Manipulating the Crystallization and Phase Transition for High‐Performance CsPbI2Br Solar Cells

Abstract The inorganic perovskite solar cells (IPSC) are promising in the context of simultaneously delivering high efficiency and good stability. Developing a high‐performance and larger band gap IPSC is particularly in demand for commercialization due to their suitability to match with the prevailing silicon solar cells for tandem devices, while this is hindered by the poor morphology and phase stability of inorganic perovskite films. To address this issue, this work develops a combined method of nonstoichiometric composition and post‐cation exchange to improve the morphology and phase stability of the CsPbI 2 Br IPSCs, and achieves a record efficiency of 17.80%. This work finds that excessive PbI 2 regulates the CsPbI 2 Br film crystallization, and thus, a high‐quality perovskite film with enlarged grains is obtained. Further depositing the formamidinium iodide on top of the CsPbI 2 Br perovskite induces cation exchange during the post‐annealing process, which increases the phase stability of the perovskite film and significantly improves the device efficiency and stability. Therefore, this work provides an avenue toward high‐performance IPSCs, via the nonstoichiometric and ion exchange method.