Perovskite Solar Cells Tweaked to Withstand Heat

According to Vikram Dalal, a professor in engineering and in electrical and computer engineering at Iowa State University and director of Iowa State's Microelectronics Research Center, a problem with the current generation of silicon solar cells is their relatively low efficiency at converting solar energy into electricity. In the laboratory, the best silicon solar cells achieve an efficiency of about 26%, while commercial cells are only about 15% efficient. That means bigger systems are necessary to produce a given amount of electricity, and bigger systems mean higher costs.

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

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

Unfortunately, hybrid perovskite solar cells also break down when exposed to high temperatures. That's a problem when putting 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 120–130°F and solar cell temperatures can hit 200°F.

Engineers at Iowa State University, in a project partially supported by the US National Science Foundation, have now found a way to take advantage of perovskite's useful properties while stabilizing the cells at high temperatures. They report their discovery in a paper in Applied Energy Materials.

"These are promising results in pursuit of the commercialization 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 as an engineer for ON Semiconductor in Boise, Idaho.

Dalal, the corresponding author of the paper, said there are two key developments in this new solar cell technology. First, the engineers made some tweaks to the makeup of the perovskite material. They did away with organic components in the material – particularly positively charged ions, or cations – and substituted inorganic materials such as cesium, which made the material stable at higher temperatures.

Second, they developed a fabrication technique that builds up the perovskite material one thin layer – just a few billionths of a meter – at a time. This vapor deposition technique is consistent, leaves no contaminants and is already used in other industries, meaning it can be scaled up for commercial production.

The result of these changes? "Our perovskite solar cells show no thermal degradation even at 200°C (390°F) 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," added Dalal. "So this is a major advance in the field."

The new inorganic perovskite solar cells have a photoconversion efficiency of 11.8%, which means there's more work ahead for the engineers. "We are now trying to optimize 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."

For example, the engineers also 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. And so the tweaks and trials will continue.