Rotational Dynamics of Organic Cations in CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ Perovskite

Methylammonium lead iodide (CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}})$ 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 CH$_{\mathrm{3}}$NH$_{\mathrm{3}}^{\mathrm{+}}$ cations and their impact on relevant processes are still poorly understood. Using elastic and quasi-elastic neutron scattering techniques and group theoretical analysis, we studied rotational modes of the CH$_{\mathrm{3}}$NH$_{\mathrm{3}}^{\mathrm{+}}$ cation in CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$.$^{\mathrm{[1]}}$ Our results show that, in the cubic and tetragonal phases, the CH$_{\mathrm{3}}$NH$_{\mathrm{3}}^{\mathrm{+}}$ ions exhibit four-fold rotational symmetry of the C-N axis (C$_{\mathrm{4}})$ along with three-fold rotation around the C-N axis (C$_{\mathrm{3}})$, while in orthorhombic phase only C$_{\mathrm{3}}$ rotation is present. Around room temperature, the characteristic relaxation time for the C$_{\mathrm{4}}$ rotation is found to be 5ps while for the C$_{\mathrm{3}}$ rotation is 1ps. The T-dependent rotational relaxation times were fitted with Arrhenius equations to obtain activation energies. Our data show a close correlation between the C$_{\mathrm{4}}$ rotational mode and the temperature dependent dielectric permittivity. Our findings on the rotational dynamics of CH$_{\mathrm{3}}$NH$_{\mathrm{3}}^{\mathrm{+}}$ and the associated dipole have important implications on understanding the low exciton binding energy and slow charge recombination rate in CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ which are directly relevant for the high solar cell performance. [1] T. Chen et al., Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP05348J.