Interface dependent electrical properties of organic light emitting devices in ultra high vacuum

We present electroluminescence and electrical measurements on organic light emitting diodes (OLEDs) which were produced and operated inside a contamination-free ultra high vacuum (UHV) system, at a purity level corresponding to a pressure of 10−9 mbar. We characterized the devices with a simple magnesium (Mg) cathode, where Mg atoms diffuse into the 8-hydroxyquinoline aluminum (Alq3) layer during evaporation, and devices where a 1-nm thin silver (Ag) layer acts as a diffusion barrier between Mg and Alq3. All OLEDs could be operated in UHV during 1 day at a brightness level of 1000 cd/m2, without the appearance of any “black spots”. When exposed to air however, a significant degradation of the device performance was observed. The lifetime in air was a few seconds for the device with the simple Mg cathode, and a few hours for the device with the Ag layer, indicating the importance of Mg diffusion when the device is operated in air.