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Browsing by Author "Sites, James R., committee member"

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    A study of oxide/CdTe interfaces for CdTe photovoltaics using atomistic modeling
    (Colorado State University. Libraries, 2021) Thiyagarajan, Aanand, author; Sampath, W. S., advisor; Weinberger, Christopher, advisor; Martinez Pozzoni, Umberto, committee member; Sites, James R., committee member; Popat, Ketul, committee member
    Solar photovoltaics (PV) has undergone a dramatic transformation over the past few decades and is now a widespread electricity generation source. Among currently existing PV technologies, the thin film sector led by cadmium telluride is the most promising. Cadmium Telluride (CdTe) PV has experienced unprecedented growth and is now a major commercial player. However, the field has a few challenges to overcome until it reaches its full potential. The focus of this study is the interface between the CdTe-based absorber and the front window layer. Traditionally, cadmium sulfide has been used as the window layer in such devices. At the Next Generation Photovoltaics (NGPV) center in Colorado State University, superior devices have been demonstrated using magnesium zinc oxide (MgxZn1-xO or MZO) as the window layer. This is attributed to the larger bandgap of MZO causing a pickup in the current and the open circuit voltage. A magnesium to zinc atomic ratio of 23:77 has shown optimal performance characteristics. Alloying CdTe with Se to form cadmium selenium telluride (CdSexTe1-x or CST) has resulted in further improvements. One way to determine the quality of an interface is to study the electronic band alignment at that interface. Existing band alignment models show only limited features and hence there is a need for a more sophisticated approach to investigate complex characteristics. This study uses atomistic modeling based on Density Functional Theory (DFT) to investigate certain structural and electronic properties of the oxide and the oxide/absorber interface. The technique solves for electronic structures of materials based on electron density and predicts the structural properties of materials to a high degree of accuracy. Electronic characteristics are determined using a semi-empirical method known as DFT-1/2. A mathematical formulation called Green's Function (GF) has been incorporated within the model to simulate device structures. The bulk properties of MZO such as lattice constant, band gap, band edges and electron effective mass are established and compared to experiment. Following this, the band alignment at the MZO/CdTe and MZO/CST interfaces is determined, along with band offsets and interface states. The influence of chlorine in the deposition process is also investigated. This work is the first of its kind to study the oxide-CdTe and oxide/CST interfaces using DFT+GF and provides new insights into the electronic characteristics at the interface. Bulk properties of the MZO match experimental reports. Termination chemistry plays a significant role in the band bending and in the presence of defect states at the oxide/absorber interface. Calculations indicate that a Mg/Zn-Te interface is energetically preferred, with experimental reports pointing to the same. Moreover, varying the magnesium composition in the MZO alloy affects the magnitude of the band offsets. The interface band alignment results are close to those seen experimentally. A small amount of chlorine may help alleviate interface defect states by chemical passivation, possibly due to the removal of dangling bonds.
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    CdTe alloys and their application for increasing solar cell performance
    (Colorado State University. Libraries, 2016) Swanson, Drew E., author; Sampath, W. S., advisor; Sites, James R., committee member; Williams, John D., committee member; Popat, Ketul, committee member
    Cadmium Telluride (CdTe) thin film solar is the largest manufactured solar cell technology in the United States and is responsible for one of the lowest costs of utility scale solar electricity at a purchase agreement of $0.0387/kWh. However, this cost could be further reduced by increasing the cell efficiency. To bridge the gap between the high efficiency technology and low cost manufacturing, a research and development tool and process was built and tested. This fully automated single vacuum PV manufacturing tool utilizes multiple inline close space sublimation (CSS) sources with automated substrate control. This maintains the proven scalability of the CSS technology and CSS source design but with the added versatility of independent substrate motion. This combination of a scalable deposition technology with increased cell fabrication flexibility has allowed for high efficiency cells to be manufactured and studied. The record efficiency of CdTe solar cells is lower than fundamental limitations due to a significant deficit in voltage. It has been modeled that there are two potential methods of decreasing this voltage deficiency. The first method is the incorporation of a high band gap film at the back contact to induce a conduction-band barrier that can reduce recombination by reflecting electrons from the back surface. The addition of a Cd1-xMgxTe (CMT) layer at the back of a CdTe solar cell should induce this desired offset and reflect both photoelectrons and forward-current electrons away from the rear surface. Higher collection of photoelectrons will increase the cells current and the reduction of forward current will increase the cells voltage. To have the optimal effect, CdTe must have reasonable carrier lifetimes and be fully depleted. To achieve this experimentally, CdTe layers have been grown sufficiently thin to help produce a fully depleted cell. A variety of measurements including performance curves, transmission electron microscopy, x-ray photoelectron spectroscopy, and energy-dispersive x-ray spectroscopy were performed to characterize these cells. Voltage improvements on the order of 50 mV are presented at a thin (1 μm) CdTe absorber condition. However an overall reduction in fill factor (FF) is seen, with a strong reduction in FF as the magnesium incorporation is increased. Detailed material characterization shows the formation of oxides at the back of CdMgTe during the passivation process. A CdTe capping layer is added to reduce oxidation and help maintain the uniformity of the CdMgTe layer. A tellurium back contact is also added in place of a carbon paint back contact, reducing the impact of the valance band offset (VBO) from the CMT. With the addition of the capping layer and tellurium back contact a consistent 50 mV increase is seen with improved FF. However this voltage increase is well below modeled Voc increases of 150 mV. CMT double hetero-structures are manufactured and analyzed to estimate the interface recombination at the CdTe/CMT interface. The CdTe/CMT interface is approximated at 2*105 cm s-1 and modeling is referenced predicting significant reduction in performance based on this interface quality. To improve interface quality by removing the need for a vacuum break, the deposition hardware is incorporated into the primary deposition system. Second, CdTe has a somewhat higher band gap than optimal for single-junction terrestrial solar-cell power generation. A reduction in the band gap could therefore result in an overall improvement in performance. To reduce the band gap, selenium was alloyed with CdTe using a novel co-sublimation extension of the close-space-sublimation process. Co-sublimated layers of CdSeTe with various selenium concentrations were characterized for optical absorption and atomic concentrations, as well as to track changes in their morphology and crystallinity. The lower band-gap CdSeTe films were then incorporated into the front of CdTe cells. This two-layer band-gap structure demonstrated higher current collection and increased quantum efficiency at longer wavelengths. Material characterization shows the diffusion of selenium through the CdTe during passivation resulting in improved in lifetime and a reduced voltage deficit at lower band gaps.
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    Characterization and modeling of CdCl2 treated CdTe/CdS thin-film solar cells
    (Colorado State University. Libraries, 2010) Maxwell, Graham Lane, author; Manivannan, Venkatesan, advisor; Sampath, W. S., committee member; Sites, James R., committee member
    CdTe photovoltaic technology has the potential to become a leading energy producer in the coming decades. Its physical properties are well suited for photovoltaic energy conversion. A key processing step in the production of high efficiency CdTe/CdS solar cells is a post-CdTe deposition heat treatment with CdCl2, which can improve performance by promoting CdTe recrystallization, QE response, defect passivation and others. Understanding the effects of the CdCl2 treatment is crucial in order to optimize processing conditions and improve performance. This study investigates the effects of variations of CdCl2 treatment duration on CdTe/CdS solar cells manufactured at Colorado State University. In order to investigate the optimal time of CdCl2 treatment, sample solar cells were tested for microstructural and performance properties. Device microstructure was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Device performance was analyzed using current density-voltage (J-V) measurements, time-resolved photoluminescence (TRPL), quantum efficiency (QE), and laser beam induced current (LBIC) measurements. Little change in microstructure was observed with extended CdCl2 treatment and is attributed to the high CdTe deposition temperatures used by heat pocket deposition (HPD). This deposition technique allows for large initial grains to be formed with low lattice strain energy which prevents recrystallization and grain growth that is often seen with other deposition techniques. The CdCl2 treatment initially improves performance significantly, but it was shown to that extending the CdCl2 treatment can reduce performance. Overall performance was reduced despite an increase in minority carrier lifetime values. The mechanism of reduced performance is suggested to be the formation of a low bandgap CdTe layer resulting from sulfur diffusion from the CdS layer. Sulfur diffusion primarily occurs during the CdCl2 treatment and also leads to thinning of the CdS layer. Solar cell modeling was employed to investigate possible mechanisms for performance degradation. Modeling was done with AMPS and SCAPS modeling software. Models were created to investigate the effects of minority carrier lifetime, CdS thickness, and a low bandgap CdTe layer. Modeling results showed that the formation of a low bandgap CdTe layer combined with CdS thinning reduces device performance. Further research is needed using a statistically significant number of samples to investigate other possible degradation mechanisms associated with extended CdCl2 treatment.
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    Defect tolerance, anharmonicity, and organic-inorganic coupling in hybrid organic-inorganic semiconductors
    (Colorado State University. Libraries, 2018) Maughan, Annalise E., author; Neilson, James R., advisor; Prieto, Amy L., committee member; Reynolds, Melissa M., committee member; Sites, James R., committee member
    Implementing and improving sustainable energy technologies is predicated upon the discovery and design of new semiconducting materials. Perovskite halides represent a paradigm shift in solar photovoltaic technologies, as devices utilizing perovskites as the active semiconductor can achieve power conversion efficiencies rivaling those of commercial solar cells after less than a decade of dedicated research. In contrast to conventional semiconductors, perovskites are unique in that they exhibit excellent photovoltaic performance despite the presence of significant materials disorder. This disorder manifests as (1) a large concentration of crystallographic defects introduced by low-temperature processing, and (2) as dynamic disorder due to the deformable metal-halide framework and the presence of dynamic organic species within the crystalline voids. Vacancy ordered double perovskites of the general formula A2BX6 are a defect-ordered variant of the archetypal perovskite structure comprised of isolated [BX6] units bridged by cationic species at the A-site. The presence of ordered vacancies and relatively decoupled octahedral units presents an ideal system to investigate defects and lattice dynamics as they pertain to optical and electronic properties of perovskite halide semiconductors. This work aims to illuminate the fundamental structure-dynamics-property relationships in vacancy-ordered double perovskite and hybrid organic-inorganic semiconductors through a combination of advanced structural characterization, optical and electrical measurements, and insight from computation. We begin with a study of the Cs2Sn1-xTexI6 series of vacancy-ordered double perovskites to inform the chemical and bonding characteristics that impact defect chemistry in vacancy-ordered double perovskites. While the electronic properties of Cs2SnI6 are tolerant to the presence of crystallographic defects, introducing tellurium at the B-site yields an electronic structure that renders Cs2TeI6 defect-intolerant, indicating the importance of the B-site chemistry in dictating the optoelectronic properties in these materials. Next, we elucidate the interplay of the A-site cation with the octahedral framework and the subsequent influence upon lattice dynamics and optoelectronic properties of several tin-iodide based vacancy-ordered double perovskites. The coordination and bonding preferences of the A-site drive the structural and dynamic behavior of the surrounding octahedra and in turn dictate charge transport. A-site cations that are too small produce structures with cooperative octahedral tilting, while organic-inorganic coupling via hydrogen bonding yields soft, anharmonic lattice dynamics characterized by random octahedral rotations. Both regimes yield stronger electron-phonon coupling interactions that inhibit charge transport relative to undistorted analogs. The final study presented here details the discovery of two hybrid organic-inorganic semiconductors containing the organic tropylium cation within metal iodide frameworks. In C7H7PbI3, the tropylium electronic states couple to those of the lead iodide framework through organic-inorganic charge transfer. Electronic coupling between the organic and inorganic sublattices within a singular material provides an avenue to elicit unique optical and electronic properties unavailable to either components individually. The above work is then placed in context of other recent studies of vacancy-ordered double perovskite semiconductors, and a set of design principles are constructed. Future avenues of research are proposed. These structure-dynamics-property relationships represent an important step towards rational design of vacancy-ordered double perovskite semiconductors for potential optoelectronic applications.
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    Development of Cd1-xMgxTe thin films for application as an electron reflector in CdS/CdTe solar cells
    (Colorado State University. Libraries, 2014) Kobyakov, Pavel S., author; Sampath, W. S., advisor; Sites, James R., committee member; Olsson, N. Anders, committee member; Williams, John D., committee member
    Efficiencies of CdS/CdTe photovoltaic cells significantly lag behind their theoretical limit, primarily because open-circuit voltage (VOC) of record efficiency cells (872 mV) is well below what is expected for the CdTe band gap (1.5 eV). A substantial VOC improvement can be achieved through addition of an electron reflector (ER) layer to CdTe devices. The ER layer forms a conduction-band barrier that reflects minority-charge carriers (i.e. electrons in p-type CdTe) away from the back surface. Similar to back-surface fields in c-Si, III-V, and CIGS solar cells, the ER strategy is expected to reduce back-surface recombination and is estimated to increase CdTe VOC by about 200 mV based on numerical simulation. The presented research investigates the addition of a thin layer of wider band gap Cd1-xMgxTe (CMT) to achieve a CdTe ER structure. First, a novel co-sublimation process was developed for deposition of Cd1-xMgxTe thin films that demonstrates excellent experimental capabilities, commercial viability, and improved alloy control over other techniques. Next, the effects of processing on material properties of CMT deposition onto CdS/CdTe structures were investigated. It was discovered that substrate temperature during CMT deposition is a critical parameter for achieving uniform CMT film coverage on polycrystalline CdTe. Furthermore, CMT film growth was found to be epitaxial on CdTe where the CMT films retain the same microstructural features as the underlying CdTe grains. Despite film uniformity, significant Mg loss from the CMT film, oxide formation, and a reduction of the optical band gap was found after CdCl2-based passivation treatments. Preliminary process optimization found that band gap degradation can be minimized by utilizing MgCl2 in addition to CdCl2 as a treatment source material. Finally, development of CdS/CdTe/Cd1-xMgxTe electron reflector devices demonstrated a barrier behavior at high voltage bias and improved voltage when CdTe thickness is held below 1 μm. Additional electro-optical characterization and device modeling was used to understand the source of this device behavior. The results suggest the CdTe/Cd1-xMgxTe interface is likely free of detrimental electronic defects and the barrier behavior comes from a larger than expected valence band offset for the material system. Finally, future work to improve ER device performance is suggested.
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    Distinguishing homogeneous and heterogeneous water oxidation catalysis when beginning with cobalt polyoxometalates
    (Colorado State University. Libraries, 2013) Stracke, Jordan J., author; Finke, Richard G., advisor; Chen, Eugene Y.-X., committee member; Elliott, C. Michael, committee member; Ferreira, Eric M., committee member; Sites, James R., committee member
    Development of energy storage technologies is required prior to broad implementation of renewable energy sources such as wind or solar power. One of the leading proposals is to store this energy by splitting water into hydrogen and oxygen--that is, to store energy in chemical bonds. A major obstacle en route to this overall goal is the development of efficient, cost-effective water oxidation catalysts (WOCs). Due to the highly oxidizing environment needed to drive this reaction, one question which has arisen when dealing with homogeneous precatalysts is whether these precursors remain as intact, homogeneous WOCs, or whether they are transformed into heterogeneous metal-oxide catalysts. This problem, reviewed in Chapter II, addresses the methods and literature studies related to distinguishing homogeneous and heterogeneous water oxidation catalysts. Chapters III through V further develop the methodology for distinguishing homogeneous and heterogeneous water oxidation catalysis when beginning with the cobalt polyoxometalate [Co4(H2O)2(PW9O34)2]10- (Co4POM). In Chapter III, the investigation of Co4POM using electrochemical oxidation at a glassy carbon electrode reveals that under the conditions therein, an in-situ formed, heterogeneous cobalt-oxo-hydroxo (CoOx) material is the dominant catalyst and is formed from Co2+ leached from the Co4POM. In Chapter IV, investigation of whether the intact Co4POM could be a catalyst under other, more forcing conditions of higher electrochemical potentials and lower Co4POM concentrations is reported. Although the Co4POM shows different electrochemical properties relative to CoOx controls, the possibility that the Co4POM is being transformed into a meta-stable heterogeneous catalyst cannot be ruled out since the Co4POM degrades during the experiment. Lastly, Chapter V presents a kinetic and mechanistic study of the Co4POM when using a ruthenium(III)tris(2,2'-bipyridine) (Ru(III)(bpy)33+) chemical oxidant to drive the water oxidation reaction (i.e., rather than electrochemically driven oxidation). In this study, it was found that Co4POM catalyzes the oxidation of water as well as oxidation of the 2,2'-bipyridine ligand. In contrast, controls with in-situ formed CoOx catalysts more selectively promote the catalytic oxidation of water. The difference in reactivity and kinetics between the Co4POM and CoOx systems indicates that the active catalysts are fundamentally different when a chemical oxidant is employed. Overall, these studies demonstrate the need for careful experimental controls and highlight the importance which reaction conditions--in particular the source and electrochemical potential of the oxidant--can play in determining the active oxidation catalyst in water oxidation reactions.
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    Hot injection synthesis and characterization of copper antimony selenide non-canonical nanomaterials toward earth-abundant renewable energy conversion
    (Colorado State University. Libraries, 2018) Agocs, Daniel B., author; Prieto, Amy L., advisor; Buchanan, Kristen, committee member; Sambur, Justin, committee member; Sites, James R., committee member; Van Orden, Alan, committee member
    Renewable and carbon-free energy generation has become a critically important field as the global population continues to increase. Further, the ample supply afforded by natural resources such as sunlight and geothermal heat are attractive options that can be harnessed using technologies like photovoltaics and thermoelectrics. There is a growing interest in searching for novel materials that exhibit high efficiencies in these devices, ideally composed of earth abundant, non-toxic materials. This search is aided by theory, which has identified several families of compounds with interesting structure types that may exhibit properties amenable to incorporation in high efficiency devices. However, many of these materials have not yet been thoroughly evaluated for photovoltaics or thermoelectrics. This dissertation is focused on developing the synthesis and describing the basic characterization of nanoparticles of members of the compounds in the Cu-Sb-Se series, of which syntheses have been developed for Cu3SbSe4 and Cu3SbSe3 and are described in this dissertation. Herein, we describe a hot-injection route for the formation of Cu3SbSe4 and Cu3SbSe3 nanocrystals. In order to place this work in context, the first chapter of this dissertation provides a detailed summary of the literature investigating the Cu-Sb-Se family of compounds. Here, the highest thermoelectric efficiencies have been achieved for Cu3SbSe4 while Cu3SbSe3 is not yet comparable thermoelectrically to Cu3SbSe4 nor as efficient as the photovoltaic material CuSbSe2. The second chapter details the development of a hot injection synthesis of Cu3SbSe4 nanocrystals. In order for these materials to be applied as electronic materials in real devices, their stability and function under ambient conditions is of interest. Therefore, we studied the changes in electronic conductivity as a function of exposure to atmosphere. The conductivity increase was attributed to a hole mobility increase, and this was further correlated to structural oxidations. Chapter 3 details development of a synthesis for phase-pure Cu3SbSe3 nanodiscs. This material has become of interest recently for photovoltaic applications due to its acceptable band gap for solar absorption. While the synthesis of nanoscale Cu3SbSe3 has been reported, these results have not been reproduced, and property measurements among these limited works vary. Therefore, a robust synthesis was developed and initial optical and photoelectrochemical properties were measured and are reported in this dissertation that demonstrate photoactivity in thin films of the Cu3SbSe3 nanodiscs. In the fourth chapter, a more vigorous exploration of the nanodisc morphology observed in Cu3SbSe3 is reported. As a degree of self-assembly is observed in stacks of the nanodiscs, the morphology is investigated to understand how tuning nanocrystal morphology, size, and surface might affect the resulting particle interactions. To this end, a double injection synthesis was developed wherein the products exhibit optoelectronic properties similar to those of the original single injection reaction. Chapter 5 entails the electrochemical investigation of the copper antimony selenide nanostructures. Electrochemical measurements to experimentally elucidate the electronic structure are reported, and a photovoltaic architecture is proposed for a Cu3SbSe3-absorber layer device. Further, the presence of a thiol has been demonstrated to be critical to not only morphology within the Cu3SbSe3 synthesis but also the product phase formation. Therefore, initial measurements and challenges with in-situ electrochemical exploration of precursor reactivity are reported. Finally, chapter 6 briefly emphasizes the major findings within this dissertation. The experimental results for both Cu3SbSe4 and Cu3SbSe3 syntheses are reiterated. Further, additional directions for future work with this system are suggested. These primarily focus on fabrication of a Cu3SbSe3 photovoltaic cell to begin understanding photogenerated carrier transport. This can be extended through applying knowledge gained by understanding disc stacking to improve film deposition and electronic properties within Cu3SbSe3 materials. Finally, development of an electrochemical measurement system for use in oleylamine media would allow a new perspective on investigation of colloidal nanocrystalline formation. These proposed experiments would contribute to their respective fields in the broader context of expanding search criteria for novel photovoltaic materials, addressing the challenge of grain boundary recombination sites in photovoltaic nanocrystals, and providing tools for exploring nanoparticle synthesis.
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    Impact of thermal management on vertical-cavity surface-emitting laser (VCSEL) power and speed
    (Colorado State University. Libraries, 2011) Safaisini, Rashid, author; Lear, Kevin L., advisor; Marconi, Mario C., committee member; Reising, Steven C., committee member; Sites, James R., committee member
    Increasing the modulation bandwidth and output power of vertical-cavity surface-emitting lasers (VCSELs) are of great importance in a variety of applications such as data communication systems. The high temperature generated in the active region of VCSELs is one of the main limiting factors in achieving high power and high speed operation. This work is focused on investigating the effects of thermal management on improving AC and DC properties of VCSELs and achieving higher thermal performance devices. Thermal heatsinking is obtained by surrounding the VCSEL mesas with high thermal conductivity materials such as copper and also using passive heatsinking by flip-chip bonding the laser dies on a GaAs heat spreader. The research includes fabricating and characterizing 980 nm bottom-emitting and 670 nm top-emitting oxide-confined VCSELs. This dissertation is divided into three main parts: high-power, high-speed 980 nm VCSEL arrays, low thermal resistance 670 nm VCSELs, and temperature dependent dynamics of 980 nm VCSELs. Experimental work performed on fabricating and characterizing 980 nm, bottom-emitting, oxide-confined VCSEL arrays and single elements is presented first. The result of DC and AC characterization confirms the effectiveness of Cu electroplating of mesas and flip-chip bonding in reducing VCSELs' thermal resistance to obtain lower operating temperatures. Uniformity of frequency response and operating wavelength across the arrays also motivates managing thermal issues and is an indication of uniform distribution of current and heat flux on the array. This research resulted in record VCSEL arrays with frequency response of approximately 8 GHz and operating CW power of 200 mW. These 28-element, 18µm aperture diameter arrays represent the highest power reported for a VCSEL or VCSEL array with greater than 1 GHz modulation bandwidth. The second part of this dissertation details the fabrication steps and DC characterization of visible, 670 nm, top-emitting, oxide-confined VCSELs. Since achieving high operating temperatures is one of the main challenges in realizing improved red VCSELs, the effect of mesa heatsinking on improving their DC behavior using copper electroplating of mesas is studied. Thermal modeling of the copper plated VCSELs also facilitates better understanding and analysis of the experimental results. A photomask and process flow were designed to fabricate VCSELs with a variety of mesa diameters and inner and outer plating sizes to investigate the major direction of heat flow in the VCSELs and decrease VCSEL thermal resistance and thus increase the output power. Although copper plating significantly reduces thermal resistance, it did not substantially increase maximum operating temperature of the red devices and also put the mesas under stress that might not be desired. This study led us to analyzing the effects of stress on the VCSEL mesas which is induced by the copper films. Finally, the temperature dependence of 980 nm VCSEL dynamics is investigated using noise spectra measurement. This analysis provides some useful insights in understanding how temperature alters VCSEL properties and how these properties can be improved. A VCSEL with 7 µm aperture diameter was fabricated from the same epitaxial material and followed the same processing steps as the VCSEL arrays. Relaxation oscillation frequencies and damping factors as functions of bias current and stage temperature were extracted. These results along with the VCSEL DC measurement were used to estimate the laser differential gain as a function of temperature. The differential gain was shown to be relatively temperature independent over a temperature range of 10 °C to 70 °C with an average value of approximately 12×10-16 cm2. This research led us to the conclusion that improving the output power at elevated temperatures should yield better frequency response in this case. The VCSEL output power reduction was observed to be the major cause of bandwidth reduction at elevated temperatures for the device under test. This work is the first report on the measurement of temperature dependence of VCSEL dynamics.
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    Improving thin-film polycrystalline CdSeTe/CdTe solar cell efficiencies through statistical design of experiments
    (Colorado State University. Libraries, 2022) Lustig, Zachary F., author; Sampath, Walajabad S., advisor; Sites, James R., committee member; Popat, Ketul C., committee member
    In recent decades, cadmium telluride (CdTe) solar photovoltaic (PV) technology has become increasingly popular to meet global energy demands. Its high throughput industrial fabrication methods, low material usage, recyclability, longevity, and theoretical maximum efficiency have led to its widespread integration in the PV sector. Most of the CdTe PV research reported in literature utilizes one-factor-at-a-time (OFAT) experiments. This work leverages statistical design of experiments (DOE) and statistical analysis of data to study the relationships between multiple processing factors and solar cell performance metrics. OFAT only indicates the primary effect of the chosen variable, whereas DOE determines the primary effect as well as the interaction effects. DOE determines both critical and insignificant factors, whereas OFAT assumes everything is a critical factor. DOE also requires fewer experiments, has more sophisticated predictive capabilities, and streamlines process optimization in comparison to OFAT. Since DOE is most effective with large data sets, the unique high throughput capability of the Advanced Research Deposition System (ARDS) at Colorado State University makes our lab a perfect candidate to utilize DOE for CdTe solar cell research. In this study, DOE and statistical analysis were used to investigate copper (Cu) doping, electrode painting, absorber deposition rate and temperature, p-doping of CdSe0.4Te0.6 (CST40) through arsenic (As) incorporation and tellurium (Te) overpressure, and oxide deposition at the back of the cell. Multiple linear regression (MLR) and analysis of variance (ANOVA) were conducted on all DOE's. An improved process was identified for the baseline high efficiency Cu-doped solar cells in which total process time was reduced by 33%. A thick 6µm structure of 18.5%+ efficiency was developed following statistical model suggestions. A standard procedure for electrode painting was developed. As a result of DOE, several 19%+ cells were fabricated achieving the highest efficiency of 19.44%. The best performing As doped CST40 graded CdTe cells of 18.5%+ were also fabricated using these methods. Carrier concentration versus voltage plots indicated successful p-doping of CST40 with As. Annealing the absorber with cadmium arsenide (Cd3As2) and depositing tellurium oxide (TeOx) at the back of the cell improved performance, yielding 80%+ fill factors. Decreasing thickness of CdTe behind CST40:As increased short-circuit current density to 30 mA/cm2+. Lastly, thinner absorbers yielded higher performance when backed with NiO:Cu.
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    Investigation of Group V doping and passivating oxides to reduce the voltage deficit in CdTe solar cells
    (Colorado State University. Libraries, 2022) Danielson, Adam H., author; Sampath, W. S., advisor; James, Susan P., committee member; Popat, Ketul C., committee member; Sites, James R., committee member
    Thin film cadmium telluride is one of the most successful photovoltaic technologies on the market today. Second only to silicon in yearly output and accounting for 40% of U.S. utility-scale photovoltaic installation, CdTe is known for its ease of manufacture, ideal bandgap, and low levelized cost of energy. Despite its commercial success, CdTe underperforms compared to its theoretical potential. The current world record CdTe device is only 21.0% compared to a theoretical maximum of 33.1%. This significant discrepancy in efficiencies can mostly be attributed to the poor open-circuit voltage of CdTe devices. Compared to silicon technologies, CdTe has a large voltage deficiency, exceeding 250 mV. While copper doping has traditionally been used for CdTe devices, it has proven to be incapable of sufficiently doping CdTe. Copper typically dopes CdTe in the 1014 to 1015 holes/cm3 range where most models predict that 1016–1017 is needed. Additionally, interstitial copper is a fast diffuser in CdTe, and can lead to numerous stability issues. As an alternative to copper, this work explores arsenic as a dopant for CdTe. Using a novel arsenic doping technique, hole concentrations greater than 1015 cm-3, microsecond lifetimes, and increased radiative efficiency are achieved. These are important prerequisites to achieving higher voltages. Achieving high doping levels alone is not sufficient to achieve higher device performance. A well-passivated and carrier selective contact is needed to ensure that electron-hole pairs do not recombine and are extracted as useable energy. Aluminum oxide has been shown to passivate CdTe surfaces. This work illustrates the explorations of using Al2O¬3 as a passivation layer, pairing it with highly doped amorphous silicon as a hole contact, resulting in excess-carrier lifetimes up to 8 µs, the highest reported to date for polycrystalline Cd(Se)Te. Although the inclusion of arsenic doping and an aluminum oxide back contact are each explored separately, the combination of both methods result in massive improvements to the carrier lifetime, interface passivation and radiative efficiency. Through this combination, microsecond lifetime and External Radiative Efficiency of over 4% are achieved. The excellent ERE values measured here are indicative of large quasi-Fermi level splitting, leading to an implied voltage with multiple device structures of nearly 1 V and an implied voltage of 25%. Finally, while CdSeTe serves as a more promising photovoltaic absorber candidate compared to CdTe, certain difficulties remain which must be addressed. Careful selection of processing conditions is shown to create a dense and large-grained film while eliminating wurtzite-phase crystal growth, which has been shown to degrade device performance. Surprisingly, as-deposited CdSeTe is shown to be n-type or nearly intrinsic rather than the previously supposed p-type. This necessitates additional steps to account for very poor hole conductivity, which can produce zero-current devices if not addressed. Challenges notwithstanding, CdSeTe absorbers are shown to be a key component in devices capable of a photovoltaic conversion efficiency of greater than 25%.
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    Investigation of processing, microstructures and efficiencies of polycrystalline CdTe photovoltaic films and devices
    (Colorado State University. Libraries, 2017) Munshi, Amit Harenkumar, author; Sampath, W. S., advisor; Sites, James R., committee member; James, Susan P., committee member; Powell, Rick C., committee member; Holland, Troy B., committee member
    CdTe based photovoltaics have been commercialized at multiple GWs/year level. The performance of CdTe thin film photovoltaic devices is sensitive to process conditions. Variations in deposition temperatures as well as other treatment parameters have a significant impact on film microstructure and device performance. In this work, extensive investigations are carried out using advanced microstructural characterization techniques in an attempt to relate microstructural changes due to varying deposition parameters and their effects on device performance for cadmium telluride based photovoltaic cells deposited using close space sublimation (CSS). The goal of this investigation is to apply advanced material characterization techniques to aid process development for higher efficiency CdTe based photovoltaic devices. Several techniques have been used to observe the morphological changes to the microstructure along with materials and crystallographic changes as a function of deposition temperature and treatment times. Traditional device structures as well as advanced structures with electron reflector and films deposited on Mg1-xZnxO instead of conventional CdS window layer are investigated. These techniques include Scanning Electron Microscopy (SEM) with Electron Back Scattered Diffraction (EBSD) and Energy dispersive X-ray spectroscopy (EDS) to study grain structure and High Resolution Transmission Electron Microscopy (TEM) with electron diffraction and EDS. These investigations have provided insights into the mechanisms that lead to change in film structure and device performance with change in deposition conditions. Energy dispersive X-ray spectroscopy (EDS) is used for chemical mapping of the films as well as to understand interlayer material diffusion between subsequent layers. Electrical performance of these devices has been studied using current density vs voltage plots. Devices with efficiency over 18% have been fabricated on low cost commercial glass substrates with processes suitable for mass production. These are the highest efficiencies reported by any university or national laboratory for polycrystalline thin-film CdTe photovoltaics bettered only by researchers at First Solar Inc. Processing experiments are traditionally designed based on simulation results however in these study microscopic materials characterization has been used as the primary driving force to understand the effects of processing conditions. Every structure and efficiency reported in this study has been extensively studied using microscopic imaging and materials characterization and processing conditions accordingly altered to achieve higher efficiencies. Understanding CdCl2 passivation treatment out of this has been critical to this process. Several observations with regard to effect of CdCl2 passivation have allowed the use to this treatment to achieve optimum performance. The effects of deposition temperature are also studied in rigorous details. All of these studies have played an important role in optimization of process that lead to high efficiency thin-film CdTe photovoltaic devices. An effort is made in this study to better understand and establish a 3-way relationship between processing conditions, film microstructure and device efficiency for sublimated thin-film CdTe photovoltaics. Some crucial findings include impact of grain size on efficiency of photovoltaic devices and improvement in fill-factor resulting from use of thicker CdTe absorber with larger grain size. An attempt is also made to understand the microstructure as the device efficiency improves from ~1% efficiency to over 18% efficiency.
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    Investigations to improve CdTe-based solar cell open circuit voltage and efficiency using a passivation and selectivity theoretical framework
    (Colorado State University. Libraries, 2022) Reich, Carey, author; Sampath, Walajabad S., advisor; Holman, Zachary C., committee member; Kuciauskas, Darius, committee member; Sites, James R., committee member
    The voltage of CdTe-based solar cells has remained conspicuously low despite years of efforts focused directly on its improvement. The efforts here have been primarily in increasing the equilibrium carrier concentration of the CdTe or its alloys which are used to absorb the light. This direction has been guided by a theory of solar cells that views the cell only as a single p/n junction. The modelling which has been used to confirm this as an appropriate direction indicated that with a moderate carrier lifetime, relatively small front interface recombination velocity, and large equilibrium carrier concentration in the absorber, efficiencies greater than the current record of 22.1% will be possible with open circuit voltages reaching over 1V. However, cells with these properties have been measured and increases in Voc and efficiency have not been attained. In the c-Si community, notably, the "passivation – selectivity" framework has been developed. In particular, it rejects the view that a singular p/n junction is responsible for the function of a solar cell. Instead, this framework operates with the understanding that the potential in the cell which can be turned into useful electrical energy and an increase in open circuit voltage comes only from the excess carriers generated by sunlight forcing a deviation from the equilibrium condition. As such there are two main components: 1) passivation – which refers to the recombination behavior in the cell and development of a large internal potential difference and 2) selectivity – which refers to the asymmetry of conduction in the cell that allows for production of a unidirectional current and an external voltage approaching that within the cell. This framework tends to break the cell into 3, sometimes overlapping, regions: an absorber region that is used to produce as large a potential difference as possible, and two contact regions in which the transport properties are modified to prefer transport of one carrier or the other. Here this framework is applied to CdTe-based solar cells to determine what limits current cells and how to overcome these limitations. In the investigation of passivation, first the electron contact interface is evaluated, resulting in the determination that this interface is not currently limiting the recombination in the cell. As a result, the current baseline is compared to structures hypothesized to provide improvement in the recombination behavior. It is found that cells with CdSeTe as the only material in the bulk exhibit more ideal recombination behavior when compared to a CdSeTe/CdTe structure as is currently used. This comparison demonstrates a pathway for cells to overcome their current limitation due to recombination, with the possibility of reaching up to 25% efficiency and 970 mV Voc with the material that currently is produced at CSU. A native oxide of TeOx is found to passivate the surface, reducing the rate non-radiative recombination, and forms during dry air exposure, providing a pathway to passivate contacts that would be ideal if not for the recombination at the interface. In the investigation related to selectivity, the electron contact is evaluated and it is demonstrated that MgZnO is appropriately selective when deposited with the correct conditions. It therefore is expected that hole selectivity is the primary loss to open circuit voltage in structures determined to have longer excess carrier lifetimes and large radiative efficiencies. Efforts to investigate novel routes to hole selectivity by use of heterojunction contacts are presented. Such routes did not yield improvements in cell Voc and efficiency, and through this work it was determined that a major source of selectivity losses in these cells is the high resistance to hole transport through the bulk semiconductor. Increasing hole concentration or thinning the absorber provide pathways to overcome this specific limitation, but it is modelled that such cells will require structures with hole selective materials that internally cause a reduction of electron current to see improvement in Voc and efficiency.
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    Ion extraction from a plasma
    (Colorado State University. Libraries, 1980) Aston, Graeme, author; Kaufman, Harold R., advisor; Wilbur, Paul J., advisor; Fairbank, William M., Jr., committee member; Sites, James R., committee member
    An experimental investigation of the physical processes governing ion extraction from a plasma is presented. The screen hole plasma sheath of a multi-aperture ion accelerator system is defined by equipotential plots for a variety of accelerator system geometries and operating conditions. A sheath thickness of at least fifteen Debye lengths is shown to be typical. The electron density variation within the sheath satisfies a Maxwell-Boltzmann density distribution at an effective electron temperature dependent on the discharge plasma primary-to-Maxwellian electron density ratio. Plasma ion flow up to and through the sheath is predominately one dimensional and the ions enter the sheath with a modified Bohm velocity. Low values of the screen grid thickness to screen hole diameter ratio give good ion focusing and high extracted ion currents because of the effect of screen webbing on ion focusing.
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    Optimization of the front contact to minimize short-circuit current losses in CdTe thin-film solar cells
    (Colorado State University. Libraries, 2015) Kephart, Jason Michael, author; Sampath, W. S., advisor; Sites, James R., committee member; Williams, John D., committee member; McCamy, James M., committee member
    With a growing population and rising standard of living, the world is in need of clean sources of energy at low cost in order to meet both economic and environmental needs. Solar energy is an abundant resource which is fundamentally adequate to meet all human energy needs. Photovoltaics are an attractive way to safely convert this energy to electricity with little to no noise, moving parts, water, or arable land. Currently, thin-film photovoltaic modules based on cadmium telluride are a low-cost solution with multiple GW/year commercial production, but have lower conversion efficiency than the dominant technology, crystalline silicon. Increasing the conversion efficiency of these panels through optimization of the electronic and optical structure of the cell can further lower the cost of these modules. The front contact of the CdTe thin-film solar cell is critical to device efficiency for three important reasons: it must transmit light to the CdTe absorber to be collected, it must form a reasonably passive interface and serve as a growth template for the CdTe, and it must allow electrons to be extracted from the CdTe. The current standard window layer material, cadmium sulfide, has a low bandgap of 2.4 eV which can block over 20% of available light from being converted to mobile charge carriers. Reducing the thickness of this layer or replacing it with a higher-bandgap material can provide a commensurate increase in device efficiency. When the CdS window is made thinner, a degradation in electronic quality of the device is observed with a reduction in open-circuit voltage and fill factor. One commonly used method to enable a thinner optimum CdS thickness is a high-resistance transparent (HRT) layer between the transparent conducting oxide electrode and window layer. The function of this layer has not been fully explained in the literature, and existing hypotheses center on the existence of pinholes in the window layer which are not consistent with observed results. In this work numerous HRT layers were examined beginning with an empirical optimization to create a SnO₂-based HRT which allows significantly reduced CdS thickness while maintaining diode quality. The role of this layer was explored through measurement of band alignment parameters via photoemission. These results suggest a negative correlation of work function to device open-circuit voltage, which implies that non-ideal band alignment at the front interface of CdTe is in large part responsible for the loss of electronic quality. Several scenarios explored through 1-dimensional modeling in the SCAPS program corroborate this theory. A sputter-deposited (Mg,Zn)O layer was tested which allows for complete elimination of the CdS window layer with an increase in open-circuit voltage and near complete transmission of all above-bandgap light. An additional window layer material--sputtered, oxygenated CdS--was explored for its transparency. This material was found only to produce high efficiency devices with an effective buffer layer such as the optimized SnO₂-base HRT. The dependence of chemical, optical, electrical, and device properties on oxygen content was explored, and the stability of these devices was determined to depend largely on the minimization of copper in the device. Both sputter-deposited alloy window layers appeared to have tunable electron affinity which was critical to optimizing band alignment and therefore device efficiency. Several scenarios explored through 1-dimensional modeling in the SCAPS program corroborate this theory. Both window layers allowed an AM1.5G efficiency increase from a baseline of approximately 13% to 16%.
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    Passivation studies on Cd0.6Zn0.4Te films using CdCl2, MgCl2 and ZnCl2 for top cell application in a multijunction solar cell
    (Colorado State University. Libraries, 2018) Shimpi, Tushar M., author; Sampath, W. S., advisor; Sites, James R., committee member; Kota, Arun K., committee member; Popat, Ketul C., committee member
    Passivation treatment with the chloride compounds is an important step in the fabrication of II-VI solar cells for improving the device performance. In cadmium telluride solar cells, cadmium chloride passivation treatment incorporates chlorine along the grain boundaries and helps in recrystallization, grain growth, removal of stacking faults and doping grain boundaries as n-type. In cadmium zinc telluride solar cells, the retention of zinc after the cadmium chloride passivation treatment is one of the challenges incurred in fabricating the top cell in a multijunction solar cell. During the passivation treatment, the loss of zinc occurs in the form of volatile zinc chloride compound. The depletion or complete loss of zinc reduces the higher band gap ternary alloy into lower band gap binary compound of CdTe. This impedes the purpose of fabricating a high band gap top cell in a multijunction solar cell. The focus of this study is on passivating Cd0.6Zn0.4Te (CdZnTe) films using three different chloride compounds separately and understanding the effects by studying the material properties of the passivated films and electrical performance of the fabricated devices. In the preliminary experiments, CdZnTe films were deposited by RF sputtering from a single target. Initial characterization of CdZnTe films deposited on plain glass indicated that the films had a strong preferred orientation along {111} plane with a band gap of ~1.72eV. In the cadmium chloride passivation treatment, loss of zinc from the surface and no chlorine along the grain boundaries was observed from transmission electron microscope images and X ray diffraction measurements. No loss of zinc was observed after the magnesium chloride and zinc chloride passivation treatments. Increase in the grain size of the CdZnTe films after magnesium chloride treatment and decrease of the preferred orientation after zinc chloride treatment were the benefits of the individual passivation treatments. Modifying the test structure by adding a cadmium telluride film as a capping layer on the back of RF sputtered CdZnTe and then carrying out the cadmium chloride passivation treatment helped in retaining the zinc. Heavy diffusion of zinc into cadmium sulphide due to cadmium telluride deposition at high temperature and difficulty to isolate the photo current generated by cadmium telluride were few drawbacks of this test structure. Based on the insights gained from the preliminary experiments, two sets of experiments were conducted. In the first set, cadmium sulphide cap as a barrier was deposited on the back of RF sputtered CdZnTe and co-sublimated cadmium telluride and zinc films with a band gap of 1.72 eV. The bulk composition was maintained after the cadmium chloride passivation treatment in the films deposited by both the methods. However the device performance of co-sublimated films was better than the RF sputtered CdZnTe devices. The transmission electron image obtained from the cross section of co-sublimated film fitted with cadmium sulphide cap and then treated with cadmium chloride showed presence of chlorine along the grain boundaries. The zinc chloride passivation treatments with higher substrate temperature compared to the source were the second set of experiments. The zinc loss from RF sputtered CdZnTe films after the cadmium chloride treatment did not occur. The fabricated devices exhibited diode like behavior. The images under scanning electron microscopy showed that the grain size did not increase after the zinc chloride treatment.
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    Photo-induced electron transfer in cu(i) bis-phenanthroline based assemblies. Part I: Chromophore-acceptor diads. Part II: Donor-chromophore-acceptor triads
    (Colorado State University. Libraries, 2013) Lazorski, Megan, author; Elliott, C. Michael, advisor; Shores, Matthew P., committee member; Chen, Eugene, committee member; Bailey, Travis S., committee member; Sites, James R., committee member
    The photophysical behavior of [Cu(I)P2] (P=2,9-disubstituted-1,10-phenanthroline ligands) in donor-chromophore-acceptor (D-C-A) triads and chromophore-acceptor (C-A) diads is a complex and fascinating area of under developed, yet fundamental, electron transfer chemistry. In metal polypyridyl D-C-A and C-A triads/diads, metal polypyridyl chromophores (C) in which the polypyridyl ligands are covalently linked to acceptor (A) and/or donor (D) moieties, photo-excitation of the chromophore initiates a series of electron transfer events that result in the formation of a charge separated (CS)/charge transfer (CT) state, respectively. The majority of high-performing metal polypyridyl D-C-A/C-A complexes, on which [Cu(I)P2] D-C-A/C-A research is based, incorporate ruthenium (as [Ru(II)L3] where L=polypyridyl ligand) or other rare, expensive, and sometimes toxic metals such as osmium, rhenium and platinum. Although [Ru(II)L3] D-C-A/C-A's have historically set the benchmark for metal polypyridyl D-C-A/C-A performance, it is clear that these complexes are not a practical choice if D-C-A's or C-A's were incorporated into a device for large scale production. However, bisphenanthroline complexes of copper, a much more earth abundant, cheaper and less toxic metal, exhibit very similar photophysical properties to [Ru(II)L3] and have thus gained recognition as promising new materials for D-C-A/C-A triad/diad construction. In order to understand the electron transfer (ET) events occurring in [Cu(I)P2] D-C-A/C-A triads/diads, a complex must be synthesized that is capable of forming a CS with high quantum efficiency (Φcs/ct) and a long CS/CT lifetime (τcs/ct). Therefore, the intent of the research reported herein is to synthesize novel, yet functional heteroleptic [Cu(I)P2] D-C-A/C-A triads/diads and study their fundamental, photo-initiated electron transfer chemistry, specifically the formation of a CS/CT state. Many challenges, which are not present for [Ru(II)L3], make the design and synthesis of [Cu(I)P2] D-C-A/C-A assemblies an art in itself. Therefore, a significant amount of effort was spent on fabricating ligand architectures that (1) are appended with acceptor and/or donor moieties capable of being reduced/oxidized resulting in the formation of a CS/CT, (2) are able to be easily modified so the amount of energy stored in the CS/CT can be tuned, (3) favor the self-assembly of [Cu(I)P2] complexes, (4) are able to facilitate processes that maximize the Φcs/ct. Once the ligands were obtained, the complexation equilibria behavior of these [Cu(I)P2] triads and diads were studied. Despite efforts to design ligand architectures that favor heteroleptic formation, the thermodynamic driving force for heteroleptic D-C-A triad formation is less favor-able than expected. Thus, mixing stoichiometric quantities of D, C and A results in a statistical mixture of C-A, C-D and D-C-A products. Furthermore, since the ligands are labile and will re-arrange to the most thermodynamically stable configuration of products when these complexes are dissolved, isolation of the D-C-A product is impossible. However, recent advances in ligand design have shown promise for resolving this on-going issue. Despite having a mixture of products with the D-C-A, the electron transfer processes of the [Cu(I)P2] D-C-A triads and C-A diads were investigated. Using Transient Absorption (TA) laser spectroscopy, the CT state in the constructed C-A diads and the CS state in the D-C-A triads were detected and the lifetimes were determined. However, it was found that those lifetimes could be modulated to a small degree by solvent in the C-A diads (c.a. 6x longer in polar solvents), and drastically via the application of a magnetic field in D-C-A triads (c.a. 60x longer). The ability to modulate the lifetimes enabled the deconvolution of the effects due to the C-A diad vs D-C-A triad in the statistical product mixtures. Although the response in a magnetic field was a somewhat expected result, as similar effects occur in the [Ru(II)L3 D-C-A/C-A's, the magnitude of change in the lifetime and the quantum efficiency offers new insight into the electron transfer events that occur in the CS/CT formation process for [Cu(I)P2] D-C-A/C-A complexes.
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    Pinholes and morphology of CdS films: the effect on the open circuit voltage of CdTe solar cells
    (Colorado State University. Libraries, 2012) Tashkandi, Mohammed Abdulaziz, author; Sampath, W. S., advisor; James, Susan P., committee member; Sites, James R., committee member; Olsson, Anders, committee member
    Cadmium telluride (CdTe) solar cells are among the many types of solar cells that have the potential to harness solar energy. CdTe has a band gap of ~1.5 eV that very closely matches the spectrum of the sun. In addition, being a thin film solar cell, the entire thickness of the solar device is a few microns and the energy required to manufacture thin film solar cells is much less than some of the more widely used solar cells. Nevertheless, CdTe solar cells lag behind solar cells of similar band gap materials in open circuit voltage. This voltage deficit can be attributed to many factors among which perfecting the window layer material can be a very important key. The best window layer material in CdTe solar cells was found to be cadmium sulfide (CdS). Usually thick CdS layers on the order of 125nm are used to ensure that the voltage of the solar device is as high as possible, this thickness causes some photons to be absorbed in the window layer and thus reduce the photocurrent output of the solar device and consequently its efficiency. The remedy is then to deposit thin CdS layers, as a result, the photocurrent is increased but the open circuit voltage of the device (VOC) tends to decrease especially when the thickness of the deposited CdS film is less than 80nm. The reduction of VOC as the CdS thickness is reduced may be attributed to discontinuities and defects in the window layer material. Such defects and discontinuities that go through the entire thickness of the CdS film expose the underlying Transparent Conductive Oxide (TCO) surface and thus allow the formation of weak CdTe/TCO diodes that are known to reduce the voltage output of the device. These defects and discontinuities are otherwise known as pinholes. Pinholes can be either of natural or artificial origin. Natural sources of pinholes include CdS grain coalescence and TCO surface roughness and artificial sources include scratches, scuffing marks, cleaning residues and dust and particulates in open lab environment. There has been no detailed study that discussed the following: (i) whether these sources of pinholes can be eliminated especially in CdS films deposited via closed space sublimation, (ii) whether these pinholes are actually the reason why CdTe solar cells made with thin CdS layers have less open circuit voltage, and (iii) estimate the size effect of pinholes in CdTe solar cells, i.e., how large an area of the device is affected compared to the size of pinholes. This study focused on studying CdS films of different thicknesses deposited on TEC10 glass substrates cleaned with different cleaning methods. These films were then surveyed for pinholes using Blue-Light Transmission Optical Microscopy for pinhole observation and analysis of the artificial sources of pinholes. The natural sources of pinholes were analyzed and studied via Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). It was possible to determine the size effect of pinholes by combining images obtained from Electroluminescence (EL) as well as Light Beam Induced Current scans (LBIC). In addition, computer models and simulations using PSpice® and MATLAB® allowed further studying the effects of varying the area of pinholes on the open circuit voltage and compare such results to literature as well as identifying possible pinhole area limits via diode voltage profiles. The results indicated that natural sources of pinholes are not major sources of pinholes in CdS films deposited via closed space sublimation. Cleaning residues was found to be the major source of pinholes in these CdS films. Also, cleaning the glass substrates with plasma prior to CdS film deposition is the key to significantly reduce pinholes in CdS films of thicknesses between 50nm and 200nm. Moreover, cleaning the glass substrates within a class 1 mini-environment did not reduce pinholes in CdS films due to the quality of the cleaning process inside such environment. Nevertheless, maintaining cleaned glass substrates in such environment may help reduce pinholes in CdS films deposited on glass substrates cleaned by standard cleaning or plasma cleaning. On the other hand, it was also found out that pinholes could affect an area that is a much as 15 times larger. The PSpice® and the MATLAB® models showed acceptable agreement with literature findings. Finally, diode voltage profile constructed via PSpice® simulations indicated that a total pinhole area corresponding to 0.001% of the total device area has negligible effects in terms of number of diodes being affected in the solar cell and a corresponding VOC loss of about 30mV.
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    Synthesis and evaluation of fluorous polycyclic aromatic hydrocarbon derivatives for organic electronics
    (Colorado State University. Libraries, 2019) Rippy, Kerry C., author; Strauss, Steven H., advisor; Prieto, Amy L., committee member; Sites, James R., committee member; Szamel, Grzegorz, committee member
    Advances in the performance of electron acceptor materials for organic electronics critically depend on the efficiency of synthetic routes for new materials and fundamental understanding of the correlation between molecular structure and electronic and solid-state properties. The research presented here endeavors to address both of these needs, by developing original methods for synthesis of new organic electron acceptor materials, and by characterizing relevant properties of the resulting materials. In total, synthesis and analysis of more than 60 new molecules is presented in this work. These molecules are derivatives of polycyclic aromatic hydrocarbons (PAHs) and hetero-PAHs functionalized with fluorous moieties, synthesized via development of substrate-specific efficient, single-step direct-substitution methods. Investigation of solid-state and electronic properties focused upon effects of structural motifs including (i) the type, number, and position of electron withdrawing fluorous substituents (ii) the size and shape of aromatic π systems, and (iii) presence of hetero atoms within the aromatic core. The first chapter of this work details the development and optimization of a gas-phase radical reaction between the perfluoroalkyl diiodide 1,4-C4F8I2 and the PAH triphenylene (TRPH). The perfluoroalkyl diiodide, with two C–I bonds, one at either end, has the unique ability to bind to vicinal C atoms, forming a 6-membered ring. A family of TRPH derivatives functionalized with such rings was synthesized, and the reaction was optimized. Additionally, reductive partial defluorination of the perfluoroalkyl ring was achieved, leading to aromatization of the fluorous substituent (RD/A). The extension of the π-system, as well as the effect of fluorine atoms bound directly to the aromatic system, was examined with respect to solid-state packing and electronic levels. In Chapter 2, results of screening of 13 new PAH and n-hetero PAH substrates with respect to their reactivity towards 1,4-C4F8I2 are described. Pure compounds derived from these reactions are presented, adding several new families to the library of fluorous PAH derivatives. Unique reactivities and interesting potential applications are discussed for several of these families. Solid-state packing and electronic properties are analyzed for selected derivatives. A particularly promising family of fluorous acceptors is presented and analyzed in greater depth in Chapter 3. It is based on the substrate phenazine (PHNZ). This family of molecules is notable because several derivatives exhibit enhanced acceptor strength and linearly-fused molecular structures resembling the acene class of PAHs, a high performing class of materials widely used in organic electronics. Results suggest that the molecules investigated in this chapter would be suitable for applications as air-stable n-type semiconductors in electronic devices. Finally, in Chapter 4, the characterization of a family of trifluromethylated acridine (ACRD) derivates is described. This investigation yields new insights into the reactivity of ACRD. Furthermore, detailed structural, spectroscopic and electronic property analysis combined with computational data revealed that not only the number of substituents, but also the position of substituents affects electronic energy levels. This finding not only expands basic understanding of how molecular structure affects electronic properties of PAHs, but also provides a valuable new tool for molecular design of acceptors with desirable properties.
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    Understanding the amide-assisted synthesis and olivine structure-directed twinning of Fe₂GeS₄ nanoparticles
    (Colorado State University. Libraries, 2020) Miller, Rebecca Caroline, author; Prieto, Amy L., advisor; Shores, Matthew P., committee member; Sites, James R., committee member; Ackerson, Christopher J., committee member
    The reality of detrimental anthropogenic effects on the environment requires the development of a number of sustainable practices and technologies. The Prieto Group strives to advance the synthesis and understanding of materials for use in energy conversion and storage. Advances in computational solid-state chemistry have resulted in the identification of a number of earth-abundant, relatively non-toxic compounds as promising photovoltaic absorber materials. However, the synthesis of solids remains a step behind, requiring empirical exploration of precursors and conditions. As reaction intermediates and mechanisms are discovered, general synthetic strategies can be translated from one material system to the next. Inorganic nanoparticle (NP) syntheses rely on the interdisciplinary expertise of solid-state, organometallic, and organic chemistry and show interesting complexity. The work herein has advanced the understanding of amide-assisted NPs syntheses and examined the microstructure of twinned Fe2GeS4 NPs. Chapter 1 presents a history of solution-based, amide-assisted NP reactions. As scientists understand the in situ speciation of precursors, more efficient reactions can be designed. This understanding allows the use of more benign and safe (both in terms of human and environmental) precursors and provides higher synthetic control over the end products. The presence of amide bases has generally provided access to higher NP nucleation rates and accessed smaller, more monodisperse particles. The increased monomer reactivity has also allowed the formation of ternary NPs free from binary or unary impurities by balancing the reactivity of cations of different valency. The most common amide base is LiN(SiMe3)2, and I relate this field to the use of its conjugate acid, hexamethyldisilazane or HMDS, in NP syntheses. Its addition has aided the production of NPs, but its chemical role remains unclear. This chapter was written utilizing a portion of an invited review paper written by myself, Jennifer M. Lee, Lily J. Moloney, and Amy L. Prieto in the Journal of Solid State Chemistry (2019, 273, 243-286.). Section 2.2 of the review outlined the evolution of understanding of amide-assisted NP syntheses and was adapted and expanded upon herein. In Chapter 2, I report the redesign of a Fe2GeS4 NP synthesis. In 2013, the Prieto group was the first to report a NP synthesis for the compound, which had been predicted to be a promising photovoltaic absorber material in 2011. The original reaction relied on HMDS as an additive and employed the highly-reactive S precursor, hexamethyldisilathiane. Herein, I speculate on these precursors' roles and exchange their use for LiN(SiMe3)2 and S powder, eliminating the formation of an Fe1–xS intermediate and reducing the growth time from 24 h to 10 min. I thoroughly map the reaction landscape of this system and provide structural, compositional, and optical characterization of the particles. This work was published in the Journal of the American Chemical Society (J. Am. Chem. Soc. 2020, 142 (15), 7023–7035.). The Fe2GeS4 NPs show an interesting star-shaped morphology, so I examine the microstructure via electron microscopy and identify the presence of crystal twinning in Chapter 3. The particles exist as three sets of stacked nanoplates intersecting at 60˚ angles, which forms a triplet of twins or trillings. In the products, 98% of the particles are twinned. Because crystal twinning, and especially trilling formation, in macroscopic crystals is rare, a synthetic route to a massive collection of twinned particles stands as a valuable resource for understanding the fundamentals of crystal twinning in olivine compounds. I relate the twinning to the underlying hexagonal pseudosymmetry of the orthorhombic, olivine crystal structure. Because of the ratio of the unit cell dimensions (a_Pnma/b_(Pnma )≈√3), the compound is susceptible to forming twins with growth of the [010] direction off the {310} faces. This can occur for other olivine compounds of similar unit cell dimension ratios, so I rank all of the olivine compounds listed in the Inorganic Crystal Structure Database according to this metric in Appendix A. This chapter is a manuscript prepared for submission. Finally, Chapter 4 outlines our recommendations for future work to advance the understanding of amide-assisted NP syntheses and translate this synthetic system to other compounds. I suggest the systematic development of SnS NP reactions utilizing each of the precursors: Sn silylamide, alkali silylamides, and HMDS. I outline a set of complementary techniques to characterize the reaction intermediates and mechanisms. This type of investigation has been done by the Kovalenko group for the formation of unary Sn0 NPs, but the interaction of the chalcogen species remains unknown. Further, no systematic mechanistic study exists for the use of HMDS in NP synthesis. This work would advance the understanding and use of amide-assisted syntheses for all metal chalcogenide compounds. In addition, I present preliminary data in our extrapolation of the Fe2GeS4 NP synthesis to the following solid solutions: Fe2GeS4–xSe (including the end member Fe2GeSe4) and Fe2–xMnxGeS4. One composition of each solid solution was formed and characterized by powder X-ray diffraction, and I present electron microscopy to show twinning in the Fe2GeS4–xSex (x = 0.96, 24 mol% Se) NPs. Lastly, I consider the possibility for twinning in an important olivine compound for battery science, LiFePO4, which is a common cathode material. The crystal structure shows a high degree of hexagonal pseudosymmetry, indicating that the energetics of forming twin domains may be favorable. I discuss the possible ramifications this may have on battery cycling performance. Thus, the scope of this work focuses on one compound, Fe2GeS4, but investigation into its synthesis and microstructure has opened a number of avenues for promising research. This compound itself presents a promising material for both photovoltaic and thermoelectric energy conversion, and the syntheses herein provide a launching point for property measurement and application evaluation. Further, the general examination of twinning in olivine compounds identifies questions for evaluating the function of other compounds useful for a number of applications. Lastly, analogous calculations to the geometrical evaluation done for orthorhombic olivine compounds could be carried out for other crystal structure types with unit cells that exist close to higher orders of symmetry. The advances presented herein on understanding the reactivity and roles of NP precursors are fundamental for progressing the field of NP synthesis. The reproducible formation and structural characterization of these twinned NPs provide a promising system for future explorations in crystal twinning and its effect on material properties.