Homologous post-treatment strategy enabling phase-pure α-FAPbI3 films.

Formamidinium lead triiodide (FAPbI3) is considered as the prospective light-absorbing layer on account of the close-to-ideal bandgap of the α-phase, wide optical absorption spectrum and good thermal stability. Therefore, how to realize δ to α-phase transition to obtain phase-pure α-FAPbI3 without additives is important for FAPbI3 perovskite films. Herein, a homologous post-treatment strategy (HPTS) without additives is proposed to prepare FAPbI3 films with pure α-phase. The strategy is processed along with dissolution and reconstruction process during the annealing. The FAPbI3 film has tensile strain with the substrate, and the lattice keeps tensile, and the film maintains in an α/δ hybrid phase. The HPTS process releases the tensile strain between the lattice and the substrate. The process of strain release realizes the phase transition from δ to α-phase during this process. This strategy can accelerate the transformation from hexagonal δ-FAPbI3 to cubic α-FAPbI3 at 120℃. As a result, the acquired α-FAPbI3 films exhibit better film quality in optical and electrical properties, accordingly achieving device efficiency of 19.34% and enhanced stability. This work explores an effective approach to obtain additive-free and phase-pure α-FAPbI3 films through a homologous post-treatment strategy to fabricate uniform high-performance α-FAPbI3 perovskite solar cells.