A Systematic Study of the Interactions in the Top Electrode/Capping Layer/Thin Film Encapsulation of Transparent OLEDs

Abstract

Development of flexible transparent organic light-emitting devices (TOLEDs) still requires a number of advancements in transparent conducting electrodes with low reflection and absorption, a capping layer (CL) acting as refractive index-matching, and thin film encapsulation (TFE) with high water vapor barrier properties, among others. While substantial research has been reported on isolated examples in each area, there has been no detailed and systemic research related to the overall interactions of top electrodes, CLs, TFE, and their interfaces. In this work, TOLEDs have been fabricated with a thin Ag top electrode and CLs of different surface energy, which was encapsulated with high water vapor barrier property (1.35x10(-4) gm(-2)day(-1) at 37.8 degrees C and 100 % RH). The encapsulation barrier was comprised of 50-nm-thick Al2O3 thin films deposited using a low-temperature (95 degrees C) ALD process. Once prepared, the TOLEDs were studied using a variety of techniques to determine the enhancements to electrical, optical, and water vapor barrier properties. Although the nature of the CL materials affects the film formation on the top electrode layer, there is no impact on the properties of the Al2O3 thin films. In the formation process of the Al2O3 TFE, the device incorporating a tris(8-hydroxyquinoline)aluminum (Alq(3)) CL showed superior performance, whereas device performance degredation was noted with the 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN) CL. The lack of degradation in the Alq(3) CL device during ALD processing is attributed to the high thermal stability of Alq3, which exhibits a high glass transition temperature of 175 degrees C. However, in the HATCN CL device, it is expected that formation of aromatic radical anions [HAT(CN)(6)](center dot) and dianions [HAT(CN)(6)](2-), and/or band bending of fermi energy of the HATCN at the interface of the thin Ag layer, results in the observed degraded performance. (C) 2020 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.