Halide perovskites under polarized light: Vibrational symmetry analysis using polarized Raman

TL;DR

This study combines polarized Raman spectroscopy and density functional perturbation theory to analyze vibrational mode symmetries in methylammonium-lead iodide perovskites, revealing detailed atomic motion insights at low temperature.

Contribution

It provides a comprehensive methodology for symmetry analysis of vibrational modes in halide perovskites using polarized Raman and theoretical calculations, overcoming previous challenges.

Findings

Mode symmetries assigned at 10 K

Crystal orientation determined via polarization analysis

Birefringence effects characterized and accounted for

Abstract

In the last decade, hybrid organic-inorganic halide perovskites have emerged as a new type of semiconductor for photovoltaics and other optoelectronic applications. Unlike standard, tetrahedrally bonded semiconductors (e.g. Si and GaAs), the ionic thermal fluctuations in the halide perovskites (i.e. structural dynamics) are strongly coupled to the electronic dynamics. Therefore, it is crucial to obtain accurate and detailed knowledge about the nature of atomic motions within the crystal. This has proved to be challenging due to low thermal stability and the complex, temperature dependent structural phase sequence of the halide perovskites. Here, these challenges are overcome and a detailed analysis of the mode symmetries is provided in the low-temperature orthorhombic phase of methylammonium-lead iodide. Raman measurements using linearly- and circularly- polarized light at 1.16 eV excitation are combined with density functional perturbation theory (DFPT). By performing an iterative analysis of Raman polarization-orientation dependence and DFPT mode analysis, the crystal orientation is determined. Subsequently, accounting for birefringence effects detected using circularly polarized light excitation, the symmetries of all the observed Raman-active modes at 10 K are assigned.