Stabilizing Black-Phase Formamidinium Perovskite Formation At Room Temperature And High Humidity

The stabilization of black-phase formamidinium lead iodide (alpha-FAPbI(3)) perovskite under various environmental conditions is considered necessary for solar cells. However, challenges remain regarding the temperature sensitivity of alpha-FAPbI(3) and the requirements for strict humidity control in its processing. Here we report the synthesis of stable alpha-FAPbI(3), 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 alpha-FAPbI(3). 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 degrees C and continuous light stress, respectively.