New transparent metal films for smartphone displays

Washington, Dec 16: Scientists have developed a new transparent and electrically conductive material that could make large screen displays, smart windows, touch screens and solar cells more affordable and efficient. Indium tin oxide (ITO), the transparent conductor that is currently used for more than 90 per cent of the display market, has been the dominant material for the past 60 years.

However, in the last decade, the price of indium has increased dramatically. Manufacturers have searched for a possible ITO replacement, but until now, nothing has matched its combination of optical transparency, electrical conductivity and ease of fabrication.

Researchers led by Roman Engel-Herbert, assistant professor at Pennsylvania State University in US used thin - 10 nanometre - films of an unusual class of materials called correlated metals in which the electrons flow like a liquid.

While in most conventional metals, such as copper, gold, aluminium or silver, electrons flow like a gas, in correlated metals, such as strontium vanadate and calcium vanadate, they move like a liquid.

According to the researchers, this electron flow produces high optical transparency along with high metal-like conductivity.

"We are trying to make metals transparent by changing the effective mass of their electrons," Engel-Herbert said.

"We are doing this by choosing materials in which the electrostatic interaction between negatively charged electrons is very large compared to their kinetic energy," he said.

"As a result of this strong electron correlation effect, electrons 'feel' each other and behave like a liquid rather than a gas of non-interacting particles," he said.

"This electron liquid is still highly conductive, but when you shine light on it, it becomes less reflective, thus much more transparent," he said.

Currently indium costs around USD 750 per kilogramme, whereas strontium vanadate and calcium vanadate are made from elements with orders of magnitude higher abundance in the Earth's crust.

Vanadium sells for around USD 25 a kilogramme, less than 5 per cent of the cost of indium, while strontium is even cheaper than vanadium.

"Our correlated metals work really well compared to ITO," said Engel-Herbert. Along with display technologies, Engel-Herbert and his group are excited about combining their new materials with a very promising type of solar cell that uses a class of materials called organic perovskites.

Developed only within the last half dozen years, these materials outperform commercial silicon solar cells but require an inexpensive transparent conductor.

Strontium vanadate, also a perovskite, has a compatible structure that makes this an interesting possibility for future inexpensive, high-efficiency solar cells. The study was published in the journal Nature Materials.

PTI