Eine Reihe 3-(p-X-phenyl)-Thiophenmonomeren (X = -H, -CH3, -OCH3, -COCH3, -COOC2H5, -NO2) wurde elektrochemisch polymerisiert, um Filme zu erhalten, die umkehrbar reduziert und oxidiert werden konnten (n-und p-dotiert wurden). Die Oxidationspotentiale der Monomere und die formalen Potentiale der n und p-Dotierprozesse der Polymere wurden mit Resonanz- und induktiven Effekten der Substituenten (Hammett konstanten) am Phenylring sowie semiempirisch errechneten Bildungswärmen der Monomereradikalkationen korreliert. Außerdem wurden die Oxidationspotentiale mit den Ionisierungspotentialen der Monomere verglichen, die über die Dichtefunktionialtheorie (DFT) errechnet wurden, die der Energie für das Erzeugen der Radikalkationen entsprechen. Um theoretische Grundlagen für die Einstufen-Bildung regioregulär -konjungierter Oligo- und Polythiophene zu erhalten, wurden die elektronischen Zustände von 3-Phenylthiophen-Derivaten anhand von Molekülorbitalberechnungen auf Grundlage der Dichtefunktionaltheorie mit Becke’s Drei-Parameter-Funktion (B3LYP), sowie mit den Basissätzen 6-31G(d) und 3-21G(d) erklärt. Die Reaktivität der Verknüpfung von mono- und oligo-3-Phenylthiophenen wurde von den berechneten ungepaarten Elektronenspindichten der entsprechenden Radikal-Anionen abgeleitet. Die Ionisierungspotentiale, die den Energien zur Erzeugung der Radikal-Anionen während der Oxidation entsprechen, wurden abgeschätzt. Die aus den 3-Phenylthiophenen entstandenen regioselektiven Hauptprodukte können gut durch die Größe der Spindichten erklärt werden. Da die Verknüpfungsreaktion an der zwei-Position des Thiophnrings (C-2) sterisch durch die Phenylgruppe und den Thiophenring gehindert ist, startet die Initiierung der 3-Phenylthiophene über die Bildung eines Kopf-Schwanz-Dimers. Folglich spielt das Kopf-Schwanz-Dimer eine wichtige Rolle bei den Wachstumsreaktionen der 3-Phenylthiophene. Die Ursache dafür liegt darin, dass das Kopf-Schwanz-Dimer in 5-Position die höchste Spin-Dichte besitzt und die Wahrscheinlichkeit einer Kopf-Kopf-Verknüpfung aufgrund der sterischen Hinderung zwischen dem Thiophenring und der Phenylgruppe gering ist. Polymerfilme von 3-Phenylthiophenderivaten, die durch elektrochemische Polymerisation synthetisiert wurden, sind in situ und ex situ durch Resonanz-Raman-Spektroskopie bei verschiedenen Anregungswellenlängen, sowie durch in situ und ex situ UV-Vis Spektroskopie analysiert wurden. Die Entwicklung der in situ UV-Vis-Spektren der Polymer von 3-Phenylthiophene nach der Dotierung wird durch ähnliche Eigenschaften gekennzeichnet, wie für viele Polythiophene mit einem hohen Grad der Konjugation beobachtet. Während der schrittweisen Oxidation der Poly-3-phenylthiophen Filme verringert sich die Intensität der Absorption wegen des Überganges bei 450-566 nm und ein neues ausgedehntes Absorptionsband, das auf (bi)polaron Zustände bezogen wird erscheint bei ungefähr 730-890 nm. Andererseits wird während der Oxidation (p-Dotierung) des Poly3-phenylthiophen Filmes eine blau/hypsochrome Verschiebung für beide Absorptionsbänder beobachtet . Es wird durch die Tatsache erklärt, dass ein Polymer eine Verteilung der Kettenlängen enthält und die längste Polymer kette (dessen Absorption bei niedriger Energie auftritt), bei niedrigeren Potentialen zu oxidieren beginnt. Die elektrochemischen Bandlücken der Derivate von 3-Phenylthiophen sind durch zyklische Voltametrie gemessen worden. Der Effekt der Substituenten auf den Oxidations-/Reduktions- potentiale wird besprochen. Bei Bandlücken, die durch zyklische Voltammetrie erhalten wurden, hat sich herausgestellt, dass sie im Allgemeinen höher liegen als optische Bandlücken. Erste Resultate der in situ Resonanz-Raman-Spektroskopie, von dem elektrochemisch erzeugten Polymerderivate von 3-Phenylthiophen Filmen auf einer Platinelektrode, in einer organischen Elektrolytlösung, werden berichtet. Beobachtete Raman Banden werden zugewiesen; gegründet auf diesen Resultaten werden die zuvor angenommenen molekularen Strukturen diskutiert. / A series of 3-(p-X-phenyl) thiophene monomers (X= –H, –CH3, –OCH3, –COCH3, –COOC2H5, –NO2) was electrochemically polymerized to furnish polymer films that could be reversibly reduced and oxidized (n- and p-doped). The oxidation potentials of the monomers and formal potentials of the n- and p-doping processes of polymers were correlated with resonance and inductive effects (Hammett constants) of the substituents on the phenyl ring as well as the semiempirically calculated heats of formation of the monomer radical cations. Moreover, the oxidation potentials of the monomers were correlated with the ionization potentials of the monomers calculated via density functional theory (DFT), which correspond to the energies for generating radical cations during oxidative processes. For obtaining a theoretical basis for the one-step formation of regioregular –conjugated oligo-and polythiophenes, the electronic states of 3-phenylthiophene derivatives were elucidated by molecular orbital calculations using density functional theory with the Becke-type three parameters functional (B3LYP), the 6-31G(d), and 3-21G(d) basis sets. The reactivity for coupling reaction of mono- and oligo-3-phenylthiophenes are inferred from the calculated unpaired electron spin densities of the respective radical cations, and the ionization potentials which correspond to the energies for generating radical cations during oxidative processes were estimated. The major regioselective products of the oligomerization of 3-phenylthiophene can be well understood in terms of the magnitude of spin densities. Since the steric hindrance between the phenyl group and thiophene ring interferes with the coupling reaction occurring between 2-postions (C–2) of thiophene rings, the initiating reaction of 3-phenylthiophene is generaton of a head-to-tail (HT) dimer. Thus, the head-to-tail (HT) dimer plays an important role in the propagation reactions of 3-phenylthiophene. This originates from the highest spin density at the 5- position of the HT dimer and low probability of the HH coupling due to the steric hindrance between thiophene ring and phenyl group. Polymer films of the 3-phenylthiophene derivatives prepared by electrochemical polymerization were analyzed, in situ and ex situ, with resonance Raman spectroscopy using several excitation wavelengths as well as in situ and ex situ UV-Vis-spectroscopy. The evolution of the in situ UV-Vis-spectra of poly 3-phenylthiophene derivatives upon doping is characterized by similar features as observed for many polythiophenes with high degree of conjugation. During stepwise oxidation of the poly-3-phenylthiophene films the intensity of the absorption due to the transition around 450–566 nm decreases and a new broad absorption band related to (bi)polaron states appears around 730-890 nm. On the other hand, during the oxidation (p-doping) of the poly-3-phenylthiophene films a blue/hypsochromic shift is observed for both absorption bands. It is explained by the fact that a polymer contains a distribution of chain lengths, and the longest polymer chains (the absorption of which occurs at lower energies) start to oxidize at lower potentials. The electrochemical bandgaps of 3-phenylthiophene derivatives have been measured by cyclic voltammetry. The effect of substituents on the oxidation / reduction potentials is discussed. Bandgaps obtained by cyclic voltammetry have been found to be in general higher than optical bandgaps. Preliminary results of in situ resonance Raman spectroscopy of electrochemically generated poly-3-phenylthiophene derivatives films on a platinum electrode exposed to an organic electrolyte solution are reported. Observed Raman bands are assigned; based on these results previously suggested molecular structures are discussed.

Title: Transport and optical properties of CdTe/CdZnTe single crystals Author: Štěpán Uxa Department: Institute of Physics of Charles University Supervisor: Assoc. Prof. Eduard Belas, PhD, Institute of Physics of Charles University Abstract: The thesis is focused on a study of internal electric field and temperature dependence of the absorption edge of CdTe and CdZnTe samples. In the first part of the thesis the transient-current technique (TCT) is used for investigations of electric fields within planar radiation detectors. The original comprehensive theory that links together TCT measurements with measurements of detector's charge-collection efficiency (CCE) is presented. This approach results in the development of two new iterative methods for processing of experimental data which can be used in any situation when the internal electric field can be approximated by a linear profile. In the second part of the thesis high temperature measurements of transmittance of thin polished CdTe samples are presented, leading to the estimation of the temperature dependence of the bandgap energy from calculated spectra of absorption coefficient. This is for the first time when measurements in a Cd overpressure have been performed which significantly reduces sample sublimation. Keywords: CdTe, transient-current technique,...

>Magister Scientiae - MSc / Currently, non-renewable sources are mostly used to meet the ever-growing demand for energy. However, these sources are not sustainable. In addition to these energy sources being not sustainable, they are bad for the environment although the energy supply sectors highly depend on them. To address such issues the use of renewable energy sources has been proven to be beneficial for the supply of energy for the global population and its energy needs. Advantageous over non-renewable sources, renewable energy plays a crucial role in minimizing the use of fossil fuel and reduces greenhouse gases. Minimizing use of fossil fuels and greenhouse gases is important, because it helps in the fight against climate change. The use of renewable energy sources can also lead to less air pollution and improved air quality. Although solar energy is the most abundant source of renewable energy that can be converted into electrical energy using various techniques, there are some limitations. Among these techniques are photovoltaic cells which are challenged by low efficiencies and high costs of material fabrication. Hence, current research and innovations are sought towards the reduction of costs and increasing the efficiency of the renewable energy conversion devices.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18593号 / 工博第3954号 / 新制||工||1608(附属図書館) / 31493 / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 伊藤 紳三郎, 教授 木村 俊作, 教授 辻井 敬亘 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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Philosophiae Doctor - PhD (Chemistry) / Renewable energy has become the centrepiece of research in resolving the energy crisis. One of the forms of renewable energy is solar energy. This form of energy is costly to develop. Organic molecules are promising materials for the construction of next generation photovoltaic cells considering their advantage of lower cost compared to crystalline silicon that is currently used in solar cells. This forms the basis of this research, which focused on the synthesis and characterisation of poly(3- hexylthiophene) P3HT, stannum (Sn) nanoparticles and stannum-based bimetallic stannum-titanium (SnTi), stannum-chromium (SnCr) and stannum-vanadium (SnV) nanoparticles for the application in the construction of heterojunction photovoltaic cells (PVCs).

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The third quadrant (3rd-quad) conduction (or reverse conduction) of power transistors is critical for synchronous power converters. For power metal-oxide-semiconductor field-effect-transistors (MOSFETs), there are two current paths in the 3rd-quad conduction, namely the MOS channel path and the body diode path. It is well known that, for 1.2 kV silicon carbide (SiC) planar MOSFETs, the conduction loss in the 3rd-quad is reduced by turning on the MOS channel with a positive gate bias (VGS) and keeping the dead time as small as possible. Under this scenario, the current is conducted through both paths, allowing the device to take advantage of the zero 3rd-quad forward voltage drop (VF3rd) of the MOS channel path and the small differential resistance of the body diode path. However, in this thesis work, this popular belief is found to be invalid for power MOSFETs with higher voltage ratings (e.g., 3.3 kV and 10 kV), particularly at high temperatures and current levels. The aforementioned MOS channel and body diode paths compete in the device’s 3rd-quad conduction, and their competition is affected by VGS and device structure. This thesis work presents a comparative study on the 3rd-quad behavior of 1.2 kV to 10 kV SiC planar MOSFET through a combination of device characterization, TCAD simulation and analytical modeling. It is revealed that, once the MOS channel turns on, it changes the potential distribution within the device, which further makes the body diode turn on at a source-to-drain voltage (VSD) much higher than the built-in potential of the pn junction. In 10 kV SiC MOSFETs, with the MOS channel on, the body diode does not turn on over the entire practical VSD range. As a result, the positive VGS leads to a completely unipolar conduction via the MOS channel, which could induce a higher VF3rd than the bipolar body diode at high temperatures. Circuit test is performed, which validates that a negative VGS control provides the smallest 3rd-quad voltage drop and conduction loss at high temperatures in 10 kV SiC planar MOSFET. The study is also extended to the trench MOSFET, another major structure of commercial SiC MOSFETs. Based on the revealed physics for planar MOSFETs, the optimal VGS control for the 3rd-quad conduction in different types of commercial trench MOSFETs is discussed, which provides insights for the design of high-voltage trench MOSFETs. These results provide key guidelines for the circuit applications of medium-voltage SiC power MOSFETs. / M.S. / Recent years, the prosperity of power electronics applications such as electric vehicle and smart grid has led to a rapid increase in the adoption of wide bandgap (WBG) power devices. Silicon Carbide (SiC) metal-oxide-semiconductor field-effect transistor (MOSFET) is one of the most attractive candidates in WBG devices, owing to its good tradeoff between breakdown voltage and on resistance, capability of operation at high temperatures, and superior device robustness over other WBG power devices. In most power converters, power device is required to conduct current in its third quadrant (3rd-quad) (i.e., conduct reverse current) either for handling current during the dead time or acting as a commutation switch. In a SiC MOSFET, there are two current paths in the 3rd-quad conduction, namely the MOS channel path and the body diode path. It is widely accepted that by turning on the MOS channel with a positive gate-to-source bias (VGS), both paths are turned on in parallel such that the 3rd-quad conduction loss can be reduced. In this thesis work, it is shown that this long-held opinion does not hold for SiC MOSFETs with high voltage ratings (e.g., 3.3 kV and 10 kV). Through a combination of device characterization, TCAD simulation, and analytical modeling, this thesis work unveils the competing current sharing between the MOS channel and the body diode. Once the MOS channel turns on, it delays the turn-on of the body diode and suppresses the diode current. This effect is more pronounced in MOSFETs with higher voltage ratings. In 10 kV SiC MOSFETs, with the MOS channel on, the body diode does not turn on in the practical operation conditions. At high temperatures, as the bipolar diode path possesses the conductivity modulation, which can significantly lower the voltage drop and is absent in the MOS channel, it would be optimal to turn off the MOS channel. Circuit test is also performed to validate these device findings and evaluate their impact on device applications. Finally, the study is also extended to the commercial SiC trench MOSFET, the other mainstream type of SiC power MOSFETs. These results provide key guidelines for the circuit applications of medium-voltage SiC power MOSFETs.

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