Multifunctional Phosphonic Acid-Based Passivation: A Pathway to Enhance Efficiency and High-Temperature Durability in FAPbI₃ Perovskite Solar Cells

Perovskite solar cells (PSCs) are gaining a lot of attention in the field of renewable energy as the most promising photovoltaic technology, owing to their high performance, low-cost production, and low-temperature processability. However, long-term stability remains one of the critical challenges. This work demonstrates that the introduction of 1,4-phenylenebis(phosphonic acid) (1,4-PBPA) in formamidinium lead triiodide (FAPbI3)-based PSCs provides notable enhancements in moisture resistance, thermal stability, and power conversion efficiency (PCE). The 1,4-PBPA acts as a cross-linker and enables strong interaction between perovskite grains, yielding high-quality films with reduced defects, as evidenced by morphological and structural analyses. The encapsulated FAPbI3 PSC devices with 1,4-PBPA retain 100% of the initial PCE even after 2350 h of aging at 95% relative humidity and 40 °C. Notably, the devices exhibit 70% of the initial PCE after 1250 h of thermal stability tests at 85 °C. Compared to carboxylic acids, the phosphonic acid additives demonstrate superior stability enhancements, highlighting their potential to overcome the key limitations of PSCs.