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

Abstract

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.