Optical properties of organometallic perovskite: An ab initio study using relativistic GW correction and Bethe-Salpeter equation

In the development of highly efficient photovoltaic cells, solid perovskite systems have demonstrated unprecedented promise, with the figure of merit exceeding nineteen percent of efficiency. In this paper, we investigate the optical and vibrational properties of organometallic cubic perovskite CH₃NH₃PbI₃ using first-principles calculations. For accurate theoretical description, we go beyond conventional density functional theory (DFT), and calculate optical conductivity using relativistic quasi-particle correction. Incorporating these many-body effects, we further solve Bethe-Salpeter equations (BSE) for excitons, and found enhanced optical conductivity near the gap edge. Due to the presence of organic methylammonium cations near the center of the perovskite cell, the system is sensitive to low-energy vibrational modes. We estimate the phonon modes of CH₃NH₃PbI₃ using a small displacement approach, and further calculate the infrared (IR) absorption spectra. Qualitatively, our calculations of low-energy phonon frequencies are in good agreement with our terahertz measurements. Therefore, for both energy scales (around 1.5 eV and 0-20 meV), our calculations reveal the importance of many-body effects and their contributions to the desirable optical properties in the cubic organometallic perovskites system.