Studies of Alq/Mg: Ag Interface in Organic Light-Emitting Diodes by XPS
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Studies of Alq/Mg: Ag Interface in Organic Light-Emitting Diodes by XPS X. D. Feng, D. Grozea, A. Turak, Z. H. Lu*, H. Aziz1, and A-M. Hor1 Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario M5S 3E4, Canada 1 Xerox Research Centre of Canada, Mississauga, Ontario L5K 2L1, Canada * To whom all correspondence should be addressed; [email protected] ABSTRACT The organic/cathode interface plays an important role in device degradation of organic light-emitting diodes (OLEDs). The interface between 8-hydroxyquinolino aluminium (Alq) and Mg:Ag cathode in OLEDs, operated for some time, was characterized using Xray photoemission spectroscopy (XPS). An in-vacuum peel-off method was used to separate the buried interfaces. XPS results indicate that Alq molecules break down, resulting in formation of fragmented hydroxyquinolino, Mg oxides, and metallic Al at the interface. It is also found by XPS depth-profiling measurement that metallic Al diffuses into the cathode electrode, and that the fraction of oxidized Mg decreases gradually from the interface but extended very deep into the cathode. INTRODUCTION The organic/cathode interface in organic light-emitting diodes (OLEDs) can affect device performance such as turn-on voltage, quantum efficiency, and lifetime [1-3]. Recent studies on the formation of dark spots have also revealed that it plays an important role in device degradation under ambient conditions [3-5]. Considerable research efforts using xray and ultraviolet photoemission spectroscopy (XPS and UPS) have been made on 8hydroxyquinolino aluminium (Alq)/single metal interface studies [6-8]. However, all these previous works were focused on initial formation of interface, which may have no relevance to practical devices. Surprisingly, little work has been reported on buried Alq/ Mg: Ag device interfaces, which is still of great importance since a Mg:Ag alloy is one of the most commonly used cathodes ever since its first use by Tang and Van Slyke [9]. A deep insight on the Alq/Mg:Ag interface in a real device (operation or storage) will provide valuable information for further improving device reliability. In this paper, we report the detailed investigation on Alq molecule breakdown and the degree of oxidation of cathode at the Alq/Mg:Ag interface in OLEDs by XPS. EXPERIMENTAL DETAILS The OLED samples have a structure of ITO / CuPc (15nm) /NPB (60nm) / Alq (75nm) / Mg:Ag (200nm), and were fabricated by vapor-phase deposition in a vacuum chamber with a pressure ~10–6 Torr. The Mg:Ag electrode was co-evaporated at a rate of 3 Å/s for Mg and 0.3 Å/s for Ag, respectively. There are 10 diodes on one substrate, and each diode has an active area of ~ 2×2 mm2. Some of them were operated in a dry N2 atmosphere continuously for two weeks after fabrication, and then left in air for one P4.8.1 Downloaded from https://www.cambridge.org/core. University of Texas Libraries, on 10 Jan 2020 at 10:43:35, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.
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