Asian Journal of Physics Vol. 33, Nos 1 & 2 (2024) 29-38

Raman linewidths as a probe of lattice anharmonicity and dynamic disorder in metal halide perovskites

Alejandro R Goñi†,‡
Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus UAB, 08193 Bellaterra, Spain
ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Spain

Metal halide perovskites (MHPs) have burst into the field of photovoltaics as a base for the production of solutionprocessed solar cells, reaching nowadays power-conversion efficiencies in excess of 26%. Raman spectroscopy can characterize in-situ chemical environments in materials and reveal the nature of lattice vibrations (phonons).This is so mainly because the Raman spectrum, i.e. the characteristic frequencies of the phonon modes, is a vibrational finger print of the material. Raman linewidths, despite being less used, contain valuable information about structural quality and lattice anharmonicity. This is demonstrated here by careful inspection of the Raman spectra of methylammonium (MA) lead halide single crystals (MAPbX3, X = I, Br, Cl) obtained at temperatures between 80 and 370K. Although of general validity for other A-site cations, it will be shown that the dynamic disorder induced by jump-like reorientation of the MA cations, unlocked in their cavities at high temperatures above the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. From the analysis of the spectral linewidths, it is found that strong broadenings are produced in both homogeneous as well as inhomogeneous Raman peak widthsof the inorganic-cage phonon modes. At the origin of these effects are large anharmonicities  or bond inhomogeneities, respectively, caused by the coupling with the roto-translational dynamics and/or vibrations of the MA molecules. ©Anita Publications. All rights reserved
Keywords: Raman linewidths, Metal halide perovskites, Lattice vibrations, photovoltaics.

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