Australian researchers have used plasma to make a material that could replace a scarce element used in solar cells, touch screens and a number of other high-tech manufacturing areas.
In order to work, solar cells and phone and tablet screens need to contain a material that is transparent and can conduct electricity. The material in screen dimmers in cars and smart windows also needs to be electrochromic – that is, able to change colour or transparency depending on an externally applied voltage.
Until now, the primary substance for the job has been indium tin oxide, or ITO. As the name suggests, this substance is made of indium, tin and oxygen – and indium is a scarce resource.
“A very small amount of it is available,” says Dr Behnam Akhavan, a senior lecturer in engineering at the University of Sydney. Demand is growing for indium because of increasing production of touchscreen devices but, even though only tiny amounts are needed, there are fears supply can’t keep up.
“It’s also very hard to mine, because we don’t have any indium-specific mines,” says Akhavan. “It comes as a by-product of zinc.”.
Materials scientists have been looking for alternatives to ITO that are transparent, conductive and electrochromic. Two years ago, Akhavan’s team created a material that ticked all of these boxes, consisting of four very thin layers of tungsten and silver on glass. They’ve now been able to refine it down to three layers, simplifying production. And the whole thing has been made using plasma.
While plasma’s not common on the Earth’s surface, “it’s the most common state of matter in the universe,” according to Akhavan. “The sun, stars, lightning – they’re all made of plasma.
“In my research, I create it in the lab to bring in some really fascinating features that other states of matter don’t have, and use it to create new materials.”
The team used a sputtering technique, known as high power impulse magnetron sputtering (HiPIMS), to create nanometre-sized coats of atoms on surfaces.
“It detaches atoms from the target, and it deposits them on to any material that we want to be coated, such as glass,” says Akhavan.
In this case, the researchers covered glass with a deposit deposited 30 nanometres of tungsten oxide, followed by 10 nanometres of pure silver and then another 50 nanometres of a “nanocomposite” of tungsten oxide and silver (nanoparticles of silver mixed into tungsten oxide). The result was a clear 90-nanometre-thick coat on the glass (or about a tenth of the size of a small bacterium) that is both conductive and electrochromic.
Tungsten and silver, while not exactly abundant, are much less rare than indium.
Akhavan says an immediate use of the technology is as an anti-reflection coating for mirrors. It could also be used in smart windows, which change their transparency to prevent the in-flow of sunlight. Touchscreens could be another potential avenue for the material – although, as these devices usually don’t need to be electrochromic, Akhavan suggests that tungsten could be swapped out for more abundant titanium.
Another advantage of the technique is that it’s effectively waste-free.
“It’s a dry process,” says Akhavan. “No solvents or bench chemistry is involved. That makes it very environmentally friendly, because the amount of waste produced is almost zero.”
The plasma doesn’t deposit materials onto the glass with 100% efficiency, scattering some around the rest of the vessel during the coating process. But these mis-deposited materials remain in an unaffected state and can be re-used with ease, once taken from the vessel.
“You don’t have to extract them from a solution,” says Akhavan.
A paper describing the material is published in Solar Energy Materials and Solar Cells.
The Royal Institution of Australia has an Education resource based on this article. You can access it here.
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