A low-cost non-toxic post-growth activation step for CdTe solar cells

• Published in: Nature (London) 2014-07, Vol.511 (7509), p.334-337
• Main Authors: Major, J. D., Treharne, R. E., Phillips, L. J., Durose, K.
• Format: Article
• Language: English
• Subjects: 639/301/299/946; 639/638/298/917; Annealing; Cadmium; Cadmium telluride; Design and construction; Efficiency; Environmental impact; Environmental risk; Fabrication; Humanities and Social Sciences; letter; Manufacturing industry; Materials; multidisciplinary; Photovoltaic cells; Photovoltaics; R&D; Research & development; Science; Sodium chloride; Solar batteries; Solar cells; Solar energy; Thin films
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature13435

MgCl 2 is shown to be a cheap and non-toxic replacement for the costly and environmentally unfriendly salt CdCl 2 that has long been used as the ‘activation’ step in the production of cadmium telluride solar cells. Just add salt for cheaper solar cells Solar cells based on cadmium telluride, CdTe, are among the most cost-efficient photovoltaic systems currently in use. But according to Jonathan Major et al ., there is still plenty of room for improvement. Specifically, they show that it is possible to replace the costly and environmentally unfriendly cadmium-containing salt (CdCl 2 ), which has long been used to 'activate' the CdTe during processing, with a cheap and non-toxic alternative, MgCl 2 . This change does not appear to be detrimental to device performance, yet shows great potential for reducing processing costs and environmental risk. Cadmium telluride, CdTe, is now firmly established as the basis for the market-leading thin-film solar-cell technology. With laboratory efficiencies approaching 20 per cent 1 , the research and development targets for CdTe are to reduce the cost of power generation further to less than half a US dollar per watt (ref. 2 ) and to minimize the environmental impact. A central part of the manufacturing process involves doping the polycrystalline thin-film CdTe with CdCl 2 . This acts to form the photovoltaic junction at the CdTe/CdS interface 3 , 4 and to passivate the grain boundaries 5 , making it essential in achieving high device efficiencies. However, although such doping has been almost ubiquitous since the development of this processing route over 25 years ago 6 , CdCl 2 has two severe disadvantages; it is both expensive (about 30 cents per gram) and a water-soluble source of toxic cadmium ions, presenting a risk to both operators and the environment during manufacture. Here we demonstrate that solar cells prepared using MgCl 2 , which is non-toxic and costs less than a cent per gram, have efficiencies (around 13%) identical to those of a CdCl 2 -processed control group. They have similar hole densities in the active layer (9 × 10 14  cm −3 ) and comparable impurity profiles for Cl and O, these elements being important p-type dopants for CdTe thin films. Contrary to expectation, CdCl 2 -processed and MgCl 2 -processed solar cells contain similar concentrations of Mg; this is because of Mg out-diffusion from the soda-lime glass substrates and is not disadvantageous to device performance. However, treatment with oth