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efficiencies cannot be achieved unless the solar cell is also designed to also be a good light emitting diode (LED).   The light trapping structure requires two components, a back mirror and a textured glass.  This requires to internally trap radiation by reflecting any lost radiation back, extending the time and probability that this energy can be reused, improving photon recycling. The optical design is important so that the only loss mechanism is photons exiting at the front surface.  The back mirror can be implemented by cheap and common material SiO_2 \cite{Zhao_2010} \cite{Andreani_2012}. The design and number of mirrors can be optimized according to any particular situation via simulation. simulation \cite{Wang_2013}.  Different possible  mirror geometries are illustrated in Figure . Particularly encouraging is a numerical simulation of a proposed cell structure \cite{Khosroabadi_2014} with back mirrors had an open-circuit voltage of 1.062V, and overall efficiency of 21.02%.   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.  ## CdCl2 treatment Post-growth treatments  The "CdCl2" treatment, 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. 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, MgCl_2\cite{Major_2014},  which is non-toxic and equally effective. Alternative chemicals to be used Other alternative chlorides such  as post-growth treatments 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}