Suppressed ionic migration in FA0.91Cs0.09PbI3 is consistent with suppressed electron-phonon coupling upon Cs incorporation. Fig. shows the full-width half-maximum (FWHM) of the PL peak of FA0.91Cs0.09PbI3 and FAPbI3 perovskites (fig. ) ranging from 110 K to 296 K. The wide broadening of the PL linewidth in FAPbI3 perovskites arises from strong electron-phonon coupling . The electron-phonon interaction is dominated by high energy LO phonons in the high-temperature region, where the measured FWHM data could be fitted by the Boson model (Fig. , fig. , and Table ). Compared with FAPbI3, both electron-LO phonon coupling coefficient (ΓLO) and LO phonon energy (hω) in the FA0.91Cs0.09PbI3 are significantly reduced, indicating that the fluctuation of the PbI6 octahedra cage in FA0.91Cs0.09PbI3 is associated with much smaller energies upon the Cs sequential incorporation. This is consistent with the previous theoretical investigations, which indicate that mixed A-site cations could reduce the lattice fluctuations in halide perovskites . As such, the suppressed lattice fluctuations and electron-phonon coupling in FA0.91Cs0.09PbI3 rationalize suppressed ionic migration and hence enhanced stability in FA0.91Cs0.09PbI3 PSCs.