Uniform incorporation of Cs ions into FAPbI3 has two positive effects: enhanced phase stability of the photoactive black phase and decreased trap/defect densities. The enhanced phase stability is evidenced from the absence of color/structural changes under humid conditions. Under 60% relative humidity, FA0.91Cs0.09PbI3 maintains the black phase for 7 days without any changes (fig. ), showing significant improvement compared with FAPbI3. The decreased trap/defect densities are demonstrated from photophysical measurements. The photoluminescence (PL) intensities of FA0.91Cs0.09PbI3 perovskites are much stronger than those of pure FA perovskite films (fig. ). In addition, the time-resolved PL (TRPL) spectra in Fig. show that the PL lifetime (τ) of FA0.91Cs0.09PbI3 is much longer (413.84 ns) than that of FAPbI3 (165.49 ns).