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A Low-cost Post-growth Activation Step For CdTe Solar Cells

Magenesium chloride could replace toxic cadmium chloride say researchers
Chemists at the University of Liverpool have shown that magnesium chloride, commonly  used in bath salts, can replace a toxic, expensive chemical in the manufacture of cadmium telluride (CdTe) solar cells.

Cadmium telluride thin-film solar-cell technology has laboratory efficiencies approaching 20 percent, but a central part of its manufacturing process involves doping the polycrystalline thin-film CdTe with cadmium chloride, which costs  around 30 cents per gram and is a toxic water-soluble source of cadmium ions.

Cadmium chloride is used to form the photovoltaic junction at the CdTe/CdS interface, and to passivate the grain boundaries, making it essential to achieving high device efficiencies. But as the rsearchers detailed in the Nature letter 'A low-cost non-toxic post-growth activation step for CdTe solar cells' they have now demonstrated CdTe solar cells prepared using magnesium chloride, which is non-toxic and costs less than a cent per gram. Efficiencies of around 13 percent, they say, are identical to those of a cadmium chloride-processed control group.

Hole densities in the active layer are also similar and comparable impurity profiles for Cl and O, these elements being important p-type dopants for CdTe thin films. Contrary to expectation, cadmium chloride-processed and magnesium chloride-processed solar cells contain similar concentrations of magnesium; this is because of magnesium out-diffusion from the soda-lime glass substrates and is not disadvantageous to device performance. However, treatment with other low-cost chlorides such as NaCl, KCl and MnCl2 leads to the introduction of electrically active impurities that do compromise device performance.

The team believes the results demonstrate that magnesium chloride can directly  replace cadmium chloride in the existing  process,  both minimising the environmental risk and reducing the cost of CdTe solar-cell production.

Derived from, 'A low-cost non-toxic post-growth activation step for CdTe solar cells' by JD Major et al, Nature (2014) doi:10.1038/nature13435

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