| Achoimre: | Halide double perovskites with alternating silver and pnictogen cations are an emerging family of
photoabsorber materials with robust stability and band gaps in the visible range. However, the nature of optical excitations in these systems is not yet well understood, limiting their utility. Here, we
use ab initio many-body perturbation theory within the GW approximation and the Bethe-Salpeter
equation approach to calculate the electronic structure and optical excitations of the double perovskite series Cs2AgBX6, with B=Bi3+, Sb3+, X = Br−, Cl−. We find that these materials exhibit
strongly localized resonant excitons with energies from 170 to 434 meV below the direct band gap.
In contrast to lead-based perovskites, the Cs2AgBX6 excitons are computed to be non-hydrogenic,
with anisotropic effective masses and sensitive to local field effects, a consequence of their chemical
heterogeneity. Our calculations demonstrate the limitations of the Wannier-Mott and Elliott models for this class of double perovskites and contribute to a detailed atomistic understanding of their
light-matter interactions.
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