Longitudinal Optical Phonons Modified by Organic Molecular Cation Motions in Organic-Inorganic Hybrid Perovskites.

Masaya Nagai,Masaaki Ashida,Takuya Tomioka,Yasuhiro Yamada,Mizuki Hoyano,Ryo Akashi,Yoshihiko Kanemitsu,Tomoko Aharen

doi:10.1103/physrevlett.121.145506

Abstract

We performed terahertz time-domain spectroscopy for methylammonium (MA) lead halide perovskite single crystals and characterized the longitudinal optical (LO) phonons directly. We found that the effective LO phonon wave number does not change in the wide temperature range between 10 and 300K. However, the coupling between MA cation modes and the LO phonon mode derived from lead halide cages induces a mode splitting at low temperatures and a damping of the LO phonon mode at high temperatures. These results influence the interpretation of electron-LO phonon interactions in perovskite semiconductors, as well as the interpretations of mobility, carrier diffusion, and polaron formation.

Highlights

The organic-inorganic hybrid lead halide perovskites with the APbX3 (A 1⁄4 monovalent cation and X 1⁄4 I, Br, Cl) structure are a class of promising materials for solutionprocessed high-efficiency solar cells [1]
The coupling between MA cation modes and the longitudinal optical (LO) phonon mode derived from lead halide cages induces a mode splitting at low temperatures and a damping of the LO phonon mode at high temperatures
Fast rotation of the MA cation has been revealed with NMR spectroscopy [31], twodimensional spectroscopy [32], and quasielastic neutron scattering measurements [33], and the reported timescale of picoseconds is comparable to the period of the LO phonon

Summary

Introduction

The organic-inorganic hybrid lead halide perovskites with the APbX3 (A 1⁄4 monovalent cation and X 1⁄4 I, Br, Cl) structure are a class of promising materials for solutionprocessed high-efficiency solar cells [1]. The second type includes the weak peaks that appear in the low-temperature orthorhombic phase in the higher wave number region from 80 to 160 cm−1. La-o-vorakiat et al reported two phonon modes related to the lead halide cage at 32 and 63 cm−1 for the room-temperature MAPbI3 thin film [14], (a)

Results

Conclusion