Critical Role of Methylammonium Librational Motion in Methylammonium Lead Iodide (CH₃NH₃PbI₃) Perovskite Photochemistry

Raman and photoluminescence (PL) spectroscopy are used to investigate dynamic structure-function relationships in methylammonium lead iodide (MAPbI₃) perovskite. The intensity of the 150 cm^-1 methylammonium (MA) librational Raman mode is found to be correlated with PL intensities in microstructures of MAPbI₃. Because of the strong hydrogen bond between hydrogens in MA and iodine in the PbI₆ perovskite octahedra, the Raman activity of MA is very sensitive to structural distortions of the inorganic framework. The structural distortions directly influence PL intensities, which in turn have been correlated with microstructure quality. Our measurements, supported with first-principles calculations, indicate how excited-state MA librational displacements mechanistically control PL efficiency and lifetime in MAPbI₃-material parameters that are likely important for efficient photovoltaic devices.