Suppressing X‐Migrations and Enhancing the Phase Stability of Cubic FAPbX₃ (X = Br, I)

Abstract Chemical bonding of formamidinium (FA) with the inorganic perovskite skeleton of FAPbX₃ (X = Br, I) is studied with emphasis on the differences to methylammonium: stronger hydrogen bonding, the presence of π‐anion bonding, and more sterically hindered motion inside the perovskite inorganic cage. Organic cation dopants fitting in the perovskite cubic cell and being capable of hydrogen and halogen bonding with overall doubled bonding strength as compared to FA are proposed. They are shown to suppress not only X‐migrations but also the undesirable α–δ phase transition of FAPbI₃. In addition, a possible atomistic explanation of the champion solar cell efficiency achieved experimentally is developed.