First-Principles Calculations of the Rotational Motion and Hydrogen Bond Capability of Large Organic Cations in Hybrid Perovskites

The organic cation dynamics in organic–inorganic hybrid perovskites affect the unique physical properties of these materials. To date, the rotational dynamics of methylammonium (CH3NH3+) and formamidinium (CH(NH2)2+) have been studied both experimentally and from first-principles calculations. Recently, a novel hybrid perovskite with large organic cation guanidinium (C(NH2)3+, GA), which exhibited extraordinarily long carrier lifetimes, was reported. In order to analyze physical properties of GA, we examined the detailed rotational potential energy surfaces and rotational energy barriers of GA in cubic-phase GASnI3 and alternative perovskites using first-principles calculations. The analysis revealed that the principal rotations of GA involve six hydrogen bonds between the organic cation and the inorganic framework in the crystals. Our results suggest that GA can effectively passivate under-coordinated iodine ions using its high hydrogen bond capability, which is consistent with the experimental speculation that GA can suppress iodine defects by the hydrogen bonds.