Browsing by Subject "Photovoltaics"

Now showing 1 - 10 of 10
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    Buffer and Barrier Layers for CIGS Based Tandem Photovoltaics
    (2019-02) Bontrager, Timothy
    As the world’s energy consumption increases, a technology for energy production that does not release greenhouse gases has become desirable. Of these technologies, solar devices with an absorber composed of copper, indium, gallium, and selenium (CIGS) have become industrially viable. It is possible to increase the efficiency of a CIGS based device by implementing a so-called “tandem” configuration consisting of a wide bandgap device stacked on top of a narrow bandgap device, but actual demonstration of these devices remains elusive. This work seeks to address two concerns that arise during fabrication of this device. The first of these is the diffusion of cadmium from the buffer layer into the narrow bandgap absorber during top cell deposition. In this work, a diffusion barrier has been shown to be a partially effective mechanism for limiting the damages caused by this diffusion. The procedure for depositing the diffusion barrier and the electrical and chemical effects of the diffusion barrier are discussed. The second limitation to a tandem configuration discussed in this work is the optimization of the band structure between the top-cell wide bandgap absorber and the top-cell buffer layer. This interaction is measured directly, and the effect of varying the buffer layer on device efficiency is examined.
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    Driven by Light: An Ultrafast Look into the Bright Future of Photosensitizers
    (2024-04) Schaffner, Jacob
    This thesis investigates various strong light-absorbing molecules that have potential applications in furthering our progress into replacing fossil fuels with clean energy resources and remediating harmful chemicals in the environment. The research presented in this thesis employs a range of spectroscopic techniques, complemented with computational predictions, to characterize the light absorption and excited state dynamics of newly developed chromophores that have shown promise in these various applications. Chapter 3 investigates a BODIPY-fullerene dyad designed to be used in organic photovoltaics as a triplet sensitizer to form longer-lived excitons. This triplet sensitization occurs via a ping-pong energy transfer mechanism between the BODIPY and fullerene, resulting in a long-lived BODIPY triplet (>1 µs). Chapters 4 and 5 investigate the MB-DIPY chromophore that could potentially displace fullerene as a strong and more versatile electron acceptor in organic photovoltaics. In Chapter 4, the redox potentials and photophysics of four MB-DIPY analogs are explored. The MB-DIPYs had comparable reduction potentials to fullerene and demonstrated efficient intersystem crossing to form long-lived triplet states (>10 µs). In Chapter 5, the MB-DIPY is functionalized with ferrocene, a strong electron donor, and demonstrated sub-ps charge-transfer from the ferrocene to the MB-DIPY followedby charge recombination in 12 ps. Chapters 6 and 7 investigate the Rh-Ga and Co-Ga heterobimetallic photocatalysts that can access challenging bonds via a photoredox mechanism. The excited state nature of these photocatalysts is first explored in Chapter 6. The results were consistent with the naked anionic catalyst being the active participant in the photocatalytic cycle. Chapter 7 investigates the reactivity of the photocatalysts with a chloroadamantane substrate. The results suggested that the substrate binds to the anionic rhodium photocatalyst and that the photocatalytic reactivity is not diffusion-limited. In contrast, the anionic cobalt catalyst was converted into the chlorinated precatalyst upon the addition of the substrate, demonstrating that the chemical reactivity of the rhodium and cobalt photocatalysts differ with this substrate.
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    Evaluating utility benefits of custom owned and sited photovoltaics
    (Hubert H. Humphrey Institute of Public Affairs, 2009-11-18) Miller, Stacy
    There is growing interest in grid-connected, customer owned solar photovoltaic (PV) systems and considerable disagreement about how to determine the value of grid- connected PV. The solar industry asserts that utilities should support customer-sited PV systems because of the high correlation between solar energy production and peaking loads. Some utilities maintain that a utility realizes no net benefit from PV above wholesale value of the electricity because the utility must still maintain adequate infrastructure to meet the PV owner’s peak demand. This paper evaluates the benefits of solar energy delivered by customer owned and sited PV systems on a monetary basis from the utility’s perspective by examining the capacity of the solar resource to deliver during times of high spot market prices. The analysis is completed for the Minneapolis Saint Paul electricity market using a single PV system’s electricity production data correlated with regional wholesale pricing data to identify whether PV can reduce utility exposure to spot market pricing, thereby creating value to the utility to purchase power from solar producers.
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    Exciton Transport in Organic Semiconductors
    (2015-06) Menke, Stephen
    Photovoltaic cells based on organic semiconductors are attractive for their use as a renewable energy source owing to their abundant feedstock and compatibility with low-cost coating techniques on flexible substrates. In contrast to photovoltaic cells based traditional inorganic semiconductors, photon absorption in an organic semiconductor results in the formation of a coulombically bound electron-hole pair, or exciton. The transport of excitons, consequently, is of critical importance as excitons mediate the interaction between charge and light in organic photovoltaic cells (OPVs). In this dissertation, a strong connection between the fundamental photophysical parameters that control nanoscopic exciton energy transfer and the mesoscopic exciton transport is established. With this connection in place, strategies for enhancing the typically short length scale for exciton diffusion (LD) can be developed. Dilution of the organic semiconductor boron subphthalocyanine chloride (SubPc) is found to increase the LD for SubPc by 50%. In turn, OPVs based on dilute layers of SubPc exhibit a 30% enhancement in power conversion efficiency. The enhancement in power conversion efficiency is realized via enhancements in LD, optimized optical spacing, and directed exciton transport at an exciton permeable interface. The role of spin, energetic disorder, and thermal activation on LD are also addressed. Organic semiconductors that exhibit thermally activated delayed fluorescence and efficient intersystem and reverse intersystem crossing highlight the balance between singlet and triplet exciton energy transfer and diffusion. Temperature dependent measurements for LD provide insight into the inhomogeneously broadened exciton density of states and the thermal nature of exciton energy transfer. Additional topics include energy-cascade OPV architectures and broadband, spectrally tunable photodetectors based on organic semiconductors.
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    Impurities in silicon nanocrystals: the intentional and the inherent
    (2013-04) Rowe, David J
    Silicon nanocrystals (SiNCs) have become an important class of materials in the fields of photovoltaics, thermoelectrics, lighting, and medicine. Impurities within SiNCs dramatically alter the electrical and optical properties of the host material, whether the impurity is intentionally added in an attempt to manipulate properties, or is inherent to the material and its natural state. Despite such remarkable changes, impurity incorporation within SiNCs remains poorly understood, since concepts applied to understanding impurities in bulk materials may not completely translate to nanomaterials. Understanding the effect of SiNC impurities requires new technologies to produce materials suitable for study combined with new insights to expound the differences in the nanoscale physics. Nonthermal plasma-assisted gas-phase synthesis provides an excellent route to producing and investigating impurities within SiNCs due to the unique chemical reaction environment of the plasma. The robustness of such a technique allows for the production of very pure SiNCs or SiNCs with added impurities simply by adding different chemicals to the plasma. The chapters in this document focus on the effect that different impurities have on the properties of SiNCs. Chapter 2 focuses on heavily P-doped SiNCs exhibiting the first known observation of a unique electrical and optical property known as localized surface plasmon resonance (LSPR) within free-standing SiNCs. Chapter 3 explains the synthesis of B- and P-doped SiGeNC alloys and their deposition into thin films for thermoelectric applications. Chapter 4 highlights research which uses P-doped SiNCs to form emitter layers for pn-junction type solar cells, including device fabrication and optical characterization. Chapter 5 examines inherent impurities in the form of dangling bond defects which may be responsible for the quenching of SiNC photoluminescence, and their evolution during the process of air-ambient oxidation. Several appendices at the end of the document detail some of the fabrication processes used throughout this work, as well as brief reports of some side projects that may be of interest to researchers intent on studying SiNC synthesis and deposition technologies.
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    Light Management and Optical Loss Mitigation for Photovoltaics: Downshifting, Downconversion, and Tandem Solar Cell Designs
    (2023-07) Keil, John
    The efficiency of single-junction Si photovoltaic cells has continually increased over the past several decades, but is approaching fundamental thermodynamic limits. Holding over 95% of the solar module market share, Si modules will continue to be an integral part of the rapidly expanding photovoltaic industry, so different device technologies that increaseSi cell efficiencies beyond thermodynamic limits, or that expand the available installation sites for solar cells, are needed. In this thesis, three types of technologies are discussed that use optical design to more efficiently use the high energy solar spectrum for Si PV: downshifting, downconversion, and tandem solar cells. We first discuss the design of downshifting and concentrating devices called luminescent solar concentrators (LSCs). Tandem LSC architectures, which combine multiple luminophores to broaden the absorption spectrum, are one potential route to increase the efficiency of these devices. We first describe an analytical model to develop luminophore selection criteria for tandem LSCs. We find that luminophores with high photoluminescent quantum yield, minimal overlap between the absorption and photoluminescence spectrum, and an absorption onset closely matched to the band gap of the chosen photovoltaic cell yield the best LSC performance. We then create bilayer LSCs, which combine CdSe/CdS and Si nanocrystals in a monolithic waveguide. Through a combination of transmission measurements, position-dependent photoluminescence measurements, and ray-tracing simulations, the bilayer LSC was found to sensitize Si nanocrystal absorption and enhance the optical efficiency by 30% relative to a single layer LSC. We discuss the use of the bilayer device in agrivoltaic applications, and then explore this use further using a thin-film stack optimization methods to direct emission out one LSC side toward the plant species. The LSC extraction efficiency is increased from 13.9% to 15.1%. We next consider optical designs for downconversion, a process by which one high energy photon is converted into two lower energy photons. We consider the coupling efficiency from the downconverter to a realistic Si solar module in several different configurations, finding an optical coupling efficiency of 95.25% by placing the downconverting film directly on the Si cell. This enhances the power conversion efficiency by 2% absolute. Lastly, CdTe/Si four-terminal tandem solar cells are studied to improve the sub-band gap transparency of CdTe solar cells. We find that the surface texture of the CdTe significantly impacts light transmission into the Si bottom cell, and that the losses are dominated by the transparent conductive oxide absorption. An optical design solution is proposed that mitigates transparency loss and enhances the short circuit current density of the Si cell by 2.5 mA/cm2 , which enhances the tandem efficiency by a relative increase of 5.6%.
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    Modeling, Design, and Fabrication of Spectrally-Selective Mirrors for Photovoltaic Thermal Management
    (2020-07) Slauch, Ian
    A typical c-Si photovoltaic module will operate 20-30K above ambient temperature due to waste heat generated as it converts incident sunlight into electrical power. As temperature increases, the conversion efficiency drops by ~0.4%/K, reducing overall power output. Reducing the total amount of waste heat generated during operation would both lower the module operating temperature and improve its efficiency and energy yield. Waste heat is generated in the module in part due to parasitic absorption of sub-bandgap light that does not have enough energy to be useful for power conversion. Sub-bandgap reflection offers a method of preventing parasitic absorption, cooling the module, and increasing its efficiency. In this thesis, a time-independent matrix model is introduced to calculate module energy yield and waste heat generation through parasitic absorption, recombination, and electronic losses. The model considers the spectral and angular dependence of the optical properties of the module including modification by photonic structures, and is used to characterize and optimize the design of aperiodic photonic mirrors which selectively reflect sub-bandgap light from the module and enhance its energy yield. Importantly, these mirrors are designed considering weather and irradiance conditions typical for outdoor fixed-tilt module installations. As a result, it is shown that these mirrors are omnidirectional, achieving the required spectral selectivity regardless of the angle of incidence of sunlight or the geographic location of installation. Low-complexity mirror designs which are simple to fabricate offer the most potential for reducing the cost of energy. These designs are primarily anti-reflection coatings, but also avoid a rise in operating temperature while increasing energy output. Two simple designs are fabricated, integrated into modules, and tested outdoors. The fabricated mirrors have the desired spectral selectivity, and reduce module operating temperature by over 1K. Alternative strategies to reject sub-bandgap light, including reflection from the cell surface or cell rear contact, and backscattering from near the cell are also modeled and compared to result for reflection from the glass. Designing for the glass interface in particular allows maximization of the dual benefit, optical and thermal, of the mirrors.
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    Study of Heat Losses in Crystalline Silicon and Perovskite Solar Cells
    (2023-08) Tisha, Zakia Tamanna
    Energy from the sun is plentiful and sustainable, making it an excellent alternative to fossil fuels. Photovoltaic (PV) solar cells can directly convert this solar energy into electricity. However, PV solar cells face challenges in achieving high efficiency as some of the captured energy is lost as heat or through other means, reducing efficiency and performance. Researchers are constantly trying to improve the efficiency of solar cells. Silicon-based solar cells are widely used and have practical efficiency that keeps improving, reaching close to the theoretical limit of around 30%. One approach to increase the output of solar cells is converting the heat losses back into electricity, consequently boosting the overall efficiency of solar conversion. This heat recycling can be achieved by integrating photovoltaic (PV) devices with thermoelectric materials, which capture and recycle wasted heat. This thesis aims to lay the groundwork required for achieving this objective by studying the heat loss mechanisms and conducting evaluations of some of those mechanisms.This research focuses on understanding and categorizing the losses in solar cells, particularly the below bandgap energy and thermalization losses, which are responsible for more than half of the total losses. Two types of solar cells, crystalline silicon (c-Si) and CH3NH3PbI3 perovskite (C-P), are studied to analyze their loss characteristics.
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    Synthesis, Characterization and Electronic Transport Properties of Thin Film Iron Pyrite for Photovoltaic Applications
    (2015-08) Zhang, Xin
    The pyrite form of FeS2 has long been recognized as an earth-abundant and non-toxic material with exceptional properties as a solar absorber for inexpensive photovoltaic devices. However, a significant research effort from the mid 1980’s achieved power conversion efficiencies of only less than 3 %. The reasons for such low efficiencies have not been fully elucidated yet, primarily because the electronic transport and doping mechanisms of pyrite are poorly understood. One classic example is well-known puzzle remaining in pyrite, where bulk single crystals are almost exclusively n-type based on Hall effect measurements, whereas polycrystalline thin films are typically deduced to be p-type, mostly from thermopower measurements. The fundamental reason(s) for this are not understood, and identifying the unintentional dopants in FeS2 remains an outstanding challenge. In this work we address, using ex situ sulfidation synthesis, this long-standing problem of understanding conduction mechanisms and doping in FeS2 films. This is done by systematically exploring the effects of film synthesis conditions on microstructure, surface morphology, chemical stoichiometry, electronic transport mechanisms, charge carrier mobility and charge density. More than a hundred of FeS2 thin films and synthetic crystals were probed in this study. In addition to conventional diffusive transport, hopping transport was also frequently observed in FeS2 thin films. This hopping transport was discovered to be caused by nanoscale inhomogeneity (e.g. nanoscale Fe or FeS clusters), which has been overlooked by the pyrite community until now. This hopping transport may explain the poor performance of some FeS2-based solar cells, since the carrier mobility and lifetime are significantly reduced in hopping. More importantly, accompanying the crossover from diffusive to hopping transport, we find significant suppression, and sign inversion from electron-like to hole-like, of Hall and themopower signals in FeS2 thin films. The results indicate that thin films with diffusive transport show n-type conduction, just like single crystals, which implies that the major n-type dopants may be the same for both FeS2 thin films and single crystals. As the transport crosses over to hopping, both Hall and thermopower measurements indicate sign inversions, which are not caused by real p-type doping, but are rather an artifact of hopping conduction. These findings provide the first potential resolution for the “doping puzzle” in FeS2, and emphasize that understanding the electronic transport mechanisms is mandatory for interpreting the sign of Hall and thermopower coefficients in FeS2. In the last part of this work, some preliminary results for identifying the unintentional dopant(s) in FeS2 are presented. The results suggest the major n-type dopants in FeS2 are unlikely to be metal impurities or oxygen. S vacancies are a genuine possibility however, although further study is still required to settle this issue. These findings answer several critical questions for understanding the electronic transport and doping mechanisms in pyrite FeS2 thin films. They also have important implications for FeS2 solar cell development, emphasizing the need for (a) nanoscale chemical homogeneity, (b) caution in interpreting carrier types and densities, and (c) doping control in pyrite FeS2 films.
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    Thin-Film Synthesis of Metal Halide Perovskites for Optoelectronics
    (2020-08) Clark, Catherine
    Metal halide perovskites (MHPs), like the archetypal methylammonium lead iodide (MAPbI3), have emerged in the last decade as promising materials for efficient, low-cost optoelectronics. MHP solar cells have already reached efficiencies >25%, rivaling established technologies like single-crystal Si. Yet several challenges prevent the widespread commercialization of MHPs, including their instability in ambient conditions, their toxicity, and the need for scaleable fabrication techniques. Fundamentally, the origins of important material properties relating to carrier transport and recombination are still not well understood. Thin film deposition techniques that enable detailed study of process-structure-property relationships and are commercially relevant are consequently becoming increasingly essential. This thesis seeks to address these challenges through the design, implementation, and utilization of a carrier-gas assisted vapor deposition (CGAVD) method that can grow MHP films with highly tunable stoichiometries and morphologies. Alongside the design of a CGAVD system with six independently controllable experimental parameters, an analytical model is developed and experimentally validated that allows the determination of robust and repeatable growth regimes and the prediction of material deposition rates. Harnessing this technique, we demonstrate the ability to deposit MASnI3 and MASnBr3 films and to systematically vary their compositions across a wide range, and realize corresponding changes in film microstructures (grain size, coverage) and electronic properties (resistivity, carrier concentration, mobility). Control of grain size and film texturing is also achieved independent of stoichiometry via modulation of chamber pressure and substrate temperature. The benefits of CGAVD are further highlighted by the successful growth of novel all-MHP heterojunctions. Two stable pairings are identified: MAPbBr3/MASnBr3 and CsPbBr3/MASnBr3. Design rules to control the mixing of heterojunctions are developed by exploring the dependence of mixing rate on MHP layer composition and grain size. Finally, through a collaboration with Physical Electronics, we optimize the use of XPS depth-profiling for MHPs and investigate which ions are diffusing in a layered structure that exhibits mixing. Moving forward, the incorporation of CGAVD-grown heterojunctions and Pb MHPs into optoelectronic devices will harness the tunability of this system towards a deeper understanding of process-structure-property relationships in MHP thin films and novel layered structures.