Crystalline silicon method could benefit the solar industry

Researchers have developed a method of making crystalline silicon that is claimed to be less energy intensive than current methods of production.

Silicon dioxide, found in nature as sand, makes up about 40 per cent of the Earth’s crust, but the industrial method for converting it into crystalline silicon is expensive and has a major environmental impact due to the extreme processing conditions.

‘The crystalline silicon in modern electronics is currently made through a series of energy-intensive chemical reactions with temperatures in excess of 2,000°F that produces a lot of carbon dioxide,’ said Stephen Maldonado, professor of chemistry and applied physics at the University of Michigan.

Recently, Maldonado and chemistry graduate students Junsi Gu and Eli Fahrenkrug discovered a way to make silicon crystals directly at 180°F.

Maldonado and colleagues made a solution containing silicon tetrachloride and layered it over a liquid gallium electrode. Electrons from the metal are said to have converted the silicon tetrachloride into raw silicon, which then dissolved into the liquid metal.

‘The liquid metal is the key aspect of our process,’ Maldonado said in a statement. ‘Many solid metals can also deliver electrons that transform silicon tetrachloride into disordered silicon, but only metals such as gallium can additionally serve as liquids for silicon crystallisation without additional heat.’

The researchers reported dark films of silicon crystals accumulating on the surfaces of their liquid gallium electrodes.

So far, the crystals are about 1/2,000th of a millimetre in diameter, but Maldonado hopes to improve the technique and make larger silicon crystals, tailored for applications such as converting light energy to electricity or storing energy. The team is exploring several variations on the process, including the use of other low-melting-point metal alloys.

An electron microscope image shows the pure silicon crystals, generated with the new technique
An electron microscope image shows the pure silicon crystals, generated with the new greener technique

If the approach proves viable, the implications could be significant, particularly for the solar energy industry as crystalline silicon is presently the most-used solar energy material. However, the cost of silicon has driven some researchers to seek alternative semiconductors.

‘It’s too premature to estimate precisely how much the process could lower the price of silicon, but the potential for a scalable, dramatically less expensive and more environmentally benign process is there,’ Maldonado said. ‘The dream ultimately is to go from sand to crystalline silicon in one step. There’s no fundamental law that says this can’t be done.’

The study, which appears in the Journal of the American Chemical Society, was funded by the American Chemical Society Petroleum Research Fund.

The university is pursuing patent protection for the intellectual property and is seeking commercialisation partners to help bring the technology to market.