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_xMA_1-xPbI_xAc_3-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.