Improved Phase Stability of Formamidinium Lead Triiodide Perovskite by Strain Relaxation

Though formamidinium lead triiodide (FAPbI₃) possesses a suitable band gap and good thermal stability, the phase transition from the pure black perovskite phase (α-phase) to the undesirable yellow nonperovskite polymorph (δ-phase) at room temperature, especially under humid air, hinders its practical application. Here, we investigate the intrinsic instability mechanism of the α-phase at ambient temperature and demonstrate the existence of an anisotropic strained lattice in the (111) plane that drives phase transformation into the δ-phase. Methylammonium bromide (MABr) alloying (or FAPbI₃-MABr) was found to cause lattice contraction, thereby balancing the lattice strain. This led to dramatic improvement in the stability of α-FAPbI₃. Solar cells fabricated using FAPbI₃-MABr demonstrated significantly enhanced stability under the humid air.