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), YbQ 3 and ErQ 3, and the related materials GdQ 3 and DyQ 3, 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), AlQ 3. Given the documented differences in the photoluminscence spectra compared to AlQ 3, this implies that the visible light emission from YbQ 3 and ErQ 3 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 AlQ 3 on Ag, suggesting that the wealth of published band alignment studies for AlQ 3 may be directly applicable to these materials. YbQ 3 and ErQ 3 are found to be highly susceptible to radiation damage, particularly at photon energies <100 eV.