Free exciton and bound excitons on Pb and I vacancies and O and I substituting defects in PbI2: Photoluminescence and DFT calculations

A complete study of structure, chemical composition, morphology and photoluminescence (PL) is performed for multilayer PbI2 flakes. The reduction of thickness results in uneven spatial mapping of PL and Raman intensities, while the positions and shapes of the spectra do not undergo notable changes. The most intense peaks in the PL spectra are the direct free exciton (FX) peak at 2.44 eV (508 nm) and the bound exciton (BX) at 2.41 eV (516 nm). The variation of above/below-bandgap excitation has allowed to find three more defect-related peaks at 2.35, 2.19 and 2.04 eV. So far, these three BXs have not been studied. Our DFT calculations combined with the experimental results allow to clarify the origins of BX peaks as the recombination from the optical conduction band edge to the defect levels located above the valence band: 2.41 eV to O substituting I (OI), 2.35 eV to Pb vacancy (VPb), and 2.19/2.04 eV to IPb with split level. We have defined that the known absorption at 1.55 eV (800 nm) is related to VI nonradiative recombination center. The temperature behaviors allow to determine the thermal dissociation energy of FX of 0.22 eV. The dependences also show a co-interaction between defect levels.