Regular Article

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

¹ The National Institute for Theoretical Physics, School of Physics and Mandelstam Institute for Theoretical Physics, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa
² Department of Scientific Laboratories, Sudan University of Science and Technology, Khartoum, Sudan
³ Department of Basic Sciences, Imam Abdulrahman Bin Faisal University, P.O. Box 1982, Dammam, Kingdom of Saudi Arabia

^a e-mail: ibraphysics@gmail.com

Received: 23 February 2019
Received in final form: 27 May 2019
Published online: 11 September 2019

Abstract

The structural, mechanical, dynamical stability, electronic and optical properties of orthorhombic perovskite CH₃NH₃SnI₃ 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 CH₃NH₃SnI₃ perovskite is dynamically unstable, while at P = 0.7 GPa, the orthorhombic CH₃NH₃SnI₃ perovskite is dynamically stable. Our calculations show that CH₃NH₃SnI₃ 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 G₀W₀ 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 G₀W₀ 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 CH₃NH₃SnI₃ has potential as a low gap solar cell absorber.