David Moore wields a cheap paintbrush. With a steady, purple-gloved hand, he paints a yellowish liquid onto a specially prepared piece of glass about the size of a half-dollar. And as simple as that, a perovskite solar cell is born. According to Joseph Berry, a senior scientist at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), “There isn’t another technology you can do this with.”    Berry, who leads the laboratory’s perovskite and...