CaF₂ Nanoparticle-Induced γ-CsPbI_2.81Br_0.19 Heterogeneous Crystallization for High-Efficiency Flexible All-Inorganic Perovskite Solar Cells

All-inorganic CsPbI₃ films necessitate higher annealing temperatures for high-quality crystallization. Consequently, the conventional low-temperature solution approach often results in poor crystallization in flexible CsPbI₃ films, significantly degrading the optoelectronic performance and stability of flexible perovskite solar cells (f-PSCs). Herein, a heterogeneous CaF₂ nanocrystal seed-induced strategy has been successfully utilized to achieve enhanced crystallization of a flexible CsPbI_2.81Br_0.19 film. Due to their good lattice match with the perovskite material, CaF₂ nanoparticles can decrease the critical Gibbs free energy of CsPbI_2.81Br_0.19 perovskite nucleation, thereby accelerating γ-phase CsPbI_2.81Br_0.19 crystallization at low temperatures. This leads to an improved crystalline quality of the flexible perovskite film at low temperatures, which minimizes defects and enhances the stability of f-PSCs. The CsPbI_2.81Br_0.19 f-PSCs achieved a champion power conversion efficiency of 15.03% and demonstrated mechanical stability, retaining 98.1% of their initial efficiency even after 60 000 bending cycles with a curvature radius of 5 mm.