Tuning the Optical Properties of Already Crystalized Hybrid Perovskite

The optical properties of halide perovskite are already tuned in the initial solution by choosing the preferred precursors. However, it is not obvious that the optical properties change following perovskite crystallization; moreover, the mechanism in this case is not clear. Herein, it is shown that the optical properties are not necessarily “fixed” following perovskite crystallization. The phenomenon using the halide exchange process in the solid phase of an already crystalized perovskite is demonstrated. The effects of formamidinium iodide (FAI) and formamidinium bromide (FABr) are investigated post‐treatment on the physical, optical, and PV properties of Cs 0.2 FA 0.8 Pb(I 0.75 Br 0.25 ) 3 perovskite. Various FAI/FABr ratios in the post‐treatment solutions show that a halide exchange occurs in the solid‐state phase following the crystallization of the perovskite film. Energy‐dispersive spectroscopy line scan made on focused ion beam samples show that the bromide is present deep inside the film post‐treatment, which supports the fact that the reaction is not occurring only on the surface. Charge extraction and voltage decay measurements of the complete solar cells support the structural changes due to the halide exchange. This work enhances the knowledge of halide exchange in already crystalized perovskite.