Mountain Scholar :: Browsing by Author "Sites, James, committee member"

Browsing by Author "Sites, James, committee member"

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Open Access
Addressing absorber quality in CD1-xMGxTE wide bandgap solar cells for tandem applications
(Colorado State University. Libraries, 2018) Reich, Carey L., author; Sampath, Walajabad S., advisor; Sites, James, committee member; Popat, Ketul, committee member
Tandem photovoltaic (PV) solar cells, which use multiple absorbing layers to convert light into electricity, have the potential to surpass the conversion efficiency limits of PV which uses a single absorber. This has been proven using epitaxially grown III-V semiconductors, but these are expensive and are only commonly used for extra-terrestrial applications. To realize terrestrial, cost effective tandem PV, low cost production of these highly efficient cells is required. Using absorbers which are similar to cost effective, mass produced PV such as CdTe, Si, or CIGS, this is possible. Si and CIGS have appropriate properties for the IR absorbing layer in a tandem cell, but there is no common PV material with the ideal properties for the UV/Visible light absorbing layer, although CdTe is quite close. Even better, CdTe's properties can be altered to those of ideal with the addition of ternary elements such as Mg, Zn, and Mn. Issues still remain however as the quality of solar cells produced using ternary alloys of CdTe is much lower than that of the base material. These quality issues seem to stem from the CdTe bulk passivation process, which involves a thermal treatment in the presence of Cl (commonly CdCl2 is used as a source) to passivate the grain boundaries and catalyze the recrystallization and grain growth process which annihilates detrimental planar crystalline defects in the absorbing material. The work presented in this thesis addresses issues with the absorber quality of solar cells using Cd1-xMgxTe by using concurrent Cl sources with CdCl2, diffusion barriers during CdCl2, and tweaking the absorber material with the addition of quaternary elements or novel layers in the device stack. This work culminated in the production of a 10.6% efficient device, a record for devices using CdMgTe as an absorber, and concludes with paths for future improvements in device performance.
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 member
As 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.
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 member
CdTe 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.
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 member
A 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.
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 member
There 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.
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 member
CdTe 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.
Open Access
Characterization of tellurium back contact layer for CdTe thin film devices
(Colorado State University. Libraries, 2018) Moffett, Christina, author; Sampath, W. S., advisor; Sites, James, committee member; Popat, Ketul, committee member
Cadmium Telluride (CdTe) thin film photovoltaic technology has shown favorable progress due to inexpensive and efficient processing techniques. However, efficiencies have yet to reach the overall projected CdTe device efficiency, with the back contact being a main source of CdTe performance limitations. Tellurium (Te) applied as a back contact has led to significant increases in fill factor and an overall progress in device efficiency. Devices deposited with Te show significant improvement in uniformity, even without intentional Cu doping, when compared to devices without Te. In current - density measurements, Te shows stability even at low temperatures, which is indicative of a low barrier developed at the CdTe/Te interface. X-ray and ultra-violet photoelectron spectroscopy were carried out to examine the valence band offset at the CdTe/Te back contact interface. The valence band offset was shown to be highly dependent on the Te thickness and was largely affected by oxidation and contamination at the surface. Capacitance measurements were carried out to study the effect Te has on the absorber depletion width. Data indicate a decreased depletion width with Te applied at the back of thin film CdTe devices, which agrees with increased device performance. Te thickness was varied in all studies to understand the effect of application thickness on device performance and material characteristics. With a thicker Te layer leading to overall improvement in device performance and favorable device characteristics.
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 member
The 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.
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 member
A 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.
Open Access
Density functional theory and Green's function approach to investigate cadmium telluride based thin film photovoltaics
(Colorado State University. Libraries, 2020) Nicholson, Anthony P., author; Sampath, Walajabad S., advisor; Weinberger, Chris, advisor; Popat, Ketul, committee member; Sites, James, committee member; Martinez, Umberto, committee member
In recent years, cadmium telluride (CdTe) based thin film photovoltaics (PV) have exhibited remarkable improvements in overall efficiency and device performance. As the most notable thin film PV technology, CdTe PV is developed and manufactured in the U.S. as the leading cost-competitive option for electricity generation in comparison to other PV technologies such as silicon and CIGS PV. However, CdTe PV faces major challenges that limit its achievable performance during the solar energy conversion process. It has become increasingly evident that to improve PV efficiency, an understanding of surfaces and interfaces is necessary. Therefore, high-fidelity quantum-based atomistic simulations will be used to calculate energy band alignment of CdTe thin film surfaces and interfaces to resolve the issues found in such problematic areas and advance PV efficiency. Ab initio simulation models implement density functional theory (DFT) coupled with Green's function (GF) for investigating the electronic and structural properties of thin film surfaces and interfaces within CdTe PV device configurations. Comprehensive studies on spatially-dependent energy band alignments with respect to plane orientation, terminated surfaces, carrier concentrations and elemental composition were computationally evaluated to determine their possible effect on CdTe solar cell device performance. A total of 14 unique CdTe-based surfaces and 3 different CdTe/Te interfaces were simulated to determine their effect on energy band alignment. A number of key insights were gained that include: 1) the band bending directions dictated by the termination layer based on surface theory; 2) the role of surface reconstruction in flattening the CdTe surface energy band alignments while neutralizing surface states due to the fulfillment of the electron counting rule; 3) the formation of a cusp energy potential feature along the CdTe{111} plane oriented energy band alignments as observed by external literature studies within the CdTe/Te interface. Results to date indicate that the DFT+GF atomistic modeling approach to constructing energy band alignments matches closer to experiments than conventional band alignment methods. State-of-the-art DFT+GF calculations on CdTe-based thin film surfaces and interfaces provide a methodology for studying quantum mechanical effects in thin film PV devices such as high-efficiency single junction CdTe solar cells and tandem solar cells.
Open Access
Design, simulation, and prototyping of wavelength-shifting plate light collector for a large water Cherenkov detector
(Colorado State University. Libraries, 2014) Johnston, William Albert, author; Buchanan, Norm, advisor; Wilson, Robert J., committee member; Sites, James, committee member; Menoni, Carmen, committee member
A wavelength-shifting plate light collector has been investigated for a proposed water Cherenkov detector for the Long-Baseline Neutrino Experiment. Experimental prototypes were fabricated from four different wavelength-shifting plastics and tested under uniform illumination as well as with a point source scanner. These laboratory tests were used to study the wavelength and position dependence of the plate's light collection. These results were then used to develop an optical model for the plates that was then used to estimate their effect on measuring neutrino events in the full water Cherenkov detector simulation. These results showed that it was possible to guide between 34% and 49% extra light to a 12" hemispherical PMT. In addition the plates were not found to adversely affect the particle identification abilities of the detector.
Open Access
Development of plasma cleaning and plasma enhanced close space sublimation hardware for improving CdS/CdTe solar cells
(Colorado State University. Libraries, 2012) Swanson, Drew, author; Williams, John, advisor; Sampath, W. S., advisor; Sites, James, committee member
A scalable photovoltaic manufacturing process that employs a heated pocket deposition technique has been developed at Colorado State University. It allows for the economical manufacturing of single-junction thin-film CdTe solar cells with efficiencies over 13%. New techniques that further increase cell efficiency and reduce production expenses are required to make solar energy more affordable. To address this need a hollow-cathode plasma source was added to the load-lock region of the CSU single-vacuum in-line CdTe-cell fabrication system. This plasma source is used to clean the transparent-conductive-oxide layer of the cell prior to the deposition of the CdS and CdTe layers. Plasma cleaning enables a reduction in CdS thickness by approximately 20 nm, while maintaining an improved cell voltage. Cell current was improved and cell efficiency was increased by 1.5%. Maps generated by scanning white-light interferometry, electroluminescence, and light-beam-induced current all show uniformity improvement with plasma cleaning treatment. To further increase cell efficiency a hollow-cathode plasma-enhanced close space sublimation (PECSS) source was utilized to modify the CdS window layer material as it was being deposited. This was done by integrating PECSS into the CSU inline CdS/CdTe-cell fabricating system and by sublimating the CdS semiconductor material through a plasma discharge. To date oxygenated CdS (CdS:O) cells have been grown by sublimating CdS through a PECSS source operated on oxygen. Data are presented showing that PECSS CdS:O films have increased the band gap of the window layer therefore reducing absorption loss, increasing cell current, and improving efficiency by 1.2%.
Open Access
DFT calculations for cadmium telluride (CdTe)
(Colorado State University. Libraries, 2019) Pochareddy, Sai Avinash, author; Walajabad, Sampath, advisor; Weinberger, Chris, advisor; Sites, James, committee member
Cadmium Telluride (CdTe) thin film photovoltaics (PV) has demonstrated low Levelized Cost of Energy (LCOE). CdTe technology also counted for half the thin film market in 2013 [3]. CdTe PV has the smallest carbon footprint a nd the energy payback time (less than one year) is the shortest of any current photovoltaic technology. The modules made of CdTe can also be recycled at the end of their lifetime. The attractiveness of these materials comes from their bandgap value (1.5 eV), which falls within the solar spectrum, thereby enabling the efficient creation of electron-hole pairs (or excitons) by solar photons. This has led to the research that dates back to 1950's and is currently ongoing in many parts of the world. A simple h eterojunction cell design was evolved in which p-type CdTe was matched with n-type Cadmium Sulfide (CdS) and by adding the top and bottom contacts. Today, multiple crystalline layers, of thicknesses ranging from a few nanometers (nm) to tens of micrometers (μm), are added to improve the efficiencies of the CdTe PV cells. The highest cell efficiency recorded to date is over 22%. Different computational tools and methods are used to study these effects, with Quantum ESPRESSO and VASP being used for many years now. QuantumATK, built in 2008 by the company Atomistix and acquired by Synopsys in 2017, is a simulation tool that uses Density Functional Theory (DFT) for atomistic-scale modelling of nanostructures. In this work, QuantumATK was used to predict the structural properties of bulk CdTe. Different exchange-correlation (XC) functionals were used to perform the calculations. Firstly, the crystal structure of bulk CdTe was predicted using the tool. Later the properties like lattice parameter, were calculated. In addition to structural properties, the electrical properties were also predicted using different XC functionals. Also, the XC functionals that correct the bandgap obtained from the standard functionals were used to predict the bandgaps and the results were also compared to the experimental values again to see how accurately does QuantumATK predicts the electrical properties of bulk CdTe. The LDA and GGA XC functionals, predicted the band gap for bulk CdTe with error percentages of 57% and 40% respectively, when compared to the experimental value. The more accurate MGGA predicted the band gap with a 24% error while HSE06 (hybrid functional) predicted within 4% of experimental value. The LDA-1/2 and GGA-1/2 predicted the band gap most accurately within 2% of the compared experimental value. All the different XC functionals predicted the crystal structure correctly and the lattice parameter was within 2.2% of the experimental value.
Open Access
Experimental and theoretical investigations of selenium graded cadmium telluride-based solar cells
(Colorado State University. Libraries, 2022) Shah, Akash, author; Sampath, Walajabad S., advisor; Weinberger, Chris, advisor; Sites, James, committee member; Martinez, Umberto, committee member
In the past few years, cadmium telluride (CdTe) based solar cells have emerged as an important photovoltaic (PV) technology for electricity generation. The optimal bandgap of the absorber material and high throughput manufacturing methods make CdTe PV a leading cost-competitive alternative to silicon solar cells. However, the performance of CdTe PV-devices strongly depends on the processing of the absorber layer. This work uses experiments in conjunction with two-probe atomistic models based on density functional theory (DFT) to understand the processing effects on the electronic properties of heterostructures in CdTe PV-devices and improve the efficiencies of CdTe solar cells. Various processing conditions such as cadmium chloride (CdCl2) treatment and selenium (Se) alloying of CdTe absorber layer were found to improve the PV-device efficiencies (from 0.22% to 18.3%). Atomistic simulation models utilized DFT coupled with Green's function to investigate the electronic properties of thin film surfaces, grain boundaries and interfaces within theCdSeTe/CdTe PV-device configuration. Structural and electronic properties of bulk CdSexTe1-x and CdSe0.25Te0.75 surfaces were calculated and compared with the experimental and theoretical literature to establish the modeling parameters. The results from the elemental characterization done on the CdCl2 treated CdSeTe/CdTe films formed the basis of respective CdSeTe/CdTe interface and grain-boundary atomistic models. The electronic properties calculated for Se-graded CdSeTe/CdTe interfaces showed a pick up in the conduction band (CB) energy level (creating electron reflector effect) and was verified experimentally by Ultraviolet Photoelectron Spectroscopy (UPS). DFT models also suggested that higher p-doping is required in CdSeTe-only absorber to achieve similar electron reflector effect at the CdSeTe/Te interface. The grain-boundary models further showed that presence of Se and Cl at the CdTe grain-boundary passivates the critical acceptor defect states and leads to n-type inversion at the grain-boundary. The band engineering and defect state passivation in the Cl and Se-alloyed CdTe solar cells efficiently extracted the charge carriers, thereby producing high-performing CdSeTe/CdTe PV-devices ( 19.4%). In comparison to unalloyed CdTe absorbers, Se alloying passivated the defect states in CdTe grain boundaries. It has also been observed that Alumina/CdSeTe/Alumina double heterostructures showed higher minority carrier lifetime indicating better passivation in the bulk and the oxide/absorber interface. The electronic properties calculated via DFT suggested Te/O interface formation at the CdSeTe/Alumina interface led to reduction in the interfacial defect states. This combination of lower density of defect states at the CdSeTe grain boundaries and CdSeTe/oxide interface produces high lifetime (430 ns) in the oxide/CdSeTe double heterostructures. Following this, TeOx was also used as the back passivation layer of unalloyed CdSeTe/CdTe bilayer to produce 19% efficient CdTe PV-devices. The introduction of TeOx as the potential passivation layer eliminates the requirement of copper doping in the CdTe PV-devices, thereby increasing the longevity of CdTe solar panels in the field.
Open Access
Exploring excited states of transition metal photocatalysts with time dependent density functional theory
(Colorado State University. Libraries, 2018) Nite, Collette M., author; Rappé, Anthony K., advisor; Shores, Matthew, committee member; Strauss, Steven, committee member; Sites, James, committee member
Advances in photocatalysis have led to a rise in interests in more sustainable chemistry. It has been shown that visible light can be harnessed through a photocatalyst to promote conventionally unfavorable chemical transformations. Most of these photoreactions rely on a rare metal photocomplex such as Ru(bpy)32+. However, in order to scale these reactions for industrial purposes, rare metals must be replaced with more earth abundant metals. First row transition metals provide an earth abundant alternative that open up new reaction pathways. Due to the differences between first and second and third row transition metals, catalytic design requires complex knowledge of the photophysics and photochemistry of the complex that is not easily obtained with experimental methods. Electronic structure methods can aid in catalytic design. Density functional theory (DFT) and time dependent density functional theory (TDDFT) are methods capable of calculating large molecular systems. TDDFT is a useful tool in studying excited states, providing excited state energies and intensities, probing the photochemistry of the system. However, DFT/TDDFT are by no means black box calculations, especially when calculating first row transition metal complexes with complicated spin state manifolds. Screening different metal ligand scaffolds requires a high level of benchmarking, ensuring functionals and basis sets are optimal for the given system. A higher level of analysis is required in order to go beyond the electronic spectrum to get at the vibronic character of a system. There is also a coupling between the protonation of a complex and the electronic excited state. Understanding the protonation effects of a system is very useful for tuning a catalyst to a given reaction. In addition, specific binding effects of a solvent must be understood in order to corroborate theoretical and experimental data. All of these factors must be considered when studying the character of metals and their relation to their ligand backbone. This dissertation highlights these issues associated with using TDDFT for photocatalytic development, and derives useful conclusions furthering the development of a first row transition metal photocatalyst.
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 member
The 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.
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 member
CdS/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.
Open Access
Investigation of the emissivity of graphite when it is coated with cadmium telluride (CdTe)
(Colorado State University. Libraries, 2015) Gummadala, Venkat Anirudh Karteek, author; Walajabad, Sampath, advisor; Kirkpatrick, Allan, committee member; Sites, James, committee member
Today, thin film CdTe solar cells are used commercially in many applications. Advances in this technology can aid in meeting increased energy demands. Research is focused on increasing the efficiency of solar cells. By creating more value with minimum environmental impact, Cadmium Telluride (CdTe) solar cells promise to have the potential to become one of the leading eco efficient energy solutions available in the photovoltaic (PV) market today. CdTe PV has been commercialized at the GW/year level. Many deposition systems for CdTe PV employ graphite structures and they get coated with CdTe during operation. The objective of this thesis is to investigate how the emissivity of the Photovoltaic production tool using graphite surfaces is affected when coated with CdTe at the operating conditions. According to the published literature graphite has an emissivity of 0.65-0.95 and CdTe has an emissivity ranging between 0.2 - 0.8 depending upon the radiation and wavelength range. The current study was done in three steps. In the first step, the reflection and emissivity of Graphite coated with CdTe was calculated using Kirchhoff's Law of Radiation. In the second step, graphite-CdTe layer stack was simulated using thin film optical software and reflection data for different thicknesses of CdTe coated graphite was calculated. In the third step, the Advanced Research Deposition system in the Materials Engineering Laboratory was used to deposit CdTe on graphite substrates using a Close Space Sublimation deposition process. The graphite substrates were made from material similar to those used in these applications. The substrates were purified to remove impurities. The CdTe thickness was measured using a Taylor Hobson Profilometer. Rate of heating and cooling of graphite was measured and compared to the rate of cooling of graphite substrates coated with CdTe to evaluate the emissivity. Results in all the three steps showed that CdTe coating had a negligible effect on the emissivity of graphite at the conditions simulating the production environment.
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 member
Commercial 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.
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 member
CdS/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.