  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
calculated optical conductivity using relativist quasi-particle (GW)
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
small displacement approach, and further calculate the infrared absorption (IR)
spectra. Qualitatively, our calculations of low-energy phonon frequencies are
in good agreement with our terahertz measurements. Therefore, for both energy
scales (around 2 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.
