Spectroscopic Ellipsometry Studies Of Cds Cdte Thin Films And Photovoltaic Devices

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 critical, as these can be optimized to ensure maximum coupling of light into the solar cell absorber layer. In addition, the advanced methods of real-time spectroscopic ellipsometry (RTSE) have been used to characterize a series of CdTe thin films deposited via magnetron sputtering. In this case, analysis of RTSE data provides information on the nucleation, coalescence, and growth of thin films under different deposition conditions. The time evolution of the thicknesses of the nucleating, bulk-like, and surface roughness layers provide an indication of the growth mode, and enable the distinction of initial clustering from initial layer-by-layer growth. The growth mode shows consistent trends with the deposition parameters. In addition, RTSE provides accurate complex dielectric functions of the growing films, since the films are free from oxidation, and surface roughness can be taken into account in the analysis. From the dielectric functions, information on the void fraction, electron mean free path, and stress in these films can be obtained. Also presented in this Dissertation are the results of a study of CdTe solar cells deposited by magnetron sputtering at varying argon pressures and for varying CdCl2 treatment times. These cells were studied through analyses of the current-voltage (J-V) curves and through comparisons of J-V deduced parameters for cells of varying pressures and treatment times. In two independent series of depositions, it was surprising that samples deposited at a specific argon pressure (10 mTorr) performed the best. This optimum pressure separates a regime at low pressures in which the sputtered species experience no collisions upon transit to the substrate, and a regime at high pressures in which the sputtered species experience multiple collisions and are nearly thermalized. Also obtained in this research were quantum efficiency curves for the best performing cells. The measured quantum efficiency for a 10 mTorr sample could be simulated using parameters obtained from ex-situ spectroscopic ellipsometry measurements. Assumptions made in this simulation identified the nature of current losses in the solar cells. Finally, this Dissertation presents a study of a new flexible CdTe solar cell configuration fabricated via an advanced substrate transfer process. In this process, the solar cell is deposited on a temporary superstrate of aluminum foil and transferred to a permanent polymer substrate. This process is used since flexible polymers cannot withstand the deposition temperatures used for the fabrication of CdTe solar cells via the industrial processes of vapor transport deposition and close space sublimation. This novel process has been shown previously to be useful for production of a-Si:H solar modules, and this Dissertation research showed for the first time that it may also be useful for CdTe solar cells, even though the process requires further optimization to reach commercializable efficiencies.