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

We have used photoelectron spectroscopy to study the relative binding energ ies of the highest occupied molecular orbitals (HOMOs) of the phosphorescen t guest, 2, 3, 7, 8, 12, 13, 17, 18octaethyl-21H,23H-porphine platinum (PtO EP), and the organic electron transport host, tris(8-hydroxyquinolinato) al uminum (III) (Alq(3)) in PtOEP:Alq(3) composite films. The PtOEP HOMO posit ion was found to be 0.50 +/- 0.13 eV above that of the Alq(3) host, indepen dent 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. (C) 2000 American Institute of Physics. [S0003-6951(00)00739-7] .