Abstract: Quantum dot (QD) solar cells are a fast developing area in the field of solution processed photovoltaics. Central aspects for the application of QDs in solar cells are separation and transport of charge carriers in the QD layers and the formation of charge selective contacts. Even though efficiencies of up to 7% were reached in QD solar cells, these processes are not yet fully understood. In this thesis the mechanisms of charge separation, transport and recombination in CdSe QD layers and layer systems were studied. Charge separation was measured via surface photovoltage (SPV) at CdSe QD layers with thicknesses in the range of monolayers. To determine the influence of interparticle distance of QDs and trap states on the surface of QDs on charge separation, QDs with four different surfactant layers were studied. Layers of CdSe QDs were prepared on ITO, Si, SiO2 and CdS by dip coating under inert atmosphere. The layers were characterized by Rutherford backscattering spectrometry, UV-vis spectroscopy, step profilometry and scanning electron microscopy to determine the areal density, the absorption and thickness of CdSe QD monolayers.
Charge separation in contact systems with CdSe quantum dot layers
Abstract: Quantum dot (QD) solar cells are a fast developing area in the field of solution processed photovoltaics. Central aspects for the application of QDs in solar cells are separation and transport of charge carriers in the QD layers and the formation of charge selective contacts. Even though efficiencies of up to 7% were reached in QD solar cells, these processes are not yet fully understood. In this thesis the mechanisms of charge separation, transport and recombination in CdSe QD layers and layer systems were studied. Charge separation was measured via surface photovoltage (SPV) at CdSe QD layers with thicknesses in the range of monolayers. To determine the influence of interparticle distance of QDs and trap states on the surface of QDs on charge separation, QDs with four different surfactant layers were studied. Layers of CdSe QDs were prepared on ITO, Si, SiO2 and CdS by dip coating under inert atmosphere. The layers were characterized by Rutherford backscattering spectrometry, UV-vis spectroscopy, step profilometry and scanning electron microscopy to determine the areal density, the absorption and thickness of CdSe QD monolayers.
Dreidimensionale numerische Modellierung schräger Meteoriteneinschläge
Zusammenfassung: Das Studium von Meteoriteneinschlägen verbessert maßgeblich unser Verständnis der Entwicklungsgeschichte unseres Sonnensystems und seiner Körper sowie der Evolution von Leben.
Die bei einem Einschlag freigesetzte Energie ist proportional zur Größe und Geschwindigkeit des einschlagenden Körpers. Wie viel dieser Energie in den Untergrund übertragen wird, hängt vor allem von dem Einschlagwinkel ab. Dieser beeinflusst auch die Menge und Verteilung des ausgeworfenen Materials sowie die Größe und Form des resultierenden Kraters. Eine quantitative Beschreibung dieser Effekte sowie Untersuchungen, ob der Einschlagwinkel auch den Mechanismus der Kraterentstehung beeinflusst, ist bislang noch nicht erfolgt. Dies zu ändern ist die Hauptmotivation dieser Arbeit, zumal in der Natur nahezu jeder Einschlag unter einem schrägen Winkel erfolgt.
AlGaN/GaN HEMTs Reliability: Degradation Modes and Analysis - English
Abstract : AlGaN/GaN HEMTs reliability and stability issues were investigated in dependence on epitaxial design and process modification. DC-Step-Stress-Tests have been performed on wafers as a fast device robustness screening method. As a criterion of robustness they deliver a critical source-drain voltage for the onset of degradation. Several degradation modes were observed which depend on epi design, epi quality and process technology. Electrical and optical characterizations together with electric field simulations were performed to get insight into respective degradation modes.
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