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.

The electron transport layer (ETL) is a key component of perovskite solar cells (PSCs) and must provide efficient electron extraction and collection while minimizing the charge recombination at interfaces in order to ensure high performance. Conventional bilayered TiO ETLs fabricated by depositing compact TiO (c-TiO) and mesoporous TiO (mp-TiO) in sequence exhibit resistive losses due to the contact resistance at the c-TiO/mp-TiO interface and the series resistance arising from the intrinsically low conductivity of TiO. Herein, to minimize such resistive losses, we developed a novel ETL consisting of an ultrathin c-TiO layer hybridized with mp-TiO, which is fabricated by performing one-step spin-coating of a mp-TiO solution containing a small amount of titanium diisopropoxide bis(acetylacetonate) (TAA). By using electron microscopies and elemental mapping analysis, we establish that the optimal concentration of TAA produces an ultrathin blocking layer with a thickness of ∼3 nm and ensures that the mp-TiO layer has a suitable porosity for efficient perovskite infiltration. We compare PSCs based on mesoscopic ETLs with and without compact layers to determine the role of the hole-blocking layer in their performances. The hybrid ETLs exhibit enhanced electron extraction and reduced charge recombination, resulting in better photovoltaic performances and reduced hysteresis of PSCs compared to those with conventional bilayered ETLs.

In article number https://doi.org/10.1002/adfm.201605988, Chul‐Ho Lee, Min Jae Ko, and co‐workers report the structural engineering of formamidinium lead iodide (FAPbI3) perovskite thin films by partially substituting the formamidinium cations with smaller rubidium (Rb) cations. Even traces of Rb significantly enhance photovoltaic performances and long‐term stability of perovskite solar cells. This is due to the supplement favoring complete conversion of the perovskite to its photoactive phase while offering structural stabilization.