In dieser Arbeit wird ein grundlegender Rahmen für das Verständnis von photovoltaischer Ladungserzeugung an Grenzflächen zwischen einem Metalloxid und einem organischen Halbleiter geschaffen. Dabei wird gezeigt, dass hybride Ladungstransferzustände (HCTS) eine entscheidende Rolle im Energieumwandlungsprozess spielen. Vor ihrer endgültigen Trennung bleiben Elektronen und Löcher an gegenüberliegenden Seiten der Grenzfläche durch Coulomb-Interaktion aneinander gebunden. Nur wenn die Trennung eines solchen HCTS gelingt, kann es zu einem Photostrom beitragen. Die planaren Schichtsysteme ZnO/P3HT, ZnMgO/P3HT und SnO2/P3HT dienen als Modellsystem für eine ausführliche Studie über Energiestruktur der Grenzfläche, photovoltaische Energieumwandlung und die damit verbundenen Verluste. Es wird gezeigt, dass ein HCTS aus einem Elektron im Leitungsband des Metalloxids und einem Loch im HOMO des Polymers besteht. Folglich ist seine Entstehung eine intrinsische Eigenschaft von allen derartigen Grenzflächen. Elektrolumineszenzspektroskopie (EL) stellt sich als wirksame Methode zur Untersuchung von HCTS dar. Deren strahlende Rekombination produziert ein breites Signal im nahen Infrarotbereich. Spannungsabhängige EL-Messungen zeigen den hohen Grad an Delokalisierung von beiden Ladungsträgern in einem HCTS. EL-Spektren, die über einen weiten Temperaturbereich aufgenommen wurden, zeigen, dass nichtstrahlende Prozesse mit Abstand der dominierende Zerfallsmechanismus für HCTS bei Zimmertemperatur sind. Ein Modell aus mehreren Schritten für den Stromerzeugungsprozess kann aus temperaturabhängigen photovoltaischen Messungen abgeleitet werden. Hierbei wird deutlich, dass die Bindungsenergie von Elektron und Loch in einem HCTS keine bedeutende Einschränkung für die Leistungsfähigkeit einer Solarzelle darstellt. Die einflussreiche Rolle von nichtstrahlenden Zerfallsprozessen verursacht jedoch in allen untersuchten Materialsystemen schwere Verluste. / In this work, a fundamental framework for the understanding of photovoltaic charge generation at metal-oxide/organic hybrid interfaces is established. It is shown that hybrid charge transfer states (HCTS) play a crucial role in the power conversion process. Prior to full charge separation, pairs of electrons and holes situated at opposite sides of the heterojunction remain bound to each other by Coulomb interaction. Only if an HCTS is dissociated, a contribution to a photocurrent can be made. Planar heterojunctions of the material combinations ZnO/P3HT, ZnMgO/P3HT, and SnO2/P3HT serve as model systems for a broad investigation of interface energetics, photovoltaic power conversion and the loss processes therein. It is shown that an HCTS consists of an electron in the conduction band of the metal-oxide and a hole in the HOMO of the polymer. Consequently, its formation is an intrinsic property of all heterojunctions of that kind. Electroluminescence (EL) spectroscopy proves to be a powerful tool in the analysis of HCTS. Their radiative recombination produces a broad signal in the near-infrared spectral range. Voltage-dependent EL measurements reveal a high degree of delocalization of both carriers in an HCTS, whereas EL spectra recorded over a wide range of temperatures show that non-radiative processes are by far the dominant recombination channel for HCTS at room temperature. A multistep model of the charge generation process is derived from temperature-dependent photovoltaic measurements. It becomes apparent that the binding energy of electron and hole in an HCTS does not impose a significant limitation on device performance. The strong presence of non-radiative decay processes, however, causes severe losses for all material systems that are investigated in this work.

La mise en forme de matériaux fonctionnels et leur texturation contrôlée à toutes les échelles sont des conditions sine qua non pour l’amélioration des systèmes existants. Ce projet de thèse consiste en la création de structures solides complexes en utilisant des méthodes interdisciplinaires telles que la chimie sol-gel et la physique des fluides complexes. Ainsi, il est possible d’obtenir des fibres ou des films de dioxyde de Titane poreux présentant des pores de différentes tailles organisés hiérarchiquement. Cette organisation texturale optimise le transport de matière (gaz / liquides) pour des applications telles que la dépollution d’air (photocatalyse) ou les cellules solaires à colorant. Par ailleurs, nous avons contrôlé l’alignement de nano-bâtonnets d’oxyde de Zinc au sein d’une fibre macroscopique générant ainsi des propriétés collectives d’émission de lumière (photoluminescence)anisotrope, spécificité d’utilité pour l’électronique à permutation. Enfin, nous avons synthétisé des nano objets anisotropiques et des nano-lamelles de polypyrrole afin de générer des films minces lisses avec des propriétés électriques permettant leur utilisation comme électrode ou électrolyte solide dans des cellules solaires à colorant. / The shaping of functional materials and the control of their texture at all length scales are sine qua non conditions for the improvement of current systems. This PhD project consists in creating complex solid architectures using interdisciplinary methods such as sol-gel chemistry or complex fluids physics. Therefore, it is possible to synthesize Titanium Dioxide macroscopic fibers orfilms which possess a hierarchical porosity. This organization allows the optimization of the matter transport (liquid/gaz) for air depollution application (photocatalysis) or dye-sensitizedsolar cells. In another project, we were able to control the alignment of zinc oxide nanorods within a macroscopic fiber. This alignment provides to the fiber an anisotropic photoluminescence behavior which can be useful for switching devices application. Finally, we synthesized anisotropic particles and nano-sheets of polypyrrole (conducting polymer) in order to obtain smooth thin films presenting interesting electrical properties. The objective was to use them as electrolyte and/or electrode in dye-sensitized solar cells.

The characterization and fabrication of active matrix thin film transistors (TFTs) has been studied for an addressable microfluidic electrowetting channel device as application. A new transparent semiconductor material, Amorphous Indium Gallium Zinc Oxide (a-IGZO), is used for TFT, which shows high electrical performance rather than amorphous silicon based TFT; higher mobility and even higher transparency. The purpose of this dissertation is to optimize each TFT process including the optimization of a-IGZO properties to achieve robust device for application. To minimize hysteresis of TFT curves, the gate dielectric is discussed extensively in this dissertation. By optimizing gas ratio of NH3SiH4, it is found that the TFT with NH3 rich SiNx gate dielectric deposited with NH3/SiH4 =5.1 and stoichiometric SiO2 demonstrates best condition to reduce hysteresis. a-IGZO films is investigated as a function of power and substrate bias effect which affects to electrical performance; the higher power and substrate bias increase the carrier density in the film and mainly cause threshold voltage(VT) to shift in the negative gate voltage direction and mobility to increase, respectively. In addition, the powerful method to estimate the electrical properties of a-IGZO is proposed by calculating O2 and IGZO flux during sputtering in which the incorporation ratio with O2/IGZO ≈1 demonstrates the optimized a-IGZO film for TFT. It is confirmed that both physical and chemical adsorption affects the electrical property of a-IGZO channel by studying TFT-IV characteristics with different pressure and analyzing X-ray photoelectron spectroscopy (XPS), which mainly affects the VT instability. The sputtered SiO2 passivation shows better electrical performance. To achieve electrically compatible (lower back channel current) a-IGZO film to SiO2 sputter passivated device, a-IGZO TFTs require oxygen rich a-IGZO back channel by employing two step a-IGZO deposition process (2nd 10nm a-IGZO with PO2 = 1.5mTorr on 1st 40nm a-IGZO with PO2=1mTor). Electrowetting microfluidic channel device as application using a-IGZO TFTs is studied by doing preliminary test. The electrowetting channel test using polymer post device platform is candidate for addressable electrowetting microfluidic channel device driven by active matrix type a-IGZO TFT.

The foci of this thesis are the metal oxides Cu2O, ZnO and Fe3O4 and their interaction with water and sulfur dioxide (SO2). The intention is to study SO2-induced atmospheric corrosion on a molecular level. All studies are based on photoelectron spectroscopy (PES) and scanning tunneling microscopy (STM) measurements. The band structure of Cu2O in the Γ-M direction has been probed by angle-resolved PES (ARPES). It reveals a more detailed picture of the bulk band structure than earlier data and gives the first experimental evidence of a dispersive hybridized Cu 3d-Cu 4s state. The experimental data is compared to band structure calculations. The structure of clean metal oxide surfaces and impact of sample preparation have been studied. Oxygen vacancies can form a (√3x√3)R30° reconstruction on Cu2O(111). Oxygen atoms adjacent to copper vacancies, steps or kinks are shown to be adsorption sites for both water and SO2. Annealing temperature influences the defect density and hydrogen content in ZnO, which can have large impact on the surface properties of ZnO(0001). Water is shown to adsorb dissociatively on ZnO(0001) and partly dissociatively on Cu2O(111). The dissociation occurs at undercoordinated oxygen sites on both surfaces. Water stays adsorbed on ZnO(0001) at room temperature but on Cu2O(111), all water has desorbed at 210 K. SO2 interacts with one or two undercoordinated O-sites on all studied oxide surfaces forming SO3 or SO4 species respectively. SO4 on Fe3O4(100) follows the (√2x√2)R45° reconstruction. On Cu2O(111) and ZnO(0001), SO2 adsorbs on defect sites. An SO3 to SO4 transition is observed on Cu2O(111) when heating an SO3 adsorbate layer from 150 K to 280K. Coadsorption of water and SO2 on ZnO(0001) and Fe3O4(100) has been studied briefly. Water blocks SO2 adsorption sites on ZnO(0001). On Fe3O4(100) and on one type of reduced ZnO(0001) sample, SO2 dissociation to atomic sulfur or sulfide occurs to a higher extent on water exposed surfaces than on clean surfaces. Water thus appears to increase the charge density on some surfaces. Further studies are needed to reveal the cause of this unexpected effect. / <p>QC 20110516</p>

Metal oxide nanocrystals have attracted significant interests due to their unique chemical, physical, and electrical properties which depend on their size and structure. In this study, a continuous flow microreactor system was employed to synthesize metal oxide nanocrystals in aqueous solution. Assembly of nanocrystals is considered one of the most promising approaches to design nano-, microstructures, and complex mesoscopic architectures. A variety of strategies to induce nanocrystal assembly have been reported, including directed assembly methods that apply external forces to fabricate assembled structures. In this study ZnO nanocrystals were synthesized in an aqueous solution using a continuous flow microreactor. The growth mechanism and stability of ZnO nanocrystals were studied by varying the pH and flow conditions of the aqueous solution. It was found that convective fluid flow from Dean vortices in a winding microcapillary tube could be used for the assembly of ZnO nanocrystals. The ZnO nanocrystal assemblies formed three-dimensional mesoporous structures of different shapes including a tactoid, a retangle and a sphere. The assembly results from a competing interaction between electrostatic forces caused by surface charge of nanocrystals and collision of nanocrystals associated with Dean vortices. The as synthesized colloidal ZnO nanocrystals or assembly were directly deposited onto a substrate to fabricate ZnO nanostructured surfaces. The rectangular assembly led to flower-like ZnO nanostructured films, while the spherical assembly resulted in amorphous ZnO thin film and vertical ZnO nanowire (NW) arrays. In contrast to the formation of flower structure or amorphous thin film, only colloidal ZnO nanocrystals were used as the building blocks for forming vertical ZnO NW arrays. This study demonstrates the versatility of the microreactor-assisted nanomaterial synthesis and deposition process for the production of nanostrucuturesres with various morphologies by tuning the physical parameters while using the same chemical precursors for the synthesis. ZnO flower structure was coated on a microwick structure to improve the capillary flow. The coated microwick structure showed an enhanced capillary rise, which was attributed to the hydrophilic property and geometrical modification of ZnO nanostructure. Two-phase boiling heat transfer was performed using ZnO nanostructured surfaces. ZnO nanocoating altered the important characteristics including surface roughness and wettability. Hydrophilic nature of the ZnO nanocoating generally enhanced the boiling heat transfer performance, resulting in higher heat transfer coefficient (HTC), higher critical heat flux (CHF), and lower surface superheat comparing to the bare surface. Octahedral SnO and porous NiO films, fabricated by a continuous flow microreactor system, were suggested as potential boiling surfaces for the high porosity and irregularity of their structures. / Graduation date: 2013

Im Rahmen der vorliegenden Arbeit wurden planare und drahtförmige Heterostrukturen (HS) mit ZnO- und ZnCdO-Quantengräben bezüglich ihrer Lumineszenz untersucht. Die Proben wurden mit der gepulsten Laserabscheidung (PLD) hergestellt. Bei ZnO-basierten drahtförmigen HS mit Durchmessern im Mikro- und Nanometer-Bereich handelt es sich um vielversprechende Kandidaten für miniaturisierte optoelektronische Bauelemente. Da es für viele Anwendungen notwendig ist, dass die Emission des Quantengrabens (QW) in einem breiten Spektralbereich eingestellt werden kann, muss die ZnO-Bandlücke möglichst stark verändert werden können. Durch ZnCdO und MgZnO ist dies möglich. Durch eine Optimierung der Abscheideparameter wurde der für PLD erreichte maximale Cd-Gehalt signifikant auf 0,25 erhöht. Große Mg-Gehalte konnten schon vor der Forschung zur vorliegenden Arbeit mit der PLD realisiert werden. Die planaren HS mit ZnO-Quantengräben wurden vorrangig bezüglich Ihrer Lumines-zenzeigenschaften untersucht. Aufgrund der Orientierung der QW sollten diese zusätzlich zum Quantum-Confinement Effekt den Quantum-Confined Stark Effect (QCSE) zeigen. Der QCSE wurde durch zeitabhängige und anregungsabhängige Lumineszenzmessungen nachgewiesen. In den Mikrodraht (µW)- bzw. Nanodraht (NW)-HS mit ZnO-QW wurde die Emission zwischen 3,4 eV und 3,6 eV bzw. 3,4 eV und 3,7 eV eingestellt. Um HS mit ZnCdO-QW herstellen zu können, war es notwendig, die strukturellen und optischen Eigenschaften sowie die elektronische Struktur von ZnCdO-Dünnfilmen zu untersuchen. Durch einen hohen Cd-Gehalt von 0,25 war es möglich, die Bandlücken-energie um 0,8 eV zu verringern. In planaren HS wurde ZnO bzw. MgZnO als Barriere verwendet und die QW-Emission zwischen 2,5 eV und 3,1 eV bzw. 2,5 eV und 3,65 eV eingestellt. Es wurde untersucht, ob für HS mit ZnCdO-QW ein QCSE auftritt. Die experimentellen Energien wurden dazu mit berechneten Werten verglichen, die mithilfe einer Effektiv-Masse-Näherung und dem Modell eines endlich tiefen Potentialtopfes bestimmt wurden. In entsprechenden µW- bzw. NW-HS wurde die QW-Emission infolge des Quantum-Confinement Effektes zwischen 2,7 eV und 3,1 eV bzw. 2,5 eV und 3,4 eV variiert. Da es für die Anwendung von µW- und NW-HS wichtig ist, dass diese eine homogene QW-Emission zeigen, wurde deren spektrale Position entlang der Struktur und für die verschiedenen Facetten der hexagonalen Drähte untersucht. Die Homogenität der Emission ist für die µW-HS kleiner als für die NW-HS.

In this dissertation, the electrical characteristics of generally used hydrogenated amorphous silicon (a-Si:H) TFTs in LCD and newly risen amorphous indium-gallium-zinc oxide (a-IGZO) TFTs were studied. For modern mobile display and large-size flat panel display application, the traditional thin-film transistor-liquid crystal display (TFT-LCD) technology confronts with a lot of challenges and problems. In general, flexible displays must exhibit some bending ability; however, bending applies mechanical strain to electronic circuits and affects device characteristics. Therefore, the electrical characteristics of a-Si:H TFTs fabricated on stainless steel foil substrates with uniaxial bending were investigated at different temperatures. Experimental results showed that the on-state current and threshold voltage degraded under outward bending. This is because outward bending will induce the increase of band tail states, affecting the transport mechanism at different temperatures. In addition, for practical operation, the electrical characteristics of a-Si:H TFTs under flat and bending situations after AC/DC stress at different temperatures were studied. It was found that high temperature and mechanical bending played important roles under AC stress. The dependence between the accumulated sum of bias rising and falling time and the threshold voltage shifts under AC stress was also observed. Because a-Si:H is a photosensitive material, the high intensity backlight illumination will degrade the performance of a-Si:H TFTs. Thus, the photo-leakage current of a-Si:H TFTs under illumination was investigated at different temperatures. Experimental results showed that a-Si:H TFTs exhibited a pool performance at lower temperatures. The indirect recombination rate and the parasitic resistance (Rp) are responsible for the different photo-leakage-current trends of a-Si:H TFTs under varied temperature operations. To investigate the photo-leakage current, the a-Si:H TFTs were exposed to ultraviolet (UV) light irradiation. It was found that the photo current of a-Si:H TFTs was reduced after UV light irradiation. The detail mechanisms on reducing/increasing photo-leakage current by UV light irradiation were discussed. Recently, the oxide-based semiconductor TFT, especially a-IGZO TFT, is considered as one of promising candidates for active matrix flat-panel display. However, the a-IGZO TFT exists significant electrical instability issue and manufacturing problems. As a consequence, we investigated the effect of hydrogen incorporation on a-IGZO TFTs to reduce interface states between active layer and insulator. Experimental results showed that the electrical characteristics of hydrogen-incorporated a-IGZO TFTs were improved. The threshold voltage shift (£GVth) in hysteresis loop is suppressed from 4 V to 2 V due to the hydrogen-induced passivation of the interface trap states. Finally, we reported the effect of ambient environment on a-IGZO TFT instability. As a-IGZO TFTs were stored in atmosphere environment for 40 days, the transfer characteristics accompanying strange hump were observed during bias-stress. The hump phenomenon is attributed to the absorption of H2O molecule. Additionally, the sufficient electric field is also necessary to cause this anomalous transfer characteristic.

A new set of thermal neutron detectors has been developed as a near term 3He tube replacement. The zinc oxide scintillator is an ultrafast scintillator which can be doped to have performance equal to or superior to 3He tubes. Originally investigated in the early 1950s, this room temperature semiconductor has been evaluated as a thermal neutron scintillator. Zinc oxide can be doped with different nuclei to tune the band gap, improve optical clarity, and improve the thermal neutron detection efficiency. The effects of various dopant effects on the scintillation properties, materials properties, and crystal growth parameters have been analyzed. Two different growth modalities were investigated: bulk melt grown materials as well as thin film scintillators grown by metalorganic chemical vapor deposition (MOCVD). MOCVD has shown significant advantages including precise thickness control, high dopant incorporation, and epitaxial coatings of neutron target nuclei. Detector designs were modeled and simulated to design an improved thermal neutron detector using doped ZnO layers, conformal coatings and light collection improvements including Bragg reflectors and photonic crystal structures. The detectors have been tested for crystalline quality by XRD and FTIR spectroscopy, for scintillation efficiency by photo-luminescence spectroscopy, and for neutron detection efficiency by alpha and neutron radiation tests. Lastly, a novel method for improving light collection efficiency has been investigated, the creation of a photonic crystal scintillator. Here, the flow of optical light photons is controlled through an engineered structure created with the scintillator materials. This work has resulted in a novel radiation detection material for the near term replacement of 3He tubes with performance characteristics equal to or superior to that of 3He.

A sensor is a technological device or biological organ that detects, or senses, a signal or physical condition and chemical compounds. Technological developments in the recent decades have brought along with it several environmental problems and human safety issues to the fore. In today's world, therefore, sensors, which detect toxic and inflammable chemicals quickly, are necessary. Gas sensors which form a subclass of chemical sensors have found extensive applications in process control industries and environmental monitoring. The present thesis reports the attempt made in development of Zinc oxide thin film based gas sensors. ZnO is sensitive to many gases of interest like hydrocarbons, hydrogen, volatile organic compounds etc. They exhibit high sensitivity, satisfactory stability and rapid response. In the present work the developed sensors have been tested for their sensitivity for a typical volatile organic compound, acetone. An objective analysis of the various substrates namely borosilicate glass, sintered alumina and hard anodized alumina, has been performed as a part of this work. The substrates were evaluated for their electrical insulation and thermal diffusivity. The microstructure of the gas sensitive film on the above mentioned substrates was studied by SEM technique. The gas sensitive Zinc oxide film is deposited by D.C reactive magnetron sputtering technique with substrate bias arrangement. The characterization of the as-deposited film was performed by XRD, SEM and EDAX techniques to determine the variation of microstructure, crystallite size, orientation and chemical composition with substrate bias voltage. The thesis also describes the development of the gas sensor test setup, which has been used to measure the sensing characteristics of the sensor. It was observed that the ZnO sensors developed with higher bias voltages exhibited improved sensitivity to test gas of interest. Gas sensors essentially measure the concentration of gas in its vicinity. In order to determine the distribution of gas concentration in a region, it is necessary to network sensors at remote locations to a host. The host acts as a gateway to the end user to determine the distribution of gas concentration in a region. However, wireless gas sensor networks have not found widespread use because of two inherent limitations: Metal oxide gas sensors suffer from output drift over time; frequent recalibration of a number of sensors is a laborious task. The gas sensors have to be maintained at a high temperature to perform the task of gas sensing. This is power intensive operation and is not well suited for wireless sensor network. This thesis reports an exploratory study carried out on the applicability of gas sensors in wireless gas sensor network. A simple prototype sensing node has been developed using discrete electronic components. A methodology to overcome the problem of frequent calibration of the sensing nodes, to tackle the sensor drift with ageing, is presented. Finally, a preliminary attempt to develop a strategy for using gas sensor network to localize the point of gas leak is given.

Sensors is one of the major areas of current research. Thin film micro/nano sensors are gaining attention worldwide, as there is necessity of miniaturization. There are varieties of sensors available by utilizing different materials in bulk and thin film form for measuring parameters like temperature, pressure, flow, humidity etc. Apart from these, there are various sensors available to measure impact force. Impact sensor offers potential application possibilities in robotics, aerospace, structural & mechanical engineering and related areas. Many physical principles have been explored for the realization of impact sensor. The present thesis reports the efforts made in developing impact sensor using piezoelectric thin film. The necessary brief background information on impact sensors is presented in Chapter 1. This includes the description of available literature on impact sensors and their probable applications. In Chapter 2, a review of the various techniques such as thin film deposition techniques, film thickness measurement techniques, thin film characterization techniques, used in our work are explained in detail. Chapter 3 explains the direct and indirect methods of characterization used for confirming the piezoelectric property of zinc oxide thin films. The detailed experimental work carried out in realizing the impact sensor using piezoelectric thin films is presented in chapter 4. This includes design of the sensor, calibration setup used & the procedure followed and results obtained. Finally, we present the summary of the work carried out in the thesis, conclusions arrived at and the scope for carrying out further work in the direction of making the sensor more efficient.