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 achiev-ing 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 3 10 14 cm 23