Determination of trap energies in Alq(3) and TPD

Localized states have dominant effects on injection and transport of electr onic charge in organic semiconductors. Therefore, they are of particular im portance in organic light-emitting diodes (OLEDs) and transistors. In order to identify and characterize traps participating in the organic electron a nd hole transport we investigate tris-(8-hydroxyquinoline) aluminium (Alq(3 )) and N,N'-bis(3-methylphenyl)N,N'-diphenylbenzidine (TPD) with thermally stimulated current techniques (TSC). In Alq(3), a broad distribution of tra p levels was identified with energies ranging from 0.06 eV to more than 0.5 eV. In TPD, an isolated trap level at about 0.12 eV is observed followed b y a distribution of trap energies in the range of 0.25 to 0.54 eV. The isol ated trap level can be perfectly fitted using second-order kinetics indicat ing the significance of trapping processes. (C) 2000 Elsevier Science S.A. All rights reserved.