Phase-Pure α-FAPbI for Perovskite Solar Cells.

Because of the narrow bandgap and superior thermal stability, FAPbI is considered the most promising perovskite material for high-performance single-junction PSCs. Nevertheless, the metastable properties of the photoactive α-FAPbI becomes a primary obstacle for the development of FA-based PSCs. The main reasons for the instability of α-FAPbI are the rotation disorder of the FA cation and large anisotropic lattice strain, which lead to the high formation energy of α-FAPbI. In this Perspective, we review various strategies for preparing phase-pure α-FAPbI, such as engineering, intermediate phase engineering, and dimensionality engineering. These strategies can stabilize α-FAPbI by reducing the system energy, regulating the phase transition process and energy barrier, reinforcing the lattice structure, and passivating film defects. In addition, we investigate fundamental challenges of α-FAPbI PSCs and propose our perspective on preparing high-quality and high-purity α-FAPbI.