Efficient and Stable β-CsPbI 3 Solar Cells through Solvent Engineering with Methylamine Acetate Ionic Liquid.

CsPbI 3 , 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, CsPbI 3 is susceptible to phase changes from photoactive to photoinactive in humid environments. Hence, it is crucial to achieve controllable growth of CsPbI 3 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 CsPbI 3 precursor to fabricate β-CsPbI 3 perovskite. An intermediate compound of Cs x MA 1- x PbI x Ac 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-CsPbI 3 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 N 2 and less than 30% decay after 500 h of storage in humid air without any encapsulation) were achieved.