Role of inorganic cations in the excitonic properties of lead halide perovskites

We theoretically investigate lead iodide perovskites of general formula APbI 3 for a series of metallic cations (namely Cs + , Rb + , K + , Na + and Li + ) by means of density functional theory, the GW method and the Bethe-Salpeter equation including spin-orbit coupling. We demonstrate that the low-energy edges (up to 1.3 eV) of the absorption spectra are dominated by weakly bound excitons, with binding energies E b of ∼ 30-80 meV, and the corresponding intensities increase as metallic cations become lighter. The middle parts of the spectra (1.8-2.4 eV), on the other hand, contain optical dipole transitions comprising more confined excitons ( E b ∼ 150-200 meV) located at PbI 3 . These parts of the spectra correspond to the optical-gain wavelengths which are experimentally achieved in optically pumped perovskite lasers. Finally, the higher energy parts, from about 2.8 eV (LiPbI 3 ) to 4.3 eV (CsPbI 3 ), contain optical transitions with very confined excitons ( E b ∼ 220-290 meV) located at halide atoms and the empty states of the metallic cations. In lead halide perovskites, the role of inorganic A cations is leading in the middle part of the absorption spectrum where the excitonic binding energies of dipole transitions are much stronger than those involving PbI 3 bands only.