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.

In this study, a novel and facile passivation process for a perovskite solar cell is reported. Poor stability in ambient atmosphere, which is the most critical demerit of a perovskite solar cell, is overcome by a simple passivation process using a hydrophobic polymer layer. Teflon, the hydrophobic polymer, is deposited on the top of a perovskite solar cell by a spin-coating method. With the hydrophobic passivation, the perovskite solar cell shows negligible degradation after a 30 day storage in ambient atmosphere. Suppressed degradation of the perovskite film is proved in various ways: X-ray diffraction, light absorption spectrum, and quartz crystal microbalance. This simple but effective passivation process suggests new kind of approach to enhance stability of perovskite solar cells to moisture.

Highly efficient solar cells with sustainable performance under severe mechanical deformations are in great demand for future wearable power supply devices. In this regard, numerous studies have progressed to implement flexible architecture to high‐performance devices such as perovskite solar cells. However, the absence of suitable flexible and stretchable materials has been a great obstacle in the replacement of largely utilized transparent conducting oxides that are limited in flexibility. Here, a shape recoverable polymer, Noland Optical Adhesive 63, is utilized as a substrate of perovskite solar cell to enable complete shape recovery of the device upon sub‐millimeter bending radii. The employment of stretchable electrodes prevents mechanical damage of the perovskite layer. Before and after bending at a radius of 1 mm, power conversion efficiency (PCE) is measured to be 10.75% and 10.4%, respectively. Additionally, the shape recoverable device demonstrates a PCE of 6.07% after crumpling. The mechanical properties of all the layers are characterized by nanoindentation. Finite element analysis reveals that the outstanding flexibility of the perovskite layer enables small plastic strain distribution on the deformed device. These results clearly demonstrated that this device has great potential to be utilized in stretchable power supply applications.