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

Abstract

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.