Electronics must perform and endure for 1,000 hours at 85 °C to pass a standard accelerated lifetime test. Coffee is best served at 85 °C—a fact that’s built right into the name of the fast-growing 85°C Bakery Café chain. Just another meaningless coincidence? Not for the photovoltaics researchers at the University of California in Los Angeles. For them, 85 degrees was a clue hiding in plain sight.
UCLA professor Yang Yang’s lab chock-full of coffee drinkers spent several years searching for a stability-enhancing additive to turn famously unstable perovskite PV cells into a useful product. Then, on a lark, Yang's graduate student Rui Wang suggested they try adding caffeine to the mix. To the team's surprise, caffeine produced longer lasting and more powerful solar cells.
Yang sipped his second (and final) cup of java earlier this week as he explained to IEEE Spectrum how caffeine helps perovskite solar cells. While caffeine excites the nerves of coffee drinkers—making Yang’s hands shake if he overindulges—it actually chills out perovskites.
Caffeine's calming effect starts during the creation of perovskite crystals. “Without caffeine, the crystallization process will just take 2 seconds, but with caffeine it will take 1 to 2 minutes,” says Yang. The more deliberate growth process yields a perovskite material with larger grains of defect-free crystal. They are more stable mechanically and better at moving the charges created from incoming photons.
Perovskite cells on caffeine keep cranking out power during degradation tests, while those without caffeine collapse.Image: UCLA/Joule
Caffeine also stabilizes perovskite PV cells during operation because each caffeine molecule can bind to two lead atoms at the boundaries of the crystal grains. This dual molecular lock ties the grains together and, Yang believes, hinders the movement of ions that threaten to reshape the crystal into a weaker pattern.
The lab’s best caffeine-treated cell captures incoming light with an efficiency of 19.8 percent, up from 17 percent for untreated cells, and retains 86 percent of its output after operating for 1,300 grueling hours at 85 °C. That’s remarkable endurance compared with that of the lab’s untreated cells, whose output plummeted by 40 percent after just 175 hours.
Still, Yang says they need materials that hold it together through at least one to two years of accelerated testing to provide confidence that they can pump out power for several decades on a rooftop.
So as Yang’s students continue to test their champion cell, they are simultaneously leveraging what caffeine has taught them to seek out even better perovskite calming agents. While they take advantage of design software to find the perfect molecule, the lab’s latest advance affirms that artificial intelligence still has much to learn from the “analog brain”—especially those on coffee.
85°C Bakery Café is doing its part to help, with 29 locations around Los Angeles. The chain has grown worldwide to over 1,000 locations since first overrunning Taiwan, where it has what online foodie portal Eater calls a “staggering ubiquity and rabid following.”
Eater recommends trying 85°C's iced sea salt latte and their calamari stick—a “squid ink-based bread roll with Swiss cheese inside and garlic spread on top.” Now that's innovation!
Peter Fairley has been tracking energy technologies and their environmental implications globally for over two decades, charting engineering and policy innovations that could slash dependence on fossil fuels and the political forces fighting them. He has been a Contributing Editor with IEEE Spectrum since 2003.