The Cs incorporation also significantly improves the film morphologies (Fig. ). All FA1-xCsxPbI3 films show enlarged and pinhole-free grains compared to the FAPbI3 film, which shows coarse grains and pinholes. As shown in the cross-sectional images, FA1-xCsxPbI3 perovskite films (550 to 600 nm) are composed of micrometer-sized grains comparable to the film thickness, benefiting efficient charge extraction. ). Considering that x=0.09 provides the optimal photovoltaic (PV) performance, we then chose the case of FA0.91Cs0.09PbI3 for detailed investigations on Cs incorporation and its role on film and device properties. X-ray photoelectron spectroscopy (XPS) spectra are conducted to explore the effect of sequential Cs incorporation on the chemical composition of perovskite films. All core-level peaks are assigned to Cs, Pb, I, C, and N (Fig. , 2B, and fig. ). Fig. shows that the characteristic Cs signals in the FA0.91Cs0.09PbI3 perovskite locate at 738.5 eV and 724.7 eV. For the Pb 4f spectra in FAPbI3, two peaks corresponding to Pb 4f7/2 and Pb 4f5/2 are observed at 138.3 eV and 143.2eV. However, in the case of FA0.91Cs0.09PbI3 perovskite, both Pb 4f7/2 and Pb 4f5/2 shift 0.1 eV toward higher binding energy, originating from stronger bond energies between Cs + and [PbI6] 4-than that between FA + and [PbI6] 4-. Additionally, the N and I elements representing the formamidine component have undergone a small shift (fig. ). These results further confirm that Cs has been successfully incorporated into the FAPbI3 perovskite lattice to form FA0.91Cs0.09PbI3 perovskite.