Raman Spectroscopy of Formamidinium-Based Lead Halide Perovskite Single Crystals

Abstract

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 methylammonium) 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, NH 2 torsion, and NH 2 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.

Single crystal of two-dimensional mixed-halide copper-based perovskites with reversible thermochromism

“…The Raman bands in these regions are shifted towards higher Raman frequencies with the appearance of new features with a decrease in the weight of the halide derivatives, that is, in the Br-Cl sequence, which confirms not only the alloy nature of (CH 3 NH 3 ) 2 CuCl 4Àx Br x crystals, but also the influence of the copper-halide interaction over the Raman peaks. A similar observation was also observed for mixed-halide lead-based perovskite single crystals, as reported by Ruan et al 37 In particular, the M1-Raman band region, B97 cm À1 for all bromide-perovskite (BB) samples, which may be assigned to the stretching vibration of the Cu-Br bond, is blue-shifted with an increase in Cl content until it disappears at C0.5 and CC samples. For all chloride perovskite single crystals (CC), the Raman modes at M1-167 and M2-238 cm À1 are ascribed to the equatorial and axial planar vibrations of Cu-Cl bonds, respectively.…”

The Role of Potassium in the Segregation of MAPb(Br0.6I0.4)3 Mixed‐Halide Perovskite in Different Environments

“…PL emission related to a fully segregated ‐phase appears at 530 nm in the case of addition in the form of KBr, and the mixed‐phase emission broadens toward smaller energies. [ 56 ]…”

“…PL emission related to a fully segregated Br À -phase appears at 530 nm in the case of K þ addition in the form of KBr, and the mixed-phase emission broadens toward smaller energies. [56] In summary, we evaluated the effect of K þ addition on the segregation of MAPb(Br 0.6 I 0.4 ) 3 . Using a single cation and a Br À /I À ratio far from the stable zones allowed us to observe the segregation in a controlled way.…”

Raman spectroscopy insights into the α- and δ-phases of formamidinium lead iodide (FAPbI3)

“…A more recent study by Ruan et al, considering FAPbX 3 (X = Cl, Br and I) probed the region of 1750 cm −1 down to 250 cm −1 and showed mainly the internal modes of the organic cations. 43 A study by Steele et al made use of laser exposure of the FAPbI 3 material to transform in situ from the yellow to black phase. 44 In this study, we make use of room and high temperature Raman for the δ-FAPbI 3 phase to investigate the molecular structural differences as the sample undergoes the phase transition to the black α-FAPbI 3 phase (Fig.…”