Metal Tech News - The Elements of Innovation Discovered

With a little bit of gold, scientists at Pennsylvania State University have taken another step toward the vision of turning windows into transparent solar cells that generate electricity while still offering a view of the world outside.

"Transparent solar cells could someday find a place on windows in homes and office buildings, generating electricity from sunlight that would otherwise be wasted," said Kai Wang, assistant research professor of materials science and engineering at Penn State and co-author on a study on new ultrathin gold electrodes.

With many believing that the technology behind traditional silicon photovoltaic solar cells is reaching its limits, scientists have been looking into alternatives such as perovskite cells.

Thin-film cells that use perovskites, a group of compounds with a crystal structure similar to the calcium titanium oxide mineral for which the group gets its name, have proven to be highly efficient at harnessing sunlight to generate electricity.

Perovskite solar cells are composed of five layers –perovskite, transparent conductive oxide, two conductor layers, and an electrode.

The researchers from Penn State have found that an ultrathin layer of gold makes a stable electrode for these cells that maintain high efficiencies in laboratory testing.

"We've shown we can make electrodes from a very thin, almost few atomic layers of gold," said Shashank Priya, associate vice president for research and professor of materials science and engineering at Penn State. "The thin gold layer has high electrical conductivity and at the same time it doesn't interfere with the cell's ability to absorb sunlight."

While previous research had indicated that a gold film so thin that it is measured in atoms would make an excellent transparent electrode for perovskite solar cells, creating such a film has been challenging. In previous attempts, the gold atoms were not evenly distributed, which hindered the conductivity of the film.

"Normally, if you grow a thin layer of something like gold, the nanoparticles will couple together and gather like small islands," said Dong Yang, assistant research professor of materials science and engineering at Penn State.

The Penn State team found that chromium provides an ideal surface for growing an ultrathin gold film with the requisite uniformity and conductivity to serve as an efficient electrode.

"Chromium has a large surface energy that provides a good place for the gold to grow on top of, and it actually allows the gold to form a continuous thin film," Yang added.

A perovskite solar cell with a gold electrode that was developed by the Penn State team achieved 19.8% efficiency, a record for a semitransparent cell.

Penn State

Scientists found using a chromium seed layer allowed them to grow ultrathin gold film that serves as a transparent electrode with good conductivity for perovskite solar cells.

"This is a big step – we finally succeeded in making efficient, semitransparent solar cells," said Wang.

While this perovskite cell is not clear enough to transform glass-clad skyscrapers into metropolitan solar farms, it has the potential to squeeze added electricity out of traditional photovoltaic cells.

A tandem device with the Penn State developed perovskite cell on top of a silicon cell achieved 28.3% efficiency, up from 23.3% from the silicon cell alone.

"A 5% improvement in efficiency is giant," Priya said. "This basically means you are converting about 50 watts more sunlight for every square meter of solar cell material. Solar farms can consist of thousands of modules, so that adds up to a lot of electricity, and that's a big breakthrough."

The Penn State scientists reported their findings in the journal Nano Energy. Special thanks to Matthew Carroll, who originally covered these findings in an article for Penn State News.