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Organic solar cells have shown great promise of becoming a cheaper alternative to inorganic solar cells. Additionally, they can also be made semitransparent. To avoid using expensive indium tin oxide electrodes in organic solar cells the electrodes can be made from conductive polymer, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). However, these so-called PEDOT-PEDOT solar cells are prone to short-circuiting. The work behind this thesis thus aimed to find the cause of these short circuits. The initial working hypothesis assumed the hygroscopic PSS in the bottom electrode could attract water across the active layer when the top electrode layer was applied. This would then swell the bottom electrode and cause the active layer to crack leading to short circuits. Accordingly, swelling was investigated as it was suspected to be the main cause of the shorts. This was achieved by coating reflective substrates with different layers from the solar cell, dropping water on top of the stack and then filming the thin film interference effects. SEM, AFM and IR were also used for further analysis. Although the bottom electrode swells, it was found that water does not cause permanent cracking. Instead, the research unveiled that water causes a formation of blisters, which are suspected to be made of PSS. The exact mechanism for the formation of the shorts remains unclear however.

<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)

This work presents a study on stoichiometry and structure in perovskite-type oxide thin films and investigates the role of growth–induced defects on the properties of materials. It also explores the possibility to grow thin films with properties close or similar to the ideal bulk parent compound. A novel approach to the growth of thin films, atomic layer-by-layer (ALL) laser molecular beam epitaxy (MBE) using separate oxide targets is introduced to better control the assembly of each atomic layer and to improve interface perfection and stoichiometry. It also is a way to layer materials to achieve a new structure that does not exist in nature. This thesis is divided into three sections. In the first part, we use pulsed laser deposition (PLD) to grow LaAlO3 (LAO) thin films on SrTiO3 (STO) and LAO substrates in a broad range of laser energy density and oxygen pressure. Using x-ray diffraction (θ-2θ scan and reciprocal space mapping), transmission electron microscopy (TEM) and x-ray fluorescence (XRF) we studied stoichiometry and structure of LAO films as a function of growth parameters. We show deviation from bulk–like structure and composition when films are grown at oxygen pressures lower than 10-2 Torr. We conclude that the discussion of LAO/STO interfacial properties should include the effects of growth–induced defects in the LAO films when the deposition is conducted at low oxygen pressures, as is typically reported in the literature. In the second part, we describe a new approach to atomically layer the growth of perovskite oxides: (ALL) laser MBE, using separate oxide targets to grow materials as perfectly as possible starting from the first atomic layer. We use All laser MBE to grow Ruddlesden–Popper (RP) phase Lan+1NinO3n+1 with n = 1, 2, 3 and 4 and we show that this technique enables us to construct new layered materials (n=4). In the last and main section of this thesis, we use All laser MBE from separate oxide targets to build the LaNiO3 (LNO) films as near perfectly as possible by depositing one atomic layer at a time. We study the thickness dependent metal-insulator transition (MIT) in ultrathin LNO films on an LAO substrate. In LNO, the MIT occurs in thin films and superlattices that are only a few unit cells in thickness, the understanding of which remains elusive despite tremendous effort devoted to the subject. Quantum confinement and structure distortion have been evoked as the mechanism of the MIT; however, first-principle calculations show that LaNiO3 remains metallic even at one unit cell thickness. Here, we show that thicknesses of a few unit cells, growth–induced disorders such as cation stoichiometry, oxygen vacancies, and substrate-film interface quality will impact the film properties significantly. We find that a film as thin as 2 unit cells, with LaO termination, is metallic above 150 K. An oxygen K-edge feature in the x-ray absorption spectra is clearly inked to the transition to the insulating phase as well as oxygen vacancies. We conclude that dimensionality and strain are not sufficient to induce the MIT without the contribution of oxygen vacancies in LNO ultrathin films. Dimensionality, strain, crystallinity, cation stoichiometry, and oxygen vacancies are all indispensable ingredients in a true control of the electronic properties of nanoscale strongly–correlated materials. / Physics

Organic electronics have reached the level of commercialization and are important parts of our daily life. They are integrated into portable computers, cell phones, identification cards, television, cars, etc. The organic thin film transistors (OTFTs) are the most attractive organic electronic elements that have applications as electronic flexible paper, sensors, smart cards, erasable memory devices, RF-ID tags, and in backplanes for OLED displays. Their performance has already exceeded the performance of transistors based on the amorphous silicon (α-Si). Organic thin film phototransistors (OPTs) have attracted significant research attention as functional OTFTs due to the unique structure of OTFTs (three-terminal device) complemented with the light (fourth terminal). The OTFTs structure enables modulation and amplification of the output signal (the drain current) while light gives the functionality and enhances the performance. Compared to organic photodiodes, OPTs have higher sensitivity and lower noise due to the OTFT structure. Additionally, the advantage of OPTs over inorganic PTs lays in a variety of light responsive organic materials that can be used as active channel materials. Accordingly, use of organic compounds enabled OPTs fabrication from solution, melt, and printing, over large areas of plastic substrates with which they are compatible. So far, many researchers have reported high-performance OPTs. Typically, synthesis of the new light receiving/emitting semiconducting materials is the common approach for the OPT development. Another way is to engineer the device structure by introducing new layers with different functionalities. Often, synthesis is costly, complex, lengthy, and not industrially feasible. This thesis focuses on the development of new methods and materials for OPT performance enhancement to avoid lengthy synthesis and fabrication processes. According to the layers in a typical OPT, that is, from the top to the bottom: active channel, channel/gate dielectric interface, and the gate dielectric layer, the thesis has three major focuses: engineering of the active channel for high-performance OPTs using existing small molecule and existing or new dielectric polymeric materials (Chapters 3-5), interface engineering (Chapter 6), and engineering of the gate dielectric layer (Chapter 7). Utilizing blends of a UV-A responsive 2,7-dipentyl[1]benzothieno[3,2-b][1]benzothiophene (C5-BTBT) small molecule semiconductor and various dielectric polymers (polyesters, PMMA, PVAc, PS, and PC) we developed highly photoresponsive and photosensitive OPTs. Furthermore, we designed and synthesized a new polyimide that is soluble, thermally stable, with reduced deep coloration and more importantly with the strong electron withdrawing groups. High-performance and highly photosensitive OPTs were achieved with capabilities of the application as photo memory elements characteristic of fast switching and long retention times of the persistent photocurrent. We demonstrated that by simple channel/dielectric interface modification using organosilanes with different end groups, the drain photocurrent, and hole mobility could be modulated and enhanced under the UV light illumination. In the final part, we demonstrated that both active channel and dielectric layer engineering could synergistically enhance the performance of OPTs for potential fabrication as photo memory elements. This thesis contributed significantly to fundamental knowledge of photoresponsive organic electronic devices and application of OPTs in the area of printed and flexible electronics / Thesis / Doctor of Philosophy (PhD) / Organic electronics have become a part of our daily life since they are integrated into cell phones, computers, TVs, displays, etc. Their advantage is their versatility due to a variety of organic compounds that can be used as semiconductors for the specific applications, low-cost processing methods (solution, printing, and melt) and large-area flexible substrates that can be used for their fabrication. For the same reasons, organic phototransistors are very attractive for modern optoelectronics. Generally, in this study, we developed and demonstrated new strategies of developing an organic phototransistor and enhancing/optimizing its performance. Firstly, we developed semiconducting blends that are responsive to UV-A light when integrated into an organic phototransistor. Secondly, by channel/gate dielectric interface manipulation we demonstrated control over photoelectrical properties of the organic phototransistor and discovered mechanisms of the enhancement. Thirdly, we optimized and developed reliable, high-performance, and highly UV responsive organic phototransistors with potential application as a photo memory element

The work described in this thesis involves the measurements of the optical gain and amplified spontaneous emission (A.S.E) spectrum of Pb1-xSnxTe epilayers and the establishment of conditions under which optically pumped Pb1-xSnxTe laser using a CO2 laser as pump source can be produced. Pb1-xSnxTe epilayers have been grown by a hot wall epitaxy (HWE) technique on BaF2 single crystal substrates and the optical gain which can be produced in these layers has been measured by pumping the films transversely with a N2 laser. A model for optical gain and stimulated emission as a function of pump intensity has been developed which has permitted for the first time in these materials, a direct comparison between the magnitude of the gain pumping rate, and the optical gain generated. The measured optical gain is in very good agreement with the model predicted gain. Good fits to the measured stimulated emission spectra were also obtained from the model prediction. It is shown that the gain for a given pump wavelength has a drastic dependence on the material doping density. According to the model, CO2 laser optically pumped Pb1-xSnxTe laser can be readily achieved, provided that epilayer doping densities can be reduced to values of 1017 cm-3 or less. Nevertheless, doping densities even in nominally undoped layers are generally at least an order of magnitude too high. In some initial attempts to achieve lower doping densities, using a thermal annealing technique, doping densities as low as 2 x 10 17 cm-3 have been obtained and significant pump absorption was achieved at CO2 laser wavelength, as predicted by the model. / Thesis / Master of Engineering (ME)

Pb(II) removal is imperative due to its inherent toxicity at low levels and its tendency to accumulate in ecosystems. Conductive carbonaceous nanomaterials (CCNs), such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs), have recently gained the interest of researchers due to their superior properties and ease of functionalization. The aim of this study is to utilize CCNs for Pb(II) removal within membrane technology and bioremediation strategies. Membranes have shown promise in their treatment abilities, producing excellent effluent quality while reducing plant footprints. The integration of CNTs within membrane technology provides an opportunity to couple its removal capacity with Pb(II) removal that exhibits regeneration capabilities. However, membrane fouling can be problematic for membrane longevity and regeneration. CNTs have also shown to be capable of mitigating fouling via electrostatic repulsion and pollutant degradation. However, little work has been conducted on its fabrication. In this work, CNTs were incorporated with poly(vinyl) alcohol (PVA) in thin film composites, where the effects of PVA chain length and degree of crosslinking were investigated. It was found that a pseudo-optimal coating can be obtained using 31-50kDa PVA with 10% crosslinking. This combination lead to a highly permeable, hydrophilic surface with good electrical conductivity that exhibited a molecular weight cut off of 2000kDa. Biosorption has shown promise in Pb(II) removal in the lab scale but its large-scale use is hindered from rapid saturation of binding sites and low regeneration abilities. Exoelectrogens were proposed as reactive biosorbents to couple biosorption with bioreduction in an attached growth configuration. CCNs were investigated as bacterial scaffolds, where their efficacy and Pb(II) dosage concentration was studied. It was found that CNFs were superior in removing Pb(II), exhibiting Pb(II) concentrations ≤0.10 ppm where removal increased when Pb(II) dosage increased from 0.5 to 5ppm. SEM-EDX analysis provided evidence that bioreduction dominated Pb(II) removal. A long-term study was further conducted using CNFs, revealing its robustness in long term removal over suspended growth reactors with a sustained removal of ≈ 80%. A numerical model was further proposed which exhibited a goodness of fit with an R-squared of 0.92. This model confirmed that bioreduction dominated Pb(II) removal and revealed biofilm thickness and Monod kinetics to be the main influential parameters on Pb(II) removal. / Thesis / Master of Applied Science (MASc)

In 1982, Israelachvili and Pashley reported the first measurements of a hitherto unknown attractive force between two mica surfaces hydrophobized in cetyltrimethylammonium bromide (CTAB) solutions. Follow-up experiments conducted by many investigators confirmed their results, while others suggested that the "hydrophobic force" is an artifact due to nanobubbles (or cavitation). Evidences for the latter included the discontinuities (or steps) in the force versus distance curves and the pancake-shaped nano-bubbles seen in atomic force microscopic (AFM) images. Recent measurements conducted in degassed water showed, however, smooth force versus distance curves, indicating that the hydrophobic force is not an artifact due to nanobubbles.1, 2 Still other investigators3, 4 suggested that the long-range attraction observed between hydrophobic surfaces is due to the correlation between the patches of adsorbed ionic surfactant and the patches of unoccupied surface. For this theory to work, it is necessary that the charged patches be laterally mobile to account for the strong attractive forces observed in experiment. In an effort to test this theory, AFM force measurements were conducted with gold substrates hydrophobized by self-assembly of alkanethiols and xanthates of different chain lengths. The results showed long-range attractions despite the fact that the hydrophobizing agents chemisorb on gold and, hence, the adsorption layer is immobile. When the gold surfaces were hydrophobized in a 1 Ã 10-3 M thiol-in-ethanol solution for an extended period of time, the force curves exhibited steps. These results indicate that the long-range attractions are caused by the coalescence of bubbles, as was also reported by Ederth.5 The steps disappeared, however, when the species adsorbed on top of the chemisorbed monolayer were removed by solvent washing, or when the gold substrates were hydrophobized in a 1 Ã 10-5 M solution for a relatively short period of time. AFM force measurements were also conducted between gold substrates coated with short-chain thiols and xanthates to obtain hydrophobic surfaces with water contact angles (ï ±) of less than 90o. Long-range attractions were still observed despite the fact that cavitation is thermodynamically not possible. Having shown that hydrophobic force is not due to coalescence of pre-existing bubbles, cavitation, or correlation of charged patches, the next set of force measurements was conducted in ethanol-water mixtures. The attractive forces became weaker and shorter-ranged than in pure water and pure ethanol. According to the Derjaguin's approximation6, an attractive force arises from the decrease in the excess free energy (ï §f) of the thin film between two hydrophobic surfaces.7 Thus, the stronger hydrophobic forces observed in pure water and pure ethanol can be attributed to the stronger cohesive energy of the liquid due to stronger H-bonding. Further, the increase in hydrophobic force with decreasing separation between two hydrophobic surfaces indicates that the H-bonded structure becomes stronger in the vicinity of hydrophobic surfaces. The force measurements conducted at different temperatures in the range of 10-40C showed that the hydrophobic attraction between macroscopic surfaces causes a decrease in film entropy (Sf), which confirms that the hydrophobic force is due to the structuring of water in the thin film between two hydrophobic surfaces. The results showed also that the hydrophobic interaction entails a reduction in the excess film enthalpy (Hf), which may be associated with the formation of partial (or full) clathrates formed in the vicinity of hydrophobic surfaces. The presence of the clathrates is supported by the recent finding that the density of water in the vicinity of hydrophobic surfaces is lower than in the bulk.8 / Ph. D.

Niobium thin films are expected to be free of solid inclusions commonly seen in solid niobium. For particle accelerators, niobium thin film has the potential to replace the solid niobium in the making of the accelerating structures. In order to understand and improve the superconducting performance of niobium thin films at cryogenic temperature, an energetic vacuum deposition system has been developed to study deposition energy effects on the properties of niobium thin films on various substrates. The system directly uses microwave power to create a pure niobium plasma, which can be used to extract niobium ion flux with controllable kinetic energy for direct deposition. The ultra high vacuum avoids the gaseous inclusions in thin films. A retarding field energy analyzer is developed and used to measure the kinetic energy of niobium at the substrate location. A systematic process for thin film characterization is developed and used to analyze the niobium thin films made by this energetic condensation. The properties of niobium thin films at several deposition energies are obtained, and the results show that there exists a preferred deposition energy around 115eV. / Ph. D.