Solar cells are at the frontier of renewable energy technologies. Photovoltaic energy is clean, reusable, can be used anywhere in our solar system and can be very well integrated with power distribution grids and advanced technological systems. Thin film solar cells are a class of solar cells that offer low material cost, efficient fabrication process and compatibility with advanced electronics. However, as of now, the conversion efficiency of thin film solar cells is inferior to that of thick crystalline cells. Research efforts to improve the performance bottlenecks of thin film solar cells are highly motivated. A class of techniques towards this goal is called light trapping methods, which aims at improving the spectral absorptivity of a thin film cell by using surface texturing. The precise mathematical and physical characterization of these techniques is very challenging. This dissertation proposes a numerical and computational framework to optimize, design, and fabricate efficient light trapping structures in thin film solar cells, as well as methods to verify the fabricated designs. The numerical framework is based on the important "inverse optimization" technique, which is very is widely applicable to engineering design problems. An overview of the state-of-the-art thin film technology and light trapping techniques is presented in this thesis. The inverse problem is described in details with numerous examples in engineering applications, and is then applied to light trapping optimization. The proposed designs are studied for sensitivity analysis and fabrication error, as other aspects of the proposed computational framework. At the end, reports of fabrication, measurement and verification of some of the proposed designs are presented. / text

This dissertation presents the development of methods based on microfabricated devices for combined structure and thermoelectric characterizations of individual nanowire and thin film materials. These nanostructured materials are being investigated for improving the thermoelectric figure of merit defined as ZT=S²[sigma]T/K, where S is the Seebeck coefficient, [sigma] is the electrical conductivity, K is the thermal conductivity, and T is the absolute temperature. The objective of the work presented in this dissertation is to address the challenges in the measurements of all the three intrinsic thermoelectric properties on the same individual nanowire sample or along the in plane direction of a thin film, and in correlating the measured properties with the crystal structure of the same nanowire or thin film sample. This objective is accomplished by the development of a four-probe thermoelectric measurement procedure based on a micro-device to measure the intrinsic K, [sigma], and S of the same nanowire or thin film and eliminate the contact thermal and electrical resistances from the measured properties. Additionally the device has an etched through hole that facilitates the structural characterization of the sample using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). This measurement method is employed to characterize individual electrodeposited Bi[subscript 1-x]Te[subscript x] nanowires. A method based on annealing the nanowire sample in a forming gas is demonstrated for making electrical contact between the nanowire and the underlying electrodes. The measurement results show that the thermoelectric propertied of the nanowires are sensitive to the crystal quality and impurity doping concentration. The highest ZT found in three nanowires is about 0.3, which is still lower than that of bulk single crystals at the optimum carrier concentration. The lower ZT found in the nanowires is attributed to the high impurity or carrier concentration and defects in the nanowires. The micro-device is further modified to extend its use to characterization of the in-plane thermoelectric properties of thin films. Existing practice for thermoelectric characterization of thin films is obtaining K in the cross plane direction using techniques such as the 3[omega] method or time domain laser thermal reflectance technique whereas the [sigma] and S are usually obtained in the in-plane direction. However, transport properties of nanostructured thin films can be highly anisotropic, making this combination of measurements along different directions unsuitable for obtaining the actual ZT value. Here, the micro-device is used to measure all three thermoelectric properties in the in-plane direction, thus obtaining the in-plane ZT. A procedure based on a nano-manipulator is developed to assemble etched thin film segments on the micro-device. Measurement results of two different types of thin films are presented in this dissertation. The first type is mis-oriented, layered thin films grown by the Modulated Elemental Reactant Technique (MERT). Three different structures of such thin films are characterized, namely WSe₂, W[subscript x](WSe₂)[subscript y] and (PbSe₀.₉₉)[subscript x](WSe₂)[subscript x] superlattice films. All three structures exhibit in-plane K values much higher than their cross-plane K values, with an increased anisotropy compared to bulk single crystals for the case of the WSe₂ film. The increased anisotropy is attributed to the in-plane ordered, cross-plane disordered nature of the mis-oriented, layered structure. While the WSe₂ film is semi-insulating and the W[subscript x](WSe₂)[subscript y] films are metallic, the (PbSe₀.₉₉)[subscript x](WSe₂)[subscript x] films are semiconducting with its power factor (S²[sigma]) greatly improved upon annealing in a Se vapor environment. The second type of thin films is semiconducting InGaAlAs films with and without embedded metallic ErAs nanoparticles. These nanoparticles are used to filter out low energy electrons with the introduction of Schottky barriers so as to increase the power factor and scatter long to mid range phonons and thus suppress K. The in-plane measurements show that both the S and [sigma] increase with increasing temperature because of the electron filtering effect. The films with the nanoparticles exhibited an increase in [sigma] by three orders of magnitude and a decrease in S by only fifty percent compared to the films without, suggesting that the nanoparticles act as dopants within the film. On the other hand, the measured in-plane K shows little difference between the films with and without nanoparticles. This finding is different from those based on published cross-plane thermal conductivity results. / text

In this thesis CIGS absorber layers have been deposited by gas flow sputtering with an extra supply of selenium, a method that displays promise for large scale production because of its one-step sputtering route which deposits low energy particles in a high deposition rate. In this thesis a method was developed where selenium was added to the sputtering process inside the sputter chamber in a controllable manner and in larger amount than done in previous projects. A total of five samples were manufactured with altered evaporation temperatures and an extra supply of selenium which then were finalized to solar cells using the standard baseline process of the Ångström solar center. The characteristics of the CIGS layer and solar cells were analyzed by XRF, IV- and QE measurements. A cell with a conversion efficiency of 11.6 %, Jsc of 27.9 mA/cm2, Voc of 0.63 V and fill factor of 66.2 % was obtained on a 0.5 cm2 cell area without an antireflective coating. All samples contained cells with obtained efficiencies above 10 %, but over the whole samples the efficiencies varied considerably. The samples that were deposited with moderately large selenium evaporation provided the highest efficiencies with a relatively good homogeneity over the substrate. Results show a deficiency of copper in the CIGS films compared to the target composition. The copper content was lower than 70 % expressed in Cu/(Ga+In), which probably resulted in a low diffusion length for electrons, leading to limited cell efficiencies.  Through the duration of the thesis issues that concerned the power supply- and the controls to the substrate heaters as well as the control of the evaporation temperature during the depositions arose that required problem solving and needs to be resolved for the future progression of this work. The conclusions drawn from this thesis are that it is possible to vary the temperature of the selenium source and thereby control the amount of selenium that evaporates during the deposition process even though there is a lot of additional heating in the sputter chamber from both the substrate heaters and the sputter source which could affect the ability to control the amount of selenium being evaporated. That the most likely reason for the limited efficiencies is due to the low copper content in the CIGS films and that a larger amount of evaporated selenium compared to previous work did not result in higher obtained efficiencies.

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BCxNy films were characterized as a potential pore sealing layer for low κ dielectrics. The changes in chemical bonding were studied as a function of growth temperature to aid in understanding the variation in electrical performance of these films. Thermal chemical vapor deposition of BCxNy using dimethylamine borane and ethylene were deposited on porous methylsilsesquioxane substrates at 335 °C and 1 Torr. BCxNy was able to seal the porous interface with a thickness of 3.9 nm for both blanket and patterned substrates. BCxNy films deposited over a temperature range of 300-400 °C with dimethylamine borane and either ethylene or ammonia coreactant gas were characterized. Films deposited with ethylene became more concentrated in B at the expense of C with increasing temperature. These films favored C-B intermixing over C-C and B-B bonding at higher temperature. H was detected in the form of B-H and C-H bonds. Films deposited with ammonia became concentrated in N at the expense of B, and favored B-N viii bonding at higher temperatures. H was found in the films as B-H, C-H, and N-H bonds. The amount of H in the films decreased with increasing growth temperature for both ethylene and ammonia coreacted films. The valence band offset of C-rich films increased from 0.17 ± 0.22 eV to 0.32 ± 0.22 eV when deposited at 300°C and 400 °C. For the Nrich films, the valence band offset shifted from 0.26 ± 0.28 at 300 °C to -0.15 ± 0.24 eV at the same deposition conditions. High temperature annealing from 400-800 °C in forming gas caused all BCxNy films to decrease in thickness up to 30%. At the same time, the index of refraction increased, and likely, the dielectric constant. X-ray photoelectron spectroscopy revealed little change in the constituent bonding, suggesting that volatile –H containing species were removed during the annealing process. / text

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Increasing attention has been directed towards the development of optically transparent and mechanically flexible thin film transistors (TFTs) and associated circuits based on the transition metal oxides. These flexible see-through structures offer reduced weight, potential low-cost fabrication, and high performance compared to commonly used hydrogenated amorphous silicon (a-Si:H) in applications for large-area electronics and displays. As these emerging technologies evolve towards commercialization, a thorough investigation of the impacts of the thermo-mechanical stress and strain and their effects on the electrical and mechanical stability of the flexible microelectronic devices have become increasingly necessary. However, not much progress has been reported in this area, and the numerical modeling of the flexible transistors with the Finite Element Method (FEM) would provide unique insight to the design and operation of the flexible TFTs. In this thesis, numerical models of flexible TFTs are built up by COMSOL Multiphysics and compared with analytical models to reach the best agreement between the experimental measurements and the numerical analyses. These simulations provide additional insight into the local stress induced strain within the device due to both intrinsic and applied stress. It was shown that the thermal and mechanical impacts on the TFT performance can be reduced by placing the vital active layer of the flexible device near the neutral mechanical plane or by proper designing the device structure and processing conditions based on the data derived from the numerical models. The mathematical analysis and numerical simulation will be used to improve the electrical and mechanical performance and the reliability of the transistors for flexible applications.