Caffeine Wakes Up Perovskite Solar Cells

April 26th, 2019 by Steve Hanley

Many important discoveries are the result of accidents. Researchers looking to make super strong adhesives to glue parts of airplanes together wound up creating one of the least sticky substances known to science. It got repurposed to make Post-It notes, those little pieces of paper that can hardly stick to a refrigerator. WD-40 was intended as a spray-on substance that would displace water from metal surfaces. The WD part stands for “water displacement.” The “40” part represents the other 39 attempts that didn’t work.

People used to scoff at Thomas Edison as he tried hundreds of materials for the filament of an electric light bulb. All of them failed. “That just means I am that much closer to finding something that does work,” he allegedly said to his detractors. Then he tried tungsten wire and the rest, as they say, is history.

In theory, perovskites should make solar cells that are cheaper because they use elements found in abundance in nature. The original perovskite solar cells had an efficiency of around 1%. Today, non-tandem perovskite solar cells are able to achieve up to 24.2% efficiency, but they still suffer from several drawbacks. They don’t last very long and disintegrate in the presence of water and other contaminants. There’s no way a solar panel using cells made from perovskites could perform reliably in the wild for 20 years or more the way conventional solar panels do.

Researchers at the University of California Los Angeles think they may have found a way to improve the performance of perovskites. They were sitting around drinking coffee in the cafeteria one day, looking for an energy boost, when one of them suggested trying caffeine to improve the performance of the perovskites they were working on in the lab. So they did.

It was a moment of “pure luck,” says UCLA engineer Yang Yang, the faculty adviser to the graduate students. “We needed some kind of molecule with lone electron pairs,” he tells Scientific American. As it turns out, caffeine is just the sort of molecule they were looking for. The research was published this month in the journal Joule.

Caffeine allows perovskite crystals to form without as much “disorder” as those grown without it says Joseph Berry, a physicist at the National Renewable Energy Lab, who was not involved in the research. “Generally the perspective is, ‘If you make the material more perfect, you get one that does better.’ The caffeine, at the local level, ensures you get a material that’s a bit more well-structured,” he says. “That results in a more stable device.”

“Fundamentally, these perovskite materials offer a functionality that can’t be matched. We are only just beginning to understand them well enough to begin engineering them,” he adds. “That’s why these results from Yang are so compelling.” Insights from the new research might help scientists discover or design new molecules that stabilize perovskite solar cells even better than caffeine does.

Jinsong Huang, a physicist at the University of North Carolina Chapel Hill, thinks this new discovery could help push perovskite solar cells closer to commercial sale. “Stability is the last hurdle we need to overcome [for perovskite cells] to enter the market. You can make solar cells more efficient and more stable in other ways. But this is a very good result, and it opens our minds about different materials that you never thought would work.”

The caffeine-infused perovskite cells have proven more durable than conventional perovskites and have boosted efficiency from 16% (the starting/default cells) to 20%. (Note: that’s still less than the record-holding 24.2% efficiency non-tandem cells.)

Tandem perovskites have reached nearly 30% efficiency in the laboratory. Just imagine what they could achieve if they used caffeine-based cells. “Tandem solar cells are just like a double decker bus,” Yang says. “Anything that helps the single layer bus can help the double decker.”

Making perovskites is easy, Yang says. “We just buy chemicals and blend them in our lab in a beaker. It’s like cooking, Then we put a little bit of caffeine into the liquid, and blend it all evenly.” Finally the mixture is poured over glass to create pervoskite cyrstals. “A high school kid could make a good perovskite solar cell in our lab,” Yang says.

Some may quibble that perovskites are a long way from breaking out of the lab and into commercial production. That’s true, but it wasn’t all that long ago the light from LEDs was so faint, people wondered if they would ever have any useful purpose. Never say never, in other words. Cheap solar panels made from readily available materials may be closer than we think.









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