Correlated Metal Films Eyed for Affordable Displays

A highly transparent and electrically conductive metal thin film holds promise as an alternative to indium tin oxide (ITO) in displays, windows, touch screens and solar cells.

ITO is a transparent conductor used in more than 90 percent of the display market and has been the dominant material for the past 60 years, said researchers from Pennsylvania State University. In the last decade, the price of indium has increased dramatically, and displays and touchscreen modules have become a main cost driver in smartphones and tablets, making up close to 40 percent of the cost. In other words, while memory chips and processors get cheaper, displays get more expensive from generation to generation.

The crystal structure of strontium vanadate (orange) and calcium vanadate (blue). The red dots are oxygen atoms arranged in eight octohedra surrounding a single strontium or calcium atom. Vanadium atoms can be seen inside each octahedron. Images courtesy of Lei Zhang/Penn State.

The Penn State team has reported a design strategy using 10-nm-thick films of an unusual class of materials called correlated metals.

In most conventional metals, such as copper, gold, aluminum or silver, electrons flow like a gas. In correlated metals, such as strontium vanadate and calcium vanadate, they move more like a liquid. The electron flow produces high optical transparency along with high metal-like conductivity, the researchers said.

"We are trying to make metals transparent by changing the effective mass of their electrons," said professor Roman Engel-Herbert. "We are doing this by choosing materials in which the electrostatic interaction between negatively charged electrons is very large compared to their kinetic energy.

Samples of the correlated metals strontium vanadate (two squares on left) and calcium vanadate (two squares on right) with two uncoated squares at center.

"As a result of this strong electron correlation effect, electrons 'feel' each other and behave like a liquid rather than a gas of noninteracting particles. This electron liquid is still highly conductive, but when you shine light on it, it becomes less reflective, thus much more transparent."

The correlated metals demonstrated excellent performance when benchmarked against ITO, the researchers said.

"Now, the question is how to implement these new materials into a large-scale manufacturing process," said Engel-Herbert. "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 they seek to patent, the researchers will investigate their new materials with a type of solar cell that uses organic perovskite materials. Developed recently, these materials outperform commercial silicon solar cells but require an inexpensive transparent conductor. Strontium vanadate, also a perovskite, has a compatible structure that makes it an interesting candidate for future inexpensive, high-efficiency solar cells, the researchers said.

The research was supported by the Office of Naval Research, National Science Foundation and Department of Energy.

The study was published in Nature Materials (doi: 10.1038/nmat4493).