Raman Spectroscopy of Formamidinium-Based Lead Halide Perovskite Single Crystals

Raman spectroscopy is a powerful technique for the study of materials chemistry and nanostructure. This nondestructive technique is highly sensitive to molecular and crystal lattice vibrations, which allow for a comprehensive study of the vibrational modes of molecules incorporated in photovoltaic perovskite materials. In this study, we apply Raman spectroscopy to study FAPbX(3) (X = Cl, Br, I) and FA(x)MA(1-x)PbI(3) (FA stands for formamidinium; MA for methylamrnonium) metal halide perovskite single crystals and discuss the necessary conditions to obtain reliable data. We establish a correlation between perovskite composition and their unique Raman intensities/spectral shapes. In particular, we show that tuning of the halide content results in a spectral shift of the organic features of the Raman spectrum due to changes in the strength of hydrogen bonding, while tuning of the organic cation is related more to changes in peak intensity. Moreover, the effect of temperature on the vibrational modes corresponding to NCN bending, NH2 torsion, and NH2 wagging were studied. This enables the impact of the organic composition in FA(x)MA(1-x)PbI(3) on the phase transition temperature of the material to be determined. Furthermore, we establish links between Raman spectroscopy and other conventional measurement techniques such as X-ray diffraction (XRD) and differential scanning calorimetry (DSC). This study provides insight into the interpretation of the Raman spectra of FA-based perovskites, which furthers understanding of the properties of these materials in relation to their full exploitation in solar cells.