Doctor of Philosophy, University of Toledo, 2023, Physics

Low-cost thin film absorber layer materials with high absorption coefficients (> 105 cm-1 in visible spectral range) and bandgap close to the ideal value for efficient photovoltaic conversion efficiency are leading candidates for thin film photovoltaic (PV) applications. This dissertation discusses the fabrication and optical and microstructural properties of magnetron-sputtered glancing angle deposited CdTe thin film absorber layer material and its application as an interlayer in CdS/CdTe solar cells. In addition, optoelectronic properties of non-toxic and earth-abundant absorber layer material, antimony selenide (Sb2Se3), and optimization of polycrystalline VO2 fabrication from amorphous vanadium oxide (VOx) film along with its optical properties have been discussed. Sb2Se3 is a promising candidate as an absorber layer material in PV applications. I have performed optical property characterization of thin film Sb2Se3 and identified electronic losses when used in a PV device. The indirect bandgap, direct bandgap, and Urbach energy have been determined to be 1.12 eV, 1.17 eV, and 21.1 meV, respectively using photothermal deflection spectroscopy. Optical properties of Sb2Se3 in the form of complex dielectric function (ε = ε1 + iε2) spectra in 0.75 to 4 eV spectral range is determined using spectroscopic ellipsometry. The line shape of ε is obtained using a parametric model which incorporates an Urbach tail, a band edge function, and five critical point oscillators. The optical property spectra in ε and structural parameters in terms of the thickness of solar cell layer components are used as input parameters for external quantum efficiency (EQE) simulation to investigate the electronic and optical losses in Sb2Se3-based solar cells. A carrier collection length of ~ 400 nm and a ~97 % carrier collection probability near the heterojunction in the Sb2Se3 solar cell are identified by comparing experimental and simulated EQE. Next, I describe deposition and characterizati (open full item for complete abstract)

Committee: Nikolas J. Podraza (Committee Chair); Robert W. Collins (Committee Member); Yanfa Yan (Committee Member); Song Cheng (Committee Member); Terry Bigioni (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2022, Chemistry

Perovskite-based materials provide an exemplary platform to explore fundamental structure-property relationships. The relative simplicity of the perovskite crystal structure and a plethora of interesting properties related to purposes such as optoelectronics, multiferroics, and superconductivity, has sparked immense research interest. For halide perovskite variants, research has focused primarily on APbX3 (A = Cs+, CH3NH3+, NH2CHNH2+; X = Cl−, Br−, I−) perovskites for photovoltaic applications. One of the most impressive features of the halide perovskite is the ease of chemical substitution, which has resulted in a wide variety of structural variations. Halide perovskite materials containing transition metals offer a broader range of applications due to the possibility of diamagnetic and paramagnetic electronic configurations. Here we explore halide perovskite derivatives containing transition metals to understand the optical and magnetic properties that arise in low dimensional crystal structures. Following the first introductory chapter, Chapter 2 looks to expand on the known compositional space of the (CH3NH3)2M′MʺX6 halide double perovskites by introducing Rh3+ at the Mʺ site. Here, we synthesized (CH3NH¬3)2M′RhX6 (M′ = Na+, Ag+; X = Cl−, Br−) and looked to understand the optical properties that arise from the one-dimensional chain structure of these hexagonal 2H perovskite variants. This sets the foundation for Chapter 3, which explores two-dimensional layered perovskites containing the Ag–Rh–X inorganic framework. By replacing CH3NH3+ with a bulky organic cation, the crystal structure transitions from one-dimensional chains to two-dimensional layers. The optical properties are further explored in comparison to the previously studied hexagonal perovskites. Changes in the absorption spectra are explained by changes in the octahedral connectivity and ability of orbitals to hybridize. Chapter 4 focuses on the structural phase transitions and magnetic propertie (open full item for complete abstract)

Committee: Patrick Woodward (Advisor); Yiying Wu (Committee Member); Joshua Goldberger (Committee Member) Subjects: Chemistry; Materials Science

Doctor of Philosophy, University of Toledo, 2022, Physics

In this dissertation the structural properties of bulk alloys and thin-films are studied using a variety of di erent techniques including density functional theory (DFT) and accelerated dynamics. The first part of this dissertation involves the use of DFT calculations. In particular, in Chapter 3 the stability and mechanical properties of 3d transitional metal carbides in zincblende, rocksalt, and cesium chloride crystal structures are studied. We find that the valence electron concentration and bonding configuration control the stability of these compounds. The filled bonding states of transition metal carbides enable the stability of the compounds. In the second part of this dissertation we use a variety of accelerated dynamics techniques to understand the properties of growing and/or sublimating thin-films. In Chapter 4, the results of temperature-accelerated dynamics (TAD) simulations of the submonolayer growth of Cu on a biaxially strained Cu(100) substrate are presented. These simulations were carried out to understand the e ects of compressive strain on the structure and morphology. For the case of 4% compressive strain, stacking fault formation was observed in good agreement with experiments on Cu/Ni(100) growth. The detailed kinetic and thermodynamic mechanisms for this transition are also explained. In contrast, for smaller (2%) compressive strain, the competition between island growth and multi-atom relaxation events was found to lead to an island morphology with a mixture of open and closed steps. In Chapter 5, we then study the general dependence of the diffusion mechanisms and activation barriers for monomer and dimer diffusion as a function of strain. The results of TAD simulations of Cu/Cu(100) growth with 8% tensile strain are also presented. In this case, a new kinetic mechanism for the formation of anisotropic islands in the presence of isotropic diffusion was found and explained via the preference for monomer diffusion via exchange over hopp (open full item for complete abstract)

Committee: Jacques Amar Professor (Advisor) Subjects: Physics

Doctor of Philosophy, Case Western Reserve University, 2021, Physics

The ever-increasing number of applications requiring semiconductor materials at their core is driving the need to understand certain oxide and nitride materials. In this thesis, we investigate two of such classes. The first of those is the class of wide band-gap oxides and includes materials like β-〖Ga〗_2 O_3 and the 〖(〖Al〗_x 〖Ga〗_(1-x))〗_2 O_3 alloy system. β-〖Ga〗_2 O_3 is the most stable of the five phases in which 〖Ga〗_2 O_3 is found to exist. With a significantly high experimentally measured band gap of 4.5-4.9 eV, it is touted to be an excellent material for high-power electronics and UV transparent optoelectronic applications. Using first-principles calculations, we study this material and present the electronic band structure calculations using the quasiparticle self-consistent GW method. Next, we extend this study to the alloy system 〖(〖Al〗_x 〖Ga〗_(1-x))〗_2 O_3 in which 〖Ga〗_2 O_3 is alloyed with an even higher band-gap material, 〖Al〗_2 O_3. We study the system in both the phases, α and β, present the electronic band structures for varying compositions of Al ranging from 0% to 100%, and predict the most favorable composition and phase for such an alloy to exist. The second class of materials in this thesis is the alloy system formed by the combination of group III- and II-IV nitrides, GaN and 〖ZnGeN〗_2, respectively. In particular, we study the vibrational properties of 〖ZnGeGa〗_2 N_4. 〖ZnGeGa〗_2 N_4, at 50% composition, is an octet-preserving and lowest energy superlattice of half a cell of 〖ZnGeN〗_2 and half GaN along the b-axis of 〖ZnGeN〗_2 in the 〖Pbn2〗_1 structure. Using Density Functional perturbation theory implemented in ABINIT, the phonon modes at the zone center, Γ allow us to calculate longitudinal optical-transverse optical splittings using Born effective charges. In addition, the IR and Raman spectra along with the phonon density of states, and the phonon band structure are presented. Lastly, we study the transition metal (open full item for complete abstract)

Committee: Walter Lambrecht (Advisor) Subjects: Condensed Matter Physics; Materials Science; Physics

Doctor of Philosophy, University of Akron, 2016, Polymer Engineering

Conjugated polymers (CPs) based organic thermoelectrics (OTEs) and organic photovoltaics (OPVs) are promising alternatives to their inorganic counter-parts for the next generation renewable energy sources. However, in order to commercialize them, many fundamental and technical issues need to be addressed. This dissertation will address some of these issues. In part I of this dissertation, we report enhanced OTEs performance by tuning the molecular arrangement of CPs, hence to their doping states. In particular, a novel phenomenon of the piezo-conductive effect in CPs is observed for the first time and such phenomenon is exclusive studied. In part II of this dissertation, we report enhanced OPVs device performance by controlling molecular arrangement within the bulk heterojunction composite and interfacial reengineering. . Part I. Organic Thermoelectrics In recent years, to approach high performance thermoelectrics (TEs), great efforts have been paid to reduce the thermal conductivities of inorganic semiconductors. However, inorganic semiconductors, even with sophisticated and complex nanostructures, still possess large intrinsic thermal conductivities (>1 W/mK), which highly suppress the improvements in developing inorganic TEs. Alternatively, OTEs exhibit great potential due to low thermal conductivities (<0.5W/mK) of organic materials. Among all organic semiconductors being investigated for thermoelectric application, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) stood out to be one of the most promising organic materials. It was reported that the electrical conductivity of PEDOT:PSS thin film can be improved by three orders of magnitude via single secondary dopant as compared with pristine PEDOT:PSS thin film. For instance, the electrical conductivity of PEDOT:PSS thin film was improved to 289.52 S/cm when doped with 5% (by volume) dimethyl sulfoxide (DMSO). In order to simultaneously increase the electrical conductivity and the Seebec (open full item for complete abstract)

Committee: Xiong Gong (Advisor); Thein Kyu (Committee Chair); Robert Weiss (Committee Member); Yu Zhu (Committee Member); Jie Zheng (Committee Member) Subjects: Energy; Materials Science; Physical Chemistry; Physics; Polymers

Doctor of Philosophy (PhD), Wright State University, 2014, Engineering PhD

Novel silicene-based nanomaterials are designed and characterized by first principle computer simulations to assess the effects of adsorptions and defects on stability, electronic, and thermal properties. To explore quantum thermal transport in nanostructures a general purpose code based on Green's function formalism is developed. Specifically, we explore the energetics, temperature dependent dynamics, phonon frequencies, and electronic structure associated with lithium chemisorption on silicene. Our results predict the stability of completely lithiated silicene sheets (silicel) in which lithium atoms adsorb on the atom-down sites on both sides of the silicene sheet. Upon complete lithiation, the band structure of silicene is transformed from a zero-gap semiconductor to a 0.368 eV bandgap semiconductor. This new, uniquely stable, two-atom-thick, semiconductor material could be of interest for nanoscale electronic devices. We further explore the electronic tunability of silicene through molecular adsorption of CO, CO2, O2, N2, and H2O on nanoribbons for potential gas sensor applications. We find that quantum conduction is detectibly modified by weak chemisorption of a single CO molecule on a pristine silicene nanoribbon. Moderate binding energies provide an optimal mix of high detectability and recoverability. With Ag contacts attached to a ~ 1 nm silicene nanoribbon, the interface states mask the conductance modulations caused by CO adsorption, emphasizing length effects for sensor applications. The effects of atmospheric gases: nitrogen, oxygen, carbon dioxide, and water, as well as CO adsorption density and edge-dangling bond defects, on sensor functionality are also investigated. Our results reveal pristine silicene nanoribbons as a promising new sensing material with single molecule resolution. Next, the thermal conductance of silicene nanoribbons with and without defects is explored by Non-Equilibrium Green's function method as implemented in our Th (open full item for complete abstract)

Committee: Amir Farajian Ph.D. (Advisor); Khalid Lafdi Ph.D. (Committee Member); Sharmila Mukhopadhyay Ph.D. (Committee Member); Ajit Roy Ph.D. (Committee Member); H. Daniel Young Ph.D. (Committee Member) Subjects: Materials Science; Nanoscience; Nanotechnology

Doctor of Philosophy (PhD), Ohio University, 2014, Physics and Astronomy (Arts and Sciences)

Urbach tails are the exponential band tails observed universally in impure crystals and disordered systems. Evidence has been provided that the topological origin of the Urbach tails in amorphous materials are filaments formed by short or long bonds[20]. One aspect of my work focuses on the size effects and choice of Hamiltonian with respect to the structure of the Urbach tails. The dynamical properties of filaments have been studied by performing Molecular Dynamics simulation under constant temperature. The response of filaments under external pressure has also been explored. The second portion of this dissertation is about carbon in two-dimensional sp2 phases. Carbon has shown itself to be the most flexible of atoms, crystallizing in divergent phases such as diamond and graphite, and being the constituent of the entire zoo of (locally) graphitic balls, tubes, capsules and possibly negative curvature analogs of fullerenes, the Schwartzites. In this part, we explore topological disorder in three-coordinated networks including odd-membered rings in amorphous graphene, as seen in some experimental studies. We start with the Wooten-Weaire-Winer models due to Kumar and Thorpe, and then carry out ab-initio studies of the topological disorder. The structural, electronic and vibrational characteristics are explored. We show that topological disorder qualitatively changes the electronic structure near the Fermi level. The existence of pentagonal rings also leads to substantial puckering in an accurate density functional simulation. The vibrational modes and spectra have proven to be interesting, and we present evidence that one might detect the presence of amorphous graphene from a vibrational signature. We also explore the energy landscape of amorphous graphene and report the eigenstates near the Fermi level.

Committee: David Drabold (Advisor); Gang Chen (Committee Member); Eric Stinaff (Committee Member); Jeffrey Rack (Committee Member) Subjects: Condensed Matter Physics; Physics