Cyanovinyl Phosphonic Acid Based Molecular Additives for Highly Efficient and Stable Formamidinium‐Cesium Lead Lodide Perovskite Solar Cells

Abstract Formamidinium‐cesium lead iodide perovskites (FA _1‐x Cs _x PbI ₃ , 0 < x < 0.1) are promising solar cell absorber materials with favorable bandgap and high thermal stability. However, the fabrication of high‐quality FA _1‐x Cs _x PbI ₃ films with large grain size, stable black phase, uniform cations distribution, and minimal defects remains challenging. Here, the efficacy of cyanovinyl phosphonic acid (CPA) based molecular additives in fabricating high‐quality FA _0.95 Cs _0.05 PbI ₃ films is reported. The CPA unit shows strong interactions with all species of lead iodide (PbI ₂ ), formamidinium iodide (FAI), and cesium iodide (CsI) in the precursor solution, thus significantly alleviating the inhomogeneous crystallization in this mixed‐cation system. The resulting FA _0.95 Cs _0.05 PbI ₃ films exhibit enlarged grain size and homogenized cation distribution, and the presence of CPA‐based molecules in final perovskite films enhances optoelectronic qualities and photostability owing to efficient passivation and strong interaction with perovskite. With optimizations on molecular size and adding concentrations, inverted structured perovskite solar cells based on an optimal molecular additive (Ph‐CPA) achieve power conversion efficiencies (PCEs) up to 26.25%. Moreover, the lifespans (T90, time corresponding to 90% of initial PCE retained) of the devices are unprecedentedly prolonged from hundreds of hours to over 1000 and 3000 h under light and thermal stresses (ISOS‐L‐2I, 85 °C) and operational condition (ISOS‐L‐1I), respectively.