In this work, organic solar cells of the configuration ITO/Pedot:PSS/CuPc/PTCBI/Al (Indium tin oxide/poly(3,4-ethylenedioxythiophene): polystyrene sulfonic acid/copper phthalocyanine/3,4,9,10-perylenetetracarboxylic bisbenzimidazole/aluminum) were investigated. A high open-circuit voltage (Voc) of 1.15 V was obtained when the PTCBI layer was 7 nm thick. Lower Voc values were observed for the same structure with silver, copper and gold electrodes instead of aluminum. However, short-circuit current density (Jsc) with these electrodes was much higher (4 mA/cm2) than in the case of aluminum (0.12 mA/cm2). Results were interpreted in terms of a modified CuPc/Al Schottky diode for the thin PTCBI case and a CuPc/PTCBI heterojunction for the thick PTCBI case. Also, the formation of a thin, protective aluminum oxide layer under the aluminum electrode was postulated. For devices with silver, copper and gold electrodes, absence of this protective layer was thought to be the cause of a relatively lower Voc and higher Jsc.

Organic solar cells are cheaper and much easier to fabricate than the conventional inorganic solar cells, but they suffer from low efficiencies due to low carrier mobilities in organic films. In this study Copper Phthalocyanine (CuPc) and Buckminster Fullerene (C60) based Schottky diodes were fabricated on ITO coated glass substrates to study their performance and a study of the effect of thickness on the cell parameters of CuPc Schottky diodes was made. Also, TiO2 based devices were studied to see the effect of TiO2 layer on the cell parameters. The J-V curves were analyzed for series resistance, diode ideality factor and reverse saturation current density. The devices were also characterized by SEM and XRD measurements.

This work aims to study the variation in electrical and optical properties of nanoscale Schottky diodes by varying their dimensions in the nanoscale. Experimental conditions for fabricating porous alumina with varying pore diameters and inter-pore distances were first optimized by anodizing Aluminum tape with variable currents. Cadmium Sulfide nanowires were then synthesized inside the membranes by dc electrodeposition. Finally a high work function metal was deposited on top as a contact for the metal-semiconductor Schottky diode junction. As a comparative study, Schottky diodes fabricated using the same metals and Cadmium Sulfide thin films deposited on conducting glass substrates using electro-deposition and annealed in different ambient were also studied. Device characterizations were carried out by field-emission scanning electron microscopy (FESEM), current voltage (I-V) measurements, ultraviolet-visual (UV-Vis) absorption spectroscopy and X-Ray diffraction.

In this diploma work, two versions of the discrete nonlinear Schrödinger (DNLS) equation are used to model a nonlinear layered photonic crystal system; the cubic DNLS (cDNLS) equation and the saturable DNLS (sDNLS) equation. They both have site-dependent coefficients to break mirror symmetry with respect to propagation direction, as well as to describe the linear and nonlinear properties of the system. Analytical solutions taking on plane wave form are, via the backward transfer map, used to derive a transmission coefficient as well as a rectifying factor to quantify the diode effect. The effect of varying site-dependent coefficients is studied in detail. Numerical simulations of Gaussian wave packets impinging on the system, using open boundary conditions, show the breaking of parity symmetry. Evidence of a change in the wave packet dynamics occurring in the transition between the cubic and the saturable DNLS model is presented. A saturated system prevents the wave packet from getting stuck in the nonlinear lattice layers. The transmission properties were found to be very sensitive to slight changes of the system parameters.

High power diode-pumped solid state (DPSS) lasers are a rapidly growing technology that is attractive for various applications in scientific and industrial fields. DPSS lasers are highly efficient, reliable and durable with superior beam quality when compared to flash-lamp pumped lasers. Double-tungstate crystals such as potassium yttrium tungstate Yb:KY(WO₄)₂ (Yb:KYW) are one of the most popular active materials used in DPSS lasers for generation of continuous wave radiation and ultrashort (i.e. femtosecond, 10⁻¹⁵ s) pulses with high average output power. The high pump power of laser diodes results in considerable heat generation in a laser crystal that in turn causes thermal lensing effect. Thermal lensing affects the performance and stability of a resonator, and plays an important role in limiting the output power and degrading the beam quality of solid state lasers. Despite these facts, no detailed studies of thermal effects in Yb:KYW lasers were reported to date. In this work thermal lensing in a diode-pumped Ng-cut Yb:KYW laser operating at the wavelength of 1.04 μm was characterized. A maximum output power of 3.5 W with a nearly diffraction limited output beam (M₂ < 1.2) was achieved under the absorbed pump power of 13.8 W. The focal lengths of the induced thermal lenses were obtained from the laser output beam size measurements at various incident pump power levels and ABCD matrix analysis. At maximum output power the focal length of the induced thermal lens was found to be 814 mm for the Nm direction (horizontal) and 144 mm for the Np direction (vertical). Thermal lens sensitivity factors were 1.26 m⁻¹/W and 0.32 m⁻¹/W for the Np and Nm directions, respectively. This highly astigmatic thermal lensing can be explained by strong anisotropy of thermo-optical properties of the crystal and its cooling geometry. In addition, the finite element analysis (FEA) method was employed to obtain the focal lengths of the induced thermal lens inside the crystal. Simulation results obtained from the theoretical model were compared to experimental data, and the accuracy of the model was verified. The results of this work are critical for practical design of the efficient and reliable Yb:KYW lasers with multi-Watt average output power.

The ArcCHECK dosimeter is a novel dosimetry tool that uses a helical array of silicon diode detectors to measure dose in a cylindrical plane. 3DVH is an associated software that can use ArcCHECK diode measurements along with treatment planning system (TPS) data to guide a full 3D dose reconstruction. The ArcCHECK phantom, along with 3DVH software was evaluated as a volumetric modulated arc therapy (VMAT) pretreatment verification tool. The comprehensive evaluation of the ArcCHECK and 3DVH system involved a comparison of measured dose to both ECLIPSE and Monte Carlo calculated dose for open fields and intensity modulated radiation therapy (IMRT) plans. System based confidence limits for gamma-pass rate and dose difference metrics were established through the measurement of prostate and head and neck VMAT plans. Using the system based confidence limits and clinically accepted tolerances, the sensitivity of the ArcCHECK and 3DVH system to VMAT errors was determined. Dose measured by the ArcCHECK and reconstructed in 3DVH agreed very well with dose calculated in ECLIPSE and Monte Carlo for both open fields and IMRT plans. The only results that fell outside of clinically accepted tolerances were a set of head and neck IMRT plans, however it was determined that a major factor in this result was suboptimal modelling of MLC effects in the TPS, in combination with changes in linac performance since commissioning of the TPS model. VMAT measured by the ArcCHECK and 3DVH system were in excellent agreement with ECLIPSE results and system based confidence limits were determined to be tighter than commonly used limits. ArcCHECK and 3DVH were sensitive to clinically relevant VMAT errors and insensitive to errors with little dosimetric impact, although diode measurements alone required tighter tolerances than are typically used. The ArcCHECK phantom and 3DVH software when used together have been shown to provide useful dosimetric information when used for VMAT pretreatment verification. / Graduate / 0756

Zugl.: Duisburg, Essen, Univ., Diss., 2008

Universiẗat, Diss--Kassel, 2004.

Chemnitz, Techn. Univ., Diplomarb., 2000.

Techn. Universiẗat, Diss., 2001--Berlin.