Cs2NaGaBr6: 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, B2+ = Li, Na, B3+ = 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 Cs2LiGaBr6 and Cs2NaGaBr6 were calculated with SOC + mBJ to correct the band gap values with respect to experimental value. We obtained direct bandgaps at Gamma-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 epsilon, refractive index n, extinction coefficient k, optical conduction sigma, 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.