Dissolution Modulation of Formamidinium‐Based Perovskite for Regulated Crystallinity in Printable Mesoscopic Solar Cells

The photovoltaic, luminescence, and detector fields have witnessed the robust application prospects of solution‐processed halide perovskites. Deep insights into solution can pave the way toward the precise crystallization control of halide perovskites for giving full play to the advantages of those materials. Herein, the dissolution behavior of formamidinium lead iodide (FAPbI 3 ) together with lead iodide in amide solvents with regulated coordination ability at increasing temperature and under different molar ratio between formamidinium iodide (FAI) and PbI 2 is studied. The solvent coordination ability, temperature, and FAI/PbI 2 molar ratio demonstrate equivalent influence on the dissolution, and increasing those factors tends to increase the solubility first and decrease it then for Pb compounds. It is proposed that there are interchangeable Pb solute forms including solvent‐containing lead complexes and solvated lead halide fragments in solution and the interconversion of both solutes driven by the above factors brings the solubility change. The modulated dissolution affects the crystallization behaviors of FAPbI 3 when preparing single crystals, nanocrystal dispersions, and thin films, and allows for regulated crystallinity in printable mesoscopic solar cells which demonstrate a power conversion efficiency of 18.40%.