Real time and post-deposition optical analysis of interfaces in CdTe solar cells

Real time spectroscopic ellipsometry (RT-SE) in the near-infrared to ultraviolet range, as well as pre-deposition and post-deposition mid-infrared spectroscopic ellipsometry (IR-SE) have been applied as probes of the formation of optical interfaces in sputter-deposited CdS/CdTe solar cell structures. Both optical probes are configured for reflection from the film side of the solar cell structure. One focus of this work is to assist in the development of optical models to be used for both on-line analysis and quantum efficiency modeling. Toward this goal, RT-SE during CdS deposition has provided information on (i) [transparent conducting oxide (TCO)]/CdS interface formation - the extent to which the TCO surface roughness is conformally covered by the depositing CdS film; (ii) CdS bulk layer growth, and (iii) CdS surface roughness evolution and the final roughness thickness, which influences interface formation with the overlying CdTe. Pre-deposition and post-deposition IR-SE has also proven valuable for exploring the TCO free electron characteristics and the CdS optical properties that determine their near-infrared absorption spectra. The TCO characteristics have been observed to change with the over-deposition of the semiconductor films.