X-ray photoelectron spectroscopy and atomic force microscopy investigation of stability mechanism of tris-(8-hydroxyquinoline) aluminum-based light-emitting devices

Stability is an essential issue in the application of organic light-emitting devices (OLEDs). We have investigated the indium tin oxide (ITO) surface for operated and unoperated OLEDs using x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques. The device structure consists of ITO/phenyl-diamine (NPB)/tris-(8-hydroxyquinoline) aluminum ( Alq 3 ) /Mg:Ag with NPB thickness varied from 0 to 300 Å. The ITO surface was exposed by removing the organic and metal layers with dichloromethane, an organic solvent in which NPB and Alq 3 are highly soluble. Electroluminescence characterization demonstrates that the NPB layer substantially enhanced the stability. XPS analysis shows that for the device made without NPB and after 90 h of operation, there exists an insoluble organic material on the ITO surface. This organic material is not observed on the ITO of unoperated devices. Lateral force AFM also shows a striking difference between the ITO surface of devices with and without NPB after operation. The XPS and AFM results suggest that the organic residue is the degradation product of Alq 3 that acts as quenching sites at the ITO/ Alq 3 interface, which contribute to the early failure of the single-layer devices.