Phonon Mode Transformation Across the Orthohombic-Tetragonal Phase Transition in a Lead Iodide Perovskite CH_3NH_3PbI_3: A Terahertz Time-Domain Spectroscopy Approach

We study the temperature-dependent phonon modes of the organometallic lead iodide perovskite CH_3NH_3PbI_3 thin film across the terahertz (0.5–3 THz) and temperature (20–300 K) ranges. These modes are related to the vibration of the Pb–I bonds. We found that two phonon modes in the tetragonal phase at room temperature split into four modes in the low-temperature orthorhombic phase. By use of the Lorentz model fitting, we analyze the critical behavior of this phase transition. The carrier mobility values calculated from the low-temperature phonon mode frequencies, via two theoretical approaches, are found to agree reasonably with the experimental value (∼2000 cm^2 V^(–1) s^(–1)) from a previous time-resolved THz spectroscopy work. Thus, we have established a possible link between terahertz phonon modes and the transport properties of perovskite-based solar cells