Electro-Analytical Characterization of Silicon and Dye Sensitized Solar Cells

Abstract

The present work summarizes the experimental illustration with a specific focus on direct determination of broad range of performance indicating parameters of mono- and polycrystalline Si solar cell by using DC and AC characterization techniques. The obtained result demonstrates how a DC technique coupled with AC characterization technique can be quantitatively utilized for correlating various material designs and processing conditions of Si solar cell. The key features responsible for the discrepancies in the performance of silicon solar cells fabricated from the same ingot under the identical conditions have been thoroughly investigated in chapter 3. The higher density of deep cone shaped holes, impurity precipitates over the cell surface and non-uniform shunts across the n+ -p junction are found to be as one of the possible reasons for the difference in performances of solar cells. Under moderate reverse bias or at 0 V, all the solar cells exhibit maximum impedance. A distortion in the impedance spectra at knee voltage is due to the progressive involvement of the p-p + junction to the net impedance of solar cells. A lower value of shunt resistance and minority carrier lifetime along with the higher value of series resistance contributes to the higher resistive losses and surface recombination. The experimental results along with the analytical model will provide an insight into the loss mechanisms and also provide a simple tool that can be integrated with the conventional photovoltaic testing. Mono-crystalline Si solar cell subjected to low-concentration illumination conditions were studied in chapter 4. Contributions from different solar cell parameters, diffusion capacitance, transition capacitance, diffusion resistance, recombination resistance and back surface field has been resolved using AC impedance spectroscopy technique. The feasibility of commercially available mono-crystalline Si solar cell for low concentration photovoltaic (LCPV) applications has been explored by DC current-voltage characteristics under varying illumination. The obtained results demonstrates the importance of DC and AC characterization technique for the evaluation of performance indicating parameters of Si solar cell under non-equilibrium conditions differing from 1 sun. Commercially available poly-crystalline silicon solar cells have also been studied under varying illumination conditions in chapter 5 to explore their application to a LCPV system. On the basis of this study, it can be concluded that charge carrier recombination is closely related to the increase in carrier density with the increase in illumination. The electrical performance of a poly-crystalline silicon solar cell is limited by the recombination losses which limit the attainable open-circuit voltage. The losses get proportionately larger at higher illumination. It is emphasized that under higher illumination levels the charge transport is mainly governed by diffusion process, which leads to an increased recombination of charge carriers and a decrease in the fill factor of poly-crystalline silicon solar cells. The work also highlights a detailed analytical framework for LCPV system. The study indicated that the commercially available poly-crystalline Si solar cell works efficiently under low concentration ≈ 3 suns. After the analysis of experimental and theoretical results the study is further extended for the construction, modeling and simulation of the LCPV prototype system in chapter 6. The experiments based on LCPV system were performed to investigate the performance of commercially available crystalline silicon solar cells under low concentration in actual test condition (ATC). The experimental results show that the commercially available silicon solar cells have quite good performance under concentration conditions. Some factors that affect the output performance of the commercially available silicon solar PV module are explored experimentally and by theoretical calculations. The developed theoretical model is able to predict the performance of a LCPV system under the ATC. The open-circuit voltage was found to be decreasing from 9.86 to 8.24 V with temperature coefficient of voltage ≈ -0.021 V/K under ATC. The static and dynamic parameters affecting the LCPV module performance under ATC conditions were also explored. When the CR increased from 1 sun to 5.17 suns, the Pmax of the LCPV module registered a threefold increase. The feasibility of this project is demonstrated by a positive value of NPV obtained within 8 years. The proposed model is simple and useful to predict the steady state and dynamic parameters of LCPV module and can also be used to simulate the I-V curves of the medium and high concentration solar PV module with certain considerations. This study shows that the commercially available silicon solar PV cells can be used to work under low level concentration (< 10 Sun) to have higher power output without compromising with the performance of the solar cell. Along with the analysis of the Si solar cell the study is also extended to analyze the Photoelectrochemical systems i.e. Dye sensitized solar cell in chapter 7. The DC and AC characterization techniques are employed to investigate the dye-sensitized solar cell (area: 3.78 cm2 ) with a power conversion efficiency of 4.32%. The analysis of current-voltage characteristics depicts that the loss in photocurrent density follows an exponential behavior which is about 1 to 1.5% at low forward bias, ~8% at knee voltage and ~79% at open circuit voltage. The current-voltage characteristics obtained from the impedance parameters allows separating the contribution of different resistive processes on the overall conversion efficiency. This study suggests that the photocurrent density and the output power density ix from maximum power point to open circuit voltage can be improved by reducing the recombination or by making the charge transport faster. The experimental results along with the analytical model provide an insight into the energy loss mechanism and describe the ways to improve the performance of a solar cell and the other optoelectronic devices.