Hybrid organic-inorganic FAPbX3 perovskites (FA = NH2CHNH2 +, X = Cl-, Br-, I-) are currently intensively investigated in solar cells. In this study, off-resonance Raman and far-IR absorption spectra of all single-halogen FAPbX3, as well as of mixed-halogen FAPbBr2X derivatives, are reported and analyzed. Vibrations of the PbX6 octahedra and librations of the FA cation lay at frequencies below 250 cm-1, while external and internal FA bands are identified above 200 cm-1. An increase in the frequencies of most vibrational bands is observed upon substituting lighter and more electronegative halides for heavier ones due to strengthening of the electrostatic interaction between X- and the FA NH2 + groups. Variable-temperature Raman measurements are also carried out for FAPbBr3 and FAPbCl3 in the 77-400 K range. Upon cooling, the three Pb-X Raman vibrations of the cubic structure, split into eight separate bands, signaling the transformation to the tetragonal phase at âˆ240 K and âˆ200 K for FAPbBr3 and FAPbCl3, respectively. At even lower temperatures, a successive phase transformation to low symmetry orthorhombic phases is evidenced. These temperature-induced effects are accompanied by intense narrowing, soft-mode behavior, and/or sudden frequency shift of specific Raman bands, attributed to ordering of the FA cation.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films