Occupied and unoccupied states of the organic infrared emitters Yb- and Er-tris(8-hydroxyquinoline) studied by photoemission and X-ray absorption

Abstract The electronic structure of thin films of the organic infrared emitters Yb- and Er-tris(8-hydroxyquinoline), YbQ3 and ErQ3, and the related materials GdQ3 and DyQ3, on Ag (1 1 1) is investigated using synchrotron radiation excited photoemission, and X-ray absorption at the C and N K edges. Anti-resonant photoemission is used to suppress the 4f emission. We find that, apart from a shift in the oxygen-derived highest occupied molecular orbital (HOMO), and small differences in the X-ray absorption spectra, the results are similar to those of the well-known visible light emitter aluminium tris(8-hydroxyquinoline), AlQ3. Given the documented differences in the photoluminscence spectra compared to AlQ3, this implies that the visible light emission from YbQ3 and ErQ3 is dominated by triplet states, which are not accessible to the spectroscopies used here. The alignment of the organic HOMO with the Ag Fermi level is shown to be essentially identical to that of AlQ3 on Ag, suggesting that the wealth of published band alignment studies for AlQ3 may be directly applicable to these materials. YbQ3 and ErQ3 are found to be highly susceptible to radiation damage, particularly at photon energies