These are samples of the correlated metals strontium vanadate (two squares on the left) and calcium vanadate (two squares on the right) with two uncoated squares in the center. Photo: Lei Zhang, Penn State.
These are samples of the correlated metals strontium vanadate (two squares on the left) and calcium vanadate (two squares on the right) with two uncoated squares in the center. Photo: Lei Zhang, Penn State.

A new material that is both highly transparent and electrically conductive could make large screen displays, smart windows, and even touch screens and solar cells more affordable and efficient, according to the Penn State researchers who discovered it.

Indium tin oxide (ITO), the transparent conductor that is currently used in more than 90% of the display market, has been the dominant material for the past 60 years. In the past decade, however, the price of indium has increased dramatically. This has caused displays and touchscreen modules to become a major cost driver in smartphones and tablets, making up almost 40% of the cost. While memory chips and processors get cheaper, displays get more expensive from generation to generation. Manufacturers have searched for a possible replacement for ITO, but nothing has matched its combination of optical transparency, electrical conductivity and ease of fabrication.

A team led by Roman Engel-Herbert, assistant professor of materials science and engineering, now reports in Nature Materials a new design strategy that approaches the problem from a different angle. The researchers use 10nm-thick films of an unusual class of materials called correlated metals, in which the electrons flow like a liquid.

Whereas in most conventional metals, like copper, gold, aluminum 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 explained. "We are doing this by choosing materials in which the electrostatic interaction between negatively-charged electrons is very large compared to their kinetic energy. 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. This electron liquid is still highly conductive, but when you shine light on it, it becomes less reflective, thus much more transparent."

To better understand the process that produces this fine balance between transparency and conductivity, Engel-Herbert and his team turned to a materials theory expert, Karin Rabe of Rutgers University. "We realized that we needed her help to put a number on how 'liquid' this electron liquid in strontium vanadate is," Engel-Herbert said.

Rabe helped the Penn State team put together all the theoretical and mathematical puzzle pieces they needed in order to build transparent conductors from a correlated metal. Now that they understand the essential mechanism behind their discovery, the Penn State researchers are confident they will be able to find many other correlated metals that behave like strontium vanadate and calcium vanadate.

Lei Zhang, lead author on the Nature Materials paper and a graduate student in Engel-Herbert's group, was the first to recognize what they had discovered. "I came from Silicon Valley where I worked for two years as an engineer before I joined the group," said Zhang. "I was aware that there were many companies trying hard to optimize those ITO materials and looking for other possible replacements, but they had been studied for many decades and there just wasn't much room for improvement. When we made the electrical measurements on our correlated metals, I knew we had something that looked really good compared to standard ITO."

Strontium vanadate and calcium vanadate are made from elements that are found in much higher abundances in the earth's crust than indium. Because of this, whereas indium currently costs around $750 per kilogram, vanadium sells for around $25 a kilogram, less than 5% of the cost of indium, while strontium is even cheaper than vanadium.

"Our correlated metals work really well compared to ITO," said 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 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 past few 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.

Engel-Herbert and Zhang have applied for a patent on their novel transparent technology.

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.