A new method of making semiconductor films could lead to cheaper solar cells, claim scientists in Germany.

Silicon-germanium (green) is deposited from the ionic liquid solvent in the electrochemical cell

Frank Endres and colleagues at Clausthal University of Technology electrochemically deposited silicon-germanium films onto a gold electrode using an air- and water-stable ionic liquid as the solvent. This is the first time that high purity silicon-germanium films have been made in a controlled way at room temperature and pressure.

The method has a number of advantages over alternative deposition methods, which use ultra high vacuum and high temperatures. 'Electrodeposition is much cheaper and faster,' says Endres. 'Furthermore, if the electrodeposition bath is large enough, all sizes and shapes of substrate material can be coated.'

The electronic properties of silicon-germanium can be tuned to match its intended application by varying the germanium content of the material, which can be anywhere in the spectrum from pure silicon to pure germanium. Endres showed that pure silicon, silicon-germanium alloys and pure germanium can be deposited from the ionic liquid solution.

'Our results show for the first time that it is possible to electrodeposit semiconductors from ionic liquid solutions with a quality that is competitive with physical methods, such as molecular beam epitaxy,' comments Endres. 'This opens the door to many applications, from nanowires for lithium ion batteries to solar cells.'

Ingo Krossing, who studies the applications of ionic liquids in electrochemistry at the University of Freiburg, Germany, agrees with Endres. 'This is a remarkable step forward to achieving the goal of synthesising solar cells using electrochemistry rather than expensive and high energy melt procedures,' he says.

James Hodge