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- All Subjects: photovoltaics
- Creators: Bowden, Stuart G
Description
Silicon photovoltaics (PV) is approaching its theoretical efficiency limit as a single-junction technology. To break this limit and further lower the PV-generated levelized cost of electricity, it is necessary to engineer a silicon-based “tandem” technology in which a solar cell of another material is stacked on top of silicon to make more efficient use of the full solar spectrum.
This dissertation understands and develops four aspects of silicon-based tandem PV technology. First, a new “spectral efficiency” concept is proposed to understand how tandem cells should be designed and to identify the best tandem partners for silicon cells. Using spectral efficiency, a top-cell-design guide is constructed for silicon-based tandems that sets efficiency targets for top cells with various bandgaps to achieve targeted tandem efficiencies.
Second, silicon heterojunction solar cells are tuned to the near-infrared spectrum to enable world-record perovskite/silicon tandems both in two- and four-terminal configurations. In particular, for the 23.6%-efficient two-terminal tandem, a single-side textured silicon bottom cell is fabricated with a low-refractive-index silicon nanoparticle layer as a rear reflector. This design boosts the current density to 18.5 mA/cm2; this value exceeds that of any other silicon bottom cell and matches that of the top cell.
Third, “PVMirrors” are proposed as a novel tandem architecture to integrate silicon cells with various top cells. A strength of the design is that the PVMirror collects diffuse light as a concentrating technology. With this concept, a gallium-arsenide/silicon PVMirror tandem is demonstrated with an outdoor efficiency of 29.6%, with respect to the global irradiance.
Finally, a simple and versatile analytical model is constructed to evaluate the cost competitiveness of an arbitrary tandem against its sub-cell alternatives. It indicates that tandems will become increasingly attractive in the market, as the ratio of sub-cell module cost to area-related balance-of-system cost—the key metric that will determine the market success or failure of tandems—is decreasing.
As an evolution of silicon technology, silicon-based tandems are the future of PV. They will allow more people to have access to clean energy at ultra-low cost. This thesis defines both the technological and economic landscape of silicon-based tandems, and makes important contributions to this tandem future.
This dissertation understands and develops four aspects of silicon-based tandem PV technology. First, a new “spectral efficiency” concept is proposed to understand how tandem cells should be designed and to identify the best tandem partners for silicon cells. Using spectral efficiency, a top-cell-design guide is constructed for silicon-based tandems that sets efficiency targets for top cells with various bandgaps to achieve targeted tandem efficiencies.
Second, silicon heterojunction solar cells are tuned to the near-infrared spectrum to enable world-record perovskite/silicon tandems both in two- and four-terminal configurations. In particular, for the 23.6%-efficient two-terminal tandem, a single-side textured silicon bottom cell is fabricated with a low-refractive-index silicon nanoparticle layer as a rear reflector. This design boosts the current density to 18.5 mA/cm2; this value exceeds that of any other silicon bottom cell and matches that of the top cell.
Third, “PVMirrors” are proposed as a novel tandem architecture to integrate silicon cells with various top cells. A strength of the design is that the PVMirror collects diffuse light as a concentrating technology. With this concept, a gallium-arsenide/silicon PVMirror tandem is demonstrated with an outdoor efficiency of 29.6%, with respect to the global irradiance.
Finally, a simple and versatile analytical model is constructed to evaluate the cost competitiveness of an arbitrary tandem against its sub-cell alternatives. It indicates that tandems will become increasingly attractive in the market, as the ratio of sub-cell module cost to area-related balance-of-system cost—the key metric that will determine the market success or failure of tandems—is decreasing.
As an evolution of silicon technology, silicon-based tandems are the future of PV. They will allow more people to have access to clean energy at ultra-low cost. This thesis defines both the technological and economic landscape of silicon-based tandems, and makes important contributions to this tandem future.
ContributorsYu, Zhengshan (Author) / Holman, Zachary C (Thesis advisor) / Zhang, Yong-Hang (Committee member) / Bowden, Stuart G (Committee member) / King, Richard R (Committee member) / Arizona State University (Publisher)
Created2018
Description
Metallization of solar cells is a critical process step in the manufacturing of silicon photovoltaics (PV) as it plays a large role in device performance and production cost. Improvements in device performance linked to metallization and reduction in material usage and processing costs will continue to drive next-generation silicon PV technology. Chapter 1 introduces the context for the contributions of this thesis by providing background information on silicon PV cell technology, solar cell device physics and characterization, and metallization performance for common silicon cell structures. Chapter 2 presents a thermal model that links sub-bandgap reflectance, an important metric at the rear metal interface, to outdoor module operating temperature. Chapter 3 implements this model experimentally with aluminum back-surface field (Al-BSF), passivated emitter and rear contact (PERC), and passivated emitter rear totally diffused (PERT) mini-modules, where the PERT cells were modified to include an optimized sub-bandgap reflector stack. The dedicated optical layer was a porous low-refractive index silica nanoparticle film and was deposited between the dielectric passivation and full area metallization. This created an appreciable boost in sub-bandgap reflectance over the PERC and Al-BSF cells, which directly lead to cooler operating temperature of the fielded module. Chapter 4 investigates low-temperature Ag metallization approaches to SiO2/polysilicon passivating contacts (TOPCon architecture). The low-temperature Ag sintering process does not damage TOPCon passivation for structures with 40-nm-thick poly-Si but shows higher contact resistivity than sputtered references. This disparity is investigated and the impact of Ag diffusion processes, microstructure changes, ambient gases, and interfacial chemical reactions are evaluated. Chapter 5 investigates sputtered Al metallization to silicon heterojunction contacts of both polarities. This In-free and Ag-free metallization process can achieve low contact resistivity and no passivation loss when annealed between 150-180 °C. The passivation degradation at higher temperatures was studied with high-resolution microscopy and elemental mapping, where the interdiffusion processes were identified. Lastly, Chapter 6 summarizes the contribution of this work.
ContributorsBryan, Jonathan Linden (Author) / Holman, Zachary C (Thesis advisor) / Bertoni, Mariana I (Committee member) / Bowden, Stuart G (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2022
Description
In-field characterization of photovoltaics is crucial to understanding performance and degradation mechanisms, subsequently improving overall reliability and lifespans. Current outdoor characterization is often limited by logistical difficulties, variable weather, and requirements to measure during peak production hours. It becomes a challenge to find a characterization technique that is affordable with a low impact on system performance while still providing useful device parameters. For added complexity, this characterization technique must have the ability to scale for implementation in large powerplant applications. This dissertation addresses some of the challenges of outdoor characterization by expanding the knowledge of a well-known indoor technique referred to as Suns-VOC. Suns-VOC provides a pseudo current-voltage curve that is free of any effects from series resistance. Device parameters can be extracted from this pseudo I-V curve, allowing for subsequent degradation analysis. This work introduces how to use Suns-VOC outdoors while normalizing results based on the different effects of environmental conditions. This technique is validated on single-cells, modules, and small arrays with accuracies capable of measuring yearly degradation. An adaptation to Suns-VOC, referred to as Suns-Voltage-Resistor (Suns-VR), is also introduced to complement the results from Suns-VOC. This work can potentially be used to provide a diagnostic tool for outdoor characterization in various applications, including residential, commercial, and industrial PV systems.
ContributorsKillam, Alexander Cameron (Author) / Bowden, Stuart G (Thesis advisor) / Goryll, Michael (Committee member) / Augusto, Andre (Committee member) / Rand, James (Committee member) / Arizona State University (Publisher)
Created2022
Description
This dissertation covers my doctoral research on the cathodoluminescence (CL) study of the optical properties of III-niride semiconductors.
The first part of this thesis focuses on the optical properties of Mg-doped gallium nitride (GaN:Mg) epitaxial films. GaN is an emerging material for power electronics, especially for high power and high frequency applications. Compared to traditional Si-based devices, GaN-based devices offer superior breakdown properties, faster switching speed, and reduced system size. Some of the current device designs involve lateral p-n junctions which require selective-area doping. Dopant distribution in the selectively-doped regions is a critical issue that can impact the device performance. While most studies on Mg doping in GaN have been reported for epitaxial grown on flat c-plane substrates, questions arise regarding the Mg doping efficiency and uniformity in selectively-doped regions, where growth on surfaces etched away from the exact c-plane orientation is involved. Characterization of doping concentration distribution in lateral structures using secondary ion mass spectroscopy lacks the required spatial resolution. In this work, visualization of acceptor distribution in GaN:Mg epilayers grown by metalorganic chemical vapor deposition (MOCVD) was achieved at sub-micron scale using CL imaging. This was enabled by establishing a correlation among the luminescence characteristics, acceptor concentration, and electrical conductivity of GaN:Mg epilayers. Non-uniformity in acceptor distribution has been observed in epilayers grown on mesa structures and on miscut substrates. It is shown that non-basal-plane surfaces, such as mesa sidewalls and surface step clusters, promotes lateral growth along the GaN basal planes with a reduced Mg doping efficiency. The influence of surface morphology on the Mg doping efficiency in GaN has been studied.
The second part of this thesis focuses on the optical properties of InGaN for photovoltaic applications. The effects of thermal annealing and low energy electron beam irradiation (LEEBI) on the optical properties of MOCVD-grown In0.14Ga0.86N films were studied. A multi-fold increase in luminescence intensity was observed after 800 °C thermal annealing or LEEBI treatment. The mechanism leading to the luminescence intensity increase has been discussed. This study shows procedures that significantly improve the luminescence efficiency of InGaN, which is important for InGaN-based optoelectronic devices.
The first part of this thesis focuses on the optical properties of Mg-doped gallium nitride (GaN:Mg) epitaxial films. GaN is an emerging material for power electronics, especially for high power and high frequency applications. Compared to traditional Si-based devices, GaN-based devices offer superior breakdown properties, faster switching speed, and reduced system size. Some of the current device designs involve lateral p-n junctions which require selective-area doping. Dopant distribution in the selectively-doped regions is a critical issue that can impact the device performance. While most studies on Mg doping in GaN have been reported for epitaxial grown on flat c-plane substrates, questions arise regarding the Mg doping efficiency and uniformity in selectively-doped regions, where growth on surfaces etched away from the exact c-plane orientation is involved. Characterization of doping concentration distribution in lateral structures using secondary ion mass spectroscopy lacks the required spatial resolution. In this work, visualization of acceptor distribution in GaN:Mg epilayers grown by metalorganic chemical vapor deposition (MOCVD) was achieved at sub-micron scale using CL imaging. This was enabled by establishing a correlation among the luminescence characteristics, acceptor concentration, and electrical conductivity of GaN:Mg epilayers. Non-uniformity in acceptor distribution has been observed in epilayers grown on mesa structures and on miscut substrates. It is shown that non-basal-plane surfaces, such as mesa sidewalls and surface step clusters, promotes lateral growth along the GaN basal planes with a reduced Mg doping efficiency. The influence of surface morphology on the Mg doping efficiency in GaN has been studied.
The second part of this thesis focuses on the optical properties of InGaN for photovoltaic applications. The effects of thermal annealing and low energy electron beam irradiation (LEEBI) on the optical properties of MOCVD-grown In0.14Ga0.86N films were studied. A multi-fold increase in luminescence intensity was observed after 800 °C thermal annealing or LEEBI treatment. The mechanism leading to the luminescence intensity increase has been discussed. This study shows procedures that significantly improve the luminescence efficiency of InGaN, which is important for InGaN-based optoelectronic devices.
ContributorsLiu, Hanxiao (Author) / Ponce, Fernando A. (Thesis advisor) / Zhao, Yuji (Committee member) / Newman, Nathan (Committee member) / Fischer, Alec M (Committee member) / Arizona State University (Publisher)
Created2020