Nonalloyed α-phase formamidinium lead triiodide solar cells through iodine intercalation.

Formamidinium lead triiodide (FAPbI) is considered the most promising composition for high-performing single-junction solar cells. However, nonalloyed α-FAPbI is metastable with respect to the photoinactive δ-phase. We have developed a kinetic modulation strategy to fabricate high-quality and stable nonalloyed α-FAPbI films, assisted by cogenetic volatile iodine intercalation and decalation. The intercalation of iodine facilitated the formation of corner-sharing Pb-I framework building blocks and reduced the kinetic barrier for α-FAPbI formation, whereas the iodine decalation improved the final perovskite film quality in terms of composition purity and overall homogeneity. Solar cells based on this nonalloyed α-FAPbI (free of other extrinsic composition ions) achieved a power conversion efficiency of >24%. The devices also exhibited excellent durability, retaining 99% of their original power conversion efficiency after operating for more than 1100 hours at 85° ± 5°C under illumination.