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

Formamidinium lead triiodide (FAPbI(3)) is considered as the prospective light-absorbing layer on account of the close-to-ideal bandgap of the & alpha;-phase, wide optical absorption spectrum and good thermal stability. Therefore, how to realize & delta; to & alpha;-phase transition to obtain phase-pure & alpha;-FAPbI(3) without additives is important for FAPbI(3) perovskite films. Herein, a homologous post-treatment strategy (HPTS) without additives is proposed to prepare FAPbI(3) films with pure & alpha;-phase. The strategy is processed along with dissolution and reconstruction process during the annealing. The FAPbI(3) film has tensile strain with the substrate, and the lattice keeps tensile, and the film maintains in an & alpha;/& delta; 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 & delta; to & alpha;-phase during this process. This strategy can accelerate the transformation from hexagonal & delta;-FAPbI(3) to cubic & alpha;-FAPbI(3) at 120 & DEG;C. As a result, the acquired & alpha;-FAPbI(3) 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 & alpha;-FAPbI(3) films through a HPTS to fabricate uniform high-performance & alpha;-FAPbI(3) perovskite solar cells.