Effects of process parameters on trap distributions in organic semiconductors

The effect of different process parameters on the energetic trap structure of organic light emitting diodes (OLEDs) has been investigated. For trap level detection the method of thermally stimulated currents (TSC) has been applied. Energetic trap distributions were derived by means of a fractional TSC technique. The experiments have been carried out on aluminum tris(8-hydroxyquinoline) (Alq(3)) single-layer devices that have been prepared by vacuum deposition with evaporation rates of 0.2, 2 and 10 Angstrom/s. In all cases broad trap distributions were found. However. with increasing deposition rate the mean trap energy level shifts to lower energies. This behavior indicates a structural origin of the trap states. A correlation between the I-V and L-V characteristics and the TSC spectra is shown and discussed. Further on, the TSC spectra of N,N',-di (1- naphtyl)-N,N'-diphenylbenzidine (alpha-NPD) single-layer devices processed at varying substrate temperatures have been measured. These spectra exhibit significant differences, such as the disappearance of one TSC peak, in the case of a heated sample. (C) 2003 Elsevier Science B.V. All rights reserved.