Spectroscopic ellipsometry studies of II-VI semiconductor materials and solar cells.

摘要

The multilayer optical structure of thin film polycrystalline II--VI solar cells such as CdTe is of interest because it provides insights into the quantum efficiency as well as the optical losses that limit the short-circuit current. The optical structure may also correlate with preparation conditions, and such correlations may assist in process optimization. A powerful probe of optical structure is real time spectroscopic ellipsometry (SE) that can be performed during the deposition of each layer of the solar cell. In the CdCl2 post-deposition treatment process used for thin film polycrystalline II--VI solar cells, the optical properties of each layer of the cell change during the process due to annealing as well as to the elevated temperature. In this case, ex-situ SE before and after treatment becomes a reasonable option to determine the optical structure of CdCl2-treated CdTe thin film solar cells.;CdTe solar cells pose considerable challenges for analysis by ex-situ SE. First, the relatively large thickness of the as-deposited CdTe layer leads to considerable surface roughness, and the CdCl2 post-deposition treatment generates significant additional oxidation and surface inhomogeneity. Thus, ex-situ SE measurements in reflection from the free CdTe surface before and after treatment can be very difficult. Second, SE from the glass side of the cell is adversely affected by the top glass surface which generates a reflection that is incoherent with respect to the reflected beams from the thin film interfaces and consequently depolarization if collected along with these other beams. In this research, the first problem is solved through the use of a succession of Br2+methanol treatments that smoothens the CdTe free surface, and the second problem is solved through the use of a 60° prism optically-contacted to the top glass surface that eliminates the top surface reflection. In addition, the succession of a Br2+methanol treatment not only smoothens the CdTe surface but also enables CdTe etching in a layer-by-layer fashion. In this way, it has been possible to track the optical properties of the CdTe component layer as a function of depth from the surface toward the CdS/CdTe interface in order to gain a better understanding of the film structure.;In this study, ex-situ spectroscopic ellipsometry was applied first to investigate the optical properties of the TEC-15 glass substrate, and then to extract the optical properties of thin film CdTe and CdS both as-deposited and CdCl2-treated. After obtaining all the optical properties of the solar cell component layer materials, a comprehensive ex-situ SE analysis has been applied to extract the optical structure of a single thin film of CdCl2-treated CdTe, and finally to obtain the optical structure of the CdCl2 post-deposition treated CdTe solar cell.;Based on the fundamental studies in this thesis, various aspects of the solar cell structure after the complicated CdCl2 treatment have been determined. In future work the role of the key parameters of CdCl 2 post-deposition treatment process will be explored including: the temperature and treatment time. As a result, a correlation will be established between solar cell performance and film structure. Finally, an understanding of how solar cell structure can be optimized to achieve the highest solar cell performance may be possible through improved control of the CdCl 2 post-treatment process.