Sitting in a dimly lit bar in Japan, then graduate student Michael Lee was scribbling on a beer coaster as night fell, jotting down a list of chemical ingre- dients before he forgot them. Earlier that day scientists at Toin University of Yokohama had generously shared their groundbreaking recipe for making solar cells from a new material called perovskite rather than the usual silicon. The cells were only 3.8 percent efficient in converting sunlight to electricity, so the world had not taken notice. But Lee was inspired. After the 2011 fact-finding mission, he returned to Clarendon Laboratory at the University of Oxford, where all three of us worked at the time, and made a series of tweaks to the recipe. The changes yielded the first perovskite cell to surpass 10&nsp;percent efficiency. His invention sparked the clean-energy equivalent of an oil rush, as researchers worldwide raced to push perovskite cells even higher.

The latest record, set at 20.1 percent by the Korea Research Institute of Chemical Technology in November 2014, marked a fivefold increase in efficiency in just three years. For comparison, after decades of development state-of-the-art silicon solar cells have plateaued at about 25 percent, a target that perovskite researchers like us have squarely in our sights. We are also anticipating a commercial debut, perhaps through a spin-off company such as Oxford Photovoltaics, which one of us (Snaith) co-founded. ...