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

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.