Determination of trap energies in Alq3 and TPD

Localized states have dominant effects on injection and transport of electronic charge in organic semiconductors. Therefore, they are of particular importance in organic light-emitting diodes (OLEDs) and transistors. In order to identify and characterize traps participating in the organic electron and hole transport we investigate tris-(8-hydroxyquinoline) aluminium (Alq3) and N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) with thermally stimulated current techniques (TSC). In Alq3, a broad distribution of trap 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 by a distribution of trap energies in the range of 0.25 to 0.54 eV. The isolated trap level can be perfectly fitted using second-order kinetics indicating the significance of trapping processes.