Giant excitonic effects in vacancy-ordered double perovskites

Using first-principles GW plus Bethe-Salpeter equation calculations, we identify exceptionally strong excitonic effects in several vacancy-ordered double perovskites Cs2MX6 (M = Ti, Zr; X=I,Br). Giant exciton binding energies of about 1 eV are found in these moderate-gap, inorganic bulk semiconductors, pushing the limit of our understanding of the electron-hole interaction and exciton formation in solids. Not only are the exciton binding energies extremely large compared with any other moderate-gap bulk semiconductors, but they are also larger than typical two-dimensional semiconductors with comparable quasiparticle gaps. Our calculated lowest bright exciton energies agree well with the measured optical band gaps. The low-energy excitons closely resemble the Frenkel excitons in molecular crystals, as they are highly localized in a single [MX6]2- octahedron and extended in the reciprocal space. The weak dielectric screening effects and the nearly flat frontier electronic bands, which are derived from the weakly coupled [MX6]2- units, together explain the significant excitonic effects. Spin-orbit coupling effects play a crucial role in redshifting the lowest bright exciton by mixing up spin-singlet and spin-triplet excitons, while exciton-phonon coupling effects have minor impacts on the calculated exciton binding energies.