Cs 2 NaGaBr 6 : a new lead-free and direct band gap halide double perovskite.

In this work, we have studied new double perovskite materials, A 2 1+ B 2+ B 3+ X 6 1- , where A 2 1+ = Cs, B 2+ = Li, Na, B 3+ = Al, Ga, In, and X 6 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 Cs 2 LiGaBr 6 and Cs 2 NaGaBr 6 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 Cs 2 LiGaBr 6 and 1.762 eV for Cs 2 NaGaBr 6 , which are similar to the parent organic-inorganic perovskite (MAPI) CH 3 NH 3 PbI 3 ( 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 Cs 2 NaGaBr 6 a great competitor in the Pb-free hybrid perovskite solar cells world.