title: Rotational dynamics of organic cations in the CH3NH3PbI3 perovskite.
abstract: 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 CH3NH3(+) ions exhibit four-fold rotational symmetry of the C-N axis (C4) along with three-fold rotation around the C-N axis (C3), while in the orthorhombic phase (T < 165 K) only C3 rotation is present. At around room temperature, the characteristic relaxation times for the C4 rotation are found to be τC4 ≈ 5 ps while for the C3 rotation τC3 ≈ 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 C4 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.
authors.affiliation: Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA. shlee@virginia.edu jjc6z@virginia.edu.
authors.forename: Tianran
authors.initials: T
authors.lastname: Chen
authors.affiliation: Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904, USA.
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Foley
authors.affiliation: NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA and Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, USA.
authors.forename: Bahar
authors.initials: B
authors.lastname: Ipek
authors.affiliation: NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
authors.forename: Madhusudan
authors.initials: M
authors.lastname: Tyagi
authors.affiliation: NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
authors.forename: John R D
authors.initials: JR
authors.lastname: Copley
authors.affiliation: NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA and Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, USA.
authors.forename: Craig M
authors.initials: CM
authors.lastname: Brown
authors.affiliation: Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904, USA.
authors.forename: Joshua J
authors.initials: JJ
authors.lastname: Choi
authors.affiliation: Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA. shlee@virginia.edu jjc6z@virginia.edu.
authors.forename: Seung-Hun
authors.initials: SH
authors.lastname: Lee