Browsing by Author "Sampath, Walajabad, committee member"
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Item Open Access A calcium aluminate electride hollow cathode(Colorado State University. Libraries, 2014) Rand, Lauren Paula, author; Williams, John, advisor; Reynolds, Melissa, committee member; Sampath, Walajabad, committee member; Yalin, Azer, committee memberThe development and testing of a hollow cathode utilizing C12A7 (12CaO.Al2O3) electride as an insert are presented. Hollow cathodes are an integral part of electric propulsion thrusters on satellites and ground-based plasma sources for materials engineering. The power efficiency and durability of these components are critical, especially when used in flight applications. A low work function material internal to the cathode supplies the electrons needed to create the cathode plasma. Current state-of-the- art insert materials are either susceptible to poisoning or need to be heated to temperatures that result in a shortened cathode lifetime. C12A7 electride is a ceramic in which electrons contained in sub-nanometer sized lattice cages act as a conductive medium. Due to its unique atomic structure and large size, C12A7 electride has a predicted work function much lower than traditional insert materials. A novel, one-step fabrication process was developed that produced an amorphous form of C12A7 electride that had a measured work function 0.76 eV. A single electride hollow cathode was operated on xenon for over 60 hours over a two-month period that included 20 restarts and 11 chamber vent pump-down sequences with no sign of degradation, and on iodine for over 20 hours with no apparent reactivity issues. The operations of cathodes with three different orifice sizes were compared, and their effects on the interior cathode plasma modeled in a zero- dimensional phenomenological model.Item Open Access A real-time building HVAC model implemented as a tool for decision making in early stages of design(Colorado State University. Libraries, 2015) Syed, Zaker Ali, author; Bradley, Thomas H., advisor; Anderson, Charles, committee member; Sampath, Walajabad, committee memberConstruction of buildings is one of the major sources of greenhouse gases (GHGs) and energy consumption. It would therefore be beneficial to improve the design of new buildings so that they consume less energy and reduce GHG emissions over their lifecycle. However, the design of these “green buildings” is challenging because the analyses required to design and optimize these buildings is time intensive and complicated. In response, numerous software applications have been developed over the years by various government agencies, organizations and researchers. But, recent surveys of architects have found that these energy simulation programs are used irregularly and by very few architectural firms. The utility of these programs is limited by three main factors. First, these software applications are complicated, stand-alone programs that require extensive training to be effective. Second, there are a large set of energy simulation programs available, all of which have differing metrics of building performance with differing degrees of accuracy. And lastly, these applications do not fit into the conventional workflow that architects follow for a majority of projects. To address these issues, this thesis focuses on the development of a simplified HVAC model that not only gives sufficiently accurate results but also can be easily integrated into the conventional design workflow. There are some key challenges in developing such a model. • Early in the design process (when many irreversible energy impacting design decisions are made) there is very limited information available about the building materials, heat loads, and more. • The simulation must be integrated into the design software and workflows that are currently being used by architects. This requires a near-instantaneous calculation method that can extract information from the only available data at the initial design (sketching) phase, the computer aided design (CAD) models and the location. To achieve these objectives, the Radiant Time Series (RTS) method was supplemented with real data from National Solar Radiation Database to enable a near-instantaneous annual HVAC load calculation to be integrated into preliminary CAD modelling software. This model was integrated in to the Trimble Sketch-up™ software. The combined software system is demonstrated to enable effective design feedback in early design decisions including building orientation, construction material and design of fenestration.Item Embargo Advanced nanostructured materials for enhancing bioactivity(Colorado State University. Libraries, 2024) Bhattacharjee, Abhishek, author; Popat, Ketul C., advisor; Sampath, Walajabad, committee member; Herrera-Alonso, Margarita, committee member; Wang, Zhijie, committee memberHealth hazards such as pathogenic infection, communicable diseases, and bone damage and injuries cause enormous human suffering and pain worldwide. Biomaterials such as orthopedic implants and biosensors are crucial tools to remedy these complications. Development of novel biomaterials and modifying existing materials can help enhance medical device efficacy. One of the key aspects of improving biomaterials is the utilization of nanotechnology. Nanoscale surface features can improve the interaction between materials and biological agents, thus improving their bioactivity. In this dissertation research, two different biomaterials were used for two distinct applications. Firstly, titanium, a common material for orthopedic implants, was used. Ti is a popular implant material because of its superior corrosion resistance, lightweight, and excellent biocompatibility. However, 10% of Ti implants fail each year due to pathogenic bacterial infection and poor osseointegration resulting in revision surgeries and immense suffering of the patients. Nanostructured surface modification approaches can potentially reduce the failure rate of Ti implants. In this study, TiO2 nanotube arrays (NT) were fabricated followed by zinc (Zn) and strontium (Sr) doping. These elements provide important signals to mesenchymal stem cells to differentiate into osteoblasts which helps in bone healing. Zn also reduces bacterial adhesion to the implant surface. Results showed that the modified surfaces could significantly reduce bacterial adhesion and improved osseointegration properties of the mesenchymal stem cells. Secondly, a polydiacetylene (PDA)-based electrospun nanofiber biosensor was prepared that is flexible in nature for monitoring bacterial or viral infection. The nanofiber biosensor could selectively detect Gram-negative bacteria via a vivid blue-to-red color transition. Since the color transition is visible to the naked eye, the biosensor offers immense potential to be used as a screening device for Gram-negative bacterial infection in various industries such as food packaging, medical, intelligence, and national security. During the COVID-19 pandemic, the PDA biosensing platform was utilized to detect the spike (S) protein of the SARS-CoV-2. For this, the surface chemistry of the PDA fibers was modified, and a receptor protein was conjugated at the end of the PDA polymer chain. When the modified PDA fibers were incubated with the S protein, the blue-to-red color transition happened, thus sensing the presence of S protein in the environment. This result indicated that PDA nanofiber biosensor is a flexible sensing platform for effectively detecting both bacteria and viruses. The two biomaterials investigated in this research indicated that the use of nanotechnology can help in enhancing their bioactivity.Item Open Access Applications and advanced sintering techniques of functionally graded ZnO-based thermoelectric material(Colorado State University. Libraries, 2017) Cramer, Corson Lester, author; Ma, Kaka, advisor; James, Susan P., advisor; Williams, John D., committee member; Sampath, Walajabad, committee member; Neilson, Jamie R., committee memberThermoelectric generator (TEG) materials provide a unique solid-state energy conversion from heat to electricity. Nanostructured TEGs experiencing transient thermal loads at medium to high-temperatures are susceptible to degradation due to thermal stress cracking, which subsequently causes decreased lifetime. Previous efforts to prevent the thermal degradation have led to the following approaches: geometric pinning, compositional gradients, and segmentation of different materials. In the present research, functionally graded zinc oxide (ZnO) materials with graded grain size distribution were fabricated using a water sintering strategy via spark plasma sintering (SPS) with a thermal gradient in combination with modified tooling and strategic mechanical load schedules. Samples with homogeneous grain size distribution were also fabricated as a baseline for comparison. The primary objective of the work is to investigate the correlation between the processing conditions, formation of graded microstructure, and the resultant thermoelectric (TE) output performance and lifetime of the ZnO materials. The fundamental understanding of this correlation will contribute to future design of TEG materials using the approach of graded microstructure. The hypothesis is as follows: in a TEG material with graded grain size distribution, one side that consists of coarse (micron-sized) grains is exposed to the heat source. This coarse-grained side of the material can mitigate thermal stress cracking by spreading the heat more quickly during transient heating and thus provide improved thermal stability. The other side of the TEG material consists of fine grains (submicron-sized) and still exhibits high efficiency. In the current study, both continuously graded ZnO materials and a five-layer discretely graded ZnO material were fabricated. Microstructural characterization shows that the grain size gradient of the continuously graded materials across a 10-mm thickness goes from submicron scale (average size ~ 180 nm) to micron scale (~1.2 μm). The thermoelectric properties of the baseline ZnO materials with uniform grain sizes were measured. Using the data obtained from those samples with uniform grain sizes, the peak efficiencies of the continuously graded materials and the five-layer graded materials were simulated and compared to the experimentally measured values. The lifetime of the ZnO samples was evaluated from the electrical resistance at the cycling temperature. The results of the final efficiencies suggest that the thermoelectrical performance of the ZnO materials benefit from the grain size gradation. In addition, the sintering behavior of the continuously graded ZnO system is investigated and compared to that of the isothermally sintered samples to establish a predictive model of the microstructure (density-grain size-time relation). A discrepancy is observed between the prediction of the continuously graded materials and the experimental results. This discrepancy is attributed to a stress shielding that develops during sintering due to differential sintering from the temperature gradient. The stress shielding occurs when denser, and thus stiffer material develops adjacent to less dense and less stiff material causing the stress to vary because the stress is not evenly distributed. The stress shielding effect during sintering is further investigated through theoretical sintering equations. Using the viscoelastic analogy in sintering, the stress to be added to the sample during sintering in a thermal gradient is quantified to compensate the discrepancy from the samples sintered isothermally based on an average strain rate difference.Item Open Access Computational modeling of cadmium sulfide deposition in the CdS/CdTe solar cell manufacturing process(Colorado State University. Libraries, 2013) Hemenway, Davis Robert, author; Sakurai, Hiroshi, advisor; Sampath, Walajabad, committee member; Sites, James, committee memberA thin film CdS/CdTe solar cell manufacturing line has been developed in the Photovoltaic Materials Engineering Lab at Colorado State University. This system incorporates multiple stations using NiCr embedded heaters in graphite crucibles to successively sublimate layers of different photovoltaic materials onto glass substrates. Times, temperatures and chemical compositions of these layers can be varied or excluded according to the desired characteristics of the 3" x 3" solar cell sample. Though the tool allows for flexibility and variability of materials, the uniformity of material deposition remains one of the largest sources of performance variability between samples. Computational Fluid Dynamics (CFD) programs have been used previously to predict the thermal performance of the embedded heaters and to ensure thermal uniformity in each of the heated deposition pockets. The thermal modeling used in the designing of these sources has been proven to be within 2.5% of the experimentally measured temperatures in laboratory and industrial applications. Building off of the thermal modeling effort, CFD models were created to model the sublimation, vapor transport and film deposition that occurs within the CdS source. Fluid models of the CdS source were created to accurately reflect the current deposition technique with the intent of predicting future deposition uniformity during the evaluation process for new source designs. The developed model was able to accurately predict film growth in an untested source in which the uniformity of the film deposition was increased by over 70%. These models were created using ANSYS Fluent, and utilized Arrhenius reaction rate equations to describe the sublimation and condensation reactions. Modeling results showed a strong correlation with the experimental data.Item Open Access Development and advancement of thin CdTe-based solar cells for photovoltaic performance improvements(Colorado State University. Libraries, 2020) Bothwell, Alexandra, author; Sites, James, advisor; Krueger, David, committee member; Gelfand, Martin, committee member; Sampath, Walajabad, committee member; Topič, Marko, committee memberPhotovoltaic technologies, with an essentially infinite energy source, large total capacity, and demonstrated cost competitiveness, are well-positioned to meet growing global demand for clean energy. Cadmium-telluride (CdTe) thin-film photovoltaics is advantageous primarily for its direct optical band gap (approximately 1.48 eV) which is well-matched to the standard AM 1.5G solar spectrum, and its high absorption coefficient. These advantages, in tandem with innovations in fabrication and photovoltaic design in the past decade, have significantly increased CdTe photovoltaic device performance and reduced cost. Major advances in CdTe device performance have been achieved through improved current collection and fill factor, however, the open-circuit voltage (VOC) of CdTe devices remains limited compared to the band gap-determined maximum achievable VOC. The voltage deficit could be minimized through various approaches, and this work addresses it through progressive structural changes to a thin CdTe device. Absorbers of less than 2 µm were pursued for ultimate electron-reflector devices which incorporate a wide band-gap material behind the absorber to induce a back-surface field via a back-side conduction-band offset for improved VOC. An optimized and stable base structure is necessary to quantify characteristics and improvements in progressive devices with additional material layers. Thin, 0.4-1.2 µm CdTe absorber devices were optimized and demonstrated respectable and repeatable performance parameters, and a maximum efficiency of 15.0% was achieved with only 1.2 µm CdTe. Capacitance measurements also showed that thinner devices had fully-depleted absorbers into forward bias. To improve device performance through increased current collection, a 1.4-eV band gap CdSeTe layer was introduced as an additional absorber material preceding CdTe. Prior understanding of the effects of the additional CdSeTe material was incomplete, and this work deepens and expands this understanding. Performance improvement was achieved for thin, 1.5-µm absorber devices with no intentional interdiffusion of the CdSeTe and CdTe. The importance of the CdSeTe thickness was demonstrated, where performance was consistently reduced for CdSeTe thickness greater than CdTe thickness, independent of CdSe composition in the close-space sublimation (CSS) CdSeTe source material. Longer time-resolved photoluminescence (TRPL) tail lifetimes in CdSeTe/CdTe devices compared to CdTe devices suggested better bulk properties, and current loss analysis showed that CdSeTe is the dominant absorber in 0.5-µm CdSeTe/1.0-µm devices. 1.5-µm CdSeTe/CdTe devices demonstrated increased current collection and 30-mV voltage deficit reduction due to the 100-meV narrower band gap of CdSeTe compared to CdTe and passivating effects of selenium, for an ultimate efficiency improvement to 15.6%. Lattice-constant matching to CdTe and wide, ~1.8-eV band-gap requirements directed the selection of CdMgTe as the electron-reflector layer. CdMgTe was incorporated into the CdSeTe/CdTe device structure first through CSS, but sputter deposition was found to be more favorable to address the material complexities of CdMgTe (temperature-induced magnesium diffusion and CdCl2 passivation loss, doping, and MgO formation), and produced higher performing CdMgTe electron-reflector devices. Low substrate temperature achievable in sputtered CdMgTe deposition proved the greatest advantage over CSS-CdMgTe: CdCl2 passivation and magnesium can be appropriately maintained with a corresponding maintenance of device performance, whereas temperature-induced CdCl2 passivation loss or magnesium loss will occur for CSS-deposited CdMgTe with incumbent performance reduction. Through low-temperature depositions, doping optimization, and small structural adjustments, 16.0% efficiency was achieved with CdMgTe sputtered on 0.5-µm CdSeTe/1.0-µm CdTe absorbers, the highest-known CdMgTe electron-reflector device performance. The CdMgTe and non-CdMgTe-containing device VOC's suggested that electron reflection was enacted with partial success for the sputter CdMgTe-incorporated structure, but the significant improvements expected based on simulation have not been realized due to MgO formation and a negative valence-band offset which somewhat impedes hole transport to the back contact. Suggestions to overcome or circumvent these limitations are presented and discussed in the context of progressed understanding of CdMgTe electron-reflector devices.Item Open Access Distortions to current-voltage curves of CIGS cells with sputtered Zn(O,S) buffer layers(Colorado State University. Libraries, 2013) Song, Tao, author; Sites, James R., advisor; Wu, Mingzhong, committee member; Sampath, Walajabad, committee memberSputtered-deposited Zn(O,S) is an attractive alternative to CdS for Cu(In,Ga)Se2 (CIGS) thin-film solar cells' buffer layer. It has a higher band gap and thus allows greater blue photon collection to achieve higher photon current. The primary goal of the thesis is to investigate the effects of the secondary barrier at the buffer-absorber interface on the distortions to current-voltage (J-V) curves of sputtered-Zn(O,S)/CIGS solar cells. A straightforward photodiode model is employed in the numerical simulation to explain the physical mechanisms of the experimental J-V distortions including J-V crossover and red kink. It is shown that the secondary barrier is influenced by both the internal material properties, such as the conduction-band offset (CBO) and the doping density of Zn(O,S), and the external conditions, such as the light intensity and operating temperature. A key parameter for the sputter deposition of Zn(O,S) has been the oxygen fraction in the argon beam. It is found that the CBO varies with the oxygen fraction in the argon beam at a fixed temperature. With a greater CBO (∆EC > 0.3 eV), the resulting energy barrier limits the electron current flowing across the interface and thus leads to the J-V distortion. Two different ZnS targets, non-indium and indium-doped one, were used to deposit the Zn(O,S) buffer layer. At the same oxygen fraction in argon beam, a non-In-doped Zn(O,S) buffer with a smaller amount of doping forms a greater secondary barrier to limit the electron current due to the compensation of the Zn(O,S) buffer layer. In addition, the temperature-dependent J-V crossover can be explained by the temperature-dependent impact of the secondary barrier - at lower temperature in the dark, the maximum distortion-free barrier is reduced and results in a more serious current limitation, indicating a greater J-V crossover. It is also found that, under low-intensity illumination, there is a lower doping density of Zn(O,S) due to a smaller amount of photons with hν > Eg(Zn(O,S)) which can excite the buffer layer to release the trapped electrons from the deep-level defect state. The result is a greater secondary barrier to limit the electron current through the interface and shift the light J-V curve right towards the dark J-V curve at high bias (V > VOC) which reduces the J-V crossover. Finally, the quantitative comparison of J-V distortion between simulation and experiment is employed to examine the credibility of the secondary barrier theory.Item Open Access Evaluating leafy green production in a Colorado rooftop agrivoltaic system(Colorado State University. Libraries, 2024) Villa-Ignacio, Armando, author; Bousselot, Jennifer, advisor; Uchanski, Mark, committee member; Sampath, Walajabad, committee memberCombining green roofs with solar modules can protect plants and produce energy in cities. Growing crops in this system is called rooftop agrivoltaics (RAV) and can complement current urban agriculture efforts. We evaluated a group of five leafy green crops (arugula, kale, lettuce, spinach, and Swiss chard) under different solar modules over two years at two locations. Data measurements were taken for fresh and dry weight (FW, DW) stomatal conductance (SC), plant size at harvest (PSH), and microclimate data. At the Colorado State University Foothills Campus, the treatments included a polycrystalline opaque silicon module, a cadmium telluride (CdTe) frameless opaque module, and a 40% semi-transparent CdTe module. At CSU Spur, there was an opaque module and a bifacial module. Both sites included a full sun control plot. At the Foothills campus, for of the five leafy greens produced higher FW and DW under the 40% semi-transparent modules compared to other treatments and the full sun control, except spinach. Most species also produced larger PSH under the PV module treatments compared to the full sun control. Leafy greens under the module treatments resulted in lower SC, however, lettuce and Swiss chard grown under the semi-transparent module treatment produced higher SC compared to all other treatments. At CSU Spur, plant responses were also species specific with arugula, kale, and lettuce yielding higher FW and DW in full sun. Most leafy greens resulted in lower SC, except for lettuce, which had a higher SC under solar module treatments. Spinach had no difference in FW but lower DW in the opaque treatment compared to the full sun control, and lower SC under both treatments. There was a lower FW between the bifacial treatment and the full sun control in Swiss chard. This research shows that incorporating photovoltaics on rooftop gardens influences the yield and stomatal conductance of select leafy green crops. While FW and DW mostly decreased under the deep shade treatments (opaque module, frameless module, and bifacial module) SC decreased, possibly due to less solar radiation on the leafy greens, reducing water use. Understanding the growth characteristics and growing environment of high value crops like leafy greens will increase understanding of what food crops are suitable for RAV systems.Item Open Access Investigating the suitability of existing commercial hydrophobic coatings for soiling mitigation in the photovoltaic industry(Colorado State University. Libraries, 2019) Strauss, Ben, author; Barth, Kurt, advisor; Sampath, Walajabad, committee member; Sites, James, committee memberThe global production of solar power has been increasing approximately 40% per year for the last two decades, making solar one of the quickest growing renewable energy technologies. Estimated to increase 14-fold by the year 2040, solar photovoltaic (PV) power will become a major source of electricity. Soiling, the build-up of dust and debris on the surface of a solar module, is the third largest contributor to losses in solar power output. Decreases in solar module energy production of 20-30% have been observed in arid-desert climates, regions where sunlight is most intense and abundant. Current soiling mitigation techniques involve some type of mechanical cleaning process, either manual or automated, which can be highly water, time, and cost intensive. A potentially beneficial option to reducing PV soiling involves the use of anti-soiling coatings. A number of studies have previously examined the anti-soiling properties of various hydrophobic (water-fearing) and hydrophilic (water-loving) coatings. Though studies are ongoing, research generally shows hydrophobic coatings have an advantage over hydrophilic coatings due to lower dust adhesion forces and water-repellency properties. However, existing research efforts have not conclusively shown that hydrophobic coatings can survive the harsh environmental conditions experienced by a solar module during its lifetime. Anti-soiling research on existing commercial hydrophobic coatings is also minimal. Therefore, this research aims to understand the viability of using existing hydrophobic coatings to mitigate soiling losses seen in the PV industry. A group of hydrophobic coatings were obtained from various sectors of industry, including surface refinement, electronics, ophthalmic, and automotive. An initial screening procedure, designed to characterize the hydrophobic properties of the obtained coatings, was then implemented to identify a group of candidate coatings for this study. An accelerated durability testing procedure, designed specifically for hydrophobic coatings on solar cover glass, was used to identify degradation mechanisms of the candidate coatings in the presence of environmental stressors. Utilizing a custom-built soiling chamber and various dust removal apparatuses, a testing methodology was developed to understand the anti-soiling properties of the coatings. Finally, using an outdoor solar test array, comparative tracking of coated and uncoated modules was performed over an extended period of time. Through durability and anti-soiling experimentation, results from this work led to the identification of a single commercially available hydrophobic coating that demonstrates strong potential for anti-soiling applications in PV.Item Open Access Investigation into catalyst interactions in a dye-sensitized photoelectrochemical cell for water oxidation catalysis(Colorado State University. Libraries, 2022) Jewell, Carly Francis, author; Finke, Richard, advisor; Shores, Matthew, committee member; Krummel, Amber, committee member; Sampath, Walajabad, committee memberSolar energy has the potential to contribute significantly to solving the global energy crisis. However, solar energy is both diffuse and intermittent, meaning the capture and storage of this energy is critical. One method of storing this energy is the generation of storable hydrogen fuel via photoelectrochemical water-splitting, that is, storing energy in chemical bonds, specifically the H-H bond. However, the efficiency of the water-splitting process is limited by the water oxidation reaction, a four-electron process occurring at the anode. As such, water splitting devices, and more specifically water-oxidation devices, have been the focus of research for several decades. One such strategy, employed herein, uses molecular light-harvesting dyes and associated materials to capture and convert energy from the sun into chemical bonds. The work presented in this dissertation examines one such water-oxidation dye-sensitized photoelectrochemical cell (DS-PEC) with the goal of better understanding how charge-carrier interactions in the system are impacted by varying the system's catalyst, architecture and device composition. Throughout this dissertation a photoanode consisting of nanostructured SnO2 coated in perylene diimide dye N,N'-bis(phosphonomethyl)-3,4,9,10-perylenediimide plus photoelectrochemically deposited cobalt oxide (CoOx) is examined. Chapter I provides an in-depth overview to water-oxidation catalysis with a focus on the state of DS-PECs in the literature. Chapter II looks to understand the impact of an alumina overlayer on this DS-PEC system, with the specific goal of better understanding why the addition of the CoOx catalyst decreases photocurrent and increases recombination, a so-called "anti-catalyst" effect. The studies presented in Chapter II demonstrate that the presence of an ultrathin alumina overlayer by atomic layer deposition (ALD) increases photocurrents and decreases recombination in the device, although the addition of CoOx catalyst still decreases photocurrent. Chapter III examines the same system with the continued goal of identifying the source of increased recombination and decreased photocurrents with CoOx catalyst addition. Through a series of controls, residual carbon attributable to organic stabilizer used in the nanostructured SnO2 synthesis is discovered to be the culprit of this "anti-catalysis" effect. Anodes made using more carbon-free SnO2 deposited by ALD, rather than the nanostructured SnO2 with residual carbon, show an increase in photocurrents with CoOx addition. Subsequently, Chapter IV looks at two methods of overcoming and outcompeting the recombination attributable to residual carbon in the device. The effect of the residual carbon is shown to be mitigated through both the use of a more active iridium-based catalyst, amorphous Li-IrOx, rather than CoOx, and then through the use of a more carbon-free ALD-SnO2, without organic stabilizer, rather than nanostructured SnO2. The planar ALD-SnO2 is compared to the nanostructured SnO2 both on a per dye basis and on an electrochemically active surface area basis. The results presented in this dissertation offer fundamental insights into achieving both a better understanding, and an improved performance, of DS-PECs for water-oxidation catalysis that is a critical component of solar energy capture and storage.Item Open Access Measurement of cadmium telluride bilayer solar cells(Colorado State University. Libraries, 2024) Chime, Chinecherem Agnes, author; Sites, James, advisor; Buchanan, Kristen, committee member; Sampath, Walajabad, committee memberPhotovoltaic (PV) technology is a green technology that uses devices and semiconducting materials to generate power by converting the absorbed energy from solar to electrical energy. Understanding the performance and behavior of a fabricated device is essential for enhancing their efficiency for future commercialization. Cadmium-telluride (CdTe) technology is a PV technology that uses CdTe as the semiconductor layer for absorbing and converting sunlight into electricity. Incorporating a bilayer of cadmium selenium telluride (CdSexTe1-x) alloy and CdTe into solar cell devices have shown particularly good performance, enhanced passivation, and higher efficiency. In this research, cadmium telluride solar cells were fabricated with a focus on improving the performance of the absorber layers. Radio frequency (RF) magnetron sputtering and close-space sublimation were adopted in preparing the front and back contact layers respectively. The fabricated device comprises of Tec-10 glass/100-nm magnesium-doped zinc oxide (MZO)/0.5-μm CST40/2.5-µm CdTe/ cadmium-chloride passivation/ Cu-doping/ 40-nm Te/ carbon and nickel paint back contact. As part of the performance improvement measures, the bilayer surface was passivated with cadmium chloride (CdCl2) and doped afterwards with copper. The fabricated CdSexTe1-x/CdTe device was subjected to room temperature and low temperature current density-voltage (J-V), capacitance, phase angle, quantum efficiency (QE), reflectance, electroluminescence (EL), and photoluminescence (PL) measurements. The J-V characteristics gave 15% device efficiency and showed diode curves which rolled over at lower temperatures, but were more ideal at higher temperatures. Capacitance measurements gave a hole density of 4x1014 cm-3 and a phase angle of 88o. The cells recorded high quantum efficiency of about 85% which is indicative of reduced recombination rate. Few defects were observed from the EL images while the PL emission peaks were obtained at 875 nm corresponding to an approximate energy band gap value of 1.42 eV. The measurement results show good performance for use in commercial solar cells for energy sustainability. Future implications encompass module fabrication, flexible devices, and affordability for enhancing green energy production and minimizing environmental pollution. Prospects envisage fabricating CdTe devices with higher efficiencies which would continue to compete successfully with other solar cell technologies.Item Open Access Photoelectrochemical cells employing molecular light-harvesting materials for the capture and conversion of solar energy(Colorado State University. Libraries, 2017) Kirner, Joel Thomas, author; Finke, Richard G., advisor; Reynolds, Melissa, committee member; Van Orden, Alan, committee member; Sampath, Walajabad, committee memberSolar light has the potential to be a substantial contributor to global renewable energy production. The diffuse nature of solar energy requires that commercially viable devices used to capture, convert, and store that energy be inexpensive relative to other energy-producing technologies. Towards this end, photoelectrochemical cells have been the subject of study for several decades. Particularly interesting to chemists, molecular light-harvesting materials can be employed in photoelectrochemical cells. For example, a dye-sensitized solar cell (DSSC) is a type of photoelectrochemical cell designed to capture solar energy and convert it to electricity. Alternatively, molecular light-harvesting materials have also been employed in water-splitting photoelectrolysis cells (PECs), which capture solar energy and store it in the form of chemical bonds such as H2 and O2. The work presented in this dissertation falls into two major projects. The first involves fundamental studies of water-oxidizing PECs employing a novel perylene diimide molecule as the light-harvesting unit. Background is provided in Chapter II, composed of a comprehensive literature review of water-oxidizing PEC systems that employ light-harvesting materials composed of earth-abundant elements. Chapter III describes preliminary studies of a water oxidizing PEC composed of a perylene diimide organic thin-film (OTF) and cobalt oxide catalyst, the first of its kind in the literature. Characterization of this novel device provided knowledge of the efficiency-limiting processes that would need to be addressed in order to improve device performance. Subsequently, Chapter IV describes preliminary studies of the same perylene diimide molecule in an alternative, literature-precedented, dye-sensitized photoelectrolysis cell (DS-PEC) architecture aimed at improving the efficiency-limiting processes of the first OTF-PEC. Characterization of this DS-PEC architecture reveals that the efficiency-limiting processes of the OTF-PEC were indeed improved. However, deposition of the cobalt oxide catalyst onto the DS-PEC did not successfully result in water oxidation. Alternative catalyst-deposition strategies from the literature are described as direction for future studies. The second project of this dissertation involves the study of novel high-redox-potential organometallic cobalt complexes as redox mediators in DSSCs, and is presented in Chapter V. Therein, it was found that the use of electron-withdrawing functional groups on cobalt coordinating ligands not only increased the redox potential, but also increased the lability of the ligands. The resulting complex instability caused performance-limiting electron-recombination reactions in assembled DSSCs. These results point future researchers towards the study of higher-chelating ligands for enhanced stability in high-potential cobalt complexes.Item Open Access Technical and economic evaluation of triglyceride gasoline blends as an alternative fuel for diesel engines(Colorado State University. Libraries, 2018) Lakshminarayanan, Arunachalam, author; Olsen, Daniel, advisor; Marchese, Anthony, committee member; Sampath, Walajabad, committee member; Cabot, Perry, committee memberDeveloping viable and sustainable alternative fuels is critical in addressing future energy needs. Existing fossil fuels, being limited in nature, are depleting, contribute to climate change, health effects and their markets are volatile resulting in price fluctuations. Liquid fuels comprise a significant portion (about 40%) of a nation's total energy demand and production. Transportation sector being a key contributor national growth and security consumes almost 24% of the liquid fuel, while farming consumes about 15% to 17% of the liquid fuels. Bio diesel and bio ethanol are the two most widely used alternative, renewable fuels available. This work presents the technical and economics of using Triglyceride gasoline blends (TGBs) in a diesel engine. Canola straight vegetable oil (SVO) is highly viscous and has poor flow ability in cold weather. Consequently, it cannot be used in diesel engines without modification to the fuel system. Blending regular unleaded gasoline (10% by volume) to unrefined canola oil results in the specific gravity of the blend being similar to that of diesel. This enables it to be used in off road diesel engines in cold weather without modifications to the fuel system. A series of studies were performed to examine the viability of using TGBs to fuel diesel engines. Engine experiments were conducted on a 4.5L, turbocharged, intercooled Tier-III diesel engine. Lower heating value, higher mass based fuel consumption and slightly higher thermal efficiencies were recorded using TGB10 compared to diesel. The cylinder pressure traces and location of 50% mass fraction burnt for TGB10 and diesel were similar in most load points of the ISO 8178 8-mode test cycle. The average peak pressure of TGB10 was within ±4.5% to that of diesel. The combustion duration of TGB10 was about 12% to 15% shorter than diesel. Increased weighted NOX emissions (+9.8%), slightly lower weighted PM emissions (-5.5%), significantly lower weighted CO emission (-51.7%) and higher metal content (various orders of magnitude) were observed when using TGB10 as fuel in comparison to diesel. Additional engine experiments included varying the gasoline percentage in the TGB, evaluating combustion statistics, engine ECU parameters like start of injection, turbocharger speed and emissions analysis. Overall for blends containing up to 25% gasoline, most of the combustion parameters were identical to 100% triglyceride. As the gasoline content increased up to 55%, the combustion parameters were similar to diesel. For blends containing gasoline greater than 60% the combustion parameters were significantly different than diesel. A durability study (250 hours) on three fuels – (i) off road diesel, (ii) canola based bio diesel, and (iii) canola based TGB10 was conducted on a single-cylinder, naturally aspirated Yanmar diesel engine operating at constant load. Oil samples, injector spray patterns and carbon buildup from the injector and cylinder surfaces for the three fuels were analyzed and compared. Biodiesel had a cleansing effect on the injector tip. TGB10 left behind thick sludge on piston crown while diesel fuel had the least impact on lubricating oil quality. Finally, an economic business case model was analyzed for a complete lifecycle for TGB10. The model includes growing the canola crop, setting up a crushing facility to extract unrefined canola oil to converting it to TGB10 and the cost of ownership for a farm tractor over four different lifespans. The results show that though the cost of producing TGB10 can be lower than diesel, the cost of ownership can significantly vary on the lifespan of engine and its components. Expensive diesel prices and higher engine lifespans are the key to making TGB10 economically viable.Item Open Access Thermoelectric properties of Si/SiC thin-film superlattices grown by ion beam sputtering(Colorado State University. Libraries, 2015) Cramer, Corson Lester, author; Williams, John, advisor; Sampath, Walajabad, committee member; Neilson, James, committee memberThere are many mechanical systems that convert heat to work and processes that utilize heat including power plants, automobiles, and foundries. Most of these systems expel large amounts of waste heat to the environment that goes unused. One way of recovering the waste heat is to use a solid-state energy converter based on thermoelectric processes. Nano-scaled materials are of interest for use in thermoelectric devices because their properties enhance the efficiency over those obtained using bulk materials. Some nano-scaled materials systems being considered are thin-film superlattices that utilize quantum confinement effects. Thin-film, superlattice thermoelectric devices could revolutionize traditional heat-to-work systems and heat-only processes if they are coupled to the systems to recycle a fraction of the waste heat into usable power. The advantage of thermoelectrics over traditional mechanical systems is that they use solid-state processes instead of moving parts and working fluids. As a result, they can be made to be more reliable and require less maintenance. This thesis focuses on the characterization of a thin-film, superlattice (SL) thermoelectric material formed by alternating silicon and silicon carbide layers to form an n-type quantum well. Superlattices of 31 bi-layers of Si/SiC (10 nm each) were deposited on silicon, quartz, and mullite substrates using a high-speed, ion-beam sputter deposition process, and the Seebeck coefficient and electrical resistivity are measured as a function of temperature and used to compare film performance. In addition, SL layer thicknesses of 2 and 5 nm were deposited on mullite to determine the effect layer thickness has on the thermoelectric properties.