Green Method to Prepare Pure δ‐FAPbI₃ Crystals for Fabrication of Highly Efficient Perovskite Solar Cells

The production of pure formamidinium lead iodide (FAPbI₃) is crucial to ensure high‐efficiency perovskite solar cells and such pure FAPbI₃ is obtainable from crystallization process because impurities from the reagents are removable by molecular recognition effect. In addition, it is important to develop a green crystallization process for the production of FAPbI₃ single crystals. Via broad screening of chemicals, here, triethylphosphate and acetonitrile are selected as nontoxic solvent and antisolvent, respectively, for the liquid–liquid reaction of FAI and PbI₂ and the antisolvent crystallization. The new green crystallization process produces pure δ‐FAPbI₃ single crystals, which are applicable to fabricate α‐FAPbI₃ perovskite solar cells. Due to the reduced trap states in the δ‐FAPbI₃ single crystals, the corresponding α‐FAPbI₃ perovskite solar cells have 22.61% efficiency (average = 20.65 ± 0.86% across 50 devices) and show good operational stability under continuous 1 sun illumination at 50 °C for 1000 h.