Determination of the energy levels of a phosphorescent guest in organic light emitting devices

We have used photoelectron spectroscopy to study the relative binding energies of the highest occupied molecular orbitals (HOMOs) of the phosphorescent guest, 2, 3, 7, 8, 12, 13, 17, 18octaethyl-21H,23H-porphine platinum (PtOEP), and the organic electron transport host, tris(8-hydroxyquinolinato) aluminum (III) (Alq3) in PtOEP:Alq3 composite films. The PtOEP HOMO position was found to be 0.50±0.13 eV above that of the Alq3 host, independent of the guest molecule concentration (0.8%–22% by mass). These results are consistent with the assumption that the vacuum levels of the guest and host align.