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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A solvent is vital to the control of crystallization and crystal growth in state-of-theart solution-processed hybrid organic-inorganic perovskites. We demonstrate an alternative environmentally friendly room-temperature molten salt, methylammonium acetate (MAAc), as a solvent characterized by high viscosity, negligible vapor pressure, and nonhazardous nature, which can be used to produce highly efficient perovskite solar cells (PSCs) in ambient air. The resulting PSCs exhibited excellent stability under light and dark conditions.