Efficient and Stable β‑CsPbI3 Solar Cells through Solvent Engineering with Methylamine Acetate Ionic Liquid

CsPbI3, an all-inorganic perovskite material with suitable band gap and excellent thermal stability, has garnered significant attention for its potential in perovskite solar cells (PSCs). However, CsPbI3 is susceptible to phase changes from photoactive to photoinactive in humid environments. Hence, it is crucial to achieve controllable growth of CsPbI3 perovskite thin films with the desired β-crystal phase and compact morphology for efficient and stable PSCs. Herein, MAAc was used as a solvent for the CsPbI3 precursor to fabricate β-CsPbI3 perovskite. An intermediate compound of Cs x MA1–x PbI x Ac3–x was initially formed in the MAAc solution, and during annealing, the MA+ and Ac– ions were replaced by Cs+ and I– ions, respectively. Furthermore, the incorporation of strong CO···Pb coordination stabilized the black-phase β-CsPbI3 and facilitated the growth of crystals with a narrow vertical orientation and large grain size. As a result, the PSCs with an efficiency of 18.9% and improved stability (less than 10% decay after 2000 h of storage in N2 and less than 30% decay after 500 h of storage in humid air without any encapsulation) were achieved.