The stabilization of black-phase formamidinium lead iodide (α-FAPbI3) perovskite under various environmental conditions is considered necessary for solar cells. However, challenges remain regarding the temperature sensitivity of α-FAPbI3 and the requirements for strict humidity control in its processing. Here we report the synthesis of stable α-FAPbI3, regardless of humidity and temperature, based on a vertically aligned lead iodide thin film grown from an ionic liquid, methylamine formate. The vertically grown structure has numerous nanometer-scale ion channels that facilitate the permeation of formamidinium iodide into the lead iodide thin films for fast and robust transformation to α-FAPbI3. A solar cell with a power-conversion efficiency of 24.1% was achieved. The unencapsulated cells retain 80 and 90% of their initial efficiencies for 500 hours at 85°C and continuous light stress, respectively.

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Ferroelectric capacitive memories have not achieved the commercial success originally hoped for them in large volume because the area of the capacitors ("footprint") is too large to scale them up to gigabit density devices, [ 1 ] and a restoring pulse is required after a destructive readout. The non-destructive readout of the binary information is possible from the bipolar switching between high-and low-conductance of a ferroelectric diode under two opposite polarizations, as fi rst discovered by Blom et al. in PbTiO 3 perovskite thin fi lms [ 2 ] and later reported by Choi et al. in bulk BiFeO 3 single crystals and Pb(Zr,Ti)O 3 fi lms. [3][4][5] Important properties of such memory are the ultrafast operating speed depending on the polarization fl ipping time (1-2 ps in principle) [ 6 ] and the high ratio of resistance in the forward and reverse directions (3000:1). [ 7 ] However, most ferroelectrics are insulating wide bandgap semiconductors at room temperature, which limits the maximum diode current to the order of ≈ 20 mA cm − 2 . [ 2 , 3 ] Therefore, reaching a suffi cient ferroresistive diode current for the stable detection of memory status using the sense amplifi ers in modern memory circuitry with tiny cell size is a major challenge.In such strongly insulating ferroelectrics, suffi cient diode currents can, in fact, only be observed in ultrathin fi lms, where quantum mechanical tunneling current dominates [ 8 ] and is modulated by varying the tunneling barrier height along with the polarization reversal. Although this effect has been reproducibly demonstrated through local electron transport from an atomic force microscope (AFM) tip into ferroelectric thin fi lms, [9][10][11] the local-probe-based data storage is incompatible with current complementary metal-oxide semiconductor integration processes. Meanwhile, with macroscopic capacitor-type upper and lower electrodes capping the ultrathin ferroelectric layer, an overwhelming leakage current through existing defect-mediated leakage paths could swamp the tunneling current, thereby making the switching signal unreadable. In addition, large lattice-mismatch stresses in ultrathin epitaxial fi lms prevent their use as longtime retention memories due to preferred domain orientations. [ 12 ] One solution to these diffi culties has been to more broadly consider resistive switching effects in (non-ferroelectric) metal oxides. [13][14][15][16][17] However, most of these resistive switching effects are based on a certain type of defect (ionic or electronic) mediated phenomenon, suggesting the inherent diffi culty in precise control of the switching behavior. In contrast, ferroresistive switching behavior is based on the intrinsic switching of ferroelectric domains without invoking of charged defect migration and may, therefore, possess a fundamental merit over defectmediated mechanisms for achieving reliable performance requisite for commercial production once reliable fabrication parameters are established. A critical measure of such success using ferroelectric s...

All‐inorganic lead halide perovskites are promising candidates for optoelectronic applications. However, fundamental questions remain over the component interaction in the perovskite precursor solution due to the limitation of the most commonly used solvents of N,N‐dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Here, we report an interaction tailoring strategy for all‐inorganic CsPbI3−xBrx perovskites by involving the ionic liquid solvent methylammonium acetate (MAAc). C=O shows strong interaction with lead (Pb2+) and N−H⋅⋅⋅I hydrogen bond formation is observed. The interactions stabilize the perovskite precursor solution and allow production of the high‐quality perovskite films by retarding the crystallization. Without the necessity for antisolvent treatment, the one‐step air‐processing approach delivers photovoltaic cells regardless of humidity, with a high efficiency of 17.10 % along with long operation stability over 1500 h under continuous light illumination.