Master of Science in Chemistry, Youngstown State University, 2018, Department of Chemistry

The energy we get from the sun is a key factor in electric power production on earth and in space applications. The development of photovoltaic cells has enabled a new direct method for solar electricity. However, the manufacturing cost of photovoltaic cells must be lowered to have widespread implementation. Among the leading candidates CdTe, is used for photovoltaic applications, since it has optimum band gap energy for the efficient conversion of solar energy into electricity. It is produced by a series of vacuum procedures, which is a significant part of its fabrication cost. In this work, cadmium telluride (CdTe) thin films were electrodeposited on stainless steel 304 substrates using a three-electrode system at a negative potential. Cadmium sulfate and tellurium dioxide in pH 1.8 H2SO4 were used as the cadmium and tellurium sources, respectively. Deposition conditions were adjusted to codeposit Cd and Te at the same rate. Films were deposited on stainless steel 304 as a relatively inexpensive substrate. However, to obtain a proper ohmic contact between CdTe and the steel, it was necessary to electrodeposit a thin interlayer of pure Te to achieve an ohmic contact. Electrodeposition of a thin Pt layer on top of the CdTe served to greatly increase the rate of H2 evolution. The structural and morphological properties of the resulting films were characterized using light/dark voltammetric methods, X-ray diffraction (XRD), Profilometry, Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS).

Committee: Clovis Linkous PhD (Advisor); Tom Order PhD (Committee Member); Timothy Wagner PhD (Committee Member) Subjects: Analytical Chemistry; Chemistry; Condensed Matter Physics

Doctor of Philosophy, University of Toledo, 2015, Physics

Spectroscopic ellipsometry (SE) in the mid-infrared to ultraviolet range has been implemented in order to develop and evaluate optimization procedures for CdTe solar cells at the different stages of fabrication. In this dissertation research, real time SE (RT-SE) has been applied during the fabrication of the as-deposited CdS/CdTe solar cell. Two areas of background research were addressed before undertaking the challenging RT-SE analysis procedures. First, optical functions were parameterized versus temperature for the glass substrate and its overlayers, including three different SnO2 layers. This database has applications not only for RT-SE analysis but also for on-line monitoring of the coated glass itself at elevated temperature. Second, post-deposition modifications of substrate have been studied by infrared spectroscopic ellipsometry (IR-SE) prior to the RT-SE analysis in order to evaluate the need for such modification in the analysis. With support from these background studies, RT-SE has been implemented in analyses of the evolution of the thin film structural properties during sputter deposition of polycrystalline CdS/CdTe solar cells on the transparent conducting oxide (TCO) coated glass substrates. The real time optical spectra collected during CdS/CdTe deposition were analyzed using the optical property database for all substrate components as a function of measurement temperature. RT-SE enables characterization of the filling process of the surface roughness modulations on the top-most SnO2 substrate layer, commonly referred to as the high resistivity transparent (HRT) layer. In this filling process, the optical properties of this surface layer are modified in accordance with an effective medium theory. In addition to providing information on interface formation to the substrate during film growth, RT-SE also provides information on the bulk layer CdS growth, its surface roughness evolution, as well as overlying CdTe interface formation and bulk layer g (open full item for complete abstract)

Committee: Robert Collins Dr. (Committee Chair); Nikolas Podraza Dr. (Committee Member); Yanfa Yan Dr. (Committee Member); Sanjay Khare Dr. (Committee Member); Stephen O'Leary Dr. (Committee Member) Subjects: Physics

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

Master of Science in Engineering, Youngstown State University, 2023, Department of Electrical and Computer Engineering

The use of renewable and sustainable energy sources such as solar cells have been increasing slowly and gradually. With the rapid increase and growth of solar photovoltaics (PV) production, approximately 22% from 2010 to 2020, high number of PV cells are being manufactured annually. Solar cells have been proven to be an efficient source of renewable energy because of their availability, installation, and affordability. Among all the different types of solar cells, thin-film PV has several advantages over other technologies because of its need for relatively fewer resources, ability to fully automate fabrication process, and the option to fabricate on a more affordable substrate. Nevertheless, the manufacturing process of these thin-film solar cells still involves several critical steps. For example, they still rely on conventional method of laser scribing techniques which in addition to being time consuming, has several reliability issues as well. In this project, the theory of threshold switching method is experimentally verified on Cadmium Telluride (CdTe) PV. The study was conducted by applying different voltage bias configurations to the sample. A drastic decrease in the resistance was observed. The time lapse study of decreased resistance was conducted, which proved that the resistance change is indeed a permanent phenomenon. Our experimental results of threshold switching indicates that this could potentially lead to a scribe-less technology in manufacturing of solar cells which may potentially lead to an increase in manufacturing reliability and efficiency.

Committee: Vamsi Borra PhD (Advisor); Daniel Georgiev PhD (Committee Member); Victor Karpov PhD (Committee Member); Ghassan Salim MS (Committee Member) Subjects: Aerospace Materials; Electrical Engineering; Materials Science; Nanotechnology

Doctor of Philosophy, University of Toledo, 2023, Physics

Cadmium Telluride (CdTe) has been long recognized as a semiconductor material well suitable for high-energy x-ray detection in spectroscopic, imaging, and radiation therapy applications. Its high-resolution makes CdTe an attractive material for radiation detectors in small field dosimetry. Interpretation of small-field measurements with pretty much any detector typically requires use of correction factors which depend on the detector construction and measurement conditions. A greatly preferable solution to account for perturbations compensation to achieve correction-less dosimetry is to design a water equivalent detector. In a recently proposed approach mass-density is utilized as the main factor of detector water-equivalence. We evaluated the approach for a range of geometric parameters utilizing CdTe and compared with traditional silicon (Si) diode sensitive media. Monte Carlo simulation was used to optimize dosimeter designs combining a semiconductor sensitive volume with an air-gap of various thickness, following a mass-density matching approach. It was discovered that, unlike Si, the relative response of a CdTe detector to water is energy dependent. The addition of air to the sensitive volume of a CdTe-based detector does not lead to an improved response to water. Moreover, we explored the relevant energy spectra, backscattered fluences, and their correlations with the critical detector metric of dose deposition. For this purpose, we modified electron spectra reaching and depositing energy in sensitive volume of CdTe detector using a multilayer structure of CdTe combined with metal back-reflectors of lead (Pb) and Copper (Cu). The total fluence of electrons scattered back in CdTe with a metal back reflector and CdTe alone were found to be increased by factors of 6 and 3.7 using Pb and Cu, respectively. This suggests that the detector signal can be adjusted in a multi-layered structure consisting of a sensitive volume and a metal back-reflector layer. A detailed (open full item for complete abstract)

Committee: David Pearson (Committee Chair); Nicholas Sperling (Committee Member); Suleiman Aldoohan (Committee Member); Richard Irving (Committee Member) Subjects: Physics; Radiation

Doctor of Philosophy, University of Toledo, 2023, Physics

Current manufacturing techniques allow for mass production of high-efficiency cadmium telluride (CdTe) photovoltaic (PV) modules at a low cost per watts. The robust nature of the materials and the high optical absorption coefficient with a suitable band gap for optimal photon power conversion made CdTe more attractive. Today's CdTe solar cells hold a record efficiency of 22.1%. However, the CdTe device efficiency is below the theoretical limits due to the recombination of photo-generated carriers in front/back interfaces and in the bulk of the absorber. Such recombination reduces the open circuit voltage (Voc) of the devices. Understanding the role of the different defects and defects complexes formed during absorber preparation and after post-deposition treatments is necessary to minimize carrier recombination. Also, using the front/back buffer layers for proper band alignment at interfaces are needed to reduce interfacial recombination. This dissertation focuses on materials engineering and control to minimize carrier recombination and hence improve devices performance. We fabricated high-quality CdTe absorbers with a new approach to CdTe deposition using a high vacuum close space sublimation (CSS) system. Reorganization of the defect complexes associated with Cu ion migration during light soaking of CdSe/CdTe devices is studied. A minority carrier lifetime of 656.5 ns is reported with a high-quality CdTe absorber and passivated back surface with a back buffer layer of copper aluminum oxides (CuxAlOy), resulting in ~ 860 mV Voc and ~ 17.5 % device efficiency. A problem with low-doped magnesium zinc oxide (MZO) as a front emitter layer in CdSe/CdTe devices has been resolved by increasing the doping density of MZO films with high vacuum annealing. To minimize front interfacial recombination, a new wide bandgap front emitter layer of indium gallium oxide (InxGa1-x)2O3 (IGO) has been introduced to tune the bandgap and conduction band offset (CBO) with absorber at th (open full item for complete abstract)

Committee: Michael Heben Dr. (Advisor); Michael Heben Dr. (Committee Chair); Alvin Compaan Dr. (Committee Member); Randy Ellingson Dr. (Committee Member); Richard Irving Dr. (Committee Member); Yanfa Yan Dr. (Committee Member) Subjects: Physics

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, University of Toledo, 2022, Physics

This dissertation represents a collection of related studies that employ the various capabilities of spectroscopic ellipsometry (SE) to characterize and gain insights into the properties of the materials of relevance to advanced cadmium telluride (CdTe) photovoltaic technology. The structural, optical, and electrical properties of the component layers of the CdTe solar cell have been investigated using different SE data collection modes and analysis techniques. The component layers of the CdTe solar cell are deposited on soda lime and TEC-15 glass substrates in the superstrate configuration, i.e., with the solar irradiance entering through the glass substrate. The key layers include a transparent conducting oxide front contact, typically pyrolytic fluorine-doped tin oxide (SnO2:F); a high resistivity transparent layer (HRT) of either pyrolytic SnO2, sputtered MgxZn1 xO (MZO), or both; semiconductor layers of either n-type cadmium sulfide (CdS), cadmium selenide (CdSe), or both; p-type CdTe; a p+ back contact interlayer typically copper based; and a metallic conducting back contact layer, such as gold. In this research, SE-deduced component layer properties have been correlated with the CdTe device performance parameters. Applying various SE capabilities not only identifies the correlations between the material properties and solar cell performance but also enables optimization of the preparation conditions (e.g., substrate temperature) and resulting properties (e.g., thickness) of the CdTe device components. First, we have employed UV-VIS SE and ex-situ mapping SE results to correlate between the CdSe optical and structural properties with the CdTe solar cell performance. The effects of varying CdSe layer thickness on the CdTe solar cell performance have been studied, focusing on the TEC-15/HRT/CdSe/CdTe/Cu/Au cell structure. A set of four 6.5 cm x 6.5 cm TEC-15/HRT structures were coated with different nominal thicknesses of CdSe for incorporation within the (open full item for complete abstract)

Committee: Robert Collins Dr. (Advisor); Jone Bjorkman Dr. (Committee Member); Stephen O'Leary Dr. (Committee Member); Randall Ellingson Dr. (Committee Member); Nikolas Podraza Dr. (Committee Member) Subjects: Meteorology; Optics; Physics

Doctor of Philosophy, University of Toledo, 2020, Physics

Photovoltaic (PV) solar cells have attracted great attention because of the demand for low cost renewable energy sources. Detailed information on electronic properties, such as doping, defects, gap states…etc, must be fully understood to develop the technology of solar cells. Here, we report the fundamental electronic properties of two distinct materials systems, one is based on polycrystalline cadmium telluride (CdTe) and the other is lead-halide perovskite solar cells. This investigation provides useful information to understand the fundamental nature of single junction solar cell device and material. First, we investigate the impact of back surface treatment method for cadmium sulfide (CdS)/CdTe solar cells using hydroiodic acid (HI) etching to provide an appropriate electrical back contact. The structural properties of CdTe films and electrical properties of the CdTe absorber and interfaces are characterized. Using capacitance-based techniques with the support of current–voltage measurements, we show that the barrier height of the back contact is reduced, apparent doping concentration is increased, and a defect level at 0.409eV is eliminated after the HI-treatment. More importantly, the CdTe device performance is improved. This improvement is still limited by many factors. One factor is the device window layer that limits the current generation. Therefore, we replaced CdS layer by wide bandgap material, ZnMgO (ZMO). We noticed that the electrical properties of CdS/CdTe and ZMO/CdTe solar cells depend on both buffer material and the fabrication atmosphere. Using capacitance spectroscopy-based techniques, we show that CdS/CdTe solar cells have negligible front contact barriers regardless of the fabrication atmospher, while ZMO/CdTe devices show obvious front barriers are dependent on the fabrication atmosphere. Both CdS/CdTe and ZMO/CdTe solar cells have significant back contact barriers. Additionally, we find that the energy level of defects in CdS/CdTe cells (open full item for complete abstract)

Committee: Yanfa Yan (Committee Chair); Jian Li (Committee Member); Jacques Amar (Committee Member); Xunming Deng (Committee Member); Nikolas Podraza (Committee Member) Subjects: Energy; Materials Science; Physics

Doctor of Philosophy, University of Toledo, 2020, Physics

Energy is considered one of the top problems facing humanity, and climate change clearly require3s growth in the fraction of energy provided by renewable sources in order to minimize the use of fossil fuels. Solar energy is an environmentally friendly technology which avoids nearly all generation of greenhouse gases. It is more available than hydro, wind, and other renewable energy sources. As science and technology advance, various types of solar cells are being produced to harvest solar energy. Silicon technology dominates the photovoltaic (PV) industry. Thin film solar electric technologies are evolving to reduce the cost of energy ($/Watt). Cadmium telluriude (CdTe) is one of the leading thin film technologies with a reduced energy cost and the shortest energy payback time. CdTe is a direct band gap II-VI material and has been investigated for decades to improve device performance, stability, and conversion efficiency. However, the open-circuit voltage (VOC) of CdTe devices is lower than for GaAs solar cells, despite having similar energy band gaps. Simulation and theoretical study show that the voltage deficit can be improved by making an ohmic contact to CdTe, or by reducing the back-contact potential barrier and increasing the doping level of CdTe. In this dissertation, I report on nanomaterials-based back contact interface layers, surface etching, the doping of CdTe using copper (II) chloride (CuCl2), and on the opto-electronic properties of CdTe double heterostructure samples. Nanocrystals (NCs) of FeSe2, FeTe2, NixFe1-xS2, and CuFeS2 were synthesized using hot injection colloidal methods. These materials were then characterized using X-ray diffraction, electron microscopy, Raman and UV-Vis-NIR spectroscopies. The charge transport properties of these nanomaterial-based thin films were tested in CdTe photovoltaics. Nickel iron pyrite (NixFe1-xS2) NCs showed composition-controlled conductivity, and where x = 0.05 (5%), they showed optimal device performanc (open full item for complete abstract)

Committee: Randy Ellingson (Committee Chair); Michael Heben (Committee Member); Robert Collins (Committee Member); S. Thomas Megeath (Committee Member); Mikhail Zamkov (Committee Member) Subjects: Nanoscience; Nanotechnology; Physical Chemistry; Physics

Doctor of Philosophy, University of Toledo, 2019, Physics

Thin film solar cells are promising candidates for generation of low cost and pollution-free energy. The materials used in these devices, mainly the active absorber layer, can be deposited in a variety of industry-friendly ways, so that the cost associated with manufacturing is generally lower than for competing technologies such as crystalline silicon. This dissertation will focus on the fabrication and characterization of nanocrystalline hydrogenated silicon (nc-Si:H) and polycrystalline cadmium telluride (CdTe) thin films by industrially scalable, non-toxic, and comparatively simple magnetron sputtering. The performance of the solar cells incorporating these films as an active absorber layers are discussed. In this work, spectroscopic ellipsometry is used as the primary tool for the characterization of optical and structural properties of thin films and bulk material. As a first case study, the anisotropic optical properties of single crystal strontium lanthanum aluminum oxide (SrLaAlO4) in the form of birefringence and dichroism is obtained from Mueller matrix ellipsometry. SrLaAlO4 exhibit uniaxial anisotropic optical properties and the indirect optical band gap of 2.74 eV. A parametric model consisting of parabolic band critical points (CPs) for electronic transitions and a gap function is used to describe the complex dielectric function spectra in both the ordinary and extra-ordinary directions. The modeling in this case study has applications to both nc-Si:H, an indirect band gap semiconductor, and CdTe which may exhibit microstructural anisotropy depending upon the deposition method. Fabrication and characterization of hydrogenated silicon (Si:H) thin films produced by reactive magnetron sputtering is the second case in this study. RTSE and a virtual interface analysis (VIA) are used to track the growth evolution of sputtered Si:H. From these studies, growth evolution diagrams depicting the nucleation of nanocrystallites from the amorphous phase and (open full item for complete abstract)

Committee: Nikolas Podraza (Committee Chair); Robert Collins (Committee Member); Yanfa Yan (Committee Member); Michael Cushing (Committee Member); Sylvain Marsillac (Committee Member) Subjects: Energy; Materials Science; Optics; Physics

Doctor of Philosophy, University of Toledo, 2019, Physics

Evaluation and understanding of optical properties are essential for the use and design of optoelectronic devices. This dissertation explains the evaluation of optical properties of component layers of the encapsulated photovoltaic (PV) module and uses them in device simulation focusing on thermal management. Sub-bandgap characterizations are not emphasized enough in the PV device design earlier. The examples discussed here range from ordinary glass used to cover solar cells to completed silicon (Si) wafer and thin film cadmium telluride (CdTe) solar cells. This study will focus on two key mechanisms for thermal management: radiation and sub-bandgap reflection. Commercial solar cells have light incident through the glass in solar wavelength range ~250- 2500 nm. This cover glass has an ability to re-radiate heat in the infrared (IR) region, thermal wavelength, from the device to keep the solar cells cool. In contrast to bare semiconductors, glass has a relatively high emissivity aiding in radiative cooling of solar modules. The directional thermal emissivity of solar cell cover glasses with differences in glass composition or manufacture and surface texture are evaluated using specular and specular+diffuse infrared reflectance at a different angle of incidences. Non-textured and textured glasses all exhibit similar emissivity at all angles of incidence regardless of composition and patterning. Both diffuse and specular reflectance must be included for textured glass at any angle of incidence and may be needed for planar glass at a high angle of incidences to determine emissivity accurately. Optical characterization of the semiconductor is important from the perspective of physics and application in devices. There are different features in the optical response related to different physical phenomena such as band to band electronic transitions, vibrational or phonon modes, and free carrier absorption. I have explored optical properties of an epitaxial indium pho (open full item for complete abstract)

Committee: Nikolas Podraza (Committee Chair); Robert Collins (Committee Member); Yanfa Yan (Committee Member); Sanjay Khare (Committee Member); Michael Deceglie (Committee Member) Subjects: Physics

Master of Science (MS), Bowling Green State University, 2019, Physics

Cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) based thin-film photovoltaic devices are leading candidates for cost-effective solar electricity. Inter-diffusion of atoms in device components play a critical role in the performance and degradation of a thin-film solar cell. Diffusion studies require special attention because device components are in general polycrystalline and exhibit grain boundaries. In this work, we review the physics of diffusion kinetics in solids (crystalline and polycrystalline) and develop a general numerical simulation tool for ion drift-diffusion. We validate the model against literature data and analytical expressions for the technologically important cases of copper (Cu) and phosphorus (P) in CdTe. We present empirical and numerical analysis for evaluation of sodium (Na) diffusion in CIGS. The calculations are conducted by the finite element method using COMSOL Multiphysics® software. Our study indicates that both Cu and P diffuses in crystalline CdTe using two mechanisms, slow and fast. In the case of Cu in crystalline CdTe, our model confirms the slow diffusivity values of 3.710-4 exp(- 0.67eV/kT) cm2/s and 6.610-5 exp(-0.50eV/kT) cm2/s depending on the experiment, along with the fast diffusivity of 210-8 cm2/s. In case of P in crystalline CdTe, the slow and fast diffusivities are confirmed to be 4.8610-3 exp(-2.03eV/kT) cm2/s and 2.1510-5 exp(-1.0eV/kT) cm2/s, respectively. Migration of P in polycrystalline CdTe includes a grain boundary diffusion coefficient of 6.61102 exp(-1.97eV/kT) cm2/s. Our analysis for Na in polycrystalline CIGS indicates the grain boundary diffusivity of 5.0610-9 exp(-0.09eV/kT) cm2/s.

Committee: Marco Nardone Ph.D. (Advisor); Lewis Fulcher Ph.D. (Committee Member); Alexey Zayak Ph.D. (Committee Member) Subjects: Materials Science; Physics

Doctor of Philosophy, University of Toledo, 2017, Physics

Spectroscopic ellipsometry (SE) is a powerful tool for non-destructive evaluation of thin films consisting of single layers or multilayers on substrates. For such thin films, SE can provide structural parameters and component material optical properties over a wide spectral range. Further analyses of these optical properties can provide additional information of interest on the physical and chemical properties of the materials. Installation of the SE instrument on a deposition chamber enables monitoring of the thin film during deposition. This experiment, referred to as in-situ real time SE (RTSE), enables high surface sensitivity, fast data acquisition, and non-invasive probing which lead to unique insights into the dynamics of film growth. In this dissertation research, RTSE was applied for analysis of the structural evolution of oxygenated CdS (CdS:O) films deposited on c-Si substrates using different [O2 ]/{[Ar] + [O2]} gas flow ratios. The analysis of RTSE data provides valuable information including the initial film growth mode (e.g. layer-by-layer or clustering), subsequent bulk layer and surface roughness thickness evolution, the growth rate in terms of effective thickness (or volume/area), and the final film optical properties. As an additional application of SE, ex situ through-the-glass mapping SE (TG-M-SE) has been used to study the structural properties and area uniformity of CdS/CdTe solar cells over large areas. The mapping results can be correlated with the efficiency of solar cells fabricated over the same area, exploiting inevitable non-uniformities in the process to identify the optimum structural parameters for highest efficiency solar cells. In a second component of this research, RTSE has proven to be very powerful for the development and optimization of thin film FeS2 deposited by a novel hybrid sputtering/co-evaporation method. This effort started with the sputtering of elemental Fe metal thin films and proceeded to the evaporation (open full item for complete abstract)

Committee: Robert W. Collins (Committee Chair); Nikolas J. Podraza (Committee Member); Randall J. Ellingson (Committee Member); Thomas J. Kvale (Committee Member); Stephen O’Leary (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2017, Physics

Thin film solar cells based on hybrid organic-inorganic perovskites (HOIPs) have become highly attractive over the past several years due to a high solar to electric power conversion efficiencies (PCEs). Perovskite materials based on methylammonium lead iodide (CH3NH3PbI3, MAPbI3) possess high optical absorption coefficients, long minority carrier lifetimes and diffusion lengths, and desirable optical band gaps, and carrier collection in these materials can be highly efficient when they are paired with appropriate electron and hole transport materials (ETMs and HTMs), respectively. Additionally, perovskite solar cells (PSCs) can be fabricated via a variety of solution-based routes, which are suitable for low-cost, large area manufacturing. The combination of these attributes gives PSCs an advantage over currently available commercial photovoltaic (PV) technologies. Understanding the nucleation and growth mechanisms, and controlling the grain size and crystallinity in the solution-processed fabrication of perovskite thin films are important to prepare electronic-quality materials for PV applications. We investigated the nucleation and growth mechanisms of MAPbI3 formed in a two-step solution process. To prepare the MAPbI3 films, PbI2 films were spin-coated and then were reacted with methylammonium iodide (MAI) in the isopropanol (IPA) solution at various concentrations. We showed that the conversion rate, grain size, and morphology of MAPbI3 perovskite films depend on the concentration of the MAI solution. Three distinct perovskite formation behaviors were observed at various MAI concentrations, and a tentative model was proposed to explain the reaction mechanisms. The nucleation and growth process of MAPbI3 can be significantly changed by adding divalent metal salts into the MAI solution. We showed that the incorporation of Cd2+ ions significantly improved the grain size, crystallinity, and photoexcited carrier lifetime of MAPbI3. Formation of ( (open full item for complete abstract)

Committee: Michael J. Heben Ph.D. (Committee Chair); Randy J. Ellingson Ph.D. (Committee Member); Yanfa Yan Ph.D. (Committee Member); Song Cheng Ph.D. (Committee Member); Terry P. Bigioni Ph.D. (Committee Member) Subjects: Materials Science; Physics

Doctor of Philosophy, University of Toledo, 2014, Physics

Radiation therapy has been established as a standard technique for cancer treatment. Advances in nanotechnology have enabled the application of many new approaches in the diagnosis and treatment of cancer. Achievement of selective enhancement in radiation dose deposition within a targeted tumor, while sparing surrounding normal structures, remains a challenge and one of the major objectives of cancer-related research. This objective can be realized by the insertion of high atomic number (Z) materials in the tumor site. Due to their high atomic number (Z=79) and favorable biological compatibility, gold nanoparticles (AuNPs) have been found very promising in this respect. Another candidate material, platinum (Z=78), offering very similar radiation interaction properties to gold and exhibiting additional cytotoxic effects, has been exploited in chemotherapeutic agents for a long time. A number of studies evaluating dose enhancement on the basis of an approximation of a uniform distribution of individual gold atoms rather than nanoparticles neglect the effects near the gold-tissue interface. Those studies have demonstrated high dose enhancement at low kiloelectronvolt (keV) energies, relevant only to certain brachytherapy nuclides, but have not shown a significant enhancement in clinical megavoltage (MV) radiation beams. However, recent experiments with biological systems have brought mixed results, few of them demonstrating promising cell kill effects exceeding the predictions based on pure dose deposition under MV beams. It has been a general observation in physics that many new phenomena originate from interfaces and hence it is important to closely examine the interface effects between high-Z materials, such as gold or platinum, and low-Z tissues. The small physical dimension of the radiation dose enhancement region, ~ 1 mm or less, and the very high dose gradient pose a great challenge in the investigation of the effects. Nonetheless, the interface effects (open full item for complete abstract)

Committee: Diana Shvydka PhD (Advisor) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2014, College of Natural Sciences and Mathematics

Carbon single wall nanotubes (SWNTs) are hollow cylinders of graphene. They have unique mechanical, electrical, and optical properties that are directly influenced by their diameter and chirality. SWNTs are promising candidates for photovoltaics due to their high electrical conductivity and optical transparency. They do not require costly high vacuum processing conditions. SWNTs can be deposited onto different substrates using ultrasonic spray and membrane transfer methods. SWNTs have potential applications in solar cell technology. CdTe is one of the most promising semiconductor materials used for inexpensive solar cell technology. It has demonstrated a high efficiency of 20.4% in small area cells. One of the most important technological problems in CdTe solar cells is to make a good ohmic contact with CdTe for obtaining higher efficiencies. As CdTe has a very high work function, only high work functions metals can make a good ohmic contact with CdTe. However, no such metals with high work function than the work function of CdTe exist. Cu is used as a dopant in CdTe at the back contact in order to promote quantum mechanical tunneling. The record cell uses the Cu but the use of Cu causes a degradation of the device as Cu diffusion occurs through CdTe to the underlying CdS layer. In order to avoid such problems, Cu should not be used to create an ohmic contact with p-CdTe. The application of SWNT films without Cu at the back contact for CdTe solar cells has been investigated in this work. In this thesis, the use of SWNT films as a Cu free back contact in a CdTe solar cell and the corresponding device performance were investigated and compared with the performance of a device with the standard Cu/Au back contact. The main goal of this thesis work was the investigation of the fundamental physics involved in low contact barriers and the charge transport at the SWNT/CdTe and SWNT/Au interfaces.

Committee: Michael Heben (Committee Chair); Robert Collins (Committee Member); Thomas Kvale (Committee Member); Randy Ellingson (Committee Member); Terry Bigioni (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2013, Physics

Cadmium telluride (CdTe) solar cells have been developing as a promising candidate for large-scale application of photovoltaic energy conversion and have become the most commercially successful polycrystalline thin-film solar module material. In scaling up from small cells to large-area modules, inevitably non-uniformities across the large area will limit the performance of the large cell or module. The effects of these non-uniformities can be reduced by introducing a thin, high-resistivity transparent buffer layer between the conductive electrodes and the semiconductor diode. ZnO is explored in this dissertation as a high-resistivity transparent buffer layer for sputtered CdTe solar cells and efficiencies over 15% have been achieved on commercially available Pilkington TEC15M glass substrates. The highest open-circuit voltage of 0.858V achieved using the optimized ZnO buffer layer is among the best reported in the literature. The properties of ZnO:Al as a buffer are also investigated. We have shown that ZnO:Al can serve both as a transparent conducting oxide layer as well as a high-resistivity transparent layer for CdTe solar cells. ZnO:Al reactively sputtered with oxygen can give the necessary resistivities that allow it to be used as a high-resistivity transparent layer. Glass is the most common choice as the substrate for solar cells fabricated in the superstrate configuration due to its transparency and mechanical rigidity. However flexible substrates offer the advantages of light weight, high flexibility, ease of integrability and higher throughput through roll-to-roll processing over glass. This dissertation presents significant improvements made to flexible CdTe solar cells reporting an efficiency of 14% on clear Kapton® flexible polyimide substrates. Our efficiency of 14% is, to our knowledge, the best for any flexible CdTe cell reported in literature.

Committee: Alvin Compaan (Committee Co-Chair); Dean Giolando (Committee Co-Chair); Robert Deck (Committee Member); Michael Heben (Committee Member); Upali Jayamaha (Committee Member) Subjects: Alternative Energy; Energy; Materials Science; Optics; Physics

Doctor of Philosophy, University of Akron, 2013, Polymer Engineering

Photodetectors are light responsive devices that convert optical signals into electric signals. Photodetectors have wide applications in image sensing, environmental monitoring, day- and night-surveillance, chemical and biological detection, industrial process control, communication, planetary probing and so on. Currently, photodetectors based on GaN, ZnO, Si, InGaAs and bulk PbS cover different sub-bands from UV to infrared region. These photodetectors are expensive and some of them require to be operated at low temperature, which certainly limits their applications. Polymer photodetectors made with conjugated polymers possess the unique features, including room-temperature operation, high sensitivity, low working voltage, low cost, thin profile, large area and flexibility. Ultrasensitive polymer photodetectors with high response speed and spectral response ranging from UV to near infrared have been demonstrated. However, new device structure, high responsivity and stable polymer photodetectors needs to be developed. In my dissertation, we reported various methods to enhance the performance of polymer photodetectors. By solvent annealing and post-production thermal annealing, we were able to demonstrate that polymer photodetectors possess comparable responsivity to inorganic counterparts. We have, for the first time, developed the inverted device structure for polymer photodetectors. By utilizing inorganic nanowires and quantum dots as either cathode or anode buffer layer, we were able to demonstrate robust polymer photodetectors. We also investigate the device performance versus energy offset between the workfunction of anode electrode and the valance band of conjugated polymers, band offset at the heterojunction and purity of conjugated polymers.

Committee: Xiong Gong Dr. (Advisor); Alamgir Karim Dr. (Committee Chair); Stephen Cheng Dr. (Committee Member); Matthew Becker Dr. (Committee Member); Yi Pang Dr. (Committee Member) Subjects: Electrical Engineering; Materials Science; Polymers

Doctor of Philosophy, University of Toledo, 2012, Physics

As the demand for clean, renewable energy sources increases, the development of high efficiency, low cost photovoltaic devices from thin films becomes increasingly important. Spectroscopic ellipsometry is a promising tool for the characterization and investigation of thin film photovoltaic devices and the component materials from which they are made. This tool can be applied either in-situ and in real-time using high speed multichannel instruments, or ex-situ using slower wavelength-by-wavelength scanning instruments. Spectroscopic ellipsometry is promising for use in thin film photovoltaics because it provides the thicknesses of the individual layers and their optical properties, which in turn provide insights into the light collection required for photocurrent generation. Advanced forms of ellipsometry include (i) ex-situ mapping spectroscopic ellipsometry with a multichannel ellipsometer, which provides information on thickness and optical property non-uniformities over large areas of a coated substrate, and (ii) real-time spectroscopic ellipsometry with similar high speed instrumentation, which provides information on thin film nucleation, coalescence, and growth, as well as changes in the structure of the material over time. Both advanced forms of ellipsometry have been applied in this Dissertation to analyze thin films with applications in photovoltaics. In this Dissertation research, ex-situ mapping ellipsometry has been used to study Au nanoparticle thin films. These films are useful because they can be integrated into solar cells to promote light trapping within the absorber layers, and hence, increase the overall efficiency of the cells. Studying these films with mapping spectroscopic ellipsometry provides a means for determining thickness uniformity over large areas of the sample for scale-up of the deposition processes. The uniformity of other parameters of the Au nanoparticle films such as the plasmon resonance band energy and its broadening are also (open full item for complete abstract)

Committee: Dr. Robert Collins (Advisor); Dr. Karen Bjorkman (Committee Member); Dr. Jacques Amar (Committee Member); Dr. Nikolas Podraza (Committee Member); Dr. Terry Bigioni (Committee Member) Subjects: Energy; Physics