Infrared Spectroscopic Study of Vibrational Modes in Methylammonium Lead Halide Perovskites

The
organic cation and its interplay with the inorganic lattice
underlie the exceptional optoelectronic properties of organo-metallic
halide perovskites. Herein we report high-quality infrared spectroscopic
measurements of methylammonium lead halide perovskite (CH3NH3Pb­(I/Br/Cl)3) films and single crystals
at room temperature, from which the dielectric function in the investigated
spectral range is derived. Comparison with electronic structure calculations
in vacuum of the free methylammonium cation allows for a detailed
peak assignment. We analyze the shifts of the vibrational peak positions
between the different halides and infer the extent of interaction
between organic moiety and the surrounding inorganic cage. The positions
of the NH3+ stretching vibrations point to significant
hydrogen bonding between the methylammonium and the halides for all
three perovskites.