Injection of charge into the archetype organic hole transporting material TPD at the electrical contacts with ITO and Al

A study of the current–electric field characteristics of single layer structures Quartz/ITO/TPD/Al and Quartz/Al1/TPD/Al2, containing the vacuum evaporated films of a diamine derivative (TPD) used commonly as a hole-transporting material in organic light emitting diodes, was carried out. The currents are dominated by injection of holes independent on the bias of the electrical contacts. In addition to an obvious asymmetry in the currents for ITO+ and ITO− biased contacts, a distinct asymmetry for aluminum Al1+ and Al1- biased contact is observed. The results have been interpreted in terms of the injection-limited current theory accounted for the electrode and bi-molecular recombination.