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Degradation effects are often associated with metal diffusion from the back contact of the cell, which is Cu in most cases. However, cells with stable back contact can also exhibit instability, as also all the other cell layers are potential sources of impurities causing instability.\cite{B_tzner_2004}  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 a cell are important if voltage is to be maximized.  Many novel back contacts have been proposed and simulated numerically \cite{Matin_2010}.  high-work function back contact materials;  Titanium Selenide   Potential layered compound  candidates are the selenides TiSe2, VSe2, NbSe2 and TaSe2, as well as their corresponding  sulfides.   \cite{Ferekides_2011}  ## Efficient Light Trapping Structure  This strategy is influenced in part by the success of GaAs photovoltaics, the highest  efficiencies cannot be achieved unless the solar cell is also designed to also be a good light emitting diode (LED).  
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While ultra-thin (<1µ) semiconductor layers are less effective at absorbing sunlight, they are cheaper from a material point view. If efficent light trapping techniques are used, the absortion efficency can be increased allowing for minimal loss. Intead of 1-2 µm thin films, 100nm could be adequete for a direct bangap material as CdTe.  ## Post-growth treatments  The "CdCl2" treatment, treatment or “activation” process,  while poorly understood is a treatment that 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. (a)  CdTe grain enhancement; (b) enhanced interdiffusion between CdTe and CdS (that leads to the  formation of a mixed Cd1-XSXTe crystal at the junction interface); (c) defect passivation/lifetime  improvement via the formation of Cl-related complexes in CdTe.  This acts to form the photovoltaic junction at the CdTe/CdS interface3, 4 and to passivate the grain boundaries5, making it essential in achieving high device efficiencies  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.\cite{Dharmadasa_2014}  There has been alternative treatments that involve MgCl_2\cite{Major_2014}, which is non-toxic and equally effective. Other alternative chlorides such as NaCl, KCl and MnCl2 have been used but compromise device performance. Alternative chemical treatements should be explored.  With MgCl_2, there is possibility of the formation of a intermixed layer of the possibility of forming a highly doped field at the back of the device that could act as an electron reflector that could boost device performance by directing electrons back into the absorber layer and increasing the voltage while limiting recombination at the back of the device  hus reducing the back-surface recombination and improving the Voc\cite{Hsiao_2011}