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

Internal photoemission has been used to determine the energy barriers for e lectrode/organic-layer interfaces in organic light-emitting diodes. The dio des 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 photoemissio n yield follows the Fowler theory and the energy barriers exhibit a small l owering 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 t he naive expectations from the difference between the energy levels taken f rom the literature. Interface dipole effects and uncertainties concerning t he electrochemical determination of the highest occupied molecular orbital levels are the most plausible origins of this difference. (C) 2001 American Institute of Physics.