Effect of halogen on the luminescence properties of two-dimensional double-perovskite (C6H16

Two-dimensional (2D) metal halide perovskites have garnered significant attention in the field of light-emitting diodes due to their high photoluminescence quantum yield and tunability. In comparison with the widely studied 2D Ruddlesden-Popper (RP) perovskites, the 2D Dion-Jacobson (DJ) double perovskites, which offer higher stability and nontoxicity, have received relatively less attention. In addition, the regulatory effects of halogens on the luminescence mechanisms and performance of 2D DJ double perovskites remain unknown. Utilizing density functional theory, the stability and luminescence properties of 2D DJ double-perovskite $({\mathrm{C}}_{6}{\mathrm{H}}_{16}{\mathrm{N}}_{2}{)}_{2}\mathrm{Ag}\mathrm{Bi}{X}_{8}\ensuremath{\cdot}{\mathrm{H}}_{2}\mathrm{O}$ ($X$ = $\mathrm{Cl},\phantom{\rule{0.2em}{0ex}}\mathrm{Br},\phantom{\rule{0.2em}{0ex}}\mathrm{I}$) are effectively modulated through halogen substitution. The findings indicate that the dissociation energy incrementally increases as the halogen changes from $\mathrm{I}$ to $\mathrm{Br}$ to $\mathrm{Cl}$, suggesting that the incorporation of lighter halogens enhances structural stability. The minimal self-trapping formation energy (i.e., 0.08 eV) of $({\mathrm{C}}_{6}{\mathrm{H}}_{16}{\mathrm{N}}_{2}{)}_{2}{\mathrm{Ag}\mathrm{Bi}\mathrm{I}}_{8}\ensuremath{\cdot}{\mathrm{H}}_{2}\mathrm{O}$ facilitates easy detrapment of self-trapped excitons, leading to a tendency toward free exciton luminescence. The self-trapping formation energies of $({\mathrm{C}}_{6}{\mathrm{H}}_{16}{\mathrm{N}}_{2}{)}_{2}{\mathrm{Ag}\mathrm{Bi}\mathrm{Br}}_{8}\ensuremath{\cdot}{\mathrm{H}}_{2}\mathrm{O}$ and $({\mathrm{C}}_{6}{\mathrm{H}}_{16}{\mathrm{N}}_{2}{)}_{2}{\mathrm{Ag}\mathrm{Bi}\mathrm{Cl}}_{8}\ensuremath{\cdot}{\mathrm{H}}_{2}\mathrm{O}$ are 0.77 eV and 0.96 eV, respectively, indicating substantial self-trapping depths; thus, favoring self-trapped exciton luminescence. In addition, the transition dipole moments of $({\mathrm{C}}_{6}{\mathrm{H}}_{16}{\mathrm{N}}_{2}{)}_{2}{\mathrm{Ag}\mathrm{Bi}\mathrm{Br}}_{8}\ensuremath{\cdot}{\mathrm{H}}_{2}\mathrm{O}$ and $({\mathrm{C}}_{6}{\mathrm{H}}_{16}{\mathrm{N}}_{2}{)}_{2}{\mathrm{Ag}\mathrm{Bi}\mathrm{Cl}}_{8}\ensuremath{\cdot}{\mathrm{H}}_{2}\mathrm{O}$ are substantially higher than those of $({\mathrm{C}}_{6}{\mathrm{H}}_{16}{\mathrm{N}}_{2}{)}_{2}{\mathrm{Ag}\mathrm{Bi}\mathrm{I}}_{8}\ensuremath{\cdot}{\mathrm{H}}_{2}\mathrm{O}$, suggesting that substituting I with $\mathrm{Cl}$ and $\mathrm{Br}$ enhances the luminous efficiency of 2D DJ double-perovskite $({\mathrm{C}}_{6}{\mathrm{H}}_{16}{\mathrm{N}}_{2}{)}_{2}\mathrm{Ag}\mathrm{Bi}{X}_{8}\ensuremath{\cdot}{\mathrm{H}}_{2}\mathrm{O}$ ($X$ = $\mathrm{Cl},\phantom{\rule{0.2em}{0ex}}\mathrm{Br},\phantom{\rule{0.2em}{0ex}}\mathrm{I}$). These results indicate that halogen substitution can not only affect the stability of 2D DJ perovskites but also modulate their luminescent properties. Our research provides theoretical insights for the experimental design of superior luminescent materials.