A Novel Approach to Dynamical Phonon Properties in Organic-Inorganic Perovskites for Tunable IR Absorption of FAPbI_xBr_3−x

Organic-inorganic hybrid perovskites (OIHPs) are emerging as promising materials for optoelectronic devices, including solar cells, LEDs, and, more recently, THz sensors. Understanding their phonon properties is critical for predicting their infrared (IR) absorption spectra, but conventional methods often fall short in capturing the dynamical phonons of OIHPs, mainly due to the rotational motion of organic molecules at room temperature. In this study, we employed the temperature-dependent effective potential (TDEP) method to account for these dynamic effects, enabling accurate phonon calculations for FAPbI3 and FAPbBr3. Furthermore, we extended this approach to model the mixed compound FAPbI_xBr_3−x, which offers tunable properties based on the composition. Since the TDEP method requires large supercells, it becomes computationally expensive for mixed systems. To address this, we introduced the molecular rotation spherical approximation (MRSA), which models the rotational behavior of molecules as spheres while accounting for internal molecular interactions. This approach enabled us to efficiently compute the phonon properties and simulate the IR spectra of FAPbI_xBr_3−x as a function of its composition. Our results demonstrate that this method offers a promising framework for phonon calculations in OIHPs, facilitating the exploration of their dynamic optical properties.