Enhanced efficiency and durability of organic electroluminescent devices by inserting a thin insulating layer at the Alq3/cathode interface

We report the effect of a thin (about 10 إ) MgF2 layer at the cathodic interface on the quantum efficiency and durability of organic electroluminescent devices with the emitting layer of tris-(8-hydroxyquinoline) aluminum (Alq3). In the device with a thin MgF2 layer the quantum efficiency increases without increase of threshold voltage, the leakage current is substantially reduced, and the half-lifetime of the light output under the same current injection increases from about 1 to about 7 h. Furthermore, the drive voltage is initially stable and rises much slower under constant current injection in devices with a thin MgF2 layer, indicating a suppression of the cathode degradation. Thus, the MgF2 layer at the cathodic interface seems to improve the morphology and adhesion of the cathode. It is also considered that the MgF2 tunneling barrier at the cathodic interface decreases the hole current leakage through the cathode, and injected holes accumulate at the Alq3/ MgF2 interface. Then the electron injection rate at the cathode is enhanced by a space charge field across an interfacial MgF2 layer, which leads to the enhancement of quantum efficiency.