Origin of damages in OLED from Al top electrode deposition by DC magnetron sputtering

In this study, we examine organic light emitting diodes (OLEDs) having Al top electrodes deposited on organic layers by direct-current magnetron sputtering. The OLEDs consisted of electronically doped transport layers and phosphorescent emission layer were characterized by typical current-voltage-luminance measurement. They showed higher leakage currents, decreased forward currents, and corresponding increases of driving voltage after the sputter deposition on the organic layers. The OLEDs exhibited randomly distributed bright spots on the active area, and the bright spots were investigated by scanning electron microscopy/energy-dispersive X-ray spectroscopy. In order to prove the origins of sputter damage, simple organic/Al layer samples were made and investigated by ellipsometry and laser-induced desorption/ionization time-of-flight mass spectrometry. The results are compared with previous works addressing the fundamental phenomena of magnetron sputtering. We conclude that the high leakage current originated from a penetration of sputtered metal atoms into the underlying organic layers, and the decrease of forward current resulted from an interface degradation caused by the radiation of plasma, which reduces charge carrier injection preferentially at the Al/organic layer interface. (C) 2009 Elsevier B.V. All rights reserved.