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

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Highlights

  • There has been no detailed and systemic research related to the overall interactions of top electrodes, capping layers (CLs), thin film encapsulation (TFE), and their interfaces in OLEDs.

  • In the formation process of the Al2O3 TFE, the device incorporating the tris(8-hydroxyquinoline)aluminum (Alq3) 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 Alq3 CL device during ALD processing is attributed to the high thermal stability of Alq3, which exhibits a high glass transition temperature of 175 °C.

  • In the HATCN CL device, it is expected that formation of aromatic radical anions [HAT(CN)6]•− 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.

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−2day−1 at 37.8 °C and 100 % RH). The encapsulation barrier was comprised of 50-nm-thick Al2O3 thin films deposited using a low-temperature (95 °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 (Alq3) 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 Alq3 CL device during ALD processing is attributed to the high thermal stability of Alq3, which exhibits a high glass transition temperature of 175 °C. However, in the HATCN CL device, it is expected that formation of aromatic radical anions [HAT(CN)6]•− 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.

Section snippets

INTRODUCTION

Flexible transparent organic light-emitting devices (TOLEDs) and top-emitting organic light-emitting diodes (TEOLEDs) have attracted increasing interest for use in next-generation displays [1], [2], [3]. In these OLED devices, the transparent top electrode still requires many improvements, and significant research efforts have been devoted to its development. Generally, top electrode materials are composed of transparent conducting oxides (TCOs) or thin metal films as well as combinations of

Fabrication of TOLEDs with both Al2O3 TFE and various CL layers

Fig. 1 shows a schematic of the TOLED structure. The TOLEDs containing N, N′‐diphenyl‐N, N′‐bis [1‐naphthyl‐(1, 1′‐biphenyl]‐4, 4′‐diamine (NPB), 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN), and tris (8‐hydroxyquinoline) aluminum (Alq3) CLs were fabricated with the following structure: glass as a substrate/ITO (70 nm) as an anode/HATCN as a hole injection layer (HIL) (10 nm)/NPB (40 nm) as a hole transport layer (HTL)/ 4,4′,4″-tris(Ncarbazolyl)-triphenylamine (TcTa) (10 nm) as an

RESULTS AND DISCUSSION

As shown in Fig. 1, a transparent conducting electrode with low reflection and absorption, a CL acting as refractive index-matching, and a TFE with high water vapor barrier properties are all critical individual components of TOLEDs. By using a low-temperature (95 °C) thermal ALD process for the encapsulation of the TOLEDs, this allows for detailed and systemic studies related to top electrodes, CLs, TFE, and their interfaces of electro-optically controlled TOLEDs.

Fig. 2(a)–(c) show the

CONCLUSION

Herein, TOLEDs with both Al2O3 TFE and various CL layers were fabricated. The top electrode (thin Ag), CLs (NPB, HATCN, and Alq3), TFE (Al2O3), and their interfaces were studied using various characterization techniques. The CLs exhibited different surface properties: The NPB and Alq3 formed hydrophobic surfaces with water contact angles of 97.65° and 64.62°, but the HATCN formed a hydrophilic surface with a water contact angle of 48.53°. However, the Al2O3 film density grown on the CLs with

Declaration of interest statement

None declared.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgement

This work was partly supported by Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korea government [20ZB1100, Development of Creative Technology for ICT] and an Institute for Information & communications Technology Promotion (IITP) grant funded by the Korea government (MSIT) [2018-0-00202, Development of core technologies for transparent flexible display integrated biometric recognition device], and an Institute for Information & Communications Technology

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