First principles study of structural, electronic, elastic and optical properties of Cs[formula omitted]LiTlBr[formula omitted] and Cs[formula omitted]NaTlBr[formula omitted

Abstract

New double perovskites materials, Cs2B+1B+3X6, where B+1 = Na, Li B+3 = Tl, Sb, Bi and X6 = Cl, Br are investigated under the framework of density functional theory (DFT) by applying all electron full-potential linearized augmented plane wave (FP-LAPW+lo) technique. Without and with spin–orbit coupling (SOC) band structures are calculated. Electronic charge density graph reveal the ionic nature of Cs-atom with Br-atom and strong covalent bonding between Tl-atom and Br-atom. While band structures for Cs2LiTlBr6 and Cs2NaTlBr6 are calculated with mBJ+SOC for the correction of value of the bandgap relative to experimental value of MAPI. Values of direct bandgap at Γ-point are 1.817 eV for Cs2LiTlBr6 and 1.827 eV for Cs2NaTlBr6 that are close to the bandgap of 1.55 eV of parent organic–inorganic perovskite CH3NH3PbI3 (MAPI). Effective mass of electron (m*e) for Cs2LiTlBr6 and Cs2NaTlBr6 are 0.671 and 0.724 respectively which are less than that of Si (1.09). Therefore it is expected that the carrier mobility of both materials is greater than Si. In order to study the contribution of orbits of Cs, Li, Na, Tl and Br close by the Fermi level, we examined the total and partial density of states. The optical properties were study to investigate the absorption coefficient of both materials. Studies of the thermodynamics and elastic characteristics along with phonon dispersion confirms that these two materials are stable. Because of stable, direct bandgap and a value of the bandgap closer as MAPI the materials Cs2LiTlBr6 and Cs2NaTlBr6 are significant competitors in the world of photovoltaic solar cells.