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Perovskite solar cells (PSCs) based on organic monovalent cation (methylammonium or formamidinium) have shown excellent optoelectronic properties with high efficiencies above 22%, threatening the status of silicon solar cells. However, critical issues of long-term stability have to be solved for commercialization. The severe weakness of the state-of-the-art PSCs against moisture originates mainly from the hygroscopic organic cations. Here, rubidium (Rb) is suggested as a promising candidate for an inorganic-organic mixed cation system to enhance moisture-tolerance and photovoltaic performances of formamidinium lead iodide (FAPbI 3 ). Partial incorporation of Rb in FAPbI 3 tunes the tolerance factor and stabilizes the photoactive perovskite structure. Phase conversion from hexagonal yellow FAPbI 3 to trigonal black FAPbI 3 becomes favored when Rb is introduced. The authors find that the absorbance and fluorescence lifetime of 5% Rb-incorporated FAPbI 3 (Rb 0.05 FA 0.95 PbI 3 ) are enhanced than bare FAPbI 3 . Rb 0.05 FA 0.95 PbI 3 -based PSCs exhibit a best power conversion efficiency of 17.16%, which is much higher than that of the FAPbI 3 device (13.56%). Moreover, it is demonstrated that the Rb 0.05 FA 0.95 PbI 3 film shows superior stability against high humidity (85%) and the full device made with the mixed perovskite exhibits remarkable long-term stability under ambient condition without encapsulation, retaining the high performance for 1000 h. Figure 6. Fluorescence lifetime imaging microscopy (FLIM) images of a) FAPbI 3 and b) Rb 0.05 FA 0.95 PbI 3 films deposited on glass. c) Time-resolved photoluminescence (TRPL) spectra of FAPbI 3 and Rb 0.05 FA 0.95 PbI 3 films. Scale bar: 2 µm.