A main reason for enhanced stability PSCs is attributed to suppressed ionic migration. In Fig. and 4D, we compare the I -ions distribution at the ~300 nm depth of the perovskite layer for PSCs based on FAPbI3 and FA0.91Cs0.09PbI3 after 240 h MPP test. In the FAPbI3-based device, strong aggregation of I -clusters is observed in the perovskite absorber layers; in contrast, I -ions distribute uniformly in the FA0.91Cs0.09PbI3 based devices. This sharp contract indicates that, the ionic migration in the FA0.91Cs0.09PbI3 is much suppressed upon Cs sequential incorporation.