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

Abstract

Solar perovskites have received phenomenal attention and success over the past decade, due to their high power conversion efficiencies (PCE), ease of fabrication and low cost which has enabled the...

Temperature-Dependent Luminescence and Anisotropic Optical Properties of Centimeter-Sized One-Dimensional Perovskite Trimethylammonium Lead Iodide Single Crystals

“…The polarized Raman spectra exhibit only one peak located at ∼104 cm −1 , and other peaks found via conventional Raman spectroscopy are too weak to be observed in the polarized spectra, which is similar to the case for δ-FAPbI 3 (111 cm −1 ). 69 This peak can be mainly attributed to Pb−I vibration by comparing the Raman spectra of TMAPbI 3 and TMAI (Figure S8b), which also agrees with the Pb−I vibration peak position in MAPbI 3 (97 cm −1 ). 70 Other Raman peaks with higher wave numbers of TMAPbI 3 are attributed to the organic cation vibration, but these peaks are very weak and cannot be observed in polarized Raman spectra.…”

“…As the angle between the laser polarization direction and the c axis (crystal axis) of the single crystal changes, the peak intensity of the Raman scattering peak also changes greatly. The polarized Raman spectra exhibit only one peak located at ∼104 cm –1 , and other peaks found via conventional Raman spectroscopy are too weak to be observed in the polarized spectra, which is similar to the case for δ-FAPbI 3 (111 cm –1 ) . This peak can be mainly attributed to Pb–I vibration by comparing the Raman spectra of TMAPbI 3 and TMAI (Figure S8b), which also agrees with the Pb–I vibration peak position in MAPbI 3 (97 cm –1 ) .…”

High-Pressure Behavior of δ-Phase of Formamidinium Lead Iodide by Optical Spectroscopies

“…Differently from the molecular modes, whose peaks are scarcely affected by cage symmetry changes, the Raman spectrum of the octahedron vibrations below 200 cm –1 undergoes significant variations passing from α-FAPbI 3 to δ-FAPbI 3. As shown in the inset of Figure b, the cubic phase, obtained by heating our δ-FAPbI 3 powder up to ∼200 °C, displays a nearly featureless profile, , coherent with that expected for Pm m space group of the crystal lattice, while the spectrum of the hexagonal phase is characterized by four distinct peaks at 36, 58, 87, and 108 cm –1 .…”

“…Because the tolerance factor of FAPbI 3 is very close to 1 ( t ∼ 0.99 if one uses the Shannon ionic radii for Pb 2+ and I – and the FA + radius calculated by Kieslich et al), at ambient conditions the compound crystallizes in a non-perovskite hexagonal geometry (δ-FAPbI 3 ), yellow in color, in which the neighboring octahedra are organized in a face-sharing arrangement. The α-FAPbI 3 perovskite, black in color, with cubic symmetry and corner-sharing octahedra, is achievable by heating δ-FAPbI 3 around 150 °C. − The compound, however, slowly transforms back to the yellow phase with times dependent on the synthesis method. − The thermally stable δ-FAPbI 3 is known for not being photoactive because of its low absorption and larger bandgap (2.14 eV), which implies low efficiency in solar absorption and energy conversion. Moreover, the crystal configuration with face-sharing octahedrons arranged in long chains interferes with electron transport, making δ-FAPbI 3 less suitable for photovoltaic applications compared to its cubic counterpart …”

Polymer‐Wrapped Perovskite Single Crystal for Surface Passivation and Robust Stability

“…The FAMACs single crystal shows two characteristic peaks at 49 and 85 cm –1 , which can be assigned to the I‐Pb‐I bending mode and Pb‐I stretching mode of [PbI 6 ] 4– . [ 31 ] The PMMA‐FAMACs single crystal exhibits a new peak at 133 cm –1 , which can be ascribed to the bond between Pb 2+ and the C=O group in PMMA, [ 32,33 ] providing further evidence for the interaction between PMMA and FAMACs single crystal.…”