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instability.Cell degradation due to generation of defects from external influences like particle irradiation, e.g.in space, is another  reason for instability.The development of stable back contacts as well as sources of instability in the cell performance are  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 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.