Generation of Coherent Optical Phonons in Methylammonium Lead Iodide Thin Films

A long-lived hot carrier in organic-inorganic hybrid perovskite materials is possibly assisted by the large polaron and hot phonon bottleneck. The phonon plays a significant role in the properties of hybrid perovskites. Coherent phonon refers to the in-phase vibration of the crystal lattice. Here, temperature-dependent, fluence-dependent, and polarization-dependent transient absorption spectroscopy is utilized to study the coherent phonon in methylammonium lead iodide. A coherent optical phonon is observed due to the existence of Pb-I-Pb angular distortion at 24-33 cm^-1 in both tetragonal and orthorhombic phases. Oscillation from the excited-state absorption is also observed, suggesting coupling between the coherent phonon and photoexcited state. Oscillation of the coherent phonon attenuates with increasing environmental temperature. The dependence of the coherent phonon on polarization of the pulse and quantitative analysis indicates that the generation mechanism is impulsive stimulated Raman scattering and transient depletion field screening.