Effects of multi doped zones on the current-voltage characteristics and EL spectra in organic light emitting diodes

The narrow rubrene-doped zones were located in the emitting layer, AlQ3. The effects of doped-zone width, position and zone number on the electroluminescence (EL) intensity of organic light emitting diodes (OLEDs) were studied. The current-voltage characteristics were also studied according to the trap charge limited (TCL) transport models. When the doped zone width increased, the EL intensity decreased dramatically due to the large trapping defects introduced by rubrene molecules. When a single doped zone is placed adjacent to the TPD/AlQ3 interface, the EL spectra have higher intensity. As the doped zone location moved away from the TPD/AlQ3 interface to the cathode, the conduction current decreased, the turn-on voltage increased and the EL efficiency decreased due to insufficient amount of holes can diffuse to the doped zone to recombine with trapped electrons in doped regions. Double and triple doped zones structures were used to fabricate the devices. The double doped-zone structure has the highest EL intensity. The emission intensity was more than 4 times higher than that of a single doped diode.