Formamidinium's (FAI) Impact on α‐CsPbI3 Perovskite Stability in Ambient Air: A Path for Highly Efficient Perovskite–Perovskite Tandem Solar Cells

Cesium lead triiodide (CsPbI3) perovskites are known for their instability, particularly under ambient conditions, where they often degrade from the preferred black‐phase (α‐phase) to the nonperovskite yellow‐phase (δ‐phase). This phase transition causes a loss of optical performance, which drastically lowers the solar cell's durability and efficiency. To solve this problem, we explored adding formamidinium iodide (FAI) as a CsPbI3 stabilizer agent. By adding FAI, we facilitate the transition from the less stable δ phase to the more stable and optically active α‐phase. This modification enhances the crystallinity of the material, reduces the density of defects, and improves the mobility of charge carriers, all of which improve device performance. Our results show a noticeable increase in solar cell efficiency after FAI incorporation. Theoretical calculations have shown that with single‐junction devices, the PCE was enhanced from 23.12% to 26.9%. Furthermore, the material becomes more stable over time, especially as compared to its original unstable structure. Finally, we integrated CsPbI3 into tandem perovskite–perovskite solar cells for the first time, achieving a ground‐breaking efficiency of 32%. These advancements represent a significant leap forward for perovskite‐based solar technologies. The promising outcomes of this research are under active consideration for commercialization, paving the way for the practical use of CsPbI3‐based solar technologies.