Perovskite solar cells (PSCs) have attracted considerable attention as promising next-generation photovoltaic technologies due to their low-cost solution processing, strong light absorption, and favorable charge-transport properties, which enable high power conversion efficiencies (PCEs). Among various perovskite compositions, FAPbI₃ is particularly attractive as a light-absorbing material owing to its suitable bandgap and excellent optical properties.

 Nevertheless, the formation of defects during FAPbI₃ crystallization remains a major challenge. Iodide vacancies and undercoordinated Pb²⁺ sites at the surfaces and grain boundaries can act as non-radiative recombination centers. Furthermore, uncontrolled crystallization and interfacial defects can impede charge transport and extraction, ultimately reducing photovoltaic performance and long-term device stability.

 To overcome these limitations, iodotriphenylphosphonium iodide (ITPPI) was incorporated as a multifunctional ionic additive into FAPbI₃ perovskites. PSCs were fabricated using an n-i-p architecture consisting of FTO/TiO₂/FAPbI₃/Spiro-OMeTAD/Au. The introduction of ITPPI effectively modulates perovskite crystallization and suppresses defect-mediated recombination. Spectroscopic analyses and density functional theory (DFT) calculations indicate that iodide ions supplied by ITPPI contribute to iodide vacancy passivation, while the phosphonium moiety interacts with the perovskite framework and modifies the local electronic environment near defect sites. These interactions facilitate the formation of highly crystalline FAPbI₃ films with larger grains and lower defect densities. Consequently, the ITPPI-modified films exhibit extended charge-carrier lifetimes, reduced non-radiative recombination, and improved energy-level alignment for efficient charge extraction.

 As a result, the ITPPI-modified PSC achieves a PCE of 24.81%, exceeding the 22.95% obtained for the control device, while exhibiting negligible hysteresis. Furthermore, the modified device demonstrates enhanced long-term, thermal, and environmental stability, retaining 86.84% of its initial PCE after 2,455 h of storage under dry-room conditions. These findings highlight the potential of ionic phosphonium additives for simultaneously controlling perovskite crystallization, passivating defects, and improving the efficiency and stability of PSCs.