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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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.

Electron-transport-layer free perovskite solar cells (ETL-free PSCs) have attracted great attention due to their low cost and simple manufacturing process. However, an additional interface layer has to be introduced, and the currently achieved efficiency remains far from full-structure PSCs. Here, we report an in situ interface engineering strategy by the methylammonium acetate (MAAc) ionic liquid perovskite precursor. We found that a dipole layer was in situ constructed through the physical adsorption of the residual MAAc polar molecules on the indium tin oxide electrode, which is significantly different from the treatment by the interface layer in previous reports. This allows a decrease of the effective work function and enables in situ band bending in the perovskite semiconductor. The in situ band bending facilitates charge collection and hinders interfacial charge recombination, leading to ETL-free PSCs with a maximum power conversion efficiency of 21.08%, which is the highest report to date.