LAUSR

We study the optical properties of ${\mathrm{Cs}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}$ nanoplatelets using a combination of first-principles density functional theory, $\mathit{GW}$ plus Bethe-Salpeter equation calculations, and spectroscopic experiments. We show that the material exhibits flat bands and hence high effective masses. This manifests itself in the lowest-energy transition in the absorption spectrum arising from excitons with a high binding energy of 300 meV and a Bohr radius smaller than 6 nm. Due to the indirect band gap, electrons and holes are efficiently separated in reciprocal space and recombine slowly across the band gap, leading to very weak photoluminescence. Our results resolve inconsistencies in previous studies on ${\mathrm{Cs}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}$ and lay the groundwork for further applications of this material, reliant on charge separation.