Insufficient phase stability has hindered the use of all-inorganic cesium lead iodide perovskite in light-emitting devices. Cheolmin Park and his team have demonstrated that adding a small amount of poly(ethylene oxide) promotes the formation of the desirable black α-phase while inhibiting the unwanted yellow δ-phase. This method for controlling perovskite crystallization will enable the production of high-quality films with excellent characteristics.