Coulomb Screening and Coherent Phonon in Methylammonium Lead Iodide Perovskites

Methylammonium lead iodide (CH₃NH₃PbI₃) hybrid perovskite in the tetragonal and orthorhombic phases have different exciton binding energies and demonstrate different excitation kinetics. Here, we explore the role that crystal structure plays in the kinetics via fluence dependent transient absorption spectroscopy. We observe stronger saturation of the free carrier concentration under high pump energy density in the orthorhombic phase relative to the tetragonal phase. We attribute this phenomenon to small dielectric constant, large exciton binding energy, and weak Coulomb screening, which results in difficult exciton dissociation under high light intensity in the orthorhombic phase. At higher excitation intensities, we observe a coherent phonon with an oscillation frequency of 23.4 cm^-1 at 77 K, whose amplitude tracks the increase of the first-order lifetime.