Effects of Halogen Substitution on the Optoelectronic Properties of Two-Dimensional All-Inorganic Double Perovskite Cs4AgBiX8 ( X

Due to the influence of dimension, two-dimensional (2D) all-inorganic double perovskites may be superior to their three-dimensional (3D) counterparts as environmentally friendly and efficient optoelectronic materials. Nevertheless, how the halogens affect the optoelectronic performance of 2D all-inorganic double perovskite is still unknown. Here, the photoelectric properties, including band structures, optical absorption spectra, carrier mobilities, and exciton binding energies, of 2D all-inorganic double perovskites ${\mathrm{Cs}}_{4}\mathrm{Ag}\mathrm{Bi}{X}_{8}$ ($X$ = $\mathrm{Cl}$, $\mathrm{Br}$, $\mathrm{I}$) with the Ruddlesden-Popper structure are studied via density-functional theory along with the spin-orbit coupling effect. Considering the influence of the exciton effect in low-dimensional materials, we also calculate light absorption by using the GW Bethe-Salpeter equation method. The obtained results show that the substitution of $\mathrm{Cl}$ with $\mathrm{Br}$ or $\mathrm{I}$ atoms decreases the band gap (from 2.706 eV to 2.221 and 1.715 eV) for ${\mathrm{Cs}}_{4}{\mathrm{Ag}\mathrm{Bi}\mathrm{Cl}}_{8}$, enhances the light-absorption performance, increases the mobility of carriers, and reduces the exciton binding energy (from 1529.90 meV to 1268.70 and 941.71 meV). Moreover, the outcomes demonstrate that 2D all-inorganic double perovskites ${\mathrm{Cs}}_{4}\mathrm{Ag}\mathrm{Bi}{X}_{8}$ ($X$ = $\mathrm{Cl}$, $\mathrm{Br}$, $\mathrm{I}$) might be better candidates for luminescent devices than photovoltaic materials, and the excellent performance of ${\mathrm{Cs}}_{4}{\mathrm{Ag}\mathrm{Bi}\mathrm{I}}_{8}$ makes it the optimal among the three. Our study will expand theoretical explorations for research into 2D all-inorganic double perovskite materials for potential luminescent or photovoltaic applications.