Solar cells based on metal halide perovskite compounds draw widespread attention because these low-cost devices’ efficiency for generating electricity from sunlight has grown by leaps and bounds in the past few years. Aiming to further boost the conversion efficiency, scientists have examined the influence of reagent concentration, temperature, and other variables on the reaction that forms the crystalline light-harvesting perovskite film. Yet that process remains poorly understood and difficult to control because scientists have not yet examined some variables systematically. One of them can now be checked off the list—light (Nature 2017, DOI: 10.1038/nature22072). A team led by Michael Grätzel of the Swiss Federal Institute of Technology, Lausanne, used microscopy techniques and other methods to examine the influence of light on common procedures for reacting solution-phase PbI2 with CH3NH3I to form CH3NH3PbI3, a standard perovskite solar-cell material. The team observed that when the reagents are deposited sequentially on a TiO2 substrate, light accelerates perovskite formation. It also yields smaller and more numerous crystallites, causes other film-morphology differences, and doubles solar-cell efficiency. In contrast, perovskite films made via the single-step version of the reaction perform better when made in the dark.