podrien convertir en fonts d’energia molt interessants mitjançant l’ús adequat de la tecnologia. Els dispositius termoelèctrics son conversors d’estat sòlid capaços de jugar aquest important paper, ja que son capaços de transformar diferències de temperatura en energia elèctrica i vice-versa. Poden ser instal·lats a qualsevol emplaçament si son adaptats a l’aplicació en qüestió, ja sigui a escala domèstica o industrial, per millorar la seva eficiència energètica o, per exemple, alimentar altres dispositius de baix cost. Si, a més a més, el conjunt del procés de fabricació és de baix cost i fàcilment escalable per la seva producció en massa, els dispositius termoelèctrics resultants tindran la possibilitat d’entrar dins de nous mercats, fins ara impossibles degut a una barreja fatal d’alts preus i baixes eficiències dels productes comercials disponibles actualment. El primer pas cap a la fabricació de mòduls termoelèctrics més efectius en tots els sentits, és la millora de la seva eficiència a través de la recerca de nous o més efectius materials dels quals estan constituïts. Tanmateix, però, aquesta millora no pot ser a qualsevol cost. És necessari que aquests nous materials mantinguin alhora l’eficiència i baix cost en la seva fabricació. En aquest sentit, les tècniques de processat en solució son una gran alternativa per la producció de materials i dispositius termoelèctrics, i, en particular, la utilització de nanopartícules col·loïdals, amb mida, forma, fase i composició controlada. No hi ha cap altra tecnologia que aconsegueixi el seu nivell de control sobre el disseny de materials funcionals sense la necessitat de costosos equipaments o procediments complexes, no només per termoelèctrics sinó per un ampli ventall d’aplicacions. No obstant això, algunes limitacions encara han de ser superades per tal de poder explotar plenament el potencial que les tècniques de processat en solució ofereixen. Els dos majors reptes als quals la tecnologia s’enfronta son: primer, millorar l’eficiència dels materials, i, segon, en el desenvolupament de nous models de dispositius. En aquest treball, fem un viatge des del desenvolupament del material fins la fabricació d’un dispositiu.

During the last decades, silicon nanocrystals have focused great attention due to the size-dependent physical properties they present, attributed to the quantum confinement effect. This, added to the bulk silicon compatibility with the well-established microelectronics technology and the low mining and manipulation costs this material presents, makes silicon a potential candidate for the growing photonics and optoelectronics fields. In particular, the tunnability of the electronic properties of silicon nanocrystals can be reached by controlling the nanocrystal size. This has been recently achieved by means of the superlattice approach, consisting of the alternated deposition of ultra-thin (2-4 nm) stoichiometric and silicon-rich layers of a given silicon-rich material. After a high-temperature annealing treatment, the silicon excess precipitates and crystallizes in the final form of nanocrystals, whose properties strongly depend on the fabrication process. Consequently, an ordered arrange of size-controlled nanocrystals (the superlattice) is obtained. In this Thesis Project, the structural, optical, electrical and electro-optical properties of silicon nanocrystal superlattices have been studied, using two different silicon-based materials as host matrices: silicon oxide and silicon carbide. The fabrication of these material systems has been carried out at different European institutions, specialists in the controlled deposition of nm¬thick films. Aiming at the nanocrystal superlattices characterization, different experimental techniques have been employed, which yield structural (transmission and scanning electron microscopies, X-ray diffraction), optical (optical absorption, photoluminescence and Raman scattering spectroscopies) and electrical / electro-optical (current versus voltage analysis in dark and under illumination, and electroluminescence, electro-optical response and light-beam induced photocurrent spectroscopies) information. From the material's point of view, the optimum structural properties that allow an almost perfect nanocrystal arrangement, size control and crystalline degree have been determined, always aiming at an optimum light emission and/or light absorption. Within this frame, fundamental studies have been performed to assess the crystalline degree of the nanostructures (confirming an atomic-thin transition layer between the crystalline nanocrystal core and the surrounding matrix), and to carefully inspect the controversial origin of luminescence within the nanocrystals when embedded in a silicon oxide matrix; as well, the structural conditions under which size-confinement of nanocrystals is reached when embedded in silicon carbide are reported. Once the best structural and optical properties from silicon nanocrystal superlattices were found, these material systems have been employed as active layers for light emitting and light converter (i.e. photovoltaic) devices. In oxide-based systems, the mechanisms that govern charge transport through the superlattices have been studied, and impact ionization has been hypothesized as the main electroluminescence excitation mechanism according to the experimental observations. In addition, the structural conditions (sublayer thicknesses, silicon-rich layer stoichiometry) that yield a maximum electroluminescence efficiency have been determined. Regarding silicon nanocrystals embedded in silicon carbide, a correlation has been established between the charge photogeneration and extraction when acting as an absorber material, which allowed assessing the structural conditions that maximize charge transport while minimizing the non-desirable recombination. Finally, via spectral response measurements, quantum confinement of excitons within silicon nanocrystals has been reported in silicon carbide matrix for the first time. In conclusion, the study on silicon nanocrystal superlattices developed within the present Thesis Project reveals the potential of silicon oxide as host matrix for silicon nanostructures to be used as light-emitting devices; instead, silicon carbide has proved a more suitable host material for photovoltaic applications, which sheds light to the future application of silicon nanocrystals as the top cell of an all-Si tandem cell.
Els nanocristalls de silici han esdevingut objecte d'estudi durant l'últim quart de segle, degut a què presenten, a causa de l'efecte de confinament quàntic, unes propietats físiques dependents de la seva mida. A més, la compatibilitat del silici massiu amb la ben establerta tecnologia microelectrònica juga en favor de la seva utilització i el seu desenvolupament per a futures aplicacions en el camp de la fotònica i l'optoelectrónica. El control del creixement de nanocristalls de silici es pot dur a terme mitjançant el dipòsit de superxarxes d'entre 2 i 4 nm de gruix, on capes de material estequiomètric basat en silici s'alternen amb altres de material ric en silici. Un posterior procés de recuit a alta temperatura permet la precipitació de l'excés de silici i la seva cristal.lització, tot originant una xarxa ordenada de nanocristalls de silici de mida controlada. En aquesta Tesi, s'han estudiat les propietats estructurals, òptiques, elèctriques i electro-òptiques de superxarxes de nanocristalls de silici embeguts en dues matrius diferents: òxid de silici i carbur de silici. Amb tal objectiu, s'han emprat tot un seguit de tècniques experimentals, que comprenen la caracterització estructural (microscòpia electrònica de transmissió i d'escombrat, difracció de raigs X), òptica (espectroscòpies d'absorció òptica, de fotoluminescència i dispersió Raman) i elèctrica / electro-òptica (caracterització intensitat-voltatge en foscor o sota il.luminació, electroluminescència, resposta electro-òptica), entre d'altres. Des del punt de vista del material, s'han estudiat les propietats estructurals òptimes per tal d'obtenir un perfecte ordenament en la xarxa de nanocristalls, una major qualitat cristal.lina i unes propietats d'emissió òptimes. L'optimització del material s'ha dut a terme en vistes a la seva utilització com a capa activa dins de dispositius emissors de llum i fotovoltaics, l'eficiència dels quals ha estat monitoritzada segons els diferents paràmetres estructurals (gruix de les capes nanomètriques involucrades, estequiometria, temperatura de recuit). Finalment, els nanocristalls de silici embeguts en òxid de silici han demostrat un major rendiment com a emissors de llum, mentre que una matriu de carbur de silici beneficia les propietats d'absorció i extracció (fotovoltaiques) del sistema.

Electronic processes in two different electroluminescent device structures, the forward biassed metal/thick insulator/semiconductor (MIS) diode and the high field metal/insulator/metal (MIM) panel, are investigated. Models are produced to explain the behaviour of two particular MIS systems which have been studied experimentally. One of these systems is the Au/cadmium stearate/n-GaP structure, where the insulator is deposited using Langmuir-Blodgett (LB) technology. The other is the Au/i-ZnS/n-ZnS structure. In the MIS devices electroluminescence occurs as a result of the recombination of electrons and holes in the semiconductor and so it is necessary to have an efficient minority carrier (hole) injection mechanism. Attention is paid to the impact excitation of the electron gas in the metal by the electrons injected from the semiconductor because this has been proposed by other workers as a process for producing holes in the metal that are energetically capable of entering the semiconductor valence band, provided they can traverse the insulator. The characteristics of the LB film devices are found to be best described by assuming the minority carrier injection to be limited by the hole transport through the insulator. Hopping between interface states on the successive LB layers is proposed as the transport mechanism. However, the device incorporating a II-VI semi-insulator is shown to be more characteristic of hole transport in the insulator valence band and a minority carrier injection which is limited by the supply of holes from the metal. In high field MIM panels the mechanism of electroluminescence is quite different with impurity centres being impact excited or impact ionised by injected electrons and subsequently luminescing. Such devices driven by a dc signal are susceptible to the formation of high current filaments which burn out and result in device failure. A model is developed which predicts that there is a voltage range over which the device can exist in either a low current state or two higher current states and the resultant instability is expected to be destructive. Current-voltage characteristics are produced using this model and their general features are found to be relatively insensitive to material and device parameters. In order to understand the evolution of the electrical state of the MIM device after switch-on, a time dependent theory of system behaviour is also developed. This is particularly important as the devices are usually driven by a pulsed signal. For an homogeneous system the current is found to converge to the lower current state of the steady state characteristic.

In this thesis, we present the first-principle calculations of inelastic electron tunneling spectroscopy(IETS) in single molecular break junctions. In a two-probe electrode-molecule-electrode setup, density functional theory(DFT) is used for the construction of the Hamiltonian and the Keldysh non-equilibrium Green's function(NEGF) technique will be employed for determining the electron density in non-equilibrium system conditions. Total energy functional, atomic forces and Hessian matrix can be obtained in the DFT-NEGF formalism and self-consistent Born approximation(SCBA) is used to integrate the molecular vibrations (phonons) into the framework once the phonon spectra and eigenvectors are calculated from the dynamic matrix. Geometry optimization schemes will also be discussed as an indispensable part of the formalism as the equilibrium condition is crucial to correctly calculate the phonon properties of the system.To overcome the numerical difficulties, especially the large computational time demand of the electron-phonon coupling problem, we develop a numerical approximation for the electron self-energy due to phonons and the error is controlled within numerical precision. Besides, a direct IETS second order I-V derivative expression is derived to reduce the error of numerical differentiation under reasonable assumptions. These two approximations greatly reduce the computation requirement and make the calculation feasible within current numerical capability.As the application of the DFT-NEGF-SCBA formalism, we calculate the IETS of the gold-octanedithiol(ODT) molecular junction. The I-V curve, conductance and IETS from ab-inito calculations are compared directly to experiments. A microscopic understanding of the electron-phonon coupling mechanism in the molecular tunneling junctions is explained in this example. In addition, comparisons of the hydrogen-dissociative and hydrogen-non-dissociative ODT junctions as well as the different charge transfer behaviors are presented to show the effects of thiol formation in the ODT molecular junction.
Dans cette thèse, nous présentons des calculs ab initio de la spectroscopie à effet tunnel par électron inélastique (IETS)appliqués à des jonctions moléculaires. Dans le cadre d'une configuration électrode-molécule-électrode,la théorie de la fonctionnelle de la densité (DFT) est utilisée pour construire l'hamiltonien et les fonctions de Green hors-équilibres(NEGF) sont employées pour déterminer la densité électroniquedans des conditions hors-équilibre. Le cadrede la DFT-NEGF nous permet de calculer des quantités telles que la fonctionnelle d'énergie totale,les forces atomiques ainsi que la matrice de Hessian. L'approximationauto-consistante de Born (SCBA) est employée afin d'intégrer les vibrations moléculaires (phonons) dans le formalisme DFT-NEGF,une fois que le spectre des phonons et les vecteurs propres ont été calculés à partir de la matrice dynamique. Des méthodes d'optimisations géométriques sont aussi discutées en tant que part indispensable du formalisme,étant donné que la condition d'équilibre mécanique est essentielle afin de calculer correctement les propriétés des phonons du système.Afin de surmonter les difficultés numériques, particulièrement concernant la grande demandecomputationnelle requise pour le calcul du couplage électron-phonon, nous développons une approximation numérique pour la self-énergie associée aux phonons. De plus, en employant quelques hypothèses raisonables, nous dérivons une expression pour l'IETS calculée à partir de laseconde dérivée de la courbe I-V dans le butde réduire l'erreur associée à la différentiation numérique. L'utilisation de ces deux approximations diminuent grandement les exigences computationnelles et rendent les calculs possibles avec les capacités numériques actuelles.Comme application du formalisme DFT-NEGF-SCBA, nous calculons l'IETS de la jonction moléculaire or-octanedithiol(ODT)-or. La courbe I-V, la conductance et l'IETS obtenues par calculs ab initio sontdirectement comparées aux données expérimentales. Une compréhension microscopique du couplage électron-phonon pour une jonction moléculaire à effet tunnel est élaborée dans cet exemple. De plus, des comparaisons entre les jonctions ODT à hydrogène dissociatif et à hydrogène non-dissociatif ainsi queles différents comportements de transfert de charges sont présentés afin de montrer les effets de la formation du thiol dans la jonction moléculaire ODT.

Metal/organic/indium tin oxide (ITO) structures, including OLEDs, are demonstrated to contain multiple nonvolatile conductance states that can be programmed by the application of an external bias above a certain threshold voltage (Vth). These conductance states are stable and in turn can be probed by the use of a bias lower in value than Vth. The unbiased retention time of states is greater than several weeks, and more than 48,000 write-read-rewrite-read cycles have been performed with minimal degradation. It is found that the programming of a continuum of conductance states is possible, and techniques to do so are outlined. The electrical conductivity of the highest and lowest states can differ by six orders of magnitude. Switching speeds below 50 ns are shown, resulting in an energy requirement of about 100 pJ to switch from one conductance state to another. The memory phenomenon is shown to be influenced by the active layer thickness and anode/surface roughness while temperature dependence is limited. The electrical characteristics of these devices are consistent with metal diffusion or filament phenomena found in metal-insulator-metal structures, suggesting a possible mechanism by which the states are stored.Electroluminescent devices employing several new organic-inorganic lumophore-functionalized macromolecules are presented. In this study, macromolecules incorporating several lumophores covalently bonded to the vertices of a cubical core structure based on Polyhedral Oligomeric Silsesquioxane (POSS) in multiple configurations are implemented as light-emitting centers. The hole-transporting polymer poly(N-vinylcarbazole) (PVK) and electron-transporting additive 2-(4-biphenylyl)-5-(4-tert-butylphenyl)1,3,4-oxadiazole (PBD) are used as a two-part host to enhance the carrier transport in these simple solution-processed single-layer devices. A study of energy transfer in several systems is carried out to understand the requirements needed to create white-light emission from a single macromolecule. A single macromolecule incorporating twenty-one blue and one yellow lumophore is shown to exhibit field-independent stable white-light electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.31, 0.37). An external quantum efficiency of 0.55 percent and a maximum brightness of 1600 cd/m2 are attained with simple solution-processed single-layer devices. High solubility and ease of purification give these macromolecule white-light emitters advantages over their small molecule and polymeric type counterparts.

Correlated double sampling (CDS) is a widely used signal processing technique for removal of the Nyquist (reset) noise which is associated with charge sensing circuits employed in a solid state imager. In this thesis work, the power spectral density at the output of a correlated double sampling circuit with first-order low-pass filtered white noise at the input is calculated. A circuit constructed with discrete elements is made to simulate the output stage of a charge-coupled device (CCD). A low-pass filtered wide-band noise from a noise generator is added to the reset reference level when the output signal from this simulator is sampled by the correlated double sampling technique. The experiment measurements show that only about 10% of the noise power measured by simple sampling is obtained when CDS is employed. An autoregressive (AR) model is assumed to fit the sampled data and a recursive algorithm, based on least-squares solutions for the AR parameters using forward and backward linear prediction, is adopted for spectrum estimation. Some conclusions on choosing the bandwidth of the low pass filter for optimum operation is also included.

Interface-dominated materials such as nanocrystalline thin films have emerged as an enthralling class of materials able to engineer functional properties of transition metal oxides widely used in energy and information technologies. In this direction, it has been recently proved that grain boundaries (GBs) in the perovskite La1-xSrxMnO3±δ (manganite) deeply impact its functional properties, boosting the oxygen mass transport while abating the electronic and magnetic order. The impact of grain boundary in nanocrystalline thin films is so relevant to radically change the behaviour of the material, transforming an electronic conductor into a mixed ionic-electronic conductor functional for redox-based solid state devices. Based on these preliminary studies, it became crucial to understand the origin of this enhancement, in order to gain engineering capabilities and potentially extend it to other functional perovskite materials. Following this approach, this thesis focuses in analysing the remarkable properties of GBs in manganites and, ultimately, investigating the possibility of engineering these interfaces. First, the structural and chemical characterization of the LSM thin films deposited by pulsed laser deposition (PLD) is presented. The compositional analysis of the layers revealed a severe Mn deficiency, ascribed to the plasma-background interactions during the deposition. The analysis of the GBs of these Mn-deficient thin films revealed a remarkable local modification of ionic composition, consisting in a Mn and O depletion along with a La and Sr enrichment (viz. GBdef). Then, through a PLD combinatorial approach, Mn was progressively inserted in the perovskite structure, altering the overall cationic ratio of the thin films (Mn/(La+Sr)). The variation of cationic chemical potential of the thin films was observed to significantly affect the GB composition, which passed from Mn depletion (La-enrichment) to Mn enrichment (La-depletion), while maintaining an O deficiency character (viz. GBrich). This behaviour suggests that through the tuning of the overall cationic concentration in the thin films the GB composition can be altered, offering an innovative way for engineering chemical defects in strained interfaces. The effect of these different GBs on the electrical conductivity and the oxygen mass transport properties of LSM thin films with different Mn content was then measured. It was found that in the layers characterized by GBdef, the lack of Mn hinders the low temperature metal insulator transition and, in its place, a variable range hopping mechanism occurs, where electrons tunnels across the GBs for reaching distant Mn atoms. Moreover, a simultaneous decrease of activation energies of both GB oxygen diffusivity and GB oxygen surface exchange coefficient was observed further decreasing the Mn concentration in these thin films, indicating a strong interdependence between the two phenomena. The results suggest that the GB accumulation of oxygen vacancies is at the origin of the large improvement of both oxygen mass transport parameters observed in LSM polycrystalline thin films. In LSM thin films characterized by GBrich, the low temperature metallic behaviour is progressively restored and an increase of electronic conductivity is observed in the entire temperature range. Additionally, in these layers relative changes of Mn do not give rise to a variation of the oxygen diffusivity, meaning that the GBs oxygen vacancy concentration is not altered anymore. Overall, the results demonstrate the possibility of engineering the functional properties of LSM polycrystalline thin films by modifying the GB cationic composition. In the third part of the thesis, the effect of Co substitution on LSMC functional properties was investigated. The LSMC thin films were produced by combinatorial PLD, which allow a direct measure of real-continuous spread LSMC system. The oxygen mass transport properties of bulk and GB were evaluated by finite element model fitting of 18O exchange profiles. The results revealed that GBs enhance the transport properties of the whole material in the range of composition under study, although for high Co concentration the GB effect is concealed by the high bulk diffusion.

In recent years there has been an increased interest in developing fault current limiters for power distribution networks. This arises from the need to cope with the ever increasing short-circuit levels and to reduce the stress on system equipment, e.g. transformers, circuit-breakers and cables. It is also due to the increased concern about power quality, where fault current limiters are expected to play an important role in mitigating voltage sags during faults. Various devices to limit the fault current have been proposed, such as controlled fuses, tuned LC circuits, solid-state and superconducting fault current limiters. This research investigate the use of a novel technique to develop a solid-state Fault Current Limiting and Interrupting device (FCLID) suitable for low voltage distribution networks. The FCLID mainly consists of a high-speed bi-directional semiconductor switch, a varistor (non-linear resistor) and a snubber circuit; all connected in parallel. The semiconductor switch and the varistor share the fault current during the period of FCLID operation. To protect the semiconductor switch and the varistor from damage due to overheating, their temperatures are indirectly monitored in order to define the maximum operating time of the FCLID. A new method for estimating the junction temperature of the switching device and the varistor under transient condition has been developed and experimental tests were carried out to validate the proposed method. The energy handling capability of varistors and associated problems due to their non-linear characteristics have also been investigated. Experimental tests were carried out to measure the energy handling capability of the varistor using thermal imaging system. A new method for improving the current sharing between parallel varistors has been implemented. A computer model of the FCLID has been developed and implemented into a typical distribution network using MATLAB/ SIMULINK. The network performance under different conditions has been analysed. An experimental single-phase 230 V prototype FCLID was developed and tested under different operating conditions. Finally, the outcome of the theoretical, simulation and experimental phases of the research was used to establish the outline design specifications of a FCLID suitable for 11 kV distribution networks.

Fundamental understanding of the electronic properties, and charge transfer mechanism of organic semiconductors and functionalized oligoacenes in particular, is of great importance for the design and fabrication of organic electronic devices. This work is devoted to the study of the electronic properties of organic semiconductors in the gas, solution, and solid phases, thus providing insights into the intra- and intermolecular electronic interactions of these materials from the isolated-molecule level to the solid-state device limit. The organic semiconductors investigated in this work are bis-triisopropylsilylethynyl-substituted (TIPS) anthracene, TIPS tetracene, TIPS pentacene, bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]-pentacene (TP-5), and 2,2,10,10-tetraethyl-6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5). This research is conducted on the basis of experimental and computational studies. The experimental analysis is based on the combination of closely-related gas-phase and solid-phase photoelectron spectroscopy measurements, along with electrochemical measurements in solution. The electronic structure quantum-mechanical computations are performed at the density functional theory level, and are in good agreement with experimental results.This dissertation reports important findings on the electronic properties of organic semiconductors and how these properties change between phases. The role of polarization effects on the electronic properties of these materials was demonstrated to be significant and strongly dependant on the molecular structure and electronic interactions at the isolated- (or single-) molecule level as well as on the molecular packing and electronic interactions in the solid state at the device limit.

Die Menschheit steht vor der großen Herausforderung der Energieversorgung des 21. Jahrhundert. Nirgendwo ist diese noch dringlicher geworden als im Bereich der Energiespeicherung und Umwandlung. Konventionelle Energie kommt hauptsächlich aus fossilen Brennstoffen, die auf der Erde nur begrenzt vorhanden sind, und hat zu einer starken Belastung der Umwelt geführt. Zusätzlich nimmt der Energieverbrauch weiter zu, insbesondere durch die rasante Verbreitung von Fahrzeugen und verschiedener Kundenelektronik wie PCs und Mobiltelefone. Alternative Energiequellen sollten vor einer Energiekrise entwickelt werden. Die Gewinnung erneuerbarer Energie aus Sonne und Wind sind auf jeden Fall sehr wichtig, aber diese Energien sind oft nicht gleichmäßig und andauernd vorhanden. Energiespeichervorrichtungen sind daher von großer Bedeutung, weil sie für eine Stabilisierung der umgewandelten Energie sorgen. Darüber hinaus ist es eine enttäuschende Tatsache, dass der Akku eines Smartphones jeglichen Herstellers heute gerade einen Tag lang ausreicht, und die Nutzer einen zusätzlichen Akku zur Hand haben müssen. Die tragbare Elektronik benötigt dringend Hochleistungsenergiespeicher mit höherer Energiedichte. Der erste Teil der vorliegenden Arbeit beinhaltet Lithium-Ionen-Batterien unter Verwendung von einzelnen aufgerollten Siliziumstrukturen als Anoden, die durch nanotechnologische Methoden hergestellt werden. Eine Lab-on-Chip-Plattform wird für die Untersuchung der elektrochemischen Kinetik, der elektrischen Eigenschaften und die von dem Lithium verursachten strukturellen Veränderungen von einzelnen Siliziumrohrchen als Anoden in einer Lithium-Ionen-Batterie vorgestellt. In dem zweiten Teil wird ein neues Design und die Herstellung von flexiblen on-Chip, Festkörper Mikrosuperkondensatoren auf Basis von MnOx/Au-Multischichten vorgestellt, die mit aktueller Mikroelektronik kompatibel sind. Der Mikrosuperkondensator erzielt eine maximale Energiedichte von 1,75 mW h cm-3 und eine maximale Leistungsdichte von 3,44 W cm-3. Weiterhin wird ein flexibler und faserartig verwebter Superkondensator mit einem Cu-Draht als Substrat vorgestellt. Diese Dissertation wurde im Rahmen des Forschungsprojekts GRK 1215 "Rolled-up Nanotechnologie für on-Chip Energiespeicherung" 2010-2013, finanziell unterstützt von der International Research Training Group (IRTG), und dem PAKT Projekt "Elektrochemische Energiespeicherung in autonomen Systemen, no. 49004401" 2013-2014, angefertigt. Das Ziel der Projekte war die Entwicklung von fortschrittlichen Energiespeichermaterialien für die nächste Generation von Akkus und von flexiblen Superkondensatoren, um das Problem der Energiespeicherung zu addressieren. Hier bedanke ich mich sehr, dass IRTG mir die Möglichkeit angebotet hat, die Forschung in Deutschland stattzufinden
Human beings are facing the grand energy challenge in the 21st century. Nowhere has this become more urgent than in the area of energy storage and conversion. Conventional energy is based on fossil fuels which are limited on the earth, and has caused extensive environmental pollutions. Additionally, the consumptions of energy are still increasing, especially with the rapid proliferation of vehicles and various consumer electronics like PCs and cell phones. We cannot rely on the earth’s limited legacy forever. Alternative energy resources should be developed before an energy crisis. The developments of renewable conversion energy from solar and wind are very important but these energies are often not even and continuous. Therefore, energy storage devices are of significant importance since they are the one stabilizing the converted energy. In addition, it is a disappointing fact that nowadays a smart phone, no matter of which brand, runs out of power in one day, and users have to carry an extra mobile power pack. Portable electronics demands urgently high-performance energy storage devices with higher energy density. The first part of this work involves lithium-ion micro-batteries utilizing single silicon rolled-up tubes as anodes, which are fabricated by the rolled-up nanotechnology approach. A lab-on-chip electrochemical device platform is presented for probing the electrochemical kinetics, electrical properties and lithium-driven structural changes of a single silicon rolled-up tube as an anode in lithium ion batteries. The second part introduces the new design and fabrication of on chip, all solid-state and flexible micro-supercapacitors based on MnOx/Au multilayers, which are compatible with current microelectronics. The micro-supercapacitor exhibits a maximum energy density of 1.75 mW h cm-3 and a maximum power density of 3.44 W cm-3. Furthermore, a flexible and weavable fiber-like supercapacitor is also demonstrated using Cu wire as substrate. This dissertation was written based on the research project supported by the International Research Training Group (IRTG) GRK 1215 "Rolled-up nanotech for on-chip energy storage" from the year 2010 to 2013 and PAKT project "Electrochemical energy storage in autonomous systems, no. 49004401" from 2013 to 2014. The aim of the projects was to design advanced energy storage materials for next-generation rechargeable batteries and flexible supercapacitors in order to address the energy issue. Here, I am deeply indebted to IRTG for giving me an opportunity to carry out the research project in Germany. September 2014, IFW Dresden, Germany Wenping Si

Los materiales conductores mixtos de electrones e iones (oxígeno o protones) son capaces de separar oxígeno o hidrógeno de los gases de combustión o de corrientes de reformado a alta temperatura. La selectividad de este proceso es del 100%. Estos materiales, óxidos sólidos densos, pueden usarse en la producción de electricidad a partir de combustibles fósiles, así como formar parte de los procesos que forman parte del sistema de captura y almacenamiento de CO2. Las membranas de transporte de oxígeno (MTO) se pueden utilizar en las plantas energéticas con procesos de oxicombustión, así como en reactores catalíticos de membrana (RCM), mientras que las membranas de transporte de hidrógeno (MTH) se aplican en procesos de precombustión. Además, estos materiales encuentran aplicación en componentes de sistemas energéticos, como electrodos o electrolitos de pilas de combustible de óxido sólido, de ambas clases iónicas y protónicas (SOFC y PC-SOFC). Los procesos mencionados implican condiciones de operación muy severas, como altas temperaturas y grandes gradientes de presión parcial de oxígeno (pO2), probablemente combinadas con la presencia de CO2 and SO2. Los materiales más que mayor rendimiento de separación presentan y más ampliamente investigados en este campo son inestables en estas condiciones. Por tanto, existe la necesidad de encontrar nuevos materiales inorgánicos estables que proporcionen alta conductividad electrónica e iónica. La presente tesis propone una búsqueda sistemática de nuevos conductores iónicos-electrónicos mixtos (MIEC, del inglés) con diferente estructura cristalina y/o diferente composición, variando la naturaleza de los elementos y la estequiometría del cristal. La investigación ha dado lugar a materiales capaces de transportar iones oxígeno, protones o cargas electrónicas y que son estables en las condiciones de operación. La caracterización de una amplia serie de cerias (CeO2) dopadas con lantánidos proporciona una comprensión general de las propiedades estructurales y de transporte, así como la relación entre ellas. Además, se estudia el efecto de la adición de cobalto a dicho sistema. Se ha completado el análisis con la optimización de las propiedades de trasporte a partir de la microestructura. Todo esto permite hacer una clasificación inicial de los materiales basada en el comportamiento de transporte principal y permite adecuar la estructura y las condiciones de operación para obtener las propiedades deseadas para cada aplicación. Algunos de los materiales extraídos de este estudio alcanzaron las expectativas. Las familias de materiales basadas en Ce1-x Tbx O2-¿ y Ce1-x Tbx O2-¿ +2 mol% Co proporcionan flujos de oxígeno bajos pero competitivos, ya que son estables en atmósferas con CO2. Además, la inclusión de estos materiales en membranas de dos fases aumenta el flujo de oxígeno. La combinación con una espinela libre de cobalto y de metales alcalinotérreos como es el Fe2 NiO4, ha dado lugar a un material prometedor en cuanto a flujo de oxígeno y estabilidad en CO2 y en SO2, que podría ser integrado en el proceso de oxicombustión. Por otra parte, se ha añadido metales como codopantes en el sistema Ce0.9-x Mx Gd0.1O1.95. Estos materiales, en combinación con la perovskita La1- x Srx MnO3 usada comúnmente como cátodo de SOFC, han sido capaces de disminuir la resistencia de polarización del cátodo. La mejora es consecuencia de la introducción de conductividad iónica por parte de la ceria. Las perovskitas dopadas basadas en CaTiO3 forman el segundo grupo de materiales investigados. La dificultad de obtener perovskitas estables y que presenten conducción mixta iónica y electrónica se ha hecho evidente. De entre los dopantes utilizados, el hierro y la combinación hierro-magnesio han sido los mejores candidatos. Ambos materiales presentan conductividad principalmente iónica a alta temperatura, mientras que a baja predomina la conductividad electrónica tipo p. CaTi0.73Fe0.18Mg0.09O3-¿ se ha mostrado como un material competente en la fabricación de membranas de oxígeno, que proporciona flujos adecuados a la par que estabilidad en CO2. Finalmente, la perovskita La0.87Sr0.13CrO3 (LSC) ha sido dopada con el objetivo de aumentar la conductividad mixta protónica electrónica. Este estudio ha llevado al desarrollo de una nueva generación de ánodos para PC-SOFC basadas en electrolitos de LWO. Las perovskitas dopadas con Ce en el sitio del La (LSCCe) y con Ni en el sitio del Cr (LSCN) son estables en condiciones de operación reductoras, así como en contacto con el electrolito. El uso de ambos materiales como ánodo disminuye la resistencia de polarización con respecto al LSC. El LSCCe está limitado por los procesos que ocurren a baja frecuencia (BF), relacionados con los procesos superficiales, y que son atenuados en el caso del LSCN debido a la formación de nanopartículas de Ni metálico en la superficie. La infiltración posterior con nanopartículas de Ni permite disminuir la resistencia a BF lo que sugiere que la reacción superficial de oxidación del H2 está siendo catalizada. La infiltración más concentrada en Ni (5Ni) elimina completamente la resistencia a BF en ambos ánodos, de forma que los procesos que ocurren a altas frecuencias son ahora limitantes. El ánodo constituido por LSCNi20+5Ni dio una resistencia de polarización de 0.26 ¿·cm 2 at 750 ºC en H2 húmedo.
Mixed ionic (oxygen ions or protons) and electronic conducting materials (MIEC) separate oxygen or hydrogen from flue gas or reforming streams at high temperature in a process 100% selective to the ion. These solid oxide materials may be used in the production of electricity from fossil fuels (coal or natural gas), taking part of the CO2 separation and storage system. Dense oxygen transport membranes (OTM) can be used in oxyfuel combustion plants or in catalytic membrane reactors (CMR), while hydrogen transport membranes (HTM) would be applied in precombustion plants. Furthermore, these materials may also be used in components for energy systems, as advanced electrodes or electrolytes for solid oxide fuel cells (SOFC) and proton conducting solid oxide fuel cells (PCSOFC) working at high and moderate temperature. The harsh working conditions stablished by the targeted processes include high temperatures and low O2 partial pressures (pO2), probably combined with CO2 and SO2 containing gases. The instability disadvantages presented by the most widely studied materials for these purposes make them impractical for application to gas separation. Thus, the need to discover new stable inorganic materials providing high electronic and ionic conductivity is still present. This thesis presents a systematic search for new mixed ionic-electronic conductors. It includes different crystalline structures and/or composition of the crystal lattice, varying the nature of the elements and the stoichiometry of the crystal. The research has yielded new materials capable to transport oxygen ions or protons and electronic carriers that are stable in the working condition to which they are submitted.
Balaguer Ramírez, M. (2013). New solid state oxygen and hydrogen conducting materials. Towards their applications as high temperature electrochemical devices and gas separation membranes [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/31654
TESIS
Premiado

In this PhD thesis I have studied through state-of-the-art quantum simulations (mainly within the density functional theory approach, DFT) triarylmethyl (TAM) based systems with potential for future nano-devices and designed a series of TAM-based 2D covalent organic frameworks (from now on TAM 2D-COFs). TAMs are organic radicals (i.e. open-shell molecules) which have been used for numerous applications during the last 20 years. In the first part of this PhD thesis I have studied a series of TAM-based systems in collaboration with the experimental groups led by Profs. Jaume Veciana and Concepció Rovira and Dr. Marta Mas-Torrent, respectively, both from the Institute of Materials Science of Barcelona (ICMAB). In such collaborative studies we have evaluated the potential of TAMs for different potential applications. In the first two works we assess the possibility of using closed-shell quinoidal TAMs which, upon being chemisorbed in metal substrates give rise to an open-shell (i.e. radical) monolayer. This is demonstrated by means of on-surface techniques such as X-ray photo-electron spectroscopy (XPS) and angle-resolved ultra-violet photo-electron spectroscopy (ARUPS) and our periodic density functional theory calculations. Complementing to this work I also present a second where a similar radical SAM is formed using, in this case, a TAM-based bi-radical compound. In a third collaborative study we study the E – Z isomerisation in a hydrogenated closed-shell TAM (the so-called H-PTM) bonded with an ethylene unit. An irreversible E to Z transformation is experimentally measured with no evident explanation. Based in DFT and ab initio molecular dynamics simulations (AIMD), I was able to provide a sensible hypothesis for such results based in a sterical blocking effect in the Z conformer. The last two chapters of this PhD thesis collect the computational works focused in making theoretical predictions of yet un-synthesized systems. In Chapter 4, I present a work were we studied how to control the unpaired electron in TAMs, finding out that in these molecules there exists a linear correlation between the aryl ring twist angles and the localization of their unpaired electron. Based on this study we then looked for TAM-based systems where the aryl rings’ twist angles could be externally manipulated. In such direction, I present TAM 2D-COFs (see above) as the only possible platform where aryl ring twist angles may be externally manipulated. As reported in the second publication of Chapter 4, uniaxially stretching the structure of our designed TAM 2D-COFs allows for a fine and reversible (i.e. elastic) twisting of all aryl rings within the 2D material. This allows controlling the localization of all unpaired electron in the network, as well as the band of the material and magnetic interactions. In the last work of this chapter we assess the possibility of having chemical persistence of TAM monomers and structural flexibility through a screening procedure based in force-field calculations. In the last chapter of this PhD thesis I present two studies where it is demonstrated that TAMs, upon being covalently bonded in para- one respect each other, present electrical conductive characteristics. In the first work, in collaboration with the experimental groups from ICMAB, this is demonstrated for a PTM dimer where one of the PTM units is reduced to the anion. The resulting negative charge is found to conduct between both PTM units at room temperature. Finally, in the last predictive work of this thesis, I present a work where we demonstrate based in hybrid DFT calculations that para-connected TAM 2D-COFs behave as semimetals with energetically close-lying semiconductor solutions.
En esta tesis doctoral presentada por artículos he estudiado mediante cálculos DFT (density functional theory, del inglés) sistemas basados en moléculas triaril-metil (TAM) para potenciales aplicaciones futuras. Las moléculas TAM son compuestos orgánicos radicales (es decir, con un electrón desapareado) que se han utilizado para construir diversos materiales durante los últimos 20 años. En la primera parte de la tesis presento los estudios llevados a cabo en colaboración con grupos experimentales del Instituto de Ciencia de los Materiales de Barcelona (ICMAB) expertos en la síntesis de tales compuestos. En los primeros estudios de esta parte se ha llevado a cabo la formación de una mono-capa auto-ensamblada de TAMs en diferentes superficies metálicas. Mediante técnicas de superficie i cálculos DFT periódicos hemos demostrado que utilizando moléculas TAM de capa cerrada (es decir, diamagnéticas) se puede generar una mono-capa radical, o de capa abierta (es decir, paramagnética). En un tercer estudio en colaboración con los mismos grupos experimentales estudiamos la isomerización E – Z (o cis- trans-) irreversible en un sistema TAM-etileno (de capa cerrada). Los cálculos computacionales han sido claves en este estudio para entender el bloqueo cinético que se da en el isómero Z (cis-), lo cual impide su isomerización al isómero E (trans-), a pesar de ser éste último más estable termodinámicamente. En la segunda parte de esta tesis doctoral presento una serie de estudios en los cuales hemos diseñado materiales 2D basados en moléculas TAM, aún no preparados en el laboratorio. En estos estudios se demuestra el gran potencial de dichas redes basadas en moléculas TAM y su gran versatilidad electrónica a la nano-escala. Nuestros resultados demuestran que dichos materiales 2D se puede comportar tanto como aislantes eléctricos, como semiconductores o como semimetales (tales como el grafeno) según su diseño molecular. Además, en dicho materiales es posible controlar sus propiedades electrónicas mediante la manipulación del ángulo de giro de los anillos aril en cada unidad TAM.

Organic semiconductors are a new key technology for large-area and flexible thin-film electronics. They are deposited as thin films (sub-nanometer to micrometer) on large-area substrates. The technologically most advanced applications are organic light emitting diodes (OLEDs) and organic photovoltaics (OPV). For the improvement of performance and efficiency, correct modeling of the electronic processes in the devices is essential. Reliable characterization and validation of the electronic properties of the materials is simultaneously required for the successful optimization of devices. Furthermore, understanding the relations between material structures and their key characteristics opens the path for innovative material and device design. In this thesis, two material characterization methods are developed, respectively refined and applied: a novel technique for measuring the charge carrier mobility μ and a way to determine the ionization energy IE or the electron affinity EA of an organic semiconductor. For the mobility measurements, a new evaluation approach for space-charge limited current (SCLC) measurements in single carrier devices is developed. It is based on a layer thickness variation of the material under investigation. In the \"potential mapping\" (POEM) approach, the voltage as a function of the device thickness V(d) at a given current density is shown to coincide with the spatial distribution of the electric potential V(x) in the thickest device. On this basis, the mobility is directly obtained as function of the electric field F and the charge carrier density n. The evaluation is model-free, i.e. a model for μ(F, n) to fit the measurement data is not required, and the measurement is independent of a possible injection barrier or potential drop at non-optimal contacts. The obtained μ(F, n) function describes the effective average mobility of free and trapped charge carriers. This approach realistically describes charge transport in energetically disordered materials, where a clear differentiation between trapped and free charges is impossible or arbitrary. The measurement of IE and EA is performed by characterizing solar cells at varying temperature T. In suitably designed devices based on a bulk heterojunction (BHJ), the open-circuit voltage Voc is a linear function of T with negative slope in the whole measured range down to 180K. The extrapolation to temperature zero V0 = Voc(T → 0K) is confirmed to equal the effective gap Egeff, i.e. the difference between the EA of the acceptor and the IE of the donor. The successive variation of different components of the devices and testing their influence on V0 verifies the relation V0 = Egeff. On this basis, the IE or EA of a material can be determined in a BHJ with a material where the complementary value is known. The measurement is applied to a number of material combinations, confirming, refining, and complementing previously reported values from ultraviolet photo electron spectroscopy (UPS) and inverse photo electron spectroscopy (IPES). These measurements are applied to small molecule organic semiconductors, including mixed layers. In blends of zinc-phthalocyanine (ZnPc) and C60, the hole mobility is found to be thermally and field activated, as well as increasing with charge density. Varying the mixing ratio, the hole mobility is found to increase with increasing ZnPc content, while the effective gap stays unchanged. A number of further materials and material blends are characterized with respect to hole and electron mobility and the effective gap, including highly diluted donor blends, which have been little investigated before. In all materials, a pronounced field activation of the mobility is observed. The results enable an improved detailed description of the working principle of organic solar cells and support the future design of highly efficient and optimized devices
Organische Halbleiter sind eine neue Schlüsseltechnologie für großflächige und flexible Dünnschichtelektronik. Sie werden als dünne Materialschichten (Sub-Nanometer bis Mikrometer) auf großflächige Substrate aufgebracht. Die technologisch am weitesten fortgeschrittenen Anwendungen sind organische Leuchtdioden (OLEDs) und organische Photovoltaik (OPV). Zur weiteren Steigerung von Leistungsfähigkeit und Effizienz ist die genaue Modellierung elektronischer Prozesse in den Bauteilen von grundlegender Bedeutung. Für die erfolgreiche Optimierung von Bauteilen ist eine zuverlässige Charakterisierung und Validierung der elektronischen Materialeigenschaften gleichermaßen erforderlich. Außerdem eröffnet das Verständnis der Zusammenhänge zwischen Materialstruktur und -eigenschaften einen Weg für innovative Material- und Bauteilentwicklung. Im Rahmen dieser Dissertation werden zwei Methoden für die Materialcharakterisierung entwickelt, verfeinert und angewandt: eine neuartige Methode zur Messung der Ladungsträgerbeweglichkeit μ und eine Möglichkeit zur Bestimmung der Ionisierungsenergie IE oder der Elektronenaffinität EA eines organischen Halbleiters. Für die Beweglichkeitsmessungen wird eine neue Auswertungsmethode für raumladungsbegrenzte Ströme (SCLC) in unipolaren Bauteilen entwickelt. Sie basiert auf einer Schichtdickenvariation des zu charakterisierenden Materials. In einem Ansatz zur räumlichen Abbildung des elektrischen Potentials (\"potential mapping\", POEM) wird gezeigt, dass das elektrische Potential als Funktion der Schichtdicke V(d) bei einer gegebenen Stromdichte dem räumlichen Verlauf des elektrischen Potentials V(x) im dicksten Bauteil entspricht. Daraus kann die Beweglichkeit als Funktion des elektrischen Felds F und der Ladungsträgerdichte n berechnet werden. Die Auswertung ist modellfrei, d.h. ein Modell zum Angleichen der Messdaten ist für die Berechnung von μ(F, n) nicht erforderlich. Die Messung ist außerdem unabhängig von einer möglichen Injektionsbarriere oder einer Potentialstufe an nicht-idealen Kontakten. Die gemessene Funktion μ(F, n) beschreibt die effektive durchschnittliche Beweglichkeit aller freien und in Fallenzuständen gefangenen Ladungsträger. Dieser Zugang beschreibt den Ladungstransport in energetisch ungeordneten Materialien realistisch, wo eine klare Unterscheidung zwischen freien und Fallenzuständen nicht möglich oder willkürlich ist. Die Messung von IE und EA wird mithilfe temperaturabhängiger Messungen an Solarzellen durchgeführt. In geeigneten Bauteilen mit einem Mischschicht-Heteroübergang (\"bulk heterojunction\" BHJ) ist die Leerlaufspannung Voc im gesamten Messbereich oberhalb 180K eine linear fallende Funktion der Temperatur T. Es kann bestätigt werden, dass die Extrapolation zum Temperaturnullpunkt V0 = Voc(T → 0K) mit der effektiven Energielücke Egeff , d.h. der Differenz zwischen EA des Akzeptor-Materials und IE des Donator-Materials, übereinstimmt. Die systematische schrittweise Variation einzelner Bestandteile der Solarzellen und die Überprüfung des Einflusses auf V0 bestätigen die Beziehung V0 = Egeff. Damit kann die IE oder EA eines Materials bestimmt werden, indem man es in einem BHJ mit einem Material kombiniert, dessen komplementärer Wert bekannt ist. Messungen per Ultraviolett-Photoelektronenspektroskopie (UPS) und inverser Photoelektronenspektroskopie (IPES) werden damit bestätigt, präzisiert und ergänzt. Die beiden entwickelten Messmethoden werden auf organische Halbleiter aus kleinen Molekülen einschließlich Mischschichten angewandt. In Mischschichten aus Zink-Phthalocyanin (ZnPc) und C60 wird eine Löcherbeweglichkeit gemessen, die sowohl thermisch als auch feld- und ladungsträgerdichteaktiviert ist. Wenn das Mischverhältnis variiert wird, steigt die Löcherbeweglichkeit mit zunehmendem ZnPc-Anteil, während die effektive Energielücke unverändert bleibt. Verschiedene weitere Materialien und Materialmischungen werden hinsichtlich Löcher- und Elektronenbeweglichkeit sowie ihrer Energielücke charakterisiert, einschließlich bisher wenig untersuchter hochverdünnter Donator-Systeme. In allen Materialien wird eine deutliche Feldaktivierung der Beweglichkeit beobachtet. Die Ergebnisse ermöglichen eine verbesserte Beschreibung der detaillierten Funktionsweise organischer Solarzellen und unterstützen die künftige Entwicklung hocheffizienter und optimierter Bauteile

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. August 2014.
The process of the computational discovery of materials for future technologies is a combination of numerical techniques and general scientific intuition to select elements and combine in order to form novel types of materials. Modern ab initio methods based on density functional theory are capable of predicting with a high level of accuracy the most stable ground state atomic configurations of any given material. Once the ground state configurations are established, the electronic, optical and mechanical properties of the novel bulk nitrides may be determined. Electronic properties of C3N4, CN2, SiN2, GeN2, C2N2(NH), Si2N2(NH), Ge2N2(NH) and Sn2N2(NH) are analysed by computing the Kohn-Sham band structures. The optical properties are investigated by calculating the real and the imaginary parts of the frequency-dependent dielectric constant. The mechanical properties are determined by calculating elastic constants, Young’s modulus, Poisson’s ratio, Vickers hardness, shear and bulk moduli.

There is significant need to improve the sensitivity and selectivity for detecting chemical and biological agents. This need exists in a myriad of human endeavors, from the monitoring of production of consumer products to the detection of infectious agents and cancers. Although many well established methodologies for chemical and biological sensing exist, such as mass spectrometry, gas or liquid phase chromatography, enzymelinked immunosorbent (ELISA) assays, etc., it is the goal of the work described herein to outline aspects of two specific platforms which can add two very important features, low cost and portability. The platforms discussed in this dissertation are organic semiconductor field-effect transistors (OFETS), in various architectural forms and chemical modifications, and ion channels immobilized in tethered lipid bilayers integrated with solid state devices. They take advantage of several factors to make these added features possible, low cost manufacturing techniques for producing silicon and organic circuits, low physical size requirements for the sensing elements, the capability to run such circuits on low power, and the ability of these systems to directly transduce a sensing event into an electrical signal, thus making it easier to process, interpret and record a signal. In the most basic OFET functionality, many types of organic semiconductors can be used to produce transistors, each with a slightly different range of sensitivities. When used in concert, they can produce a reversible chemical "fingerprint". These OFETS can also be integrated with silicon transistors - in a hybrid device architecture - to enhance their sensitivity while maintaining their reversibility. The organic semiconductors themselves can be chemically altered with the use of small molecule receptors designed for specific chemicals or chemical functional groups to greatly enhance the interaction of these molecules with the transistor. This increases both sensitivity and selectivity for discrete devices. Specially designed nanoscale OFET configurations with individually addressable gates can enhance the sensitivity of OFETS as well. Finally, ion channels can be selected for immobilization in tethered lipid bilayer sensors which are already inherently sensitive to the analyte of choice or can be genetically modified to include receptors for many kinds of chemical or biological agents.
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abstract: With the penetration of distributed renewable energy and the development of semiconductor technology, power electronic devices could be utilized to interface re- newable energy generation and the distribution power grid. However, when directly connected to the power grid, the semiconductors inside the power electronic devices could be vulnerable to the power system transient, especially to lightning strikes. The work of this research focuses on the insulation coordination of power elec- tronic devices connected directly to the power distribution system. The Solid State Transformer (SST) in Future Renewable Electric Energy Delivery and Management (FREEDM) system could be a good example for grid connected power electronic devices. Simulations were conducted in Power Systems Computer Aided Design (PSCAD) software. A simulation done to the FREEDM SST showed primary re- sults which were then compare to simulation done to the grid-connected operating Voltage Source Converter (VSC) to get more objective results. Based on the simulation results, voltage surges caused by lightning strikes could result in damage on the grid-connected electronic devices. Placing Metal Oxide Surge Arresers (MOSA, also known as Metal Oxide Surge Varistor, MOV) at the front lter could provide eective protection for those devices from power transient. Part of this research work was published as a conference paper and was presented at CIGRE US National Conference: Grid of the Future Symposium [1] and North American Power Symposium [2].
Dissertation/Thesis
Masters Thesis Engineering 2017

Undergraduate engineering students usually face difficulties understanding electric circuit concepts. Some of those difficulties regard with misconceptions students bring into the classroom and develop during the learning process. Additionally, the increasing complexity of the topics along the fundamental electric circuits course constitutes another factor to those difficulties students experience. Another component we can add to this equation consists of the need of modernize and actualize the curriculum to meet the society’s demands of the next taskforce. Therefore, it is important to investigate the conceptual difficulties students experience when they analyze complex electric circuits. In this dissertation, I identify what those conceptual difficulties are when undergraduate sophomore engineering students attempt to analyze solid-state device circuits. The context of this research comprises a modernized version of the traditional fundamental electric circuits course. This modernized version includes DC analysis, 1^st order transient analysis, AC, and solid-state device analysis.

This dissertation took the form of three individual but complementary studies. Each study contributes to partially answer the overall research question. However, each study answered its own research problem. The first study attempted for identifying what concepts beginning students find challenging regarding semiconductors physics, diodes, and transistors. The second study identified student’s misconceptions when they analyze two solid-state device circuits, one with a diode, and the other with a transistor. The final study looked for determining what misconceptions students use at both earlier and more advances stages along the course. This study also searched for understanding how students move through conceptual changes along the semester.

The general findings comprise three main points. First, students bring misconceptions into the classroom probably built from their previous experiences. Second, they also can develop those misconceptions through the learning process. This is particularly key regarding the relatively new and complex topics from student’s perspectives. Finally, language plays an important role on the kind of misconceptions students develop. How students perceive the professional community use language contributes to either consolidate or modify old misconceptions or develop new ones.

abstract: In recent years, wide bandgap (WBG) devices enable power converters with higher power density and higher efficiency. On the other hand, smart grid technologies are getting mature due to new battery technology and computer technology. In the near future, the two technologies will form the next generation of smart grid enabled by WBG devices. This dissertation deals with two applications: silicon carbide (SiC) device used for medium voltage level interface (7.2 kV to 240 V) and gallium nitride (GaN) device used for low voltage level interface (240 V/120 V). A 20 kW solid state transformer (SST) is designed with 6 kHz switching frequency SiC rectifier. Then three robust control design methods are proposed for each of its smart grid operation modes. In grid connected mode, a new LCL filter design method is proposed considering grid voltage THD, grid current THD and current regulation loop robust stability with respect to the grid impedance change. In grid islanded mode, µ synthesis method combined with variable structure control is used to design a robust controller for grid voltage regulation. For grid emergency mode, multivariable controller designed using H infinity synthesis method is proposed for accurate power sharing. Controller-hardware-in-the-loop (CHIL) testbed considering 7-SST system is setup with Real Time Digital Simulator (RTDS). The real TMS320F28335 DSP and Spartan 6 FPGA control board is used to interface a switching model SST in RTDS. And the proposed control methods are tested. For low voltage level application, a 3.3 kW smart grid hardware is built with 3 GaN inverters. The inverters are designed with the GaN device characterized using the proposed multi-function double pulse tester. The inverter is controlled by onboard TMS320F28379D dual core DSP with 200 kHz sampling frequency. Each inverter is tested to process 2.2 kW power with overall efficiency of 96.5 % at room temperature. The smart grid monitor system and fault interrupt devices (FID) based on Arduino Mega2560 are built and tested. The smart grid cooperates with GaN inverters through CAN bus communication. At last, the three GaN inverters smart grid achieved the function of grid connected to islanded mode smooth transition
Dissertation/Thesis
Doctoral Dissertation Electrical Engineering 2017

Die Menschheit steht vor der großen Herausforderung der Energieversorgung des 21. Jahrhundert. Nirgendwo ist diese noch dringlicher geworden als im Bereich der Energiespeicherung und Umwandlung. Konventionelle Energie kommt hauptsächlich aus fossilen Brennstoffen, die auf der Erde nur begrenzt vorhanden sind, und hat zu einer starken Belastung der Umwelt geführt. Zusätzlich nimmt der Energieverbrauch weiter zu, insbesondere durch die rasante Verbreitung von Fahrzeugen und verschiedener Kundenelektronik wie PCs und Mobiltelefone. Alternative Energiequellen sollten vor einer Energiekrise entwickelt werden. Die Gewinnung erneuerbarer Energie aus Sonne und Wind sind auf jeden Fall sehr wichtig, aber diese Energien sind oft nicht gleichmäßig und andauernd vorhanden. Energiespeichervorrichtungen sind daher von großer Bedeutung, weil sie für eine Stabilisierung der umgewandelten Energie sorgen. Darüber hinaus ist es eine enttäuschende Tatsache, dass der Akku eines Smartphones jeglichen Herstellers heute gerade einen Tag lang ausreicht, und die Nutzer einen zusätzlichen Akku zur Hand haben müssen. Die tragbare Elektronik benötigt dringend Hochleistungsenergiespeicher mit höherer Energiedichte. Der erste Teil der vorliegenden Arbeit beinhaltet Lithium-Ionen-Batterien unter Verwendung von einzelnen aufgerollten Siliziumstrukturen als Anoden, die durch nanotechnologische Methoden hergestellt werden. Eine Lab-on-Chip-Plattform wird für die Untersuchung der elektrochemischen Kinetik, der elektrischen Eigenschaften und die von dem Lithium verursachten strukturellen Veränderungen von einzelnen Siliziumrohrchen als Anoden in einer Lithium-Ionen-Batterie vorgestellt. In dem zweiten Teil wird ein neues Design und die Herstellung von flexiblen on-Chip, Festkörper Mikrosuperkondensatoren auf Basis von MnOx/Au-Multischichten vorgestellt, die mit aktueller Mikroelektronik kompatibel sind. Der Mikrosuperkondensator erzielt eine maximale Energiedichte von 1,75 mW h cm-3 und eine maximale Leistungsdichte von 3,44 W cm-3. Weiterhin wird ein flexibler und faserartig verwebter Superkondensator mit einem Cu-Draht als Substrat vorgestellt. Diese Dissertation wurde im Rahmen des Forschungsprojekts GRK 1215 "Rolled-up Nanotechnologie für on-Chip Energiespeicherung" 2010-2013, finanziell unterstützt von der International Research Training Group (IRTG), und dem PAKT Projekt "Elektrochemische Energiespeicherung in autonomen Systemen, no. 49004401" 2013-2014, angefertigt. Das Ziel der Projekte war die Entwicklung von fortschrittlichen Energiespeichermaterialien für die nächste Generation von Akkus und von flexiblen Superkondensatoren, um das Problem der Energiespeicherung zu addressieren. Hier bedanke ich mich sehr, dass IRTG mir die Möglichkeit angebotet hat, die Forschung in Deutschland stattzufinden.
Human beings are facing the grand energy challenge in the 21st century. Nowhere has this become more urgent than in the area of energy storage and conversion. Conventional energy is based on fossil fuels which are limited on the earth, and has caused extensive environmental pollutions. Additionally, the consumptions of energy are still increasing, especially with the rapid proliferation of vehicles and various consumer electronics like PCs and cell phones. We cannot rely on the earth’s limited legacy forever. Alternative energy resources should be developed before an energy crisis. The developments of renewable conversion energy from solar and wind are very important but these energies are often not even and continuous. Therefore, energy storage devices are of significant importance since they are the one stabilizing the converted energy. In addition, it is a disappointing fact that nowadays a smart phone, no matter of which brand, runs out of power in one day, and users have to carry an extra mobile power pack. Portable electronics demands urgently high-performance energy storage devices with higher energy density. The first part of this work involves lithium-ion micro-batteries utilizing single silicon rolled-up tubes as anodes, which are fabricated by the rolled-up nanotechnology approach. A lab-on-chip electrochemical device platform is presented for probing the electrochemical kinetics, electrical properties and lithium-driven structural changes of a single silicon rolled-up tube as an anode in lithium ion batteries. The second part introduces the new design and fabrication of on chip, all solid-state and flexible micro-supercapacitors based on MnOx/Au multilayers, which are compatible with current microelectronics. The micro-supercapacitor exhibits a maximum energy density of 1.75 mW h cm-3 and a maximum power density of 3.44 W cm-3. Furthermore, a flexible and weavable fiber-like supercapacitor is also demonstrated using Cu wire as substrate. This dissertation was written based on the research project supported by the International Research Training Group (IRTG) GRK 1215 "Rolled-up nanotech for on-chip energy storage" from the year 2010 to 2013 and PAKT project "Electrochemical energy storage in autonomous systems, no. 49004401" from 2013 to 2014. The aim of the projects was to design advanced energy storage materials for next-generation rechargeable batteries and flexible supercapacitors in order to address the energy issue. Here, I am deeply indebted to IRTG for giving me an opportunity to carry out the research project in Germany. September 2014, IFW Dresden, Germany Wenping Si

Organic semiconductors are a new key technology for large-area and flexible thin-film electronics. They are deposited as thin films (sub-nanometer to micrometer) on large-area substrates. The technologically most advanced applications are organic light emitting diodes (OLEDs) and organic photovoltaics (OPV). For the improvement of performance and efficiency, correct modeling of the electronic processes in the devices is essential. Reliable characterization and validation of the electronic properties of the materials is simultaneously required for the successful optimization of devices. Furthermore, understanding the relations between material structures and their key characteristics opens the path for innovative material and device design. In this thesis, two material characterization methods are developed, respectively refined and applied: a novel technique for measuring the charge carrier mobility μ and a way to determine the ionization energy IE or the electron affinity EA of an organic semiconductor. For the mobility measurements, a new evaluation approach for space-charge limited current (SCLC) measurements in single carrier devices is developed. It is based on a layer thickness variation of the material under investigation. In the \"potential mapping\" (POEM) approach, the voltage as a function of the device thickness V(d) at a given current density is shown to coincide with the spatial distribution of the electric potential V(x) in the thickest device. On this basis, the mobility is directly obtained as function of the electric field F and the charge carrier density n. The evaluation is model-free, i.e. a model for μ(F, n) to fit the measurement data is not required, and the measurement is independent of a possible injection barrier or potential drop at non-optimal contacts. The obtained μ(F, n) function describes the effective average mobility of free and trapped charge carriers. This approach realistically describes charge transport in energetically disordered materials, where a clear differentiation between trapped and free charges is impossible or arbitrary. The measurement of IE and EA is performed by characterizing solar cells at varying temperature T. In suitably designed devices based on a bulk heterojunction (BHJ), the open-circuit voltage Voc is a linear function of T with negative slope in the whole measured range down to 180K. The extrapolation to temperature zero V0 = Voc(T → 0K) is confirmed to equal the effective gap Egeff, i.e. the difference between the EA of the acceptor and the IE of the donor. The successive variation of different components of the devices and testing their influence on V0 verifies the relation V0 = Egeff. On this basis, the IE or EA of a material can be determined in a BHJ with a material where the complementary value is known. The measurement is applied to a number of material combinations, confirming, refining, and complementing previously reported values from ultraviolet photo electron spectroscopy (UPS) and inverse photo electron spectroscopy (IPES). These measurements are applied to small molecule organic semiconductors, including mixed layers. In blends of zinc-phthalocyanine (ZnPc) and C60, the hole mobility is found to be thermally and field activated, as well as increasing with charge density. Varying the mixing ratio, the hole mobility is found to increase with increasing ZnPc content, while the effective gap stays unchanged. A number of further materials and material blends are characterized with respect to hole and electron mobility and the effective gap, including highly diluted donor blends, which have been little investigated before. In all materials, a pronounced field activation of the mobility is observed. The results enable an improved detailed description of the working principle of organic solar cells and support the future design of highly efficient and optimized devices.:1. Introduction 2. Organic semiconductors and devices 2.1. Organic semiconductors 2.1.1. Conjugated π system 2.1.2. Small molecules and polymers 2.1.3. Disorder in amorphous materials 2.1.4. Polarons 2.1.5. Polaron hopping 2.1.6. Fermi-Dirac distribution and Fermi level 2.1.7. Quasi-Fermi levels 2.1.8. Trap states 2.1.9. Doping 2.1.10. Excitons 2.2. Interfaces and blend layers 2.2.1. Interface dipoles 2.2.2. Energy level bending 2.2.3. Injection from metal into semiconductor, and extraction 2.2.4. Excitons at interfaces 2.3. Charge transport and recombination in organic semiconductors 2.3.1. Drift transport 2.3.2. Charge carrier mobility 2.3.3. Thermally activated transport 2.3.4. Diffusion transport 2.3.5. Drift-diffusion transport 2.3.6. Space-charge limited current 2.3.7. Recombination 2.4. Mobility measurement 2.4.1. SCLC and TCLC 2.4.2. Time of flight 2.4.3. Organic field effect transistors 2.4.4. CELIV 2.5. Organic solar cells 2.5.1. Exciton diffusion towards the interface 2.5.2. Dissociation of CT states 2.5.3. CT recombination 2.5.4. Flat and bulk heterojunction 2.5.5. Transport layers 2.5.6. Thin film optics 2.5.7. Current-voltage characteristics and equivalent circuit 2.5.8. Solar cell efficiency 2.5.9. Limits of efficiency 2.5.10. Correct solar cell characterization 2.5.11. The \"O-Factor\" 3. Materials and experimental methods 3.1. Materials 3.2. Device fabrication and layout 3.2.1. Layer deposition 3.2.2. Encapsulation 3.2.3. Homogeneity of layer thickness on a wafer 3.2.4. Device layout 3.3. Characterization 3.3.1. Electrical characterization 3.3.2. Sample illumination 3.3.3. Temperature dependent characterization 3.3.4. UPS 4. Simulations 5.1. Design of single carrier devices 5.1.1. General design requirements 5.1.2. Single carrier devices for space-charge limited current 5.1.3. Ohmic regime 5.1.4. Design of injection and extraction layers 5.2. Advanced evaluation of SCLC – potential mapping 5.2.1. Potential mapping by thickness variation 5.2.2. Further evaluation of the transport profile 5.2.3. Injection into and extraction from single carrier devices 5.2.4. Majority carrier approximation 5.3. Proof of principle: POEM on simulated data 5.3.1. Constant mobility 5.3.2. Field dependent mobility 5.3.3. Field and charge density activated mobility 5.3.4. Conclusion 5.4. Application: Transport characterization in organic semiconductors 5.4.1. Hole transport in ZnPc:C60 5.4.2. Hole transport in ZnPc:C60 – temperature variation 5.4.3. Hole transport in ZnPc:C60 – blend ratio variation 5.4.4. Hole transport in ZnPc:C70 5.4.5. Hole transport in neat ZnPc 5.4.6. Hole transport in F4-ZnPc:C60 5.4.7. Hole transport in DCV-5T-Me33:C60 5.4.8. Electron transport in ZnPc:C60 5.4.9. Electron transport in neat Bis-HFl-NTCDI 5.5. Summary and discussion of the results 5.5.1. Phthalocyanine:C60 blends 5.5.2. DCV-5T-Me33:C60 5.5.3. Conclusion 6. Organic solar cell characteristics: the influence of temperature 6.1. ZnPc:C60 solar cells 6.1.1. Temperature variation 6.1.2. Illumination intensity variation 6.2. Voc in flat and bulk heterojunction organic solar cells 6.2.1. Qualitative difference in Voc(I, T) 6.2.2. Interpretation of Voc(I, T) 6.3. BHJ stoichiometry variation 6.3.1. Voc upon variation of stoichiometry and contact layer 6.3.2. V0 upon stoichiometry variation 6.3.3. Low donor content stoichiometry 6.3.4. Conclusion from stoichiometry variation 6.4. Transport material variation 6.4.1. HTM variation 6.4.2. ETM variation 6.5. Donor:acceptor material variation 6.5.1. Donor variation 6.5.2. Acceptor variation 6.6. Conclusion 7. Summary and outlook 7.1. Summary 7.2. Outlook A. Appendix A.1. Energy pay-back of this thesis A.2. Tables and registers
Organische Halbleiter sind eine neue Schlüsseltechnologie für großflächige und flexible Dünnschichtelektronik. Sie werden als dünne Materialschichten (Sub-Nanometer bis Mikrometer) auf großflächige Substrate aufgebracht. Die technologisch am weitesten fortgeschrittenen Anwendungen sind organische Leuchtdioden (OLEDs) und organische Photovoltaik (OPV). Zur weiteren Steigerung von Leistungsfähigkeit und Effizienz ist die genaue Modellierung elektronischer Prozesse in den Bauteilen von grundlegender Bedeutung. Für die erfolgreiche Optimierung von Bauteilen ist eine zuverlässige Charakterisierung und Validierung der elektronischen Materialeigenschaften gleichermaßen erforderlich. Außerdem eröffnet das Verständnis der Zusammenhänge zwischen Materialstruktur und -eigenschaften einen Weg für innovative Material- und Bauteilentwicklung. Im Rahmen dieser Dissertation werden zwei Methoden für die Materialcharakterisierung entwickelt, verfeinert und angewandt: eine neuartige Methode zur Messung der Ladungsträgerbeweglichkeit μ und eine Möglichkeit zur Bestimmung der Ionisierungsenergie IE oder der Elektronenaffinität EA eines organischen Halbleiters. Für die Beweglichkeitsmessungen wird eine neue Auswertungsmethode für raumladungsbegrenzte Ströme (SCLC) in unipolaren Bauteilen entwickelt. Sie basiert auf einer Schichtdickenvariation des zu charakterisierenden Materials. In einem Ansatz zur räumlichen Abbildung des elektrischen Potentials (\"potential mapping\", POEM) wird gezeigt, dass das elektrische Potential als Funktion der Schichtdicke V(d) bei einer gegebenen Stromdichte dem räumlichen Verlauf des elektrischen Potentials V(x) im dicksten Bauteil entspricht. Daraus kann die Beweglichkeit als Funktion des elektrischen Felds F und der Ladungsträgerdichte n berechnet werden. Die Auswertung ist modellfrei, d.h. ein Modell zum Angleichen der Messdaten ist für die Berechnung von μ(F, n) nicht erforderlich. Die Messung ist außerdem unabhängig von einer möglichen Injektionsbarriere oder einer Potentialstufe an nicht-idealen Kontakten. Die gemessene Funktion μ(F, n) beschreibt die effektive durchschnittliche Beweglichkeit aller freien und in Fallenzuständen gefangenen Ladungsträger. Dieser Zugang beschreibt den Ladungstransport in energetisch ungeordneten Materialien realistisch, wo eine klare Unterscheidung zwischen freien und Fallenzuständen nicht möglich oder willkürlich ist. Die Messung von IE und EA wird mithilfe temperaturabhängiger Messungen an Solarzellen durchgeführt. In geeigneten Bauteilen mit einem Mischschicht-Heteroübergang (\"bulk heterojunction\" BHJ) ist die Leerlaufspannung Voc im gesamten Messbereich oberhalb 180K eine linear fallende Funktion der Temperatur T. Es kann bestätigt werden, dass die Extrapolation zum Temperaturnullpunkt V0 = Voc(T → 0K) mit der effektiven Energielücke Egeff , d.h. der Differenz zwischen EA des Akzeptor-Materials und IE des Donator-Materials, übereinstimmt. Die systematische schrittweise Variation einzelner Bestandteile der Solarzellen und die Überprüfung des Einflusses auf V0 bestätigen die Beziehung V0 = Egeff. Damit kann die IE oder EA eines Materials bestimmt werden, indem man es in einem BHJ mit einem Material kombiniert, dessen komplementärer Wert bekannt ist. Messungen per Ultraviolett-Photoelektronenspektroskopie (UPS) und inverser Photoelektronenspektroskopie (IPES) werden damit bestätigt, präzisiert und ergänzt. Die beiden entwickelten Messmethoden werden auf organische Halbleiter aus kleinen Molekülen einschließlich Mischschichten angewandt. In Mischschichten aus Zink-Phthalocyanin (ZnPc) und C60 wird eine Löcherbeweglichkeit gemessen, die sowohl thermisch als auch feld- und ladungsträgerdichteaktiviert ist. Wenn das Mischverhältnis variiert wird, steigt die Löcherbeweglichkeit mit zunehmendem ZnPc-Anteil, während die effektive Energielücke unverändert bleibt. Verschiedene weitere Materialien und Materialmischungen werden hinsichtlich Löcher- und Elektronenbeweglichkeit sowie ihrer Energielücke charakterisiert, einschließlich bisher wenig untersuchter hochverdünnter Donator-Systeme. In allen Materialien wird eine deutliche Feldaktivierung der Beweglichkeit beobachtet. Die Ergebnisse ermöglichen eine verbesserte Beschreibung der detaillierten Funktionsweise organischer Solarzellen und unterstützen die künftige Entwicklung hocheffizienter und optimierter Bauteile.:1. Introduction 2. Organic semiconductors and devices 2.1. Organic semiconductors 2.1.1. Conjugated π system 2.1.2. Small molecules and polymers 2.1.3. Disorder in amorphous materials 2.1.4. Polarons 2.1.5. Polaron hopping 2.1.6. Fermi-Dirac distribution and Fermi level 2.1.7. Quasi-Fermi levels 2.1.8. Trap states 2.1.9. Doping 2.1.10. Excitons 2.2. Interfaces and blend layers 2.2.1. Interface dipoles 2.2.2. Energy level bending 2.2.3. Injection from metal into semiconductor, and extraction 2.2.4. Excitons at interfaces 2.3. Charge transport and recombination in organic semiconductors 2.3.1. Drift transport 2.3.2. Charge carrier mobility 2.3.3. Thermally activated transport 2.3.4. Diffusion transport 2.3.5. Drift-diffusion transport 2.3.6. Space-charge limited current 2.3.7. Recombination 2.4. Mobility measurement 2.4.1. SCLC and TCLC 2.4.2. Time of flight 2.4.3. Organic field effect transistors 2.4.4. CELIV 2.5. Organic solar cells 2.5.1. Exciton diffusion towards the interface 2.5.2. Dissociation of CT states 2.5.3. CT recombination 2.5.4. Flat and bulk heterojunction 2.5.5. Transport layers 2.5.6. Thin film optics 2.5.7. Current-voltage characteristics and equivalent circuit 2.5.8. Solar cell efficiency 2.5.9. Limits of efficiency 2.5.10. Correct solar cell characterization 2.5.11. The \"O-Factor\" 3. Materials and experimental methods 3.1. Materials 3.2. Device fabrication and layout 3.2.1. Layer deposition 3.2.2. Encapsulation 3.2.3. Homogeneity of layer thickness on a wafer 3.2.4. Device layout 3.3. Characterization 3.3.1. Electrical characterization 3.3.2. Sample illumination 3.3.3. Temperature dependent characterization 3.3.4. UPS 4. Simulations 5.1. Design of single carrier devices 5.1.1. General design requirements 5.1.2. Single carrier devices for space-charge limited current 5.1.3. Ohmic regime 5.1.4. Design of injection and extraction layers 5.2. Advanced evaluation of SCLC – potential mapping 5.2.1. Potential mapping by thickness variation 5.2.2. Further evaluation of the transport profile 5.2.3. Injection into and extraction from single carrier devices 5.2.4. Majority carrier approximation 5.3. Proof of principle: POEM on simulated data 5.3.1. Constant mobility 5.3.2. Field dependent mobility 5.3.3. Field and charge density activated mobility 5.3.4. Conclusion 5.4. Application: Transport characterization in organic semiconductors 5.4.1. Hole transport in ZnPc:C60 5.4.2. Hole transport in ZnPc:C60 – temperature variation 5.4.3. Hole transport in ZnPc:C60 – blend ratio variation 5.4.4. Hole transport in ZnPc:C70 5.4.5. Hole transport in neat ZnPc 5.4.6. Hole transport in F4-ZnPc:C60 5.4.7. Hole transport in DCV-5T-Me33:C60 5.4.8. Electron transport in ZnPc:C60 5.4.9. Electron transport in neat Bis-HFl-NTCDI 5.5. Summary and discussion of the results 5.5.1. Phthalocyanine:C60 blends 5.5.2. DCV-5T-Me33:C60 5.5.3. Conclusion 6. Organic solar cell characteristics: the influence of temperature 6.1. ZnPc:C60 solar cells 6.1.1. Temperature variation 6.1.2. Illumination intensity variation 6.2. Voc in flat and bulk heterojunction organic solar cells 6.2.1. Qualitative difference in Voc(I, T) 6.2.2. Interpretation of Voc(I, T) 6.3. BHJ stoichiometry variation 6.3.1. Voc upon variation of stoichiometry and contact layer 6.3.2. V0 upon stoichiometry variation 6.3.3. Low donor content stoichiometry 6.3.4. Conclusion from stoichiometry variation 6.4. Transport material variation 6.4.1. HTM variation 6.4.2. ETM variation 6.5. Donor:acceptor material variation 6.5.1. Donor variation 6.5.2. Acceptor variation 6.6. Conclusion 7. Summary and outlook 7.1. Summary 7.2. Outlook A. Appendix A.1. Energy pay-back of this thesis A.2. Tables and registers