Assessment of Additive Chemistry in FAPbI ₃ Perovskite Solar Cells: Linking Molecular Design to Performance and Long‐Term Device Stability

Perovskite solar cells (PSCs) have emerged as leading candidates for next‐generation photovoltaics owing to their low material cost, facile solution processing, tunable bandgaps, and rapidly improving power conversion efficiencies. Among perovskite compositions, formamidinium lead iodide (FAPbI 3 ) delivers an ideal bandgap and enhanced thermal stability compared to methylammonium analogs, yet its photoactive α ‐phase undergoes undesirable conversion to the nonphotoactive δ ‐phase at room temperature. Additive engineering has proven instrumental in overcoming this polymorphism by modulating crystallization kinetics, passivating defects, and stabilizing the α ‐phase. Organic cations, inorganic dopants, ionic liquids, and low‐dimensional precursors each contribute complementary mechanisms of multidentate coordination, hydrophobic surface capping, interfacial dipole formation, and seed‐assisted nucleation that synergistically enhance film quality, suppress nonradiative recombination, and extend operational lifetimes under humidity, thermal, and illumination stress. Structure–property relationships reveal that multifunctional additives achieve an optimal balance of efficiency and stability. This review examines the role of the additive in absorber composition, classifies additive strategies by mechanistic function, stability metrics, and provides guidelines for translating laboratory advances into scalable, durable PSC architectures.