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discussed with a special focus on the CdS layer and CdTeyCdS interface, which are very sensitive to the accumulation of  impurities and defects.\cite{B_tzner_2004}  ** CdCl2 **CdCl2  treatment** \cite{Dharmadasa_2014} The treatment consists of growth or post-growth heat treatment of CdTe layers in the presence of CdCl2 or halogens (mostly, chlorine, and fluorine) molecules, atoms or ions. Post-growth heat treatment is usually carried out at temperatures in the range 350–450 °C for 60–20 min duration under atmospheric conditions. This treatment drastically enhances the solar cell efficiency from about 1%–5% for as-grown materials to double-digit figures for CdCl2-treated materials. This is almost an order of magnitude increase in conversion efficiency.