Excess Cesium Iodide Induces Spinodal Decomposition of CsPbI2Br Perovskite Films

We report an exploratory study on the crystal formation behavior of CsPbI2Br perovskite films by adding excess cesium iodide (CsI). Surprisingly, facile co-crystallization of CsI and CsPbI2Br in the form of spinodal decomposition is observed. Significantly, the two phases spontaneously form morphing into a remarkably uniform bicontinuous nanoscale blend with high orientational correlation through the well matched (110) plane of CsI and the (200) plane of CsPbI2Br. The CsPbI2Br films produced by the spinodal decomposition method not only enjoy a compact surface, low defect concentration, and long carrier lifetimes, they also retain their excellent charge transport property. By employing such a CsPbI2Br film for carbon-based perovskite solar cells, power conversion efficiency exceeding 10% is achieved with remarkable thermal stability. Our results provide valuable insight into the role of CsI in perovskite crystallization and a promising approach for designing inorganic halide perovskite-based devices.