Doctor of Philosophy, Case Western Reserve University, 2015, Chemical Engineering

Metal electrodeposition is often associated with simultaneous hydrogen co-evolution. The presence of bubbles complicates the design and control of electrodeposition processes. This is particularly relevant to the electrodeposition from aqueous electrolytes of numerous metals with standard potentials that are negative to hydrogen. As shown in this study, hydrogen co-evolution enhances the transport rates of the metal deposition reaction beyond those predicted by the classical, steady-state mass transport model. Available models addressing transport in the presence of gas co-evolution are based on free convection that is enhanced by the rising bubble cloud. However, there are no models that address mass transfer enhancement by bubbles under forced convection, such as analyzed here for the commonly used, facing-down rotating disk electrode (RDE). This study characterizes experimentally the phenomenon and introduces a model for quantifying it. Experimental data was collected in plating copper at high cathodic overpotentials (-0.4 to -1.0V vs SHE) from acidified copper sulfate on a RDE. The transport enhancement (~2-6 fold) was determined by measuring the copper deposition by gravimetry. Pulse experiments, where the current decay was measured following a short bubble generation confirmed the linkage between the current enhancement and the presence of bubbles. A model based on fresh electrolyte replenishing the volume vacated by the translating bubbles and thus subjecting regions of the electrode to enhanced transient currents has been derived. The model correlates the experimental data indicating higher transport enhancement with increasing cathodic polarization and dependence of the enhancement on the rotation rate and on the bulk copper concentration.

Committee: Uziel Landau (Committee Chair); Rohan Akolkar (Committee Member); Donald Feke (Committee Member); Daniel Scherson (Committee Member); Mohan Sankaran (Committee Member) Subjects: Applied Mathematics; Chemical Engineering; Chemistry; Engineering; Materials Science; Mechanical Engineering; Physics; Technology

Doctor of Philosophy, University of Toledo, 2013, College of Arts and Sciences

Real time spectroscopic ellipsometry (RTSE), and ex-situ mapping spectroscopic ellipsometry (SE) are powerful characterization techniques capable of performance optimization and scale-up evaluation of thin film solar cells used in various photovoltaics technologies. These non-invasive optical probes employ multichannel spectral detection for high speed and provide high precision parameters that describe (i) thin film structure, such as layer thicknesses, and (ii) thin film optical properties, such as oscillator variables in analytical expressions for the complex dielectric function. These parameters are critical for evaluating the electronic performance of materials in thin film solar cells and also can be used as inputs for simulating their multilayer optical performance. In this Thesis, the component layers of thin film hydrogenated silicon (Si:H) solar cells in the n-i-p or substrate configuration on rigid and flexible substrate materials have been studied by RTSE and ex-situ mapping SE. Depositions were performed by magnetron sputtering for the metal and transparent conducting oxide contacts and by plasma enhanced chemical vapor deposition (PECVD) for the semiconductor doped contacts and intrinsic absorber layers. The motivations are first to optimize the thin film Si:H solar cell in n-i-p substrate configuration for single-junction small-area dot cells and ultimately to scale-up the optimized process to larger areas with minimum loss in device performance. Deposition phase diagrams for both i- and p-layers on 2" x 2" rigid borosilicate glass substrate were developed as functions of the hydrogen-to-silane flow ratio in PECVD. These phase diagrams were correlated with the performance parameters of the corresponding solar cells, fabricated in the Cr/Ag/ZnO/n/i/p/ITO structure. In both cases, optimization was achieved when the layers were deposited in the protocrystalline phase. Identical solar cell structures were fabricated on 6" x 6" borosilicate (open full item for complete abstract)

Committee: Robert Collins Dr. (Committee Chair); Nikolas Podraza Dr. (Committee Member); Song Cheng Dr. (Committee Member); Sanjay Khare Dr. (Committee Member); Andre Ferlauto Dr. (Committee Member) Subjects: Physics