Cs₂NaGaBr₆: a new lead-free and direct band gap halide double perovskite

In this work, we have studied new double perovskite materials, A₂ ¹⁺B²⁺B³⁺X₆ ^1-, where A₂ ¹⁺ = Cs, B²⁺ = Li, Na, B³⁺ = Al, Ga, In, and X₆ ^1-. We used the all electron full-potential linearized augmented plane wave (FP-LAPW+lo) method within the framework of density functional theory. We used the mBJ approximation and WC-GGA as exchange-correlation functionals. We optimized the lattice constants with WC-GGA. Band structures were calculated with and without spin-orbit coupling (SOC). Further, band structures for Cs₂LiGaBr₆ and Cs₂NaGaBr₆ were calculated with SOC + mBJ to correct the band gap values with respect to experimental value. We obtained direct bandgaps at Γ-point of 1.966 eV for Cs₂LiGaBr₆ and 1.762 eV for Cs₂NaGaBr₆, which are similar to the parent organic-inorganic perovskite (MAPI) CH₃NH₃PbI₃ (E _g = 1.6 eV). Total and partial density of states were analyzed to understand the orbital contribution of Cs, Na, Li, Ga and Br near the Fermi level. The optical properties in terms of real and imaginary ε, refractive index n, extinction coefficient k, optical conduction σ, absorption I, and reflectivity R were calculated. A study of the elastic and mechanical properties shows that both materials are thermodynamically stable. A stable, direct bandgap and a gap value close to those of MAPI make Cs₂NaGaBr₆ a great competitor in the Pb-free hybrid perovskite solar cells world.