Energy Level Alignment at the Metal/Alq 3 Interfaces Investigated with Photoemission Methods
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Energy Level Alignment at the Metal/Alq3 Interfaces Investigated with Photoemission Methods Li Yan1, C.W. Tang2, M. G. Mason2*, Yongli Gao1 1 University of Rochester, Rochester, New York 14627 USA. 2 Eastman Kodak Company, Rochester, New York 14650 USA. *Deceased. ABSTRACT Tris(8-hydroxyquinoline) aluminum (Alq3) based organic light emission diodes (OLED) have been a focus of material research in recent years. One of the key issues in searching for a better device performance and fabricating conditions is suitable electron-injection materials. We have investigated the energy alignment and the interface formation between different metals and Alq3 using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The interface is formed by depositing the target cathode material, such as Ca, Al or Al/LiF, onto an Alq3 film in a stepwise fashion in an ultrahigh vacuum environment. While the UPS results show the work function and vacuum level changes during interfaces formation, implying a possible surface dipole layer, XPS results show a more detailed and complex behavior. When a low work function metal such as Ca is deposited onto an Alq3 surface, a gap state is observed in UPS. At the same time, a new peak can be observed in the N 1s core level at a lower binding energy. These results can be characterized as charge transfer from the low work function metal to Alq3. The shifting of core levels are also observed, which may be explained by doping from metal atoms or charge diffusion. These interfaces are drastically different than the Al/Alq3 interface, which has very poor electron injection. At the Al/Alq3 interface there is a destructive chemical reaction and much smaller core level shifts are observed. Based on detailed analysis, energy level diagrams at the interface are proposed. INTRODUCTION Tris-(8-hydroxyquinoline) aluminum (Alq3) is one of the most widely used materials as an emitting layer for organic light-emitting devices (OLEDs) due to its excellent stability and luminescent properties[1,2 ]. A typical OLED consists of a high work function transparent anode such as indium tin oxide (ITO), one or more layers of organic films, and a metal cathode. The light output (electroluminescence) is derived from the radiative recombination of electrons and holes injected into the organic layers from the cathode and the anode[1]. It has been well established that the electroluminescence efficiency and the voltage required for the OLED are strongly dependent on the contacting electrodes and the nature of the metal/organic interface[2,3]. Hence, an understanding of the interface formation between the metal cathode and the underlying organic film is crucial. The cathode is normally a low work function metal such as Mg, Ca or Li or a metal alloy (Mg:Ag) in order to form an effective electron-injecting contact[1-3]. Because of the extremely high chemical reactivity of these metals, it would be preferable to use a more stable metal for the cathode such as Al. Unfortunately, Al forms a rather poor cathode with a high drive vo
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