Solar energy is one of the fastest growing forms of energy generation due to its low cost, lack of emissions, minimal maintenance, and excellent durability. However, like any other technology, it is also not free from defects and degradation, which limit its performance in the real world. Most of the degradation is related to metal contacts, which also happens to be one of the most expensive items in manufacturing, comprising almost half of the cost of converting a silicon wafer into a photovoltaic (PV) cell. Therefore, studying contact degradation to make them reliable and free of defects is the key to achieving high energy yields. High efficiency PV modules that are both cheap and reliable with an extended lifetime ultimately reduce the levelized cost of energy. This study aims to characterize contact degradation in solar cells to identify the root causes of performance losses and develop alternate solutions to metallization. Electrical and optical characterizations were performed on both accelerated-aged and field-exposed solar cells and modules to look for specific performance losses. Furthermore, materials characterization was performed on selected samples to understand the potential root causes and factors affecting the degradation. Unencapsulated solar cells mainly consisting of newer cell technologies and metallization were exposed to acetic acid to simulate field conditions and understand the effect on contact corrosion. Finally, a low-cost novel contact technology called the "transferred foil contact" was developed that can be used as the back contact of a highly efficient silicon heterojunction solar cell, to minimize recombination, and potentially combine cell metallization and interconnection. An overview of the solar energy history and current state-of-the-art is first discussed, followed by a chapter on solar cell device physics and contact technology. The following chapters discuss the different degradation mechanisms in terms of the process-structure-properties relationships of the PV materials.

Ultrashort-pulsed lasers have been used for high precision processing of a wide range of materials including dielectrics, semiconductors, metals, and polymers/polymer composites, enabling numerous applications ranging from micromachining, photonics to life sciences. However, there are challenges when applying this technology in the industry, which requires scale and throughput different from lab use. The goal of this research is to understand how ultrafast laser pulses interact with thin polymers/polymer composite materials and develop a method that is efficient for ultrafast laser processing of these materials. It is a common practice in industrial applications to run the laser at a high repetition rate and hence high average power. However, the heat accumulation under such processing conditions will deteriorate the processing quality, especially for polymers, which typically have a low melting temperature. An analytical solution for two-dimensional modeling of the temperature distribution has been presented and the solution is used to understand the effect of laser parameters on ultrafast laser processing of polypropylene (PP), which is an important polymer for both scientific and industrial applications. Laser cutting experiments are carried out on PP sheets to correlate with the theoretical calculation. This study shows that in laser cutting, the total energy absorbed in the material and the intensity are two important figures of merit to predict the cutting performance. It is found that heat accumulation can be avoided by a proper choice of the processing conditions and the optical properties (i.e. reflectance, transmittance, and absorptance) are important parameters to control processing with ultrafast lasers. To determine the reflectance, transmittance, and absorptance, time-resolved, single-shot measurements are performed in ultrafast laser interaction with polypropylene for a wide range of laser pulse energies. The absorptance during the ultrafast laser interaction with polymers is divided into the different linear and non-linear effective absorption channels and the absorption mechanism of ultrafast laser interaction with polymers in near-infrared wavelength are explained with a model that takes into account different effective absorption channels and suggests that the non-linear absorption originates from vibrational overtone/combination absorption. The enhancement of the absorptance has been investigated for efficiently processing thin polymers with ultrafast lasers by changing pulse duration. It is suggested from this study that the intense shorter pulse (167 fs) is more efficient for surface processing as most of the energy absorbed at the surface due to the strong nonlinear absorption, while a longer pulse (1000 fs) is more efficient for bulk processing for polymers. The results are useful for designing and controlling ultrafast laser processing of polymers and optimizing laser process parameters for the most efficient processing of polymers.

Scanning probe microscopy techniques were used to investigate the desorption of hydrogen passivated silicon to form SiO2 etch masks The application of the etch masks were planned on being used to manufacture silicon nanowires. Low concentration hydrofluoric acid was used to passivate the surface. The surface was selectively depassivated by SPM techniques. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) were used to create the masks. The STM system used was found to desorb hydrogen from the surface faster than the STM could image and was considered incapable in the configuration investigated. This led to the use of atomic force microscopy (AFM). Using a conductive tip in contact mode, lithography on the passivated surface was performed. The topography of the lithography was compared to similar works and found to be similar in size. The width was found to be 80nm and the thickness 1nm. The depassivated layers were confirmed to be oxide through electronic force microscopy (EFM). Finally, voltages were swept with the tip in contact with the surface to find the bandgap of the oxide. It was found that the voltage sweeps were severely modifying the tip along with producing inconsistent desorption thicknesses ranging from 0.2 to 12nm. Despite the results from the voltage sweeps, the lithography procedure performed using the AFM was found to be successful.

<p>Incorporating nanoparticles within a polymer to improve the mobility of the filmis one promising way of creating organic thin film transistors (OTFTs) with large mobilities that could be applicable in real world applications. Carbon nanotubes (CNTs)and graphene nanoplatelets (GNPs) are extensively studied for this application. In order to overcome their tendency to aggregate, a method for creating a stable dispersion within both the solution phase and the film is needed. Here an easy method is established for creating a stable dispersion of CNTs or GNPs within a polymer solution which results in excellent OTFT mobility.A non-percolating network of non-covalently functionalized single walled carbon nanotubes was embedded within poly[5,5’-bis(3-dodecyl-2-thienyl)-2,2’bithiophene](PQT-12) thin films for the purpose of enhancing field effect mobility in thin film transistors. The host polymer was used to stabilize the nanotubes in suspension by π orbital overlap caused by simple application of ultrasonication. The stable nanotube suspension was cast into two different device architectures both with excellent mobilities and on/off ratios. The effect of nanotube content on polymer interaction within suspension, film morphology and electrical properties are discussed. A CNT nanocomposite OTFT with enhanced mobility was also tested for applications in vapour sensing. A method is also presented for the creation of graphene nano-platelets (GNPs) for implementation in nano-composite films. Heat treatment of expandable graphite within a vacuum evaporation chamber yielded chemically pure GNPs of a few nanometer thickness. Exfoliating expandable graphite without heat treatment resulted in even higher concentrations but chemically impure GNPs. The material was non-covalently stabilizedwith PQT-12 in a similar method to CNTs and used to create OTFTs with enhanced mobility. The effect of heat treatment parameters and exfoliation conditions on GNP thickness, size and chemical purity are discussed, as well as effect of GNP content on mobility and on/off ratio.</p> / Master of Applied Science (MASc)

<p>Research in the area of improving the efficiency and manufacturability of alternative energy technologies has been of high interest due to the growing environmental concerns of energy resources. Group III-antimonide-based compound semiconductors have been sought after as excellent candidates for photovoltaic conversion of infrared radiation, outside the spectral range absorbed by the currently available crystalline Si solar cells. The major challenge is the GaSb/Si interface is highly lattice mismatched, and inherently heterovalent. This leads to a high density of structural defects, many of which have not been investigated fully. Both optical and electrical properties of such heteroepitaxy thin films are strongly dependent on the periodicity of the crystal lattice, and the presence of extended defects cause perturbations in the lattice periodicity. Therefore the nature of such extended defects must be understood, in order to better manipulate the growth process to minimize their presence. This thesis demonstrates that through the use of conventional transmission electron microscopy, further insight can be gained into understanding the origin, distribution, propagation, and interaction of various extended defects. From this, a couple of ways to systematically suppress some of the defects have also been implemented, and the mechanism by which they induce such a suppression is also discussed.</p> / Master of Applied Science (MASc)

<p>Organic thin film transistors (OTFTs) have attracted great attention in the last couple of decades due to their potential of cost reductions in manufacturing low-end electronic devices through solution processes. Currently, one of the major challenges facing the field of OTFTs is lack of high performance functional organic materials including both organic semiconductors and gate dielectrics for effective device integrations by solution deposition technologies. This thesis focuses on material designs, interfacial compatibilities, and device integrations for high performance OTFTs.</p> <p>Research progresses in the following areas are presented in this thesis. First, novel liquid-crystalline organic semiconductors, 2,5‟-bis-[2-(4-pentylphenyl)vinyl]-thieno(3,2-</p> <p><em>b</em>) thiophene and 2,5‟-bis-[2-(4-pentylphenyl)vinyl]-(2,2‟)bithiophene for OTFT applications were developed. Mobilities of the OTFTs fabricated from these semiconductors reached 0.15 cm2/V.s with high environmental stability. Such high performance is attributed to their ability to form highly ordered molecular structures. Second, a simple effective approach was developed for tuning solubility of a high mobility polythiophene system through engineering its molecular structure. OTFTs fabricated with the newly developed copolythiophenes from an environmentally benign non-chlorinated solvent showed excellent performance with mobility up to 0.18 cm2/V.s. Third, an effective approach to a solution processed gate dielectric Ph.D. Thesis – P. Liu, McMaster University, Chemical Engineering iv</p> <p>design was developed for all solution-processed flexible OTFTs. This was achieved through a dual-layer dielectric structure design comprised of a bottom layer with a UV-crosslinked poly(4-vinyl phenol-co-methyl methacrylate), (PVP-PMMA), and a top layer with a thermally crosslinked polysiloxane. This solution-processed dual-layer dielectric structure enabled all solution-processed high performance flexible OTFTs. Finally, flexible OTFTs were successfully integrated on plastic substrates (PET) from non-chlorinated solvents by using the copolythiophenes and the dual-layer dielectric. The integrated flexible devices showed good OTFT characteristics with mobility up to about 0.1 cm²/V.s.,</p> <p>well defined linear and saturated regions, and a close to zero turn-on voltage.</p> / Doctor of Philosophy (PhD)

<p>The growth of CdTe thin films on Al₂O₃(0001) substrates by pulsed laser deposition from undoped pressed powder targets was studied. Thin film crystal structure was investigated by x-ray texture analysis as a function of plume flux, growth temperature and film thickness. Crystal texture increased for a decrease in plume flux. Single crystal CdTe (111) films were obtained by optimizing the plume flux. Increasing the growth temperature demonstrated a reduction in twin density. An optimum temperature of 300°C minimized the twin density without adverse desorption effects. The twin density decreased as an inverse squared function of film thickness. Single crystal CdTe films with comparable structural quality to Bridgeman single crystal wafers were grown under optimal conditions.</p> <p>The optoelectronic properties of CdTe films were investigated by photoluminescence and photoreflectance spectroscopy. The room temperature bandgap energy of 1.51 eV was consistent between spectroscopic measurements. Broadening parameters for spectra were consistent with reference high quality material. Low temperature photoluminescence spectra had a dominant emission consistent with bound excitons found in bulk CdTe. Emissions consistent with self-compensation or doping were not found. Hall effect and conductivity measurements at 300 K demonstrated high resistivity for undoped material and electron mobilities comparable to bulk CdTe for lightly doped films. Spectroscopic and electrical measurements of high structural quality CdTe films were consistent with high optoelectronic quality.</p> <p>An as-grown ability of the films to detach from their substrate was discovered. X-ray texture analysis and photoluminescence spectroscopy of films released onto rigid secondary carriers demonstrated that they maintained their structural and optoelectronic quality proceeding lift-off. Substrates having films released from them were found to be suitable for repeated growth. The technological relevance of this discovery is likely to drive further study into the lift-off phenomena and controlled doping of CdTe thin films.</p> / Master of Applied Science (MASc)

<p>The heteroepitaxial growth of InAs self-assembled quantum wires on vicinal substrates is investigated. InGaAlAs lattice-matched to InP was first deposited onto an InP(001) substrate with and without a 0.9 degree off-cut toward the (110) direction, followed by the deposition of a strained layer of InAs. Dense InAs quantum wires were successfully grown on both nominally flat and vicinal substrates in order to observe the effect of the presence of atomic steps. The off-cut angle was chosen based on the wire spacing on a flat substrate to serve as a template for their nucleation and improve their size distribution for use as 1.55 um wavelength lasers required by the telecommunications industry. The results have shown a modest but statistically significant improvement in the width of their size distribution. In addition, a kinetic Monte Carlo simulation including full strain calculations was developed to further understand the nucleation process. The model developed here disproves the idea that InAs quantum wires are aligned towards the (-110) direction due to diffusion anisotropy. The simulation of the formation of quantum wires similar to those observed experimentally has been achieved and the Stranski-Krastanow growth mode is demonstrated.</p> / Master of Materials Science and Engineering (MMatSE)

<p>Light emitting diode (LED)-based solid state displays (SSD) have attracted growing interest due to their advantages in terms of contrast ratio, brightness, viewing angle, and response time compared to liquid crystal displays. GaN based III-nitride thin film materials are suitable materials for SSD due to their wide and tunable bandgaps. However, the large size and costly manufacturing process of commercially available GaN-based LED chips limit the potential uses of LEDs as the pixels of SSD.</p> <p>In this work, tiny single crystal beta-phase (111) oriented SiC whiskers 2 microns in diameter and 18 microns in length are proposed as the substrates for GaN growth due to their small lattice constant mismatch (3%) with GaN, their conductive nature and their small size for potential use in SSD pixels. Aligned SiC whiskers with (111) planes exposed in an alumina matrix prepared by a precise manipulation and alignment method of SiC whiskers including a series of steps was developed in this work. The alignment degree of whiskers achieved in this work is higher than conventional extrusion methods, and a sintering approach capable of forming an aligned alumina/SiC composite was developed and understood using a self-limiting oxidation reaction mechanism.</p> <p>To take advantage of the potential versatility, scalability and cost effectiveness of sputtering for SSD manufacturing, a reactive sputtering system was built for a detailed investigation of GaN thin film growth nucleation and subsequent growth behavior on SiC. 6H-SiC single crystal substrates were chosen as a reference substrate for SiC whiskers. An XRRC indicates that a high quality single crystalline GaN thin film was successfully grown epitaxially on 6H-SiC by sputtering. Two-dimensional X-ray diffraction and scanning transmission electron microscopy results demonstrated that the epitaxial growth of GaN thin films relies on the short range order and/or crystalline area of the native oxide layer in GaN/SiC interface for the first time.</p> / Doctor of Philosophy (PhD)

UV radiation is utilized in a number of environmental technologies, most notably for the disinfection of water, air, and surfaces through the use of UVC fluorescent lamps. Recently, our group developed a luminescent material that could emit germicidal UVC simply by irradiating it with a household fluorescent lamp, thus introducing a new type of antimicrobial surface powered by low-intensity visible light. The materials were doped with praseodymium ions (Pr3+) which have the unique capability of converting visible light to higher energy UV using an optical mechanism called upconversion. While visible-to-UV upconversion materials appeared promising for environmental application—particularly because solar irradiation could be used for their activation—their practical application was thwarted by low light conversion efficiencies. Herein we discuss the pursuit of new material forms and modifications designed to improve the efficiency of Pr3+-based upconversion systems. These enabled successful enhancement of antimicrobial activity and led to a proof of concept for upconversion-sensitized TiO2 photocatalysis. Correlations between material properties and optical behavior will be presented, followed by commentary on how these strategies might be used to further advance upconversion systems toward environmental application.