Browsing by Author "Sampath, Walajabad, advisor"
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Item Open Access Advanced photovoltaic module architecture for high value recycling and lower cost(Colorado State University. Libraries, 2023) Ruhle, Ryan J. E., author; Sampath, Walajabad, advisor; Sites, James, committee member; Weinberger, Crhis, committee memberAs climate concerns continue to bolster solar energy production, the need to consider how solar panels are treated at end of life as well as the cost of solar panel production is becoming a more significant issue. Traditionally, Crystalline Silicon (c-Si) solar panels are made by laminating solar cells with glass under high heat and high mechanical pressure. The most common material used for this lamination between the glass and the c-Si solar cell is Ethylene Vinyl Acetate (EVA), a copolymer of ethylene and vinyl acetate. The first and primary issue is that it requires high temperature and a significant amount of pressure to be adhered to both the glass and the c-Si cell. Another related issue is that the c-Si cell and EVA encapsulant do not have the same thermal expansion coefficients. This leads to stresses which can cause the formation and growth of microcracks which can hinder performance and reliability of the effected solar cells. End-of-life recycling is also significantly hampered by cross-linking of EVA. The Materials Engineering Laboratory (MEL) has long worked on vacuum lamination free module architectures, though this has been primarily for use for Cadmium Telluride (CdTe) solar panels. These CdTe panels have passed IEC 61215 tests and have been applied in the field. These Edge-sealed photovoltaics modules based on Insulating Glass (IG) industry technology have many advantages including lower cost, improved manufacturability, increased durability, and enable high-value recycling with the potential for material reuse. The edge-sealed modules eliminate EVA (Ethylene Vinyl Acetate) lamination, but a gap filled with air or inert gas between the glass and solar cell increases optical reflection losses. The use of edge sealed modules for c-Si was explored in this study. A prototype manufacturing system (2 ft X 4 ft substrates) has been developed at MEL and was used in this study. Many c-Si modules were fabricated with edge sealing and were studied at the National Renewable Energy Laboratory (NREL) in various tests including accelerated tests. These studies have shown that optical reflection losses can be reduced by using nanostructures made from acrylic polymers. The nanostructures are produced by hot embossing which is intrinsically a low-cost process. The edge sealed structure has demonstrated extreme robustness to moisture ingress (5000 hrs. vs 1000 hrs. in damp heat), improved mechanical robustness, significant reduction in Potential Induced Degradation (PID), survive thermal cycling and small manufacturing footprint (80% less) while improving module reliability. The edge sealed modules have demonstrated high value recycling of the components and have the potential to make recycling of c-Si PV modules economical.Item Open Access Advanced research deposition system (ARDS) for processing CdTe solar cells(Colorado State University. Libraries, 2014) Barricklow, Keegan Corey, author; Sampath, Walajabad, advisor; Sites, James, committee member; Olsson, Anders, committee memberCdTe solar cells have been commercialized at the Gigawatt/year level. The development of volume manufacturing processes for next generation CdTe photovoltaics (PV) with higher efficiencies requires research systems with flexibility, scalability, repeatability and automation. The Advanced Research Deposition Systems (ARDS) developed by the Materials Engineering Laboratory (MEL) provides such a platform for the investigation of materials and manufacturing processes necessary to produce the next generation of CdTe PV. Limited by previous research systems, the ARDS was developed to provide process and hardware flexibility, accommodating advanced processing techniques, and capable of producing device quality films. The ARDS is a unique, in-line process tool with nine processing stations. The system was designed, built and assembled at the Materials Engineering Laboratory. Final assembly, startup, characterization and process development are the focus of this research. Many technical challenges encountered during the startup of the ARDS were addressed in this research. In this study, several hardware modifications needed for the reliable operation of the ARDS were designed, constructed and successfully incorporated into the ARDS. The effect of process condition on film properties for each process step was quantified. Process development to achieve 12% efficient baseline solar cell required investigation of discrete processing steps, troubleshooting process variation, and developing performance correlations. Subsequent to this research, many advances have been demonstrated with the ARDS. The ARDS consistently produces devices of 12% ±.5% by the process of record (POR). The champion cell produced to date utilizing the ARDS has an efficiency of 16.2% on low cost commercial sodalime glass and utilizes advanced films. The ARDS has enabled investigation of advanced concepts for processing CdTe devices including, Plasma Cleaning, Plasma Enhanced Closed Space Sublimation (PECSS), Electron Reflector (ER) using Cd1-xMgxTe (CMT) structure and alternative device structures. The ARDS has been instrumental in the collaborative research with many institutions.Item Open Access Analysis of advanced vapor source for cadmium telluride solar cell manufacturing(Colorado State University. Libraries, 2013) Khetani, Tejas Harshadkumar, author; Sampath, Walajabad, advisor; Sakurai, Hiroshi, committee member; Sites, James, committee memberA thin film CdS/CdTe solar cell manufacturing line has been developed in the Materials Engineering Laboratory at Colorado State University. The original design incorporated infrared lamps for heating the vapor source. This system has been redesigned to improve the energy efficiency of the system, allow co-sublimation and allow longer run time before the sources have to be replenished. The advanced vapor source incorporates conduction heating with heating elements embedded in graphite. The advanced vapor source was modeled by computational fluid dynamics (CFD). From these models, the required maximum operating temperature of the element was determined to be 720 C for the processing of CdS/CdTe solar cells. Nichrome and Kanthal A1 were primarily selected for this application at temperature of 720 °C in vacuum with oxygen partial pressure. Research on oxidation effects and life due to oxidation as well as creep deformation was done, and Nichrome was found more suitable for this application. A study of the life of the Nichrome heating elements in this application was conducted and the estimate of life is approximately 1900 years for repeated on-off application. This is many orders of magnitude higher than the life of infrared heat lamps. Ceramic cement based on aluminum oxide (Resbond 920) is used for bonding the elements to the graphite. Thermodynamic calculations showed that this cement is inert to the heating element. An earlier design of the advanced source encountered failure of the element. The failed element was studies by scanning electron microscopy and the failure was attributed to loss of adhesion between the graphite and the ceramic element. The design has been modified and the advanced vapor source is currently in operation.Item Open Access Analysis of CuCl thin-film deposition and growth by close-space sublimation(Colorado State University. Libraries, 2016) Nicholson, Anthony, author; Sampath, Walajabad, advisor; Sakurai, Hiroshi, committee member; Sites, James, committee memberThere is a growing need to implement high fidelity, scalable computational models to various thin-film photovoltaic industries. Developing accurate simulations that govern the thermal and species-transport diffusion characteristics within thin-film manufacturing processes will lead to better predictions of thin-film uniformity at varied deposition conditions that ultimately save time, money, and resources. Thin-film deposition and growth of Copper I Chloride (CuCl) by the Close-Space Sublimation (CSS) process was investigated in an extensive range of operating and thermal conditions. A simulation model based on the ANSYS FLUENT® solver platform was developed to encompass the kinetic behavior of the CuCl species within the CSS domain while predicting the growth rate at varied system parameters. Surface physics associated with the process, notably sublimation and adsorption, were studied, quantified, and applied to the continuum-based thin-film deposition model. Experimentation of CuCl thin-film growth was performed across a range of substrate and source temperatures for verification of the model’s computational accuracy. Furthermore, characterization of the inherent growth mode exhibited by CuCl was studied in conjunction with simulation and experimental tasks. It was concluded that the simulation model provided predictions for the CuCl thickness as a function of temperature within the range of typical CSS conditions. Equally important was the elucidation of the CuCl growth mechanism, which displays a Volmer-Weber growth mode on the Fluorine-doped Tin Oxide coated layer of the substrate. Such knowledge along with the current modeling capabilities will be useful in extending the computational method to predicting the non-uniformities present in CuCl and other thin-film depositions.Item Open Access Arsenic doping, kinetic behavior and oxide formation for polycrystalline CdTe thin films photovoltaics(Colorado State University. Libraries, 2024) Mate, Mayank N., author; Sampath, Walajabad, advisor; Munshi, Amit, advisor; Sites, James, committee memberCdTe thin-film photovoltaics (PV) is one of the most promising renewable energy technologies currently in the market. Since the inception of CdTe PV in the 1950s, the technology has come a long way to be one of the most efficient solar cell technologies on the planet. However, there are still challenges associated with this technology which limit the power conversion efficiency. Currently, the most efficient CdTe solar cell to have been fabricated is still only at ~69% of the maximum possible efficiency, given by the Shockley-Queisser (SQ) limit. Performance loss analysis has suggested that the limiting electrical parameter is the open-circuit voltage (Voc) and detailed investigations have suggested that a defect state within the bandgap of Se allowed CdTe contributes significantly to this loss of Voc, which can be observed by Photoluminescence (PL). For thin-film CdTe PV to realize its full potential, research has been ongoing to improve the Voc by doping CdTe with a group V dopant, as well as passivating the surface of CdTe which causes significant recombination of charged carriers and may inhibit voltage even further. The research in this thesis focuses on group-V doping using As as the dopant and Cd3As2 as the doping material, and the formation of TeO2 as the metal oxide on the surface of CdTe. The goal of this research was to observe any improvements in Voc and overall device performance with As doping, TeO2, and a combination of both, and whether As doping mitigates the primary defect state within the bandgap of Se alloyed CdTe. To investigate the effects of these doping and surface treatments, three key hypotheses statements were formed. The first hypothesis is that As can be incorporated into CdTe using Cd3As2 without depositing it as a separate layer. This distinction of not depositing a film of Cd3As2 is important, and it aims to avoid any complexes caused by depositing Cd3As2 on the CdTe surface, creating another interface and potentially limiting the device performance, particularly due to Cd3As2 being a semi-metal. Secondary-Ion Mass Spectroscopy (SIMS) and X-Ray Photoelectron Spectroscopy (XPS) were used as the characterization methods to confirm the incorporation of As into the CdTe film by dissociation and diffusion, without depositing a layer of Cd3As2. The second hypothesis is that exposing a CdTe film to ambient air at high temperatures can lead to the enhanced formation of TeO2. Metal oxides such as Al2O3 have long been investigated to passivate the surface of CdTe, improving carrier lifetime and Voc. Traditionally, these metal oxides have been sputtered onto the surface, while native oxides on the surface of CdTe take a long time to form, on the timescale of days to weeks. However, this hypothesis investigates a novel method of forming a native layer of TeO2 by annealing a CdTe film in vacuum at elevated temperatures (~500℃) and rapidly exposing it to atmospheric oxygen while the high temperature is maintained. XPS and PL characterization was performed on the films that had undergone this 'rapid oxidation', and the formation of TeO2 was confirmed. The effect of TeO2 was combined with As doping, as well as compared in an undoped film, and it demonstrated a significant improvement in Voc when compared to control samples that were Cu doped and undoped with no intentional formation of TeO2. The third and final hypothesis is that As doping using Cd3As2 mitigates the sub-band-gap emission in photoluminescence originating from Se alloyed CdTe. To confirm or deny this theory, absolute PL was measured with an InGaAs detector, for samples that were doped with As and for samples that had a combination As doping treatments and/or TeO2 formed by using the method described for hypothesis two. The photoluminescence results showed no mitigation of the sub-band-gap emission in Se alloyed CdTe films that were doped with As. However, the addition of TeO2 demonstrated a lowering of the sub-bandgap emission intensity and overall improvement to the band-gap PL emission. Additionally, electrical parameters of six distinct device structures were measured and compared, with Cu-doped and undoped Se-alloyed CdTe being the reference/control samples. As a result of this research, a cell measuring close to 18% power conversion efficiency was measured with As doping, the native TeO2 layer, and one third of the Cu doping that went into reference samples. Finally, this research also provides some insight into the effects of high O2 presence during the As doping process using Cd3As2, and how it inhibits the ionization of As a small P-type dopant.Item Open Access Computational fluid dynamics (CFD) modeling for CdTe solar cell manufacturing(Colorado State University. Libraries, 2011) Walters, Kevin Eugene, author; Sampath, Walajabad, advisor; Sakurai, Hiroshi, advisor; Sites, James, committee memberThe CdTe solar cell manufacturing process developed at Colorado State University used a vapor source that utilized infrared heating lamps as the heating source. This was used in the initial research system that was used to develop the inline CdTe solar cell manufacturing method used in industry. This system has since been redesigned to improve its ability to function as a more versatile research tool. This thesis focuses on the modeling efforts used in the development and understanding of an embedded NiCr heating unit for the vapor source. The traditional infrared heating lamps, while effective, were inefficient. A new design consisting of a NiCr heating element embedded in to the graphite deposition crucible, was developed as a more efficient and robust replacement to the infrared lamps. Four distinct models of increasing complexity were developed using the heat transfer modeling capabilities of ANSYS Fluent. The first two models helped to determine the overall thermal uniformity and the ability of the new heating designs ability to reach the needed temperatures of the deposition processes. The third model discussed in this thesis, aided in the development of the top heater that would maintain the desired steady state temperature at the process station used to deposit CdS films. The final model developed contains a higher level of detail used to determine the validity of previous assumptions and to gain an understanding of the internal temperature profile of the completed source. This modeling effort was extended to the system used in industry. The experimental data was compared to the modeled data verifying the model accuracy. The calculated temperatures were within 2.5% of the measured temperatures. The modeling efforts of both the CSU and Abound Solar deposition systems have proved the usefulness of CFD modeling as an important tool to equipment development and characterization.Item Open Access Improvements in thin film CdTe back contact and interface layers through sputter deposition of metals and semiconductor materials(Colorado State University. Libraries, 2019) Kindvall, Anna, author; Sampath, Walajabad, advisor; Kephart, Jason, committee member; de la Venta, Jose, committee memberThe photovoltaic industry has grown at an average annual rate of 50% over the last ten years. In that time, solar module prices have dropped significantly, with the current levelized cost of electricity averaged at $0.03 per kilowatt hour. Cadmium telluride (CdTe) photovoltaics are a common commercially produced thin-film solar cell. The leader in CdTe module production and research and development is First Solar. First Solar has set the record for research scale CdTe devices, achieving an efficiency of 22.1%, far from the theoretical limit. Improving interface layers has been identified as one of the key strategies towards further improving device performance. The focus of this study is on back contact interface layers. This research explores sputtered molybdenum oxide (MoOₓ) and molybdenum nitride (MoNₓ) thin films as alternative back contact to carbon and nickel paint in a polymer binder. The MoOₓ and MoNₓ films were characterized using resistivity measurements, Hall measurements to determine carrier concentrations, x-ray photoelectron spectroscopy to determine nitrogen and oxygen incorporation into the film and x-ray diffraction to determine crystallinity. Devices were fabricated using different compositions of molybdenum, molybdenum oxide, and molybdenum nitride with an aluminum capping layer as back contacts. This structure resulted in performances very similar to the baseline with carbon and nickel paint, proving it to be a viable alternative. Additionally, this research study explored the option of zinc telluride (ZnTe) as a buffer layer between the CdTe and metal back contact. Copper doping assists in the CdTe device performance, however too much copper can be detrimental to device performance. The ZnTe layer allows for better valence band alignment and limits copper diffusion. The device structure with ZnTe resulted in a 17.6% device, which is comparable to the baseline structure. Early results indicated devices with ZnTe were more stable and robust over time.Item Open Access Investigation of gold as material for thermal radiation shielding(Colorado State University. Libraries, 2013) Munshi, Amit Harenkumar, author; Sampath, Walajabad, advisor; Kirkpatrick, Allan, committee member; Sites, James, committee memberCdS/CdTe thin film solar cells technology is one of the fastest growing carbon neutral energy sources in the world today. Manufacturing of CdS/CdTe solar modules is carried out at temperature in the range of 620°C under a vacuum of 40 millitorr using a Heated Pocket Deposition (HPD) system in the materials engineering laboratory. Since this system operates in vacuum, majority of the heat loss is due to thermal radiation. The concept here is to conserve the heat by reflecting the infrared radiation back into the deposition system thus increasing the thermal efficiency. Various metals may be used but calculations show that using a Gold thin film mirror can effectively reflect almost 97% of the incident radiation, thus conserving energy required for the manufacturing process. However, a phenomenon called thermal grooving or island formation inhibits its use. Thermal grooving occurs when the stress concentration at the grain boundaries causes grain separation. This phenomenon is observed in thin gold films that are exposed to a temperature in excess of 350°C for over 3 to 5 hours. In this study, these films are exposed to temperature up to 620°C for cycles as long as 200 hours. The goal of this research is to explore the solutions for elimination of the phenomenon of thermal grooving and thus extract maximum life out of these thin gold films for conservation of heat. After carefully exploring literature on past research and conducting experiments it was found that within the range of the films that were tested, a 2000 A° film with a 150 A° of Indium underlay showed the best performance after thermal annealing and testing.Item Open Access Mechanical studies of cadmium sulfide/cadmium telluride (CdS/CdTe) photovoltaic modules(Colorado State University. Libraries, 2015) Armijo, Mark Andrew, author; Sampath, Walajabad, advisor; Radford, Donald, committee member; Sites, James, committee memberCommercial Cadmium Sulfide (CdS) and Cadmium Telluride (CdTe) photovoltaic modules are typically 24” x 48”. The processing steps include: glass heating, Cadmium Sulfide (CdS) deposition and Cadmium Telluride (CdTe) deposition, Cadmium Chloride (CdCl2) heat treatment, back contact formation and back contact heat-treatment. The main components of the photovoltaic module under consideration in this research are the tempered front glass, an encapsulant (ethylene vinyl acetate (EVA)) interlayer, and the tempered bottom glass. During processing, the front glass loses a certain degree of temper. This results in the reduction of the residual stress within the front glass and ultimately reduces the strength of the module. The residual stress before and after processing was measured. The glass heating reduced the residual stress from 10,000 psi to approximately 2,500 psi. Even with the loss of the residual stress, the modules passed the static load test of 2,400 Pa and survived the hail impact test (1” ice balls at 50 mph). The mechanical behavior of the composite photovoltaic (PV) modules under static mechanical load and hail impact load utilizing mechanics modeling and experimental testing were characterized. The accuracy of the theoretical model is compared to the results of the experimental testing. The results will provide valuable knowledge for the mechanical characteristics of the PV module. This will contribute to the understanding of the effects of temper loss and whether the module exhibits a loss in strength.Item Open Access Optical measurement techniques for in-line process control in CdS/CdTe solar cell manufacturing(Colorado State University. Libraries, 2011) Kephart, Jason Michael, author; Sampath, Walajabad, advisor; Sites, James, committee member; Olsson, Anders, committee memberCdS/CdTe solar cells have achieved gigawatt-scale commercial production at a lower cost than traditional crystalline silicon photovoltaics. With large-scale production of semiconductor devices, process control is critical to ensuring consistent quality. While there are many classes of materials characterization techniques including scanning probe, x-ray, electronic techniques, optical techniques are particularly promising for in-line structural characterization and process control. They are fast, non-contact, occupationally safe, precise, and can be performed immediately after film deposition to detect problems early in the manufacturing process. Two such optical techniques are examined here: spectroscopic ellipsometry and scanning white light interferometry. Spectroscopic ellipsometry consists of measuring the change in polarization of light reflected from a thin film structure. Fitting a model to the data provides structural information such as layer thickness and optical properties. Visually rough films can be a major obstacle to the use of ellipsometry, and processing options are explored to reduce roughness to acceptable levels. Ellipsometry has been shown to be accurate within 4% of thickness for the CdTe absorber layer and the CdS window layer can be measured accurately under the CdTe absorber for layers thicker than about 100 nm. Scanning white light interferometry (SWLI) uses the interference of reflected light from a surface to construct an extremely precise depth profile. This technique was examined for measurement of surface morphology and roughness as well as the measurement of film step heights, which could indicate whether or not the technique can be used for scribe metrology. Though the lateral resolution is limited by the use of optical microscopy, it has been shown that surface features can be resolved. Step height measurements match stylus profilometry very closely over a wide range of device thicknesses.Item Open Access Selective removal of fluorinated tin oxide (SnO2:F) from Pilkington TEC glass(Colorado State University. Libraries, 2017) Rekow, Mathew, author; Sampath, Walajabad, advisor; Sites, James, committee member; Radford, Don, committee memberHigh power lasers are utilized in a variety of processes in solar energy production and energy storage. In the production of CdTe (Cadmium Telluride) solar cells, pulse lasers are used for the so called P1 scribe step that is the first step in process of creating discrete series connected cells on an otherwise monolithic glass panel. A standard laser process sequence is well established in the industry. The common P1 process step removes all previously deposited materials, resulting in exposure of the soda lime glass substrate along the scribe line. These previously deposited layers include three layers deposited during the fabrication of the glass panel, an intrinsic SnO2 layer, a SiO2 layer, and the conductive SnO2:F layer, the latter of which forms the transparent conductive front contact of the solar cell. If the CdTe film is applied immediately afterward, sodium diffuses from the glass into the deposited CdTe film and is detrimental to the performance of the CdTe solar cell (1). To mitigate this problem, commercial processes perform the P1 scribe after CdTe deposition and the resulting groove is filled with a photo-resist. This photo-resist application process accounts for a significant fraction of the capital equipment cost in a CdTe solar panel production line (2). A means of creating a Na barrier layer in situ would eliminate the requirement of the photo-resist application step and simplify the production process. This work is aimed at developing a laser based scribe process that removes the SnO2:F layer but preserves the intrinsic SnO2 layer and the SiO2 layer to serve as a barrier to Na diffusion and hence eliminate the need for the photoresist application step. During this work, a very unusual laser-material interaction was discovered where the laser appears to initiate a physical – chemical reaction that proceeds along an unusual and apparently undescribed pathway that has many of the characteristics of an etch process. This laser "etching" mechanism allows arbitrary reduction of the film thickness in a controlled manner on the scale of a few microns. In addition to the fine depth selection, we find that there develops a laser pulse duration dependent microstructure on the surface. The unusual characteristics of this interaction are examined and a physical model is proposed to describe it. Finally, sodium diffusion effects were measured and other potential applications of this novel process are explored.Item Open Access Stability of thin-film CdTe solar cells with various back contacts(Colorado State University. Libraries, 2021) Hill, Taylor D., author; Sites, James, advisor; Sampath, Walajabad, advisor; Bradley, Mark, committee memberWith an increasing reliance on photovoltaic energy comes an ever-increasing demand to understand the mechanisms of failure which lead one to having an under-performing solar module. Recent technological advances have proven CdTe solar cells to be competitive with traditional Si, taking up 5% of the world solar market and reaching efficiency upwards of 22.1% for small area scale and 18.6% for module scale. This thesis explores various back-contact configurations to reduce the contact barrier height as well as how they hold up under accelerated lifetime testing. Various degradation mechanisms, such as diffusion of species, drift within the built-in fields, and formations of various impurities/complexes on the surface and within the bulk were explored. The results of accelerated-lifetime experiments revealed the instability of devices with large amounts of Cu and those containing the colloidal Ni based paint solution as a metallic back contact. Sputtered films of nickel doped with vanadium (Ni:V) and chromium (Cr) demonstrated the capability to produce cells with efficiencies between 12-13% with fill factors up to 75%. Metallic bilayers containing a metallic cap of aluminum (Al) were then evaluated, demonstrating an increase in efficiency up to 15.1%. Buffer layers of NiO revealed the presence of a large back-contact barrier via the rollover effect in forward bias, leading to devices with efficiency of only 3%, but subsequent work revealed that by applying the NiO buffer prior to CdCl2 passivation reduces the back barrier and produces cells with peak efficiency of 14.8%.