Raman spectroscopy of organic-inorganic halide perovskites

Micro-Raman spectroscopy provides laterally resolved microstructural information for a broad range of materials. In this Letter, we apply this technique to tri-iodide (CH₃NH₃PbI₃), tribromide (CH₃NH₃PbBr₃), and mixed iodide-bromide (CH₃NH₃PbI_3-xBr_x) organic-inorganic halide perovskite thin films and discuss necessary conditions to obtain reliable data. We explain how to measure Raman spectra of pristine CH₃NH₃PbI₃ layers and discuss the distinct Raman bands that develop during moisture-induced degradation. We also prove unambiguously that the final degradation products contain pure PbI₂. Moreover, we describe CH₃NH₃PbI_3-xBr_x Raman spectra and discuss how the perovskite crystallographic symmetries affect the Raman band intensities and spectral shapes. On the basis of the dependence of the Raman shift on the iodide-to-bromide ratio, we show that Raman spectroscopy is a fast and nondestructive method for the evaluation of the relative iodide-to-bromide ratio. (Figure Presented).