Plasma treatment of the Mg:Ag/tris-(8-hydroxyquinoline) aluminum interface in OLEDs: effects on adhesion and performance

Abstract

The adhesion of a Mg:Ag cathode to the tris-(8-hydroxyquinoline) aluminum (Alq3) in organic light emitting devices (OLEDs) can be greatly enhanced by a remote plasma treatment of the Alq3 layer using either air or N2 prior to metal deposition. The altered surface properties which lead to increased sticking coefficients of Mg and Ag, as well as enhanced adhesion, are attributed to the introduction of new functional groups into the organic layer, as observed by X-ray photoelectron spectroscopy (XPS). The storage life of the plasma treated devices in air without any capping treatment, as judged by a visible deterioration of the cathode, was increased by approximately five to six times compared to untreated OLEDs. Current–voltage characteristics and EL efficiency, however, were shown to deteriorate for devices incorporating either an air or an N2 plasma treated Alq3 layer. For OLEDs subjected to short treatment times with an N2 plasma, only a very slight increase in the turn-on voltage, of about 0.2 V, was observed. An investigation of black spot formation revealed that an air plasma treatment resulted in a five-fold decrease in the time required for 50% of the device to become non-emissive. N2 treated devices on the other hand, developed black spots at a comparable rate to the non-treated devices. Thus, a short N2 plasma treatment of the Alq3 layer prior to metal deposition improves the adhesion at the interface, thereby reducing the oxidation and degradation of the device through exposure to ambient conditions, particularly in storage.