A-site cation effect on optical phonon modes and thermal stability in lead-based perovskite bromide single crystals using Raman spectroscopy

Lead bromide perovskites have proven to exhibit high open-circuit voltage, which is a significant factor in enhancing the overall performance of a solar cell. Single crystals of FAPbBr 3 , MAPbBr 3 and CsPbBr 3 are synthesized by solvent evaporation method. This study investigates the thermal stability, photo physical properties, and optical phonon modes of lead bromide perovskites using various spectroscopic techniques. The thermal stability measurements prove CsPbBr 3 to be more stable than two hybrid perovskites. Raman spectroscopic studies for all three perovskites are performed in a wide frequency range probing both low and high-frequency modes. Fitting analysis is done to know the exact comparison of the Raman shifts. The torsional mode shows large difference in frequency depending upon the cation. Correlation times are estimated to get more insights into the dynamics of cations and inorganic octahedral frameworks. The difference in vibrational modes upon substitution of cation, resulting in complex chemical interplay is investigated in detail.