Tuning exciton binding and photoabsorption in vacancy-ordered halide double perovskites Rb2ZrX6 (X <b>=</b> Cl, Br, I)

Vacancy-ordered halide double perovskites (VHDPs) have emerged as a promising candidate for broadband luminescence, but the underlying mechanism remains obscure. Here, we report a study of quasiparticle band structures and excitonic properties under the lattice strain of Rb2ZrX6 (X = Cl, Br, I) using G0W0 plus the Bethe–Salpeter equation calculations. Our calculations demonstrate that cubic Rb2ZrX6 have direct bandgaps and large effective masses of electrons and holes, which become smaller for larger halogens. Through the analysis of the photoabsorption spectra, we identify bright and dark excitons blueshifted for smaller halogens, finding a bright exciton at 4.96 eV in agreement with the experimental optical bandgap for Rb2ZrCl6. We find an extremely large exciton binding energy of 1.17 eV for chloride but a much smaller value of 0.22 eV for iodide. Furthermore, compressive strain reduces bandgaps, effective masses, and exciton binding energies, but tensile strain causes an inverse effect, while both strains enhance photoabsorption. Our work highlights an important role of halogen substitution and lattice strain in tuning the optoelectronic properties of VHDPs.