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Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shownimpressive power conversion efficiencies of above 20%. However, the microscopicmechanism of the high photovoltaic performance is yet to be fully understood.Particularly, the dynamics of CH3NH3+ cations and their impact on relevantprocesses such as charge recombination and exciton dissociation are stillpoorly understood. Here, using elastic and quasi-elastic neutron scatteringtechniques and group theoretical analysis, we studied rotational modes of theCH3NH3+ cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327K)and tetragonal (165K < T < 327K) phases, the CH3NH3+ ions exhibit four-foldrotational symmetry of the C-N axis (C4) along with three-fold rotation aroundthe C-N axis (C3), while in orthorhombic phase (T < 165K) only C3 rotation ispresent. Around room temperature, the characteristic relaxation times for theC4 rotation is found to be ps while for the C3 rotation ps. The -dependentrotational relaxation times were fitted with Arrhenius equations to obtainactivation energies. Our data show a close correlation between the C4rotational mode and the temperature dependent dielectric permittivity. Ourfindings on the rotational dynamics of CH3NH3+ and the associated dipole haveimportant implications on understanding the low exciton binding energy and slowcharge recombination rate in CH3NH3PbI3 which are directly relevant for thehigh solar cell performance.

Rotational Dynamics of Organic Cations in CH3NH3PbI3 Perovskite