First-Principles Simulation for Optoelectronic Characteristics and Structural Relaxation of Lead-Free Halide Double Perovskites

In this article, the optoelectronic and structural properties of lead-free halide double perovskite (HDPs) Cs2AgM3+X6 (M3+ = Sb, Bi; X = Cl, Br, I) were calculated using density functional theory (DFT) approximations. The structural analysis of Cs2AgSbX6 and Cs2AgBiX6 (X = Cl, Br, and I) indicates that the lattice constant and unit cell volume increase progressively with the increase in halides. The electronic properties reveal that all these semiconductors are good absorbers of incoming photons at the range of bandgap energy. The bandgap energies of Cs2AgSbCl6, Cs2AgSbBr6, Cs2AgSbI6, Cs2AgBiCl6, Cs2AgBiBr6, and Cs2AgBiI6 are 2.50 eV, 1.75 eV, 0.98 eV, 2.70 eV, 2.18 eV, and 1.35 eV, respectively, lying in the visible range of electromagnetic spectrum, and their electronic bandgaps decrease down the halide group. The compounds with bandgaps in the range of 1.3-1.75 eV (Cs2AgBiI6 and Cs2AgSbBr6) are suitable for solar cell applications, while the compounds with greater bandgaps are good for light-emitting diodes. Halides exhibit covalent bonding with Ag and M3+, where the bonding nature is ionic for Cs-Ag, Cs-Bi, Cs-Sb, and Cs-X. The optical properties show that, due to their suitable bandgaps, these HDPs are good absorbers of photons and most desirable for optoelectronic and photovoltaic applications. The critical energy of Cs2AgSbX6 and Cs2AgBiX6 decreases with the increase in halides from Cl to I. From the refractive index it is clear that these materials are super-luminescent for ultraviolet photons. The oscillator strength shows that transitions occur when the energy reaches the bandgap energy and the number of electrons increases rapidly up to 17 eV, after which the oscillator strength saturates at 30 eV.