Strongly Anharmonic Organic Cation Vibrations in Hybrid Lead Halide Perovskite CH3NH3PbI3 on Electronic Ground State

TL;DR

This study investigates ultrafast vibrational dynamics in hybrid lead halide perovskite CH3NH3PbI3, revealing strongly anharmonic organic cation vibrations and their impact on phonon behavior under photoexcitation.

Contribution

The paper provides the first experimental and theoretical evidence of strongly anharmonic organic cation vibrations in hybrid perovskites and links anharmonicity to observed frequency blueshifts.

Findings

Organic cation vibrations occur at 4 and 8 THz and are heavily damped.

Frequency blueshifts increase with pump density, contrary to laser heating expectations.

Ab initio simulations confirm anharmonic vibrational potentials as the cause.

Abstract

Ultrafast vibrational dynamics of inorganic-organic hybrid lead halide perovskite CH3NH3PbI$ in the room-temperature tetragonal phase is investigated under moderate photoexcitation slightly below the band gap. Time-resolved optical Kerr effect signal exhibits periodic modulations due to the libration and twisting of the methylammonium molecule at 4 and 8 THz, in addition to the inorganic sublattice deformation at 1.2 THz. These oscillations are heavily damped, and their frequencies exhibit blueshifts with increasing pump density, contrary to the expectation from conventional laser heating. Our ab initio lattice dynamics simulations reveal strongly anharmonic vibrational potentials for the zone-center optical phonon modes, and thereby confirm the anharmonicity to be the origin of the experimentally observed frequency blueshifts.