Next-gen solar designed to take the heat

A next-generation solar cell that can maintain efficiency in the heat has been developed by engineers at Iowa State University.

This solar cell takes advantage of the electro-optical properties of perovskite materials, and the materials are much more stable at high temperatures.

One problem with the current generation of silicon solar cells is their relatively low efficiency at converting solar energy into electricity, according to Vikram Dalal, the Director of Iowa State's Microelectronics Research Center.

The best silicon solar cells in the laboratory are about 26% efficient while commercial cells are about 15%. That means bigger systems are necessary to produce a given amount of electricity, and bigger systems mean higher costs.

That has researchers looking for new ways to raise efficiency and decrease costs. One idea that could boost efficiency by as much as 50% is a tandem structure that stacks two kinds of cells on top of each other, each using different, complementary parts of the solar spectrum to produce power.

The potential of perovskite

Researchers have recently started looking at hybrid organic-inorganic perovskite materials as a good tandem partner for silicon cells. Perovskite calls have efficiency rates nearing 25%, have a complementary bandgap, can be very thin (just a millionth of metre) and can easily be deposited on silicon.

However, Dalal said researchers have learned those hybrid perovskite solar cells break down when exposed to high temperatures.

This causes problems when solar arrays are placed where the sunshine is — usually hot, dry deserts in places such as the American southwest, Australia, the Middle East and India. Ambient temperatures in such places can hit 48–54°C and solar cell temperatures can hit 93°C.

Iowa State University engineers, in a project partially supported by the 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 recently published online by the scientific 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 is working in Boise, Idaho, as an engineer for ON Semiconductor.

Tweaking the material

Dalal, the corresponding author of the paper, said there are two key developments in the new solar cell technology.

Firstly, engineers made some tweaks to the make-up of the perovskite material. They removed organic components in the material — particularly cations, materials with extra protons and a positive charge — and substituted inorganic materials such as cesium. That made the material stable at higher temperatures.

Secondly, they developed a fabrication technique that builds the perovskite material one thin layer — just a few billionths of a metre — 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.

The result of those changes?

“Our perovskite solar cells show no thermal degradation even at 200°C 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,” Dalal said.

The paper reports the new inorganic perovskite solar cells have a photo-conversion efficiency of 11.8%.

“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.”

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