Decoupled Molecular and Inorganic Framework Dynamics in CH3NH3PbCl3

Published

December 30, 2019

Author(s)

M. Songvilay, Zitian Wang, Victoria Garcia Sakai, T. Guidi, M. Bari, Zuo-Guang Ye, Guangyong Xu, K. L. Brown, Peter M. Gehring, Christopher Stock

Abstract

The organic-inorganic lead halide perovskites are composed of organic molecules embedded in an inorganic framework. The compounds with general formula CH₃NH₃PbX₃ (MAPbX₂) display large photovoltaic efficiencies for halogens X=Cl, Br, and I in a wide variety of sample geometries and preparation methods. The organic cation and inorganic framework are bound by hydrogen bonds that tether the molecules to the halide anions, and this has been suggested to be important to the optoelectronic properties. We have studied the effects of this bonding using time-of-flight neutron spectroscopy to measure the molecular dynamics in CH₃NH₃PbCl₃ (MAPbCl₃). Low-energy/high-resolution neutron backscattering reveals thermally-activated molecular dynamics with a characteristic temperature of 95 K. At this same temperature, higher-energy neutron spectroscopy indicates the presence of an anomalous broadening in energy (reduced lifetime) associated with the molecular vibrations. By contrast, neutron powder diffraction shows that a spatially long-range structural phase transitions occurs at 178K (cubic to tetragonal) and 173K (tetragonal to orthorhombic). The large difference between these two temperature scales suggests that the molecular and inorganic lattice dynamics in MAPbCl₃ are actually decoupled. With the assumption that underlying physical mechanisms do not change with differing halogens in the organic-inorganic perovskites, we speculate that the energy scale most relevant to the photovoltaic properties of the lead-halogen perovskites is set by the lead-halide bond, not by the hydrogen bond.

Citation

Physical Review Materials

Volume

3

Issue

12

Pub Type

Journals

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Keywords

Photovoltaic, perovskites, lattice dynamics, neutron scattering, quasielastic scattering

Citation

Songvilay, M. , Wang, Z. , Garcia Sakai, V. , Guidi, T. , Bari, M. , Ye, Z. , Xu, G. , Brown, K. , Gehring, P. and Stock, C. (2019), Decoupled Molecular and Inorganic Framework Dynamics in CH₃NH₃PbCl₃, Physical Review Materials, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=929388 (Accessed April 18, 2024)