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Rotational dynamics of organic cations in the CH3NH3PbI3 perovskite

Chen, Tianran
Foley, Benjamin J.
İpek Torun, Bahar
TYAGI, Madhusudan
COPLEY, John R. D.
BROWN, Craig M.
Choi, Joshua J.
Lee, Seung-Hun
Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shown impressive power conversion efficiencies of above 20%. However, the microscopic mechanism of the high photovoltaic performance is yet to be fully understood. Particularly, the dynamics of CH3NH3+ cations and their impact on relevant processes such as charge recombination and exciton dissociation are still poorly understood. Here, using elastic and quasi-elastic neutron scattering techniques and group theoretical analysis, we studied rotational modes of the CH3NH3+ cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327 K) and tetragonal (165 K < T < 327 K) phases, the CCH3NH3+ ions exhibit four-fold rotational symmetry of the C-N axis (C-4) along with three-fold rotation around the C-N axis (C-3), while in the orthorhombic phase (T < 165 K) only C-3 rotation is present. At around room temperature, the characteristic relaxation times for the C-4 rotation are found to be tau(C4) approximate to 5 ps while for the C-3 rotation tau(C3) approximate to 1 ps. The T-dependent rotational relaxation times were fitted with Arrhenius equations to obtain activation energies. Our data show a close correlation between the C-4 rotational mode and the temperature dependent dielectric permittivity. Our findings on the rotational dynamics of CH3NH3+ and the associated dipole have important implications for understanding the low exciton binding energy and a slow charge recombination rate in CH3NH3PbI3 which are directly relevant for the high solar cell performance.