In only five years, the sunlight-to-energy efficiency of solar cells made from lab-grown perovskites has almost quintupled, from 3.8 to nearly 20 percent. Compare that to sluggish gains made with silicon technology, which took three decades to reach the same threshold. Today, commercial silicon panels top out at 18 percent. Perovskite cells may best that rate, and have the added advantage of being cheap and easy to manufacture. They could reduce the cost-per-watt of solar by more than half. Plus, some prototype cells are partially translucent, so researchers like Oxford University’s Henry Snaith see potential applications as windowpanes. But for all of perovskites’s promise, scientists still need to address three big challenges.