Stabilizing Formamidinium Lead Iodide Perovskite by Sulfonyl‐Functionalized Phenethylammonium Salt via Crystallization Control and Surface Passivation

Bulky organic ammonium cations have been widely used to stabilize lead halide perovskites via surface passivation or dimensionality modulation. Herein, a sulfonyl fluoride‐functionalized phenethylammonium salt (SF‐PEA) is reported as a bifunctional additive to stabilize the formamidinium lead iodide (FAPbI 3 ) perovskite. The sulfonyl group is found to interact with PbI 2 in the precursor and slow down the crystallization of FAPbI 3 during thermal annealing, leading to improved crystalline quality and decreased structural defects. After annealing, the spontaneous assembly of SF‐PEA on the crystal surface of FAPbI 3 not only passivates the surface defects, but also protects the perovskite from phase transition that is caused by strain or moisture invasion. The resulting FAPbI 3 films are extremely stable, which can maintain their black phase for more than 3 months in air with 40–50% relative humidity, much better than pristine and unsubstituted phenethylammonium (PEA)‐based samples. Because of the greatly improved phase stability and crystallization quality, a champion power conversion efficiency (PCE) of 21.25% (certified PCE of 20.70%) is achieved in planar n–i–p structured solar cells, which is the highest one among “methylammonium‐free” FAPbI 3 perovskite photovoltaics.