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 similar to 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 similar to 150-200 meV) located at PbI3. 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 (LiPbI3) to 4.3 eV (CsPbI3), contain optical transitions with very confined excitons (E-b similar to 220-290 meV) located at halide atoms and the empty states of the metallic cations.