Applications of Mapping Spectroscopic Ellipsometry in Photovoltaics: Correlations between Window Layer Thickness and Device Performance in CdSe/CdTe and CdS/CdSe/CdTe Solar Cells Prepared by Magnetron Sputtering

CdTe photovoltaic technology has traditionally used CdS as an n-type window layer in a CdS/CdTe heterojunction. The lattice mismatch between CdS and CdTe, the lack of collection from CdS, and its 2.4 eV bandgap, however, limit the short circuit current (J_SC) and open circuit voltage (V_OC) of the solar cell.  Recently, CdSe has been recognized as a promising component layer in CdTe solar cells. J_SC for CdTe devices incorporating CdSe shows an enhancement due to extended collection over both long and short wavelength ranges. For CdSe/CdTe and CdS/CdSe/CdTe solar cells, analysis of the effect of the CdSe and CdS/CdSe layer thicknesses on the CdTe solar cell parameters is critical for the purpose of optimizing the cell performance. Sets of CdTe devices were prepared by magnetron sputtering on TEC^TM15/HRT type glass with different CdS and CdSe layer thicknesses. Spectroscopic ellipsometry has been applied to generate maps for each single layer and also for the complete PV stack at different stages of processing. In this work, non-uniformity of the deposition processes leads to mapped distributions of thicknesses about the nominal values and enables accurate optimization of device performance through correlations of device parameters with the CdSe and CdS/CdSe bilayer thicknesses.