Anharmonic Vibrations of Organic Cation in Hybrid Lead Halide Perovskite CH3NH3PbI3.

Inorganic-organic hybrid lead halide perovskites, consisting of the lead halide octahedral framework and the organic molecular cations, are among the key materials for the next generation photovoltatics. Theoretical studies predicted that both the organic and inorganic components are mechanically soft with profound dynamic disorder at room temperature, which can dominate the structural stability and even affect the carrier transport. We investigate ultrafast vibrational dynamics of CH3NH3PbI3 under photoexcitation with a 1.5-eV light pulse. The optical pulse induces transient polarization anisotropy in the material and induces coherent libration and torsion oscillations of methylammonium ion (MA+) molecules. The frequencies of the MA+ oscillations exhibit blueshifts with increasing pump density and redshifts with time on sub-picosecond scale, both demonstrating the strong anharmonicity of the vibrational potential that was predicted by previous theoretical studies.