First-principles study of structural, mechanical, dynamical stability, electronic and optical properties of orthorhombic CH3NH3SnI3 under pressure

The structural, mechanical, dynamical stability, electronic and optical properties of orthorhombic perovskite CH3 NH3 SnI3 have been investigated using density functional theory (DFT) and many body perturbation theory calculations under pressure. Elastic parameters such as bulk modulus B , Young’s modulus E , shear modulus G , Poisson’s ratio ν and anisotropy value A have been calculated by the Voigt-Reuss-Hill averaging scheme at 0.7 GPa. The calculations of phonon dispersions at zero pressure showed that the orthorhombic CH3 NH3 SnI3 perovskite is dynamically unstable, while at P = 0.7 GPa, the orthorhombic CH3 NH3 SnI3 perovskite is dynamically stable. Our calculations show that CH3 NH3 SnI3 is a direct band gap semiconductor with an approximate density functional fundamental gap in the range of 0.73 eV to 1.21 eV, depending on the exchange-correlation approximation used. Many body perturbation theory at the G0 W0 level of approximation gives a fundamental band gap of 1.51 eV. In order to obtain optical spectra, we carried out Bethe-Salpeter equation calculations on top of a non-self-consistent G0 W0 calculations. Our calculated optical band gap shows anisotropy with an absorption edge of 1.27 eV in the a direction, 1.36 eV in the b direction and 1.20 eV in the c direction. Optical absorption spectra calculated at the BSE level of approximation show that the structure is a good absorber of light in the IR region, confirming that CH3 NH3 SnI3 has potential as a low gap solar cell absorber.