Next-gen Solar Cells Take The Heat And Maintain Efficiency

Iowa State University engineers use the benefits of perovskite while stabilising the cells at high temperatures

Hybrid organic-inorganic perovskite materials are showing promise as a tandem partner for silicon solar cells. Perovskite calls have efficiency rates nearing 25 percent, have a complementary bandgap, can be very thin (just a millionth of metre), and can easily be deposited on silicon.

But hybrid perovskite solar cells break down when exposed to high temperatures. That's a problem when you try to put solar arrays where the sunshine is - hot, dry deserts in places such as the American southwest, Australia, the Middle East and India. Ambient temperatures in such places can hit the 120 to 130 degrees Fahrenheit and solar cell temperatures can hit 200 degrees Fahrenheit.

Iowa State University engineers, in a project partially supported by the US National Science Foundation, have found a way to take advantage of perovskite's useful properties while stabilising the cells at high temperatures. They describe their discovery in a paper in the journal American Chemical Society Applied Energy Materials.

"These are promising results in pursuit of the commercialisation of perovskite solar cell materials and a cleaner, greener future," said Harshavardhan Gaonkar, the paper's first author who recently earned his doctorate in electrical and computer engineering from Iowa State and is now working in Boise, Idaho, as an engineer for ON Semiconductor.

Tweaking the material

Vikram Dalal, professor of Iowa State University's Microelectronics Research Centre, and corresponding author of the paper, outlined two key developments in the new solar cell technology.

First, he said the engineers made some tweaks to the makeup of the perovskite material. They did away with organic components in the material - particularly cations, materials with extra protons and a positive charge - and substituted inorganic materials such as caesium. That made the material stable at higher temperatures.

Second, they developed a fabrication technique that builds the perovskite material one thin layer at a time. This vapour deposition technique is consistent, leaves no contaminants, and is already used in other industries so it can be scaled up for commercial production.

"Our perovskite solar cells show no thermal degradation even at 200degC for over three days, temperatures far more than what the solar cell would have to endure in real-world environments," Gaonkar said.

"That's far better than the organic-inorganic perovskite cells, which would have decomposed totally at this temperature. So this is a major advance in the field," says Dalal

Raising performance

The paper reports the new inorganic perovskite solar cells have a photoconversion efficiency of 11.8 percent. That means there's more work ahead for the engineers.

"We are now trying to optimise this cell - we want to make it more efficient at converting solar energy into electricity," Dalal said. "We still have a lot of research to do, but we think we can get there by using new combinations of materials."

The engineers, for example, replaced the iodine common in perovskite materials with bromine. That made the cells much less sensitive to moisture, solving another problem with standard hybrid perovskites. But, that substitution changed the cells' properties, reducing efficiency and how well they work in tandem with silicon cells.

As they move ahead, the engineers believe they're on a proven path: "This study demonstrates a more robust thermal stability of inorganic perovskite materials and solar cells at higher temperatures and over extended periods of time than reported elsewhere," they wrote in their paper. "(These are) promising results in pursuit of the commercialisation of perovskite solar cell materials."

'Thermally Stable, Efficient, Vapor Deposited Inorganic Perovskite Solar Cells' by Harshavardhan Gaonkar et al; ACS Applied Energy Materials 2020