Browsing by Subject "thin films"

Now showing 1 - 5 of 5
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    Copper zinc tin sulfide (Cu2ZnSnS4) photovoltaic material development and thin film solar cells
    (2016-03) Zhang, Liyuan
    Copper zinc tin sulfide (Cu2ZnSnS4, or CZTS) is emerging as an alternative light absorbing material to the present thin film solar cell technologies such as Cu(In,Ga)Se2 and CdTe. All the elements in CZTS are abundant, environmentally benign, and inexpensive. In addition, CZTS has a band gap of ~1.5 eV, the ideal value for converting the maximum amount of energy from the solar spectrum into electricity. CZTS has a high absorption coefficient (>104 cm-1 in the visible region of the electromagnetic spectrum) and only a few micron thick layer of CZTS can absorb all the photons with energies above its band gap. A two-stage process of CZTS thin film synthesis is presented, which consists of sequential thermal evaporation of copper, tin and zinc layers followed by a heat treatment in the presence of sulfur vapor (sulfurization) in a sealed quartz ampoule. The metal precursor stacking order, deposition rate and thickness of each metal layer can be adjusted to give uniform metal precursor stacks of controlled morphology and composition. The effects of sulfurization temperature, time, substrate material, metal precursor stacking order, and back contact layer on the morphological and structural properties of the CZTS films are investigated. Observations of grain size changes and compositional modification are made and explained in terms of the likely secondary phases present. CZTS thin film solar cells were fabricated and the effects of chemical composition were studied both on the absorber layer properties and on the final solar cell performance. It is confirmed that CZTS thin film chemical composition affects the carrier concentration profile, which then influences the solar cell properties. Only a small deviation from the optimal chemical composition can drop device performance to a lower level, which confirms that the CZTS solar cells with high conversion efficiency existed in a relatively narrow composition region. Besides CZTS absorber chemical composition study, post deposition rapid thermal annealing (RTA) was conducted and its influence on solar cell performance was studied. It is observed that post deposition RTA would lead to an increase of device performance. Through C-V measurement results, we have shown that post RTA of CZTS solar cell affects the CZTS/CdS interfacial defect concentration and zero bias depletion depth, which means the defect-related charge at CZTS/CdS interface reduces and it improves Voc and the fill factor.
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    Going Deeper into Laser Damage: Experiments and Methods for Characterizing Materials in High Power Laser Systems
    (2016-05) Taylor, Lucas
    Laser damage is a primary limiting factor to the design of high-power laser systems. This is true for short-pulse systems as well as long-pulse and continuous-wave (CW) systems. Unlike short-pulse laser damage, CW laser damage has been much less studied. This work comprises a background of laser damage and laser heating theory, a CW laser damage experiment and an imaging technique for monitoring laser heating. The damage experiment was performed on 100 nm thick hafnia coatings deposited on fused silica. Uniformly grown films were compared to hafnia-alumina nanolaminates. While the nanolaminates are known to perform better for 1 ns pulses, we found they had worse laser damage performance in the CW regime. We found the nanolaminates reduced crystallinity. The polycrystalline uniform films are thought to have increased absorption. We measured the thermal conductivity of the nanolaminates to be approximately 1/2 that of the uniform films. A theoretical model including the absorption and thermal conductivity of the nanolaminate and uniform film agreed with the experimental data for 1 ns pulses and CW tests. During laser damage experiments, anomalous damage morphologies were observed that we were unable to explain with theoretical techniques. We then developed an experimental method to observe high-speed laser damage events at the ms time-scale. We imaged laser heating and compared it to a theoretical model with good agreement. Our measurement method captured image data from a Mach- Zender interferometer that had do be processed ex-situ. We desired a system capable of providing real-time thermal data. We developed an image processing technique at least 66 times faster than the original method.
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    Measuring the Transition from Wrinkles to Crumples in Stamped Thin Sheets
    (2023-09) Hawkins, Vanessa M
    A sufficiently thin elastic solid will exhibit changes in its topography when it is confined to a spherical curvature. As the severity of this curvature is increased, a transition from a wrinkled topography (smooth and undulating) to a crumpled topography (sharp and geometric) occurs in the sheet. Previous experiments that examined this transition featured settings where a tensional load at the sheet’s boundaries allowed it to be constrained to a curvature. However, it is not known what criteria govern this transition in a sheet without a tensional load at its boundaries. Here, we show that the point of transition from wrinkles to crumples in a thin sheet without boundary tension depends on the sheet’s thickness, the sheet’s diameter, and the imposed radius of curvature. This was investigated by using a pair of curved glass lenses to stamp a thin sheet placed between them, and the point of transition between wrinkles and crumples is represented as the distance between the lenses at which this transition occurs. We propose an empirical threshold to predict this distance based on the aforementioned parameters. This work adds to a better understanding of topographical deformations in thin sheets, which can both lead to degradation of technologies that utilize such a design and be exploited as an engineering tool for applications such as studying cell mechanics and controlling surface microfluidics.
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    Supporting Data for Circular Dichroism of Distorted Double Gyroid Thin Film Metamaterials
    (2024-11-04) McGuinness, Emily; Magruder, Benjamin; Dorfman, Kevin; Ellison, Chris; Ferry, Vivian; veferry@umn.edu; Ferry, Vivian; Department of Chemical Engineering and Materials Science, University of Minnesota
    Strong circular dichroism (CD) has been reported in triply periodic, co-continuous gyroid thin films for certain orientations and surface terminations. However, processing of gyroid thin films introduces distortions experimentally, creating a mismatch between the structures created practically and those explored computationally. This work explores the impact of compression normal to the substrate (z-compression) with conserved volume in (110)-oriented plasmonic silver double gyroid thin films on CD using finite-difference time-domain (FDTD) simulations. As compression reaches fifteen percent and above, new features emerge including termination-dependent opposite-handed CD responses and, at larger compressions, shorter wavelength responses that span many surface terminations. The longest wavelength responses of the system red-shift with increasing compression. The top surface structure contributes strongly to the emerging opposite-handed features and red-shifting of wavelengths. However, the less surface termination dependent features arise from a mixture of contributions from the top surface and interior of the films. Interplay of these leads to CD-switching phenomena as a function of compression for certain terminations and wavelengths. When alternative methods are utilized to compress the system, such as compression with a Poisson’s ratio of 0.33 (comparable to polystyrene) or the generation of compressed equilibrium structures with non-affine strut changes via self-consistent field theory, similar optical responses persist. Overall, this study highlights the significant impact experimentally relevant distortions (especially compression and some non-affine structural shifts) can have on the CD response of block copolymer templated plasmonic double gyroid thin films, and provides mechanistic insight into the film interior versus surface contributions to the CD response during compression.
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    Thin Film Growth of Black Phosphorus and Black Arsenic Phosphorus
    (2020-09) Izquierdo, Nezhueyotl
    A single-step, direct silicon-substrate growth of black phosphorus (b-P) thin films is achieved by a self-contained (ampule) short-way transport method. The synthesis reactants include tin (Sn), tin tetraiodide (SnI4), and red phosphorus (r-P). A self-generated low-pressure condition of < 1.5 MPa is reached at the maximum soak temperature of 650 ℃. A well-defined phosphorus phase dependency was determined, by adjusting the SnI4 concentration, forming either violet phosphorus (v-P) or b-P. Furthermore, in situ Sn passivation for both thin film and bulk b-P is experimentally verified, enhancing the long-term stability after 4 months of exposure to ambient conditions. A b-P hero single crystal is formed with lateral dimensions of 10 × 85 μm and 115 nm thick. Electron backscatter diffraction (EBSD) measurements determined b-P thin films do not grow epitaxially with the substrate. Cross-sectional transmission electron microscopy (CS-TEM) of a b-P thin film provides valuable insight into the growth mechanism that is difficult to achieve analyzing bulk b-P. Crystalline inclusions are discovered throughout the b-P crystal with a Sn:I ratio of 1.1-1.4, and may be responsible for the dominant mechanism for seeding vertical growth. Thin film and bulk b-P recipe crystals show an equal response below Eg dominated by free carrier absorption for IR absorption measurements. Black arsenic phosphorus (b-As1-xPx) thin films can be achieved with slight modifications to the previous method. The synthesis reactants include Sn, SnI4, grey arsenic (g-As), and red r-P. An in situ Sn passivation layer was found at the surface of the b-AsP, however, at the wafer interface an amorphous layer, with Sn0.07P0.20O0.71 composition, is found. The crystal structure and elemental composition of b-P, b-AsP, v-P, v-AsP, and c-AsP thin films were characterized using the following techniques: Raman spectroscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS), cross-sectional transmission microscopy (CS-TEM) and electron backscatter diffraction (EBSD). The data provides valuable insight into the growth mechanism which motivated the proposed growth mechanism. Thin film b-P field-effect transistors (FET) devices show improved device performance compared to unpassivated b-P films of equivalent thickness with an on/off current ratio >102. Thin film b-ASP FET’s fabricated from exfoliated bulk-b-AsP grown in the same conditions as the thin film growth process show an on-off current ratio of 102, a threshold voltage of -60 V, and a peak field-effect hole mobility of 23 cm2/V·s at Vd=-0.9 and Vg=-60 V.