X-ray photoelectron spectroscopy and atomic force microscopy investigation of stability mechanism of tris-(8-hydroxyquinoIine) 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 Angstrom. The ITO surface was exposed by removing the organic and metal layers with dichloromethane, an organic solvent in which NPB and Alq(3) 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. (C) 1999 American Vacuum Society. [S0734-2101(99)23204-6].