Determination of energy barriers in organic light-emitting diodes by internal photoemission

Internal photoemission has been used to determine the energy barriers for electrode/organic-layer interfaces in organic light-emitting diodes. The diodes were polymer or small-molecule single-layer structures made from poly-(9-vinylcarbazole), poly-(9,9-dihexylfluorene), tris-(8-hydroxyquinolinate) aluminum, and N,N′- Bis(3-methylphenyl)-N,N′-diphenylbenzidine with indium tin oxide or metallic electrodes (Al, Au, or Cu). The internal photoemission yield follows the Fowler theory and the energy barriers exhibit a small lowering for increasing internal electric field due to image-force potential. There is an offset of 0.4-0.5 eV between the actual energy barriers and the naive expectations from the difference between the energy levels taken from the literature. Interface dipole effects and uncertainties concerning the electrochemical determination of the highest occupied molecular orbital levels are the most plausible origins of this difference.