Rotational dynamics of organic cations in the CH3NH3PbI3 perovskite. Issue 46 (9th November 2015)

• Record Type: Journal Article
• Authors: Chen, Tianran; Foley, Benjamin J.; Ipek, Bahar; Tyagi, Madhusudan; Copley, John R. D.; Brown, Craig M.; Choi, Joshua J.; Lee, Seung-Hun
• Is Part Of: Physical chemistry chemical physics. Volume 17:Issue 46(2015)
• Page Start: 31278
• Page End: 31286
• Publication Date: 2015-11-09
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/c5cp05348j
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : Rotational dynamics of organic cations in the CH3 NH3 PbI3 perovskite are revealed by elastic and quasi-elastic neutron scattering and group theoretical analysis. Abstract : Methylammonium lead iodide (CH3 NH3 PbI3 ) 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 CH3 NH3 + 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 CH3 NH3 + cation in CH3 NH3 PbI3 . Our results show that, in the cubic ( T > 327 K) and tetragonal (165 K < T < 327 K) phases, the CH3 NH3 + 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 τ C 4 ≈ 5 ps while for the C 3 rotation τ C 3 ≈ 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 CH3 NH3 + and the associated dipole have important implications for understanding the low exciton binding energy and a slow charge recombination rate in CH3 NH3 PbI3 which are directly relevant for the high solar cell performance.