Fabrication of Cs x FA 1-x PbI 3 Mixed-Cation Perovskites via Gas-Phase-Assisted Compositional Modulation for Efficient and Stable Photovoltaic Devices

Over the past few years, significant attention has been focused on HC(NH ) PbI (FAPbI ) perovskite due to its reduced band gap and enhanced thermal stability compared with the most studied CH NH PbI (MAPbI ). However, FAPbI is sensitive to moisture and also encounters a serious structural phase-transition from photoactive α-phase to photoinactive δ-phase. Herein, we first develop a novel FAI gas-phase-assisted mixed-cation compositional modulation method to fabricate Cs FA PbI perovskite solar cells (PSCs), and realize the structural stabilization of α-phase FAPbI with the incorporation of smaller inorganic Cs ions. Through the setting of different Cs contents (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.50) along with a moderate FAI vapor deposition process, a series of Cs FA PbI films with consistent compositions are fabricated, which perfectly resolves the main blocking problems of the conventional solution approach, such as difficulty in compositional control and rough film morphology. Meanwhile, we find that the Cs amount is crucial for generating phase-pure Cs FA PbI (0 < x < 0.30) while higher contents result in phase segregation. Consequently, the optimum amount of Cs (x = 0.15) is verified, and Cs FA PbI shows the smallest unit cell volume and good moisture-resistant feature. Correspondingly, the highest power conversion efficiency (PCE) of 14.45% based on Cs FA PbI PSCs is successfully achieved in this work.