Theoretical Study on Rotational Controllability of Organic Cations in Organic-Inorganic Hybrid Perovskites: Hydrogen Bonds and Halogen Substitution

The organic cation dynamics in organic-inorganic hybrid perovskites strongly affect the power energy conversion and unique physical properties of these materials. To date, the first-principles rotational potential energy surface (PES) of formamidinium (FA) has not been reported. Thus, we examined the rotational energy barriers for FA in cubic-phase perovskites (FABX(3) (B = Sn/Pb; X = Cl/Br/I)) by density functional theory and compared these with those of methyl-ammonium. The calculated rotational PES of FAPbI(3) indicates that FA rotates around the N-N bond axis (phi) with a low energy barrier, whereas the energy barrier for FA rotation around the axis penetrating the C atom and the center of gravity of FA (theta) is high. Moreover, the phi and theta rotational barriers of FA increase with halogen substitution. Thus, we reveal important design rules for controlling the rotational barrier and orientation by forming hydrogen bonds and halogen substitution.