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Optical and Photovoltaic Properties of Copper Indium-Gallium Diselenide Materials and Solar Cells

Aryal, Puruswottam
http://rave.ohiolink.edu/etdc/view?acc_num=toledo1404679981

Abstract Details

2014, Doctor of Philosophy, University of Toledo, Physics.
The demand for clean and renewable energy sources in recent years has motivated research on the development of low cost, thin film photovoltaic devices. As a consequence, tools for the investigation and characterization of thin film photovoltaic component materials and devices, which can be implemented in real time as well as under in-line and off-line measurement conditions, are becoming increasingly important. Real time spectroscopic ellipsometry (RTSE) and ex-situ mapping spectroscopic ellipsometry (SE) are powerful characterization tools suitable for applications in the optimization of device performance and the evaluation of thin film photovoltaics technology scale-up from dot cell sizes in research laboratories to full module sizes in factories. These non-destructive optical probes implement multichannel spectroscopic detection for achieving high measurement speed, while simultaneously yielding high precision light-matter interaction parameters. The interaction parameters can be analyzed to obtain layer thicknesses as well as their optical properties from which material properties such as composition can be determined. The layer thicknesses and their optical properties in turn provide insights into the fraction of incident light absorbed in the active layer of the solar cell and also provide a basis for short-circuit current optimization through optical simulations. In this dissertation research, Cu(In,Ga)Se2 films with different Ga contents have been prepared by a one stage co-evaporation process. These films have been studied by spectroscopic ellipsometry (RTSE) in real time during their deposition, which has been performed at high temperature (570oC). After cooling the films to room temperature, in-situ SE measurements were undertaken in order to extract the dielectric functions of the thin film materials. An extended parameterization was established through the fitting of these dielectric functions to analytical functions, followed by the development of expressions in the free parameters that describe these analytical functions versus the Ga content. As a result of this parameterization, dielectric function spectra can be predicted for any desired composition. This capability was applied for the structural and compositional mapping of CIGS thin films and solar cells deposited over 10 cm × 10 cm substrate areas. Correlations of the deduced structural and compositional parameters with the corresponding device performance characteristics have yielded important insights with the potential to assist in the optimization of solar cell devices incorporating thin CIGS layers. In addition, a methodology of external quantum efficiency simulation (EQE) has been developed that relies on ex-situ spectroscopic ellipsometry analysis of complete thin film solar cells and so does not require free parameters. The simulations have been applied to CIGS and a-Si:H solar cells, based on the assumption that all photo-generated carriers within the active layers of these cells are collected without any recombination losses. Thus, it should be noted the predicted EQE is the maximum that the solar cell having the given structure can generate, and the difference between the predicted and measured EQE for the same device can provide insights into recombination losses in the device. Because the predicted EQE is based on specular interfaces, it can also be lower than the measured values due to light trapping caused by rough surfaces and interfaces. In another research area of interest for CIGS materials and solar cells, the role of the stage II/III substrate temperature (540oC - 640oC) in the deposition of the films by the three stage process has been studied, as has its effect on device performance, sodium diffusion, and grain size. Since standard soda-lime glass does not tolerate temperatures above 570oC due to glass softening, specially engineered high temperature soda-lime glass produced by Nippon Electric Glass Co., Japan was used as the substrate material in this study. It was found that the average device performance improves up to 620oC as a consequence of reduced shunting and improved diode quality factor which affect the fill factor of the device. At 640oC, however, these parameters have exhibited a wider distribution, and thus have yielded a lower average efficiency for the cells. SEM micrographs of these devices showed that the grain size first increased with increasing temperature up to 620oC, and then showed a bimodal distribution at 640oC. Finally, ex-situ mapping ellipsometry has been applied in the study of silver nanoparticle thin films prepared by the drop casting method. These films are important because of the plasmonic effects they exhibit. Such effects can be exploited by integrating the nanoparticle layers into solar cells in order to promote light trapping, and hence, increase the overall efficiency of the cells. A study of these films with mapping spectroscopic ellipsometry provides a means of determining thickness uniformity over large areas that is critical for scale-up of the deposition processes. The uniformity of other parameters of the films such as the plasmon resonance energy and its broadening are equally important to ensure maximum coupling of light into the solar cell absorber layer.
Robert W. Collins (Committee Chair)
Song Cheng (Committee Member)
Randall Ellingson (Committee Member)
Nikolas Podraza (Committee Member)
Terry Bigioni (Committee Member)
Physics
CIGS; ellipsometry; sodium depth profile; Ga depth profile; mapping; dielectric function

Recommended Citations

Citations

  • Aryal, P. (2014). Optical and Photovoltaic Properties of Copper Indium-Gallium Diselenide Materials and Solar Cells [Doctoral dissertation, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1404679981

    APA Style (7th edition)

  • Aryal, Puruswottam. Optical and Photovoltaic Properties of Copper Indium-Gallium Diselenide Materials and Solar Cells. 2014. University of Toledo, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=toledo1404679981.

    MLA Style (8th edition)

  • Aryal, Puruswottam. "Optical and Photovoltaic Properties of Copper Indium-Gallium Diselenide Materials and Solar Cells." Doctoral dissertation, University of Toledo, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1404679981

    Chicago Manual of Style (17th edition)

toledo1404679981
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© 2014, all rights reserved.
This open access ETD is published by University of Toledo and OhioLINK.