Doctor of Philosophy, University of Toledo, 2017, Physics

Spectroscopic ellipsometry (SE) is an important optical characterization technique, applicable to a wide variety of samples such as a single substrate material to a multilayer complex structured opto-electronic device. SE is always known to be an accurate and precise measurement of structural, optical, and electrical properties of material especially for a multilayer stack, including photovoltaic devices. This presentation demonstrates examples ranging from a gadolinium gallium garnet (Gd3Ga5O12) single crystal to environmentally unstable polycrystalline organo-halide perovskite thin films to perovskite based solar cells. A significant component of this work is real time monitoring of time sensitive perovskite thin film during the growth as well as post deposition degradation and demonstrated illustrative ideas of data analysis techniques for real time measurement. A commercially available single crystal Gd3Ga5O12 is taken as the first case. The structural and optical properties of Gd3Ga5O12 from infrared to ultraviolet (0.034 to 5.89 eV) was extracted. A simple ellipsometry model is applied to analyze the data from near infrared to ultraviolet (0.74 to 5.89 eV). Additional transmission measurement adds to the accuracy below the band gap region where the absorption coefficient is less than 1000 cm-1 and subsequently helps to detect sub band gap features near 3.95 to 5.06 eV. The band gap is identified in the ultraviolet (UV), while transverse and longitudinal optical phonon modes in the infrared (IR) region are detected. Analysis of IR spectra and quantifying these phonon modes was challenging because of nine absorption peaks present and a sufficiently large number of (>30) fitting parameters. Another case study of this research work is on (FASnI3)1-x(MAPbI3)x (x = 0.00, 0.20, 0.35, 0.40, 0.60, and 1.00) perovskite thin films formed by combining formamidinium tin iodide (FASnI3) and methylammonium lead iodide (MAPbI3). Those perovskite films are time sensitive (open full item for complete abstract)

Committee: Nikolas Podraza (Advisor); Robert Collins (Committee Member); Randall Ellingson (Committee Member); Song Cheng (Committee Member); Dean Giolando (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2016, Physics

Spectroscopic ellipsometry (SE) is a non-invasive optical probe that is capable of accurately and precisely measuring the structure of thin films, such as their thicknesses and void volume fractions, and in addition their optical properties, typically defined by the index of refraction and extinction coefficient spectra. Because multichannel detection systems integrated into SE instrumentation have been available for some time now, the data acquisition time possible for complete SE spectra has been reduced significantly. As a result, real time spectroscopic ellipsometry (RTSE) has become feasible for monitoring thin film nucleation and growth during the deposition of thin films as well as during their removal in processes of thin film etching. Also because of the reduced acquisition time, mapping SE is possible by mounting an SE instrument with a multichannel detector onto a mechanical translation stage. Such an SE system is capable of mapping the thin film structure and its optical properties over the substrate area, and thereby evaluating the spatial uniformity of the component layers. In thin film photovoltaics, such structural and optical property measurements mapped over the substrate area can be applied to guide device optimization by correlating small area device performance with the associated local properties. In this thesis, a detailed ex-situ SE study of hydrogenated amorphous silicon (a Si:H) thin films and solar cells prepared by plasma enhanced chemical vapor deposition (PECVD) has been presented. An SE analysis procedure with step-by-step error minimization has been applied to obtain accurate measures of the structural and optical properties of the component layers of the solar cells. Growth evolution diagrams were developed as functions of the deposition parameters in PECVD for both p-type and n-type layers to characterize the regimes of accumulated thickness over which a-Si:H, hydrogenated nanocrystalline silicon (nc-Si:H) and mixed phase (a+n (open full item for complete abstract)

Committee: Robert Collins (Advisor) Subjects: Materials Science; Physics

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