Stabilization of the Trigonal High-Temperature Phase of Formamidinium Lead Iodide

Formamidinium lead iodide (FAPbI(3)) has the potential to achieve higher performance than established perovskite solar cells like methylammonium lead iodide (MAPbl(3)), while maintaining a higher stability. The major drawback for the latter material is that it can crystallize at room temperature in a wide bandgap hexagonal symmetry (P6(3)mc) instead of the desired trigonal (P3m1) black phase formed at a higher temperature (130 degrees C). Our results show that employing a mixture of MM and FAI in films deposited via a two-step approach, where the Mid content is <20%, results in the exchange of FA molecules with MA without any significant lattice shrinkage. Additionally, we show with temperature dependent X-ray diffraction that the trigonal phase exhibits no phase changes in the temperature range studied (25 to 250 degrees C). We attribute the stabilization of the structure to stronger interactions between the MA cation and the inorganic cage. Finally, we show that the inclusion of this small amount of MA also has a positive effect on the lifetime of the photoexcited species and results in more efficient devices.