Optical and structural measurements of perovskite films indicate that Cs has successfully been incorporated into the lattice of FAPbI3 perovskites. Fig. shows the ultraviolet-visible (UVvis) spectra of FA1-xCsxPbI3 perovskite films, in which the absorption edges of FA1-xCsxPbI3 perovskites strongly depend on the amount of Cs incorporation. When x increases from 0 to 0.16, the absorption edge of FA1-xCsxPbI3 perovskites gradually blue-shifts from 816 nm to 802 nm, and the corresponding photoluminescence (PL) peaks shift from 809 nm to 797 nm. The X-ray diffraction (XRD) measurements are carried out to investigate the crystal structure evolution of FA1-xCsxPbI3 perovskites (Fig. ). All the FA1-xCsxPbI3 perovskites exhibit stronger peak intensity than the pure FAPbI3 film, suggesting that sequential Cs incorporation enhances the overall crystallinity of perovskite films. The inset image of Fig. shows that the (110) peak between 13.8°-14.1° shifts to a higher degree, indicating that Cs are incorporated into the perovskite lattice. The tolerance factor of FA1-xCsxPbI3 perovskites also reduced compared with pure FA perovskite, which expected to stabilize perovskite structure (fig. ).