Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Functional imaging of photovoltaic materials at the nanoscale(2018) Tennyson, Elizabeth; Leite, Marina S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ideal photovoltaic technology for global deployment must exhibit two key attributes: (i) high power-conversion efficiency, enabling a solar panel with a large power output per area and, (ii) low-cost/W, due to either being derived from earth-abundant materials and/or ease of fabrication. For the past two decades, extensive efforts have been made to boost the efficiency of some of the most promising high performance and low-cost photovoltaic materials, such as CdTe, Cu(In,Ga)Se2 (CIGS), and hybrid organic-inorganic perovskites, to achieve higher efficiency devices. However, improvement in the overall performance is still limited by the open-circuit voltage (Voc). All of the solar cell materials listed above are composed of grains and grain boundaries on the order of micro- and nanometers, respectively, and their nanoscale interfaces can cause electrical charge carriers to become trapped and recombine non-radiatively, reducing the Voc. Therefore, in this thesis, I implement high spatial resolution functional imaging techniques to resolve the local voltage variations in the thin-lm polycrystalline and hybrid perovskites materials for photovoltaic applications. First, I spectrally and spatially resolve the local photovoltage of CIGS solar cells through confocal optical microscopy to build a qualitative voltage tomography. From these photovoltage results, I discover variations in the electrical response of >20% that are also on the same length scale as the grains composing the CIGS material. Therefore, by enhancing the spatial resolution beyond the diffraction limit, the electronic properties of individual grains and the interfaces between the grains can be fully resolved. For this, I implement Kelvin probe force microscopy (KPFM), and demonstrate a universal method to directly map the Voc of any photovoltaic material with nanoscale spatial resolution. Next, we extend this ability of KPFM to rapidly image (16 sec/map) the real-time dynamics of perovskite solar cells, which are notorious for their slow and unstable electrical output. Through fast-KPFM imaging, we discover regions within a single grain that show a residual Voc response which pervades for ~9 min, likely caused by a slow ion migration process. Finally, to understand how dierent perovskite compositions influence the behavior of the nanoscale electrical response, I utilize KPFM to realize both irreversible and reversible Voc signals. Compiling all these results discussed above, throughout my Ph.D. I have yielded the following contributions: (i) evidence that the photovoltage of polycrystalline solar cell materials varies at the same length scale as the grains composing them, (ii) a nanoscale imaging platform to directly map the Voc with unprecedented spatial resolution, and (iii) a technique to map the real-time voltage response of many perovskite compositions, ultimately indicating that the elements constituting the perovskite cation and halide positions are both directly related to their reversible vs. irreversible electrical nature. From these contributions, I foresee the functional imaging methods developed in this thesis to be widely implemented as a diagnostic tool for the rational design of photovoltaics with enhanced electrical performance and lower cost.Item Structural and Chemical Factors Governing Anion Reactivity in Perovskite Oxides(2017) Taylor, Daniel Douglas; Rodriguez, Efrain E; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Topochemical reactions - those in which the composition of a material is modified while its overall structure is left largely intact - are the basis of numerous technologies such as battery electrodes and CO2 capture. This dissertation outlines our efforts to understand the underlying chemical and structural factors controlling the outcome of these reactions. With a focus on the anion sublattice in perovskite-type oxides, this research was performed as part of two projects. First, we explored the use of topochemical oxygen removal reactions for the preparation of new functional materials with potential application in next generation computing. Through this effort, we successfully synthesized the first example of ferromagnetic cubic Sr2FeMoO6 suggesting the possibility of raising the magnetic ordering temperature, and therefore the degree of spin polarization, of this material through topochemical treatments. Second, we investigated the ability to use complex transition metal oxides as oxygen storage materials for new chemical-looping technologies. For example, our studies of La(1-x)Sr(x)FeO(3-d) yielded two primary findings -- cycling reaction kinetics are strongly dependent on Sr content and each sample has an envelope of oxygen storage capacity over a set temperature and atmospheric composition. These projects both used advanced diffraction techniques, neutron and synchrotron X-ray, to study the structure of materials in-situ in order to link their structure to their properties. Furthermore, through this research we developed tools for the rapid refinement of large sets of powder diffraction patterns thus speeding up the rate at which the data from high-speed in-situ diffraction experiments can be analyzed and presented.