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.
The work, completed with collaborators at Hong Kong-based PV firm Solargiga Energy Holdings and two Chinese universities, appears today in energy research journal Joule. 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.
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 85C. 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.