Since the distribution of Cs in FA1-xCsxPbI3 perovskite has a significant effect on both the phase stability and traps/defects , we proceed to investigate the distribution of Cs in the resulting FA0.91Cs0.09PbI3 perovskite. The energy dispersive spectroscopy (EDS) mapping and time-of-flight secondary ion mass spectrometry (ToF-SIMS) establish that the incorporated Cs homogenously distributes in the surface and bulk of FA0.91Cs0.09PbI3 perovskite (Fig. and fig. ). In addition, other ions, including FA + , Pb 2+ and I -, are also uniformly distributed throughout the FA0.91Cs0.01PbI3 perovskite film (fig. ). Such uniform distribution of Cs ions is beneficial to improve the phase stability and reduce defect concentration. (Fig. ).