Solar Cell Scientist Jokes About Using Coffee to Get More Power. It Works.

Want to give your solar panels a big efficiency boost? Your morning cup of coffee could hold the answers.

In a stroke of kitchen-based genius perhaps comparable to the accidental discovery of penicillin, researchers at the University of California, Los Angeles and China-based solar energy firm Solargiga Energy have boosted the efficiency of perovskite solar cells from 17 percent to more than 20 percent by using caffeine molecules. This discovery, which started as a joke in the lab, could help this lower-cost flexible alternative compete with current solar technology.

The results were published Thursday in the journal Joule.

“It might sound like an unthinkable story, how lucky this group is. However, in reality, we have studied the perovskite for six or seven years; there is a lot of accumulated knowledge behind this ‘pure luck,’” Yang Yang, professor at the University of California, Los Angeles and leader of the research group, tells Inverse. “In addition, we have tried many of the molecules that did not work well, and we never bother to report them. The caffeine was identified, after the lucky pick, having the lone electron pairs in the oxygen atom; hence we have some ideas before we really put caffeine molecules to test.”

It comes at an exciting time for solar research as existing panels drop in price and scientists look for ways to boost efficiency. In February, a team in Switzerland announced a panel that can offer yields up to 29 percent by combining expensive space satellite cells with lenses that focus light onto a smaller area, far higher than the normal 17 to 19 percent seen in regular panels. Research last month explained how rotating solar panels could collect 32 percent more energy than fixed alternatives. Another team reached 12.25 percent efficiency with flexible layered polymer cells, which could enable more creative applications.

Perovskite cells could hold great potential. Solar comparison site EnergySage explains that unlike normal cells, which are made using crystalline silicon that requires extraction and processing, these new designs use solution processing that works similar to making a newspaper. The area has made great strides in a short space of time, and Oxford PV announced in June 2018 that it had reached a world record for efficiency in the lab of 27.3 percent.

Yang’s team saw quick results. They added the layer to 40 cells and used infrared spectroscopy to observe the outcome. The caffeine bonded with the material, and the carbonyl groups worked with the lead ions to lock. That reduced the threshold of energy required for the cells to react, boosting efficiency.

“We were surprised by the results,” Rui Wang, a Ph.D. candidate in Yang’s group, said in a statement. “During our first try incorporating caffeine, our perovskite solar cells already reached almost the highest efficiency we achieved in the paper.”

“Interestingly, at about the same time, one of the co-authors suggested a molecule which he thought should work for perovskite, and it is very similar to caffeine,” Yang says. “Therefore, even we do not have the lucky pick in the first place. We will end up identifying caffeine molecules eventually.”

Abstract: To increase the commercial prospects of metal halide perovskite solar cells, there is a need for simple, cost-effective, and generalized approaches that mitigate their intrinsic thermal instability. Here we show that 1,3,7-trimethylxanthine, a commodity chemical with two conjugated carboxyl groups better known by its common name caffeine, improves the performance and thermal stability of perovskite solar cells based on both MAPbI3 and CsFAMAPbI3 active layers. The strong interaction between caffeine and Pb2+ ions serves as a ‘‘molecular lock’’ that increases the activation energy during film crystallization, delivering a perovskite film with preferred orientation, improved electronic properties, reduced ion migration, and greatly enhanced thermal stability. Planar n-i-p solar cells based on caffeine-incorporated pure MAPbI3 perovskites, which are notoriously unstable, exhibit a champion-stabilized efficiency of 19.8% and retain over 85% of their efficiency under continuous annealing at 85C in nitrogen.