Browsing by Subject "Photovoltaics"
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Item Open Access First-Principles Studies of Electronic, Optical and Defect Properties of Photovoltaic Materials(2019) Zhu, TongThe development of the technology depends heavily on the development of materials. However, how to select the best materials for a specific purpose — i.e. materials selection, is a tricky problem in academia, industry, and our daily lives. Recently, because of the rapid development of computers, ab initio theoretical calculations can be used to aid in materials selection. However, since many approximations in the theoretical calculations exist, choosing appropriate approximations to obtain accurate and predictable materials properties is still difficult. This is the main focus of this thesis. More specifically, we will focus on the materials selection for photovoltaics, which plays a significant role in the energy field today. While modern commercial thin-film PV cells, e.g., based on metal chalcogenide zinc-blende-type materials (Cu(In,Ga)(S,Se)2 (CIGSSe), CdTe) suffer from problems like relying on elements that are either toxic or rare in the earth’s crust, a recent alternative candidate based on kesterite Cu2ZnSn(S,Se)4 (CZTS) peaked at relatively low efficiencies (12.6%) due to the limited open circuit voltage (Voc) caused by the prevalent anti-site structure disorder (e.g. Cu on Zn, Zn on Cu). A possible path forward to reduce this antisite disorder is to pursue materials in which the Cu/Zn combination is replaced by elements that are chemically less similar but that retain the same valence. Recently, Cu2 BaSnS4 ́x Sex (CBTSSe) materials with a trigonal structure (space group P31 ) and composed of only earth abundant metals have been proposed and demonstrated as emerging PV absorbers to address the above issues of CZTSSe. Results obtained as part of this thesis elucidated the band structure and electronic properties of the CBTSSe alloys. A recent device prepared from the Cu2BaSnS4 ́xSex (x « 3) has now been demonstrated with power conversion efficiency (PCE) exceeding 5%. Starting from this early prototype, many avenues remain to optimize the materials, including the underlying chemical positions, the electronic, optical and defect properties of specific compounds. In this thesis, we expand on the CBTSSe paradigm by exploring 16 related compounds, denoted I2-II-IV-VI4 (I=Cu,Ag; II=Sr,Ba; IV=Ge,Sn; VI=S,Se), and some of their alloys for their possible utility as thin-film PV absorbers.
A main methodological result of this thesis concerns the appropriate approximations we can use to obtain accurate and predictable structure, electronic, optical and defect properties for photovoltaic materials. Specifically, structure optimization using computationally expensive hybrid density functional theory is more appropriate than the normally used (semi)local functional (PBE, LDA) and can lead to reasonable and predictable structure and electronic properties. Furthermore, a detailed approach to obtain accurate carrier effective masses is pursued. For the optical properties, the effect of different broadening functions on the onset of absorption coefficients is discussed, and the correct onset behavior can be obtained using Gaussian broadening. At last, a validation of the infinite-size limit of charged defect formation energies calculated by supercell approach is given based on a benchmark study for the gallium vacancy (within charge state q = 0, -1, -2, -3) in GaAs. In general, the bare supercell approach, a supercell approach developed earlier by Freysoldt and coworkers, and a cluster approach can lead to the same infinite-size limit for the charged defect formation energies. Then, based on the appropriate approximations mentioned above, a study of materials properties is described in the I2-II-IV-VI4 (I=Cu,Ag; II=Sr,Ba; IV=Ge,Sn; VI=S,Se) 16 compound systems based on the theoretical structure, electronic and optical properties. Four compounds (Cu2BaSnS4, Cu2BaSnSe4, Cu2BaGeSe4, Cu2SrSnSe4) are identified as potential PV candidates based on their appropriate electronic, and optical properties. Then, two further re- finements are pursued for the Cu2BaSnS4 and Cu2BaGeSe4 compounds. The specific alloys Cu2BaGe1 ́xSnxSe4 (x « 3/4) and Cu2BaSnS4 ́xSex (x « 3) prove to be the best candidates for photovoltaics absorbers among the alloys of these two compounds.
Item Open Access Foundational Studies of the Deposition of Metal-Halide Perovskite Thin Films by Resonant Infrared, Matrix-Assisted Pulsed Laser Evaporation(2020) Barraza, Enrique TomasMetal-halide perovskites (MHP) comprise a diverse family of crystalline materials whose optoelectronic properties have gathered significant interest recently. Their use in transistors, solar cells, light emitting diodes, and many other applications with significant real-world impacts has been enabled by synthesis techniques that can deposit high quality MHP thin films. The simple yet powerful chemistry involved in solution-processing techniques has allowed for MHP thin films to be deposited in a variety of different ways like spin-coating, inkjet printing, and doctor blading. However, solvent in these techniques can preclude the creation of advanced MHP structures like graded composition films or all-MHP heterojunctions. Additionally, the poor solubility of complex organic moieties in the polar solvents used in solution-processing of MHP materials could prevent the creation of MHP materials with unique photophysical properties. The development of vapor-processing techniques which circumvent the use of solvent to vaporize MHP precursors and deposit thin films has shown promise in addressing these concerns with solution-processing. However, the use of highly energetic precursor vaporization mechanisms has itself raised worries about its broad applicability.
In this dissertation, the deposition of MHP thin films using Resonant Infrared, Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE) is developed as an alternative to current MHP thin film deposition techniques. The ‘dry’ deposition of materials under dynamic vacuum and the use of a low energy infrared laser directly address shortcomings of solution- and vapor-processing techniques, respectively. Using the understanding of RIR-MAPLE developed by previous studies, a double solvent approach was first developed to solubilize and deposit MHP precursors in a manner which maintained the integrity of the resulting films. The viability of this baseline approach was confirmed through the creation of MHP solar cells with competitive performance and thin films of MHP materials with complex organic moieties that demonstrate unique photophysical properties. Subsequent studies of nuanced aspects in RIR-MAPLE deposition of MHP thin films helped develop an understanding of the process-structure-property relationships in play during RIR-MAPLE deposition and in post-processing of the resulting MHP thin films.
Following these baseline studies, unique precursor delivery schemes were developed to demonstrate the versatility of RIR-MAPLE. These schemes were shown to reliably deposit continuous films of MHP materials despite differences in the state of precursors during deposition and crystallization. Finally, a comprehensive study of MHP film formation mechanisms during RIR-MAPLE deposition was undertaken. These experiments categorically described the wetting, nucleation, diffusion, and accumulation essential to MHP film development during RIR-MAPLE deposition. Overall, this work demonstrates some of the most promising aspects of the RIR-MAPLE deposition technique and develops the candidacy of RIR-MAPLE as an MHP thin film technique uniquely positioned to address the shortcomings of other currently established methods.
Item Open Access Limits and Economic Effects of Distributed PV Generation in North and South Carolina(2014) Holt, Kyra MooreThe variability of renewable sources, such as wind and solar, when integrated into the electrical system must be compensated by traditional generation sources in-order to maintain the constant balance of supply and demand required for grid stability. The goal of this study is to analyze the effects of increasing large levels of solar Photovoltaic (PV) penetration (in terms of a percentage of annual energy production) on a test grid with similar characteristics to the Duke Energy Carolinas (DEC) and Progress Energy Carolinas (PEC) regions of North and South Carolina. PV production is modeled entering the system at the distribution level and regional PV capacity is based on household density. A gridded hourly global horizontal irradiance (GHI) dataset is used to capture the variable nature of PV generation. A unit commitment model (UCM) is then used determine the hourly dispatch of generators based on generator parameters and costs to supply generation to meet demand. Annual modeled results for six different scenarios are evaluated to determine technical, environmental and economic effects of varying levels of distributed PV penetration on the system.
This study finds that the main limiting factor for PV integration in the DEC and PEC balancing authority regions is defined by the large generating capacity of base-load nuclear plants within the system. This threshold starts to affect system stability at integration levels of 5.7%. System errors, defined by imbalances caused by over or under generation with respect to demand, are identified in the model however the validity of these errors in real world context needs further examination due to the lack of high frequency irradiance data and modeling limitations. Operational system costs decreased as expected with PV integration although further research is needed to explore the impacts of the capital costs required to achieve the penetration levels found in this study. PV system generation was found to mainly displace coal generation creating a loss of revenue for generator owners. In all scenarios, CO2 emissions were reduced with PV integration. This reduction could be used to meet impending EPA state-specific CO2 emissions targets.
Item Open Access NatureWatt LLC.: A Guide to Environmentally and Socially Responsible Utility-Scale Solar Development in the Southeastern United States(2020-04-24) Oberholzer, Alicia; Schuster, AlexisUtility-scale solar photovoltaic electricity generation is growing rapidly across America, and as a result, concerning impacts on the natural environment are starting to add up. Recognizing that North Carolina has the second largest capacity of utility-scale solar generation in the country, the Nature Conservancy of North Carolina has been compelled to intervene (SEIA, 2019). Concerned about the negative impacts to the natural environment that solar development can cause, The Nature Conservancy of North Carolina has identified best practices for sustainable solar development and begun to explore ways to incentivize industry adoption (TNC, 2019). To this end, they called upon us, a Duke University Nicholas School of the Environment Masters Project team, to develop the first iteration of a voluntary-based certification system for eco-friendly solar farms. To create a certification program with the natural environment in mind, the first step was to identify core objectives for the guidebook, the reference document of the certification. This first objective was to compile negative impacts of solar development and provide mitigation and avoidance practices. The second was to estimate the qualitative costs and benefits of each practice so that stakeholders know what costs and benefits consider before they decide to pursue the certification. The last objective for the guidebook was to compile measurement tools for each practice so that the certification requirements are standardized and measurable. Together these objectives culminated in a guidebook that accomplishes the Nature Conservancy’s mission to protect and restore natural systems and biodiversity. The first step in the guidebook compilation process was to review the Principles of Low Impact Solar Siting and Design written by Elizabeth Kalies of the Nature Conservancy of North Carolina. This document is the foundation of the certification, as it lays out six principles that reflect the core practices of sustainable solar development. These principles include avoiding areas of native biodiversity, allowing for wildlife connectivity, using disturbed land, protecting water quality, restoring native plant species, and protecting and providing wildlife habitat (TNC, 2019). We aimed to further this research by reviewing additional literature regarding the impacts of solar development and other relevant topics, such as existing solar certification and other environmental certifications. In addition, informal interviews were used to gain stakeholder insight on existing mindful practices and new ideas surrounding community engagement. Once the research was complete, the writing of the guidebook began. The certification, entitled NatureWatt, is awarded through compliance with the NatureWatt Guidebook. This guidebook is comprised of four sections: Siting, Design, Social Impact, and Compliance. Each section contains principles supplemented by criteria that aim to provide a complete inventory of sustainable tactics for each step of solar project development. Each criterion contains measurement tools that must be used to measure compliance as well as qualitative, expected costs and benefits that stakeholders may incur. The NatureWatt Guidebook goes beyond the tangible aspects of solar development and includes tactics that address social impact. From promoting diversity, to community education, the guidebook offers innovative ways to address inequities in the communities where solar projects are built. The guidebook went through an extensive revision process, which included reviews by solar industry stakeholders. The NatureWatt certification draws attention to the importance of mindfulness of the surrounding environment during the siting and design of utility-scale solar generation. It is a comprehensive guidebook that allows for innovation and profitability, while allowing for the natural environment to thrive as it cohabitates with the new development. Further research opportunities will require subject matter experts who can place value on certain ecosystem services and experts who can create an implementation plan for industry adoption.Item Open Access Novel Fabrication Approaches for Optoelectronic Halide Semiconductor Thin Films and Devices(2019) Dunlap-Shohl, Wiley AlfredHalide semiconductors have recently emerged as a class of materials that unite outstanding optoelectronic properties with the ability to process device-quality thin films at low or even room temperature. Successful adoption of halide semiconductor-based technologies will, however, be contingent on the development of device architectures and film processing approaches that enable efficient, low-cost devices with stable performance and rigorous study of these materials’ photophysical properties. Herein, the goals are twofold: first, to develop low-cost device processing methods that deliver efficient solar cell performance while managing sources of instability; second, to extend existing thin film processing techniques to novel materials, enabling investigation of their optoelectronic properties.
After general introduction to halide perovskite materials, films and devices in Chapters 1 and 2, Chapter 3 confronts the first device challenge—i.e., reducing solar cell cost—by investigating cheap electron and hole transport layers (ETL and HTL) for halide perovskite solar cells. Efficient CH3NH3PbI3 perovskite solar cells are constructed using earth-abundant ETL CdS and HTL CuCrO2 that are deposited at low temperature (<100 °C). Although CuCrO2 appears to yield an inert interface with CH3NH3PbI3, X-ray photoelectron spectroscopy reveals that CdS can easily release Cd into the CH3NH3PbI3 film. X-ray diffraction (XRD) measurements show that excessive amounts of Cd cause phase segregation of insulating compounds in the perovskite, compromising solar cell performance. Nevertheless, careful optimization of device fabrication avoids this detrimental interaction, leading to solar cells with power conversion efficiency of over 15%. In addition to demonstrating efficient devices using low-cost materials, this work emphasizes the importance of managing interfacial as well as bulk stability.
Chapter 4 focuses on the second device challenge—i.e., managing instability—by developing inherently robust architectures via lamination and hot pressing. This technique circumvents the intrinsic thermal instability of perovskite thin films during processing and forms a self-encapsulating device architecture. Annealing MAPbI3 films under pressure in a specially-constructed tool allows significant grain growth at temperatures that would ordinarily decompose them rapidly to PbI2. However, these temperatures can also activate unexpected reactions with carrier transport materials previously thought to be inert, such as nickel oxide. Applying this knowledge, techniques are developed that avoid reactivity-related problems and recover the targeted solar cell performance.
Chapters 5 and 6 of this dissertation focus on developing deposition methods for new halide semiconductor films, with emphasis in this case on exploration of fundamental physical properties rather than device fabrication. In Chapter 5, resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) is first used to deposit films of the archetypal halide perovskite CH3NH3PbI3, which possesses properties comparable to those prepared by more conventional methods such as spin coating, as determined by XRD, electron microscopy and optical spectroscopy. CH3NH3PbI3 solar cells fabricated using RIR-MAPLE reach power conversion efficiency of over 12%. RIR-MAPLE is then extended to the deposition of layered lead halide perovskite films incorporating oligothiophene-derived molecular cations, which cannot be controllably deposited by other methods. By varying the number of rings in the thiophene chain and the halide component of the inorganic layers, the photoluminescence emission from these films can be tuned to originate from either the inorganic or the organic component or be quenched altogether, supporting prior computational predictions of the tunable quantum well nature of these types of perovskite structures. Carrier transfer between the inorganic and organic moieties can synergistically populate triplet states in the organic, showcasing the unique physical properties attainable in complex-organic perovskites.
Chapter 6 focuses on the halide semiconductor indium(I) iodide, which possesses elements of its electronic structure like those of halide perovskites, that are often invoked as an explanation for these materials’ remarkable defect tolerance. Indium(I) iodide is prepared in thin-film form by thermal evaporation. A photovoltaic effect is demonstrated for the first time in this material, with solar cells demonstrating ~0.4% power conversion efficiency. Overall, the results advance our scientific understanding of halide semiconductors, and provide crucial pathways by which they can be made more technologically effective, and be studied in greater depth.
Item Open Access Novel Organic Heterostructures Enabled by Emulsion-Based, Resonant Infrared, Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE)(2014) McCormick, RyanAn explosion in the growth of organic materials used for optoelectronic devices is linked to the promise that they have demonstrated in several ways: workable carrier mobilities, ease of processing, design flexibility to tailor their optical and electrical characteristics, structural flexibility, and fabrication scalability. However, challenges remain before they are ready for prime time. Deposition of these materials into ordered thin films requires that they be cast from solutions of organic solvents. Drawbacks of solution-casting include the difficulty of producing layered films without utilizing orthogonal solvents (or even with orthogonal solvents), the difficulty in controlling domain sizes in films of mixed materials, and the lack of parameter options used to control the final properties of thin films. Emulsion-based, resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) is a thin film deposition technique that is demonstrated to provide solutions to these problems.
This work presents fundamental research into the RIR-MAPLE process. An investigation of the molecular weight of deposited materials demonstrates that emulsion-based RIR-MAPLE is capable of depositing polymers with their native molecular weights intact, unlike other laser deposition techniques. The ability to deposit multilayer films with clearly defined interfaces is also demonstrated by cross-sectional transmission electron microscopy imaging of a layered polymer/quantum dot nanocomposite film. In addition, trade-offs related to the presence of surfactant in the target, required to stabilize the emulsion, are articulated and investigated by x-ray diffraction, electrical, optical, and surface characterization techniques. These studies show that, generally speaking, the structural, optical and electrical properties are not significantly affected by the affected by the presence of surfactant, provided that the concentration within the target is sufficiently low. Importantly, the in-plane mobility of RIR-MAPLE devices, determined by organic field effect transistor (OFET) characterization, rivals that of spin-cast devices produced under similar conditions.
This work also presents results of emulsion-based RIR-MAPLE deposition applied to optical coatings (gradient-refractive index antireflection coating based on porous, multilayer films) and optoelectronic devices (organic photovoltaics based on the polymer, P3HT, and small molecule, PC61BM, bulk heterojunction system). The optical coating demonstrates that RIR-MAPLE is capable of producing nanoscale domain sizes in mixed polymer blends that allow a film to function as an effective medium relevant to devices in the visible spectrum. Moreover, bulk heterojunction organic photovoltaic (OPV) devices that require nanoscale domains to function effectively are achieved by co-deposition of P3HT and PC61BM, achieving a power conversion efficiency of 1.0%, which is a record for MAPLE-deposited devices.
Results of these studies illuminate unique capabilities of the RIR-MAPLE process. Multilayer films are readily fabricated to create true bilayer OPV structures. Additionally, true gradient thin films are created by varying the ratio of two materials, including two-polymer films and a film consisting of a polymer and a small molecule, over the course of a single deposition.
Item Open Access The Growth of Solar Concentrator Photovoltaic Markets in the Southwest US(2008-04-25T20:06:48Z) Connor, SeanWorldwide solar photovoltaic (PV) markets have grown at an average rate at 38% over the past ten years. While polysilicon flat panel PV modules have traditionally dominated the overall solar market, a range of different solar energy conversion technologies are starting to gain market share. One such class of solar technologies, concentrator photovoltaics (CPV), is in its commercial infancy but offers a module manufacturing paradigm to greatly lower the cost of solar electricity production. This paper examines the attributes of CPV and analyzes how it might compete within the overall solar market. The Southwest US is used as a case study to examine specific subsidies, regulations, and business models that will affect the success of CPV. In addition, a financial model was created to examine important factors influencing retail and wholesale PV and CPV project costs under various scenarios.