A new material that is highly transparent and electrically conductive could make large-screen displays, touch screens and solar cells more affordable and efficient. Indium tin oxide (ITO) is a transparent conductor that is currently used for more than 90 percent of displays. However, in the last decade, the price of indium has increased. Manufacturers have searched for an ITO replacement, but nothing has matched ITO's combination of optical transparency, electrical conductivity and ease of fabrication.

A team led by Roman Engel-Herbert, assistant professor of materials science and engineering at Pennsylvania State University, has used 10-nanometer-thick films of a class of materials called correlated metals. While in most conventional metals—copper, gold, aluminum and 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.

Samples of the correlated metals strontium vanadate (left two squares) and calcium vanadate (right two squares) with two uncoated squares (center). Image credit: Lei Zhang/Penn State.Samples of the correlated metals strontium vanadate (left two squares) and calcium vanadate (right two squares) with two uncoated squares (center). Image credit: Lei Zhang/Penn State."We are trying to make metals transparent by changing the effective mass of their electrons," Engel-Herbert says. The researchers are doing this by choosing materials in which the electrostatic interaction between negatively charged electrons is large compared to their kinetic energy. As a result of this electron correlation effect, electrons "feel" each other and behave like a liquid rather than a gas of non-interacting particles. The electron liquid remains highly conductive, but when a light is shone on it, it becomes less reflective and much more transparent.

Currently, indium costs around $750 per kilogram. By contrast, strontium vanadate and calcium vanadate are made from elements much more abundant in the earth's crust allowing vanadium to sell for around $25 a kilogram. Strontium is even less expensive, the researchers say.

"Our correlated metals work really well compared to ITO," says Engel-Herbert. "Now, the question is how to implement these new materials into a large-scale manufacturing process. From what we understand right now, there is no reason that strontium vanadate could not replace ITO in the same equipment currently used in industry."

Along with display technologies, Engel-Herbert and his group say they believe they can combine their new materials with a type of solar cell that uses a class of materials called organic perovskites. These materials are able to 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 researchers say.

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