Electro-Analytical Characterization of Solar Cells

The studies of Si solar cells presented in this work are divided in two parts. The first part (Chapter 3) develops an analytical characterization approach for combining linear sweep voltammetry (LSV) and impedance spectroscopy (IS) for studying such cells. The second part (Chapter 4) extends this approach to further detailed examination of the effects of temperature variations on the key performance-determining parameters of a Si cell. The dye sensitized solar cell (DSSC) characterization study reported here also is based on a similar approach, focusing primarily on the development of a complete electro-analytical framework for detailed characterization of such multi-component (multi-material) solar cells.;In Chapter 3, apart from determining the standard performance parameters of the Si solar cell, the characteristic electrical features of the back-surface field (BSF, an efficiency-enhancing feature of Si based cells) have been studied rigorously. The illumination (0-1 Sun, from a solar simulator) and the temperature of the cell are independently controlled to prevent interference of photothermal and photovoltaic effects. LSV provides current-voltage plots, fill-factor, efficiency and effective cell resistances. The impedance parameters of the emitter-base junction and the BSF at different bias voltages and illuminations are obtained through complex nonlinear least square (CNLS) analysis of experimental Nyquist spectra. The IS data lead to straightforward determination of the diffusion and depletion layer capacitances, n+-p diode resistance, series resistance, concentration of majority carriers as well as lifetime of minority carriers in the base, resistance and capacitance of the p-p + BSF junction, and relaxation time of holes in the BSF. The experiments also demonstrate the importance of temperature controlled measurements, as well as the relevance of adequately normalizing the n+-p junction voltage from the measured terminal voltage of the solar cell.;The second study of Si based systems in Chapter 4 couples IS with LSV to investigate how the interplay of the temperature and voltage dependent parameters of an n+-p silicon solar cell. The underlying mechanisms of the observed temperature/voltage sensitive characteristics of the cell are examined by using (and extending) a collection of currently available theoretical models. The cell parameters measured and analyzed using this approach include: The transition layer capacitance and the built in potential of the n+-p interface, the acceptor concentration in the base, the series, shunt and recombination resistances, and the effective diode ideality factor. A typical range of temperatures (15--60 °C) for terrestrial operations of solar cells is explored in this particular study. The individual roles of minority carrier diffusion and defect-induced charge recombination are clearly manifested in the voltage and temperature dependent signatures of these parameters. The diode behavior of the Si solar cell and the effects of temperature on this particular feature of the solar cell are characterized. The implications of the commonly used single-diode model in the context of A.C. characterization of Si cells are explored in detail.;Chapter 5 focuses mostly on the diode-like D.C. current-voltage profile that plays a critical role in determining the overall performance of the cell. A forward biased dark DSSC is used to preferentially activate the interfacial reactions that govern the charge recombination (diode) characteristics of the DSSC. Rigorous IS experiments are combined with D.C. current-voltage measurements using LSV. CNLS analyses of IS data provides detailed kinetic parameters of the different active interfaces of the multi-component solar cell. Among these parameters, the electron lifetime as well as the resistance of charge transfer at the TiO2-electrolyte interface follows the same diode-like voltage dependence of the D.C. current. The observed correlation between the A.C. and D.C. data sets are explained using currently available theoretical models of electron transfer from the conduction band of the TiO2 photo-anode to I3- ions in the electrolyte.;Chapter 6 describes a basic system necessary to carry out FT-IS that was developed as part of the present thesis. The theoretical and experimental considerations for time resolved FT-IS experiments have been examined. Illustrative results are presented using a model system that supports fast surface reactions. (Abstract shortened by UMI.)...