Revealing Stability of Inverted Planar MA-Free Perovskite Solar Cells and Electric Field-Induced Phase Instability

Hybrid organic-inorganic perovskite is one of the most promising candidates to replace state-of-art silicon to fabricate low cost solar cells. However, its instability, including intrinsic and operational instability, strongly hinders its real-life applications. Methylammonium (MA)-free, formamidinium (FA)-based perovskite doped by small A-site inorganic cations was developed to tackle the intrinsic instability issue, but the operational instability, especially against the applied electric field, induced by defect mediated ion migration remains a problem. In this work, we fabricate two types of MA-free perovskites, Rb(0.05)Cs(0.1)FA(0)(.8)(5)PbI(3) and Cs(0.15)FA(0)(.8)(5)PbI(3), and investigate the effect of Rb+ on the device performance and long-term stability. We find that even with incomplete incorporation, Rb+ cation can significantly improve the device performance. We reveal the defect-mediated cation and anion migration under electric field using cross-sectional secondary electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry, and identify that RV is more vulnerable compared to Cs+. By simply mixing the precursor solution before spin coating, we significantly reduce the defect states in both types of perovskite and improve the device stability against an electric field. The modified precursor solution provides the devices with Rb(0.05)Cs(0.1)FA(0.85)PbI(3) and Cs(0.15)FA(0.85)PbI(3) active layers that retain 68% and 92% of their initial PCE, respectively, over 30 days under N-2 protection.