Flexible OLEDs: Fundamental and Novel Practical Technologies 9811935432, 9789811935435

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SpringerBriefs in Applied Sciences and Technology Display Science and Technology Mitsuhiro Koden

Flexible OLEDs Fundamental and Novel Practical Technologies

SpringerBriefs in Applied Sciences and Technology

Display Science and Technology Series Editors Karlheinz Blankenbach, FH für Gestaltung, Technik, Hochschule Pforzheim FH für Gestaltung, Technik, Pforzheim, Germany Fang-Chen Luo, Hsinchu Science Park, AU Optronics Hsinchu Science Park, Hsinchu, Taiwan Barry Blundell, St-Priest-la-Marche, France Robert Earl Patterson, Human Analyst Augmentation Branch, Air Force Research Laboratory, Wright-Patterson AFB, OH, USA Jin-Seong Park, Science and Engineering, Hanyang University, Division of Materials Science and Engineering, Seoul, Korea (Republic of)

This series presents readers with concise, readable books describing advances and the state-of-the-art in the displays field. Featuring compact volumes of 75-125 pages, it forms a companion to the Series in Display Science and Technology, covering the same range of topics, from the fundamentals of optics, color science and human factors, through display materials, electronics and driving, to advances in display technologies including LCDs, OLEDs, reflective displays, 3D displays, mobile displays and more. Related fields such as display metrology, human-computer interaction and energy are also covered.

Mitsuhiro Koden

Flexible OLEDs Fundamental and Novel Practical Technologies

Mitsuhiro Koden Innovation Center for Organic Electronics Yamagata University Yonezawa, Japan

ISSN 2191-530X ISSN 2191-5318 (electronic) SpringerBriefs in Applied Sciences and Technology ISSN 2511-1434 ISSN 2511-1442 (electronic) Display Science and Technology ISBN 978-981-19-3543-5 ISBN 978-981-19-3544-2 (eBook) https://doi.org/10.1007/978-981-19-3544-2 © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Since display is one of the important man-machine interfaces, display plays a significant role in the current IT society that creates our current life. While the most common display in the twentieth century was CRT, the last halfcentury has induced inventions and innovations of flat panel displays (FPDs). It is well known that one of FPDs making big business success is liquid crystal display (LCD), which was firstly commercialized by Sharp Corporation in 1973. The essential advantages of FPDs are thin thickness and lightweight. If we do not have FPDs, we cannot have current smartphones, notebook PCs, etc., which essentially support our current communications and lifestyles. Therefore, we can say “No FPDs, no life”. After the vigorous innovations of FPDs, in around 2010, CTRs almost disappears and LCDs have achieved a big market in the fields of TVs, PCs, mobile phones, etc. On the other hand, OLED (Organic Light Emitting Diode) displays have not caught such business success by 2017 in spite of their high potential. However, breakthroughs of OLED displays happened in 2017. The breakthroughs are penetrations into high-end TVs and smartphones. It should be noted that the development and commercialization of flexible OLED displays contributed to the penetration of OLEDs into smartphones. At present (in 2022), it is recognized that OLED is one of the major displays and that flexible OLED displays look to maintain an important position. The trend to “flexible” seems to be not only the results of technological development but also recommendations induced by current and futures IT society. Looking at the future of the IT society and current flexible device technologies, further novel technologies are necessarily required. This book reviews the fundamentals and recently developed technologies of flexible OLED display. The fundamental parts review mechanisms, materials, device structures, fabrication processes, driving methods, etc., of rigid and flexible OLEDs, accompanying the history of flexible OLED devices. The parts of developmental results review novel technologies on flexible substrates, gas barrier, encapsulation, electrodes, and on-demand patterning, all of which were developed by the collaborations between our research group (Research Group for Flexible Technologies)

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in Yamagata University (Japan) and various private companies. Therefore, these technologies are closely connected with actual businesses. I believe that this book can contribute to university students, scientists, engineers, and businesspersons to learn flexible OLED technologies and create new ideas and technologies. In the publication of this book, I would like to thank my colleagues and the collaborating companies for their contributions. In particular, I would like to thank my colleagues, who are Prof. H. Nakada, Mr. T. Furukawa, Dr. T. Yuki, Dr. H. Kobayashi, Mr. T. Moriya, Ms. M. Sugimoto, Mr. N. Kawamura, Mr. M. Abe, Ms. Y. Kosaka, Ms. C. Taguchi, Ms. M. Sakurai, Ms. R. Horie, and Ms. H. Suzuki of Innovation Center for Organic Electronics (INOEL) in Yamagata University (Japan). Yonezawa, Japan

Mitsuhiro Koden

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 Fundamentals of Flexible OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Fundamentals of OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Device Structures of Flexible OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Fabrication Processes of Flexible OLEDs . . . . . . . . . . . . . . . . . . . . . . 2.4 Required Properties for Flexible OLEDs . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 History of Flexible OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Early Research and Development of Flexible OLEDs . . . . . . . . . . . . 3.2 Development and Commercialization of Active-Matrix Flexible OLED Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Development and Commercialization of Flexible OLED Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Flexible Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Requirements for Flexible Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Candidates of Flexible Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Ultra-Thin Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Stainless Steel Foil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Barrier Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5 Gas Barrier Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Overview of Gas Barrier Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Evaluation Methods for Gas Barrier Properties . . . . . . . . . . . . . . . . . . 5.3 Dry Gas Barrier Layers Deposited by Roll-to-Roll (R2R) PE-CVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Multi-Layer Barrier Using Wet and Dry Layers . . . . . . . . . . . . . . . . .

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5.5 Multi-Layer Barrier Using Sputtering and ALD . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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6 Encapsulating Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Fundamentals of Encapsulating Technologies for Flexible OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Dam-Fill Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Thin Film Encapsulation (TFE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Laminating Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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7 Novel Electrode Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Transparent Conducting Polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Silver Nanowire (AgNW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Implanted Metal-Mesh Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Roll-to-Roll (R2R) Fabrication of Transparent Electrodes . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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8 OLEDs with On-Demand Patterns by Ink-Jet Printing . . . . . . . . . . . . . 8.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 On-Demand Patterns of Insulators Printed by Ink Jet . . . . . . . . . . . . 8.4 OLEDs Fabricated on Insulators Patterns by Ink-Jet Printing . . . . . . 8.5 OLEDs with On-Demand Patterns by Ink-Jet Printing . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1

Introduction

Abstract A flexible OLED (Organic Light Emitting Diode) is becoming one of the important key devices in the current IT society supporting our life. The business of flexible OLEDs keeps a significant portion of the current display business and is predicted to make a vigorous expansion. This chapter describes the overview of the current situation and the future’s potential of flexible OLEDs. Keywords Flexible · OLED · Display · Lighting Just 30 years after C. W. Tang and S. A. VanSlyke of Eastman Kodak Company (USA) published a paper describing a practical potential of OLED (Organic Light Emitting Diode) devices in 1987 [1], OLEDs look to turn into active business expansion in 2017. Indeed, in 2017, Apple Inc. (USA) commercialized smartphones with a flexible OLED display and plural Japanese manufacturers, which were Toshiba, Panasonic, and Sony, commercialized OLED TVs. Since 2017, the display trend is moving on the developments and businesses of OLEDs. Today, it can be said that OLED is exactly the main actor in displays. In the vigorous expansion of OLED businesses and R&Ds, flexible OLEDs play significant roles in display businesses and contribute to the IT society supporting our life. At present (in 2022), smartphones with a flexible OLED display are widely used worldwide. In addition, flexible OLED lighting and foldable smartphones with a flexible OLED display have also been commercialized. In conferences and exhibitions, various new prototypes of flexible OLEDs are being actively presented. Several commercialized products using flexible OLEDs are shown in Fig. 1.1. The number of shipments of flexible OLED displays for smartphones is reported to be about 170 million in 2019, sharing 85.4% of OLED displays for smartphones [2]. The market size of flexible OLED panels is reported to be 18.3 billion US dollars in 2019 and is predicted to be 32.7 billion US dollars in 2023 [2]. Such an expansion of flexible OLED business seems to be understood as a long-term generation change of displays and lighting as shown in Fig. 1.2. In the twentieth century, the shape of displays and lighting has been threedimensional. The typical display was CTR (Cathode Ray Tube) and the typical lighting were candescent light and fluorescent light. Since the end of the twentieth century, they have been replaced by flat panel displays (FPDs) such as liquid crystal © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Koden, Flexible OLEDs, Display Science and Technology, https://doi.org/10.1007/978-981-19-3544-2_1

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1 Introduction

Fig. 1.1 Several commercialized products using flexible OLED devices

Fig. 1.2 Generation changes in displays and lighting

display (LCD), plasma display (PDP), inorganic electroluminescent display (EL), etc., and flat lighting utilizing LEDs. Especially the appearance of FPDs gave a remarkable impact on our society and life. FPDs can realize new communication methods with mobile phones, new work styles with mobile PCs, enjoyable life with large-size TVs, etc. We can say “No FPD, no our life”. In such flat devices, a new current trend seems to be changed to organic. In mobile phone applications, LCDs are being rapidly replaced by mobile OLED displays. In TV applications, high-end models are also being rapidly replaced by large-size 4 K OLED TVs. In addition, OLED lighting has also been commercialized. The intrinsic advantages of OLEDs are high contrast ratio, wide viewing angle due to the self-emission, fast response speed, applicability to active-matrix displays

1 Introduction

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with a large number of pixels with high resolution, thin thickness with no backlight, lightweight, etc. With the extension of such generation changes, “flexible” is coming. Actually, flexible OLED displays are penetrating to markets with vigorous speed. The OLED business seems to expand with the change from rigid to flexible. In future, flexible OLED devices will be used in various application fields such as mobile communication, home entertainment, health care, wearable applications, automotive, robots, etc., which require flexible shapes intrinsically. The most important intrinsic advantage of “flexible” seems to be design flexibility, which allows a variety of final commercial products. Indeed, flexible OLEDs can realize such various shapes such as round, foldable, bendable, rollable, etc. In addition, “flexible” can realize such merits as thin thickness, lightweight, unbreakable features, etc. Moreover, “flexible” can be applied to roll-to-roll (R2R) processes that can induce a significant innovation in production. In flexible OLED devices, various key technologies are required in addition to basic rigid OLED devices fabricated on rigid substrates such as glass, etc. They are flexible substrates, gas barrier technologies, flexible encapsulating technologies, etc. In addition, several novel technologies and R2R technologies are quite useful for improvement and modifications of flexible OLED devices. This book offers a wealth of knowledge and information about the fundamental and practical aspects of flexible OLED devices. The book provides an overview of these devices by considering their merits and business potential, the history of their research and development, the fundamental technology, and the required properties for materials, devices, processes, and future trends of flexible OLED devices. The practical sections describe novel, cutting-edge expertise for flexible substrates, gas barriers, encapsulation, novel electrodes, and on-demand patterning for OLED devices. Many of the described novel technologies in this book were developed by academia-industry collaborations in “Research Group for Flexible Technologies in Yamagata University”, which were led by Prof. Hitoshi Nakada, Mr. Tadahiro Furukawa, Dr. Toshinao Yuki, and Prof. Mitsuhiro Koden. The know-how that is described here is applicable to flexible organic electronics devices which are not only for OLED but also OPV (Organic Photovoltaic), OTFT (Organic Thin Film Transistor), and other various devices such as sensors, wearable devices, robots, and healthcare devices. The information contained in this book is useful for all scientists, engineers, and managers who are interested in the field of flexible devices.

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References 1. C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51, 913–915 (1987) 2. UBI Research, Flexible & foldable OLED (2019)

1 Introduction

Chapter 2

Fundamentals of Flexible OLEDs

Abstract This chapter describes the fundamental technologies of flexible OLEDs. Sect. 2.1 describes the fundamentals of OLEDs, which include emission mechanism, device structures, encapsulations, and driving methods. Sects. 2.2 and 2.3 describe typical device structures and typical fabrication processes of flexible OLEDs, respectively. Sect. 2.4 describes required properties for flexible OLEDs. Keywords Emission mechanism · Flexible OLED · Encapsulation · Gas barrier · Coating-debonding · Bonding-debonding · Roll-to-roll

2.1 Fundamentals of OLEDs OLEDs are solid-type light-emitting devices with a simple device structure in which organic layers are sandwiched between two electrodes [1]. Figure 2.1 shows a typical device structure and the emission mechanism of OLEDs. On a substrate, a bottom electrode, plural organic layers, and a top electrode are fabricated in this sequence as shown in Fig. 2.1a. In most cases, the bottom electrode is anode and the top electrode is cathode. By applying a certain voltage to an OLED device, holes are injected from the anode and electrons are injected from the cathode. Holes and electrons are transported through organic layers and combined in an emitting layer, giving excited states of emitting materials. Emission occurs when the excited state is turned back to the ground state. Such mechanism is same as LED (Light-Emitting Diode) with inorganic materials. Therefore, OLED is called organic LED. The mechanism of OLED is illustrated by using an energy diagram as shown in Fig. 2.1b. In most OLED devices, the organic layers usually consist of plural layers which play respective roles. Such organic layers are called hole injection layer (HIL), hole transport layer (HTL), emitting layer (EM), hole blocking layer (HBL), electron transport layer (ETL), electron injection layer (EIL), etc., as shown in Fig. 2.2. The thicknesses of each organic layer are usually several to 100 nm. Therefore, the total thickness of organic layers is very thin, being tens or hundreds of nanometers. The organic layers of OLED devices are usually fabricated by vacuum depositions or wet

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Koden, Flexible OLEDs, Display Science and Technology, https://doi.org/10.1007/978-981-19-3544-2_2

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Fig. 2.1 Typical device structure and emission mechanism of OLEDs

Fig. 2.2 An example of practical device structures of OLEDs

processes of organic materials. The top electrode is usually deposited by vacuum deposition. Typical characteristics of OLED devices are shown in Fig. 2.3. The turn-on voltage, which is almost consistent with the lowest voltage giving emission, is about 2 V when materials and device architectures are optimized. The luminance over 10,000 cd/m2 is possible by applying voltage of several volts. In OLED materials, various emission mechanisms have already been known [1]. Typical emission mechanisms are summarized in Fig. 2.4. In OLEDs, a recombination of hole and electron gives two types of excited states. The one is singlet and the other is triplet. The ratio of singlet and triplet is 1:3, which is determined by spin-statistics.

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Fig. 2.3 Typical characteristics of OLED devices

Fig. 2.4 Typical emission mechanisms of OLEDs

In fluorescent materials [2], the singlet state can give an emission, but the triplet state cannot give any emission but can give only non-radiative decay. Therefore, the maximum theoretical internal quantum efficiency (IQE) of fluorescent materials is only 25%. In phosphorescent materials [3], both excited states can emit. While most phosphorescent materials are metal complexes containing a heavy metal such as Ir, Pt, Os, etc., the triplet excited state is changed from non-radiative to radiative by the spin-orbital coupling induced by the heavy metal effect. Therefore, the maximum theoretical internal quantum efficiency (IQE) of phosphorescent materials is 100%. In TADF (Thermally Activated Delayed Fluorescent) materials [4], the energy gap between the singlet excited state and the triplet excited state is very small (e.g. less

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than 0.1 eV). Therefore, electron exchange easily occurs between these excited states by the reverse intersystem crossing (RISC). The spin up-conversion from the nonradiative triplet state to the radiative singlet state occurs, giving delayed fluorescence. The maximum theoretical internal quantum efficiency (IQE) of TADF materials is 100%. OLED devices are fabricated by dry and/or wet processes. Typical fabrication processes for OLEDs are shown in Fig. 2.5. A typical dry process for OLEDs is vacuum evaporation, in which organic materials are evaporated by heating and deposited on substrates. While various wet processes are known and have been applied, a typical wet process for OLEDs seems to be ink-jet, in which a solution containing organic materials is jetted and dropped on a substrate. In OLEDs, various device structures are possible. Figure 2.6 shows three types of OLED device structures which are classified by the emitting direction. In bottomemitting OLED devices, a transparent bottom electrode, organic layers, and a reflective top electrode are stacked in that order. The emission can be observed through the

Fig. 2.5 Typical fabrication processes for OLEDs

Fig. 2.6 OLED device structures classified by emitting direction

2.1 Fundamentals of OLEDs

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Fig. 2.7 Typical RGB fabrication method in OLEDs

substrate. On the contrary, in top-emitting OLED devices, the stacking sequence on a substrate is a reflective electrode, organic layers, and a transparent top electrode in that order. The emission can be observed through the cathode. When both electrodes are transparent, OLED devices can emit in both directions as shown in Fig. 2.6c. This type of OLED is often called transparent OLED (TOLED). While OLEDs can emit various visible colors, the typical three methods for creating full-color displays are well known as shown in Fig. 2.7. The RGB side by side is the most basic method in a sense. In this method, a substrate has arranged RGB pixels of an OLED device. Each pixel emits red (R), green (G), or blue (B) light, respectively. The efficiency, lifetime, and color purity of emitted light directly reflect the OLED devices themselves. However, for fabricating RGB pixel arrangement, additional fabricating technologies, which are fine metal mask deposition, ink-jet, or other complicated technologies, are required. The other two technologies shown in Fig. 2.7 do not require RGB separation. One method is the combination of white OLED and color filter (CF). The other is blue OLED and phosphor. These methods have merit from the fabricating point of view. However, the color filter absorbs emitted light of a white OLED, reducing efficiency and lifetime. In the method of blue OLED with phosphor, efficiency and lifetime are dependent on the color conversion efficiency of phosphors. In addition, color purity is dependent on the characteristics of color filter or phosphor. As is well known, OLED devices are extremely sensitive to moisture. Indeed, bare OLED devices rapidly degrade by penetrations of moisture. Therefore, encapsulation is necessarily required. However, if the encapsulation is not enough, degradation can be easily observed by eyes as shown in Fig. 2.8, in which the OLED device has only

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Fig. 2.8 An example of degradation of OLED devices after storage in high temperature and high humidity conditions

an insufficient gas barrier. The degradation behaviors are called dark spot, dark area, black spot, etc. To protect such degradations, OLED devices necessarily require encapsulations. Figure 2.9 shows several typical encapsulating technologies. Figure 2.9a shows a metal can encapsulation, which was utilized in the world’s first OLED product of Pioneer Corporation (Japan) in 1997. In this method, an OLED device is encapsulated by a metal can and a sealant resin set in the surrounding of the device. Since moisture can penetrate through the sealant resin, desiccant should be set. The space is filled with N2 . Figure 2.9b shows an excavated glass encapsulation, which is often used in commercialized OLED devices. In this method, an OLED device is encapsulated by an excavated glass and a sealant resin set in the surrounding of the device. Since moisture can penetrate through the sealant resin, desiccant is set. The space is filled with N2 . Figure 2.9c is called frit glass encapsulation [5]. This technique was first utilized when Samsung (Korea) commercialized the world’s first mobile phone with an activematrix OLED display in 2007. In this method, a cover glass with flit glass is combined with a substrate with an OLED device by a laser irradiation. The space is filled with N2 . When the adhesion of the glass flit and both glass substrates is complete, gas penetration is protected. Figure 2.9d shows a stacking encapsulation with inorganic barrier layers, an adhesive resin layer, and an encapsulating counter substrate. The barrier property is mainly obtained by the inorganic barrier layers. Therefore, the inorganic barrier layers are required to possess sufficient barrier properties for practical applications.

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Fig. 2.9 Typical encapsulating structures of OLED devices

All of these techniques are at commercial level. However, it should be noted that these technologies can be applied to rigid OLED devices but cannot be applied to flexible OLED devices because of thick thickness and a lack of flexibility. In actual applications of OLEDs, an adequate driving method should be chosen from static-drive, passive-matrix (PM) drive, active-matrix (AM) drive, etc. These three drive methods are summarized in Fig. 2.10. In OLED lighting and OLED displays with small numbers of pixels, static-drive can be applied. In matrix-type OLED displays with medium numbers of pixels, passive-matrix drive can be used. On the other hand, full-color OLED displays with a large number of pixels, activematrix drive is commonly used. In this method, each pixel has active elements which are usually thin-film transistors (TFTs). Active-matrix OLED displays are used in various applications such as smartphones with high resolution, large-size TVs, etc. In active-matrix OLED displays, three types of TFTs are usually used. Low-temperature polysilicon (LTPS) TFTs are widely used in mainly mobile applications. The useful advantages of LTPS are high mobility (around 100cm2 /Vs) and excellent stability, which are able to drive OLED devices and make peripheral drivers. However, one of the serious disadvantages of LTPS is distributions of threshold voltage and mobility, both of which induce non-uniformity of display. As the counter technologies, compensating TFT circuits are commonly used, inducing an increase in the number of TFTs in each pixel. In addition, due to the restriction

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Fig. 2.10 Typical driving methods for OLEDs

of fabricating equipment, LPTS does not tend to be applied to large-size OLED displays. IGZO (In-Ga-Zn Oxide) TFTs are used in large-size and mobile OLED displays. The advantages of IGZO are reasonable mobility (10~30cm2 /Vs), good uniformity of threshold voltage and mobility, low leakage current, etc. Single-crystal Si TFTs are mainly applied to small displays which are used in view finder, AR (Augmented Reality), VR (Virtual Reality), etc.

2.2 Device Structures of Flexible OLEDs Figure 2.11 shows a typical device structure of flexible OLED. While basic structures of flexible OLEDs are same as rigid OLEDs shown in Fig. 2.9, substrates and encapsulations should be changed for realizing flexibility. As is shown in Fig. 2.11, gases can penetrate from all directions in flexible OLED devices. Therefore, such gas penetrations must be protected by using barrier and encapsulating technologies. The technologies of flexible substrates, gas barriers, and flexible encapsulations are described in more detail in Chaps. 4, 5, and 6, respectively. Figure 2.12 shows a typical device structure of flexible active-matrix OLED displays. On a flexible substrate, a gas barrier layer, driving lines, TFT (Thin Film Transistor) circuits, an OLED structure, encapsulation, etc., are fabricated.

2.3 Fabrication Processes of Flexible OLEDs

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Fig. 2.11 Typical device structure of flexible OLEDs

Fig. 2.12 Typical device structure of flexible active-matrix OLED displays

2.3 Fabrication Processes of Flexible OLEDs Fabrication processes of flexible OLED devices should be changed from rigid OLED devices. Several typical fabrication processes of flexible OLED devices are shown in Figs. 2.13, 2.14, 2.15, and 2.16. Figure 2.13 shows a fabrication method called “coating-debonding” method [6–8]. At present (in 2022), the “coating-debonding” method is the most common method used in mass productions of flexible active-matrix OLED displays. In this method, a solution of polymer, which is usually polyimide, is coated on a dummy glass substrate and then baked, giving a polymer film on the glass substrate. Polyimides

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Fig. 2.13 Schematic illustration of typical “coating-debonding” method for flexible OLEDs

Fig. 2.14 Schematic illustration of typical “bonding-debonding” method for flexible OLEDs

2.3 Fabrication Processes of Flexible OLEDs

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Fig. 2.15 Schematic illustration of typical “transfer” method for flexible OLEDs

Fig. 2.16 Two typical R2R fabrication methods of flexible OLEDs

have such several advantages as high thermal stability with high glass transition temperature (Tg) and excellent chemical resistance, which are necessarily required in fabrication processes of active-matrix arrays. On this substrate, bottom gas barrier layers, an OLED device which often includes active-matrix arrays, and encapsulating structures are fabricated. The dummy glass substrate works as a carrier substrate and prevents size change in the fabrication processes with high-temperature treatment. After fabricating devices, the dummy glass substrate is eliminated by laser lift-off (LLO) [9], mechanical delamination [10], etc. Figure 2.14 shows a fabrication method called “bonding-debonding” method [11]. In this method, a flexible film with or without a barrier layer is bonded to a dummy glass substrate. When the flexible film is without a barrier layer, a barrier layer is

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2 Fundamentals of Flexible OLEDs

fabricated on the flexible film after bonding to the dummy substrate. On the substrate, an OLED device which can include active-matrix arrays and encapsulating structures is fabricated. Finally, the dummy glass substrate is eliminated. This method is often used in fabrication processes of flexible OLED lighting. Semiconductor Energy Laboratory (SEL) reported another method called “transfer” method, applying the fabrication processes of various prototype flexible OLED displays [12]. Figure 2.15 shows the typical process of the “transfer” method. In this method, a separation layer, a passivation layer, and an OLED device, which often includes active-matrix arrays, are fabricated on a dummy grass substrate in this sequence. On the other hand, a separation layer and a passivation layer are fabricated on another dummy glass substrate in this sequence. A color filter layer is also fabricated if it is required. In the next step, both substrates are sandwiched and then two dummy glass substrates are eliminated. Finally, flexible films are attached using adhesive layers. In addition, roll-to-roll (R2R) methods can be applied to fabrication processes of flexible OLEDs. Figure 2.16 shows two typical R2R fabrication methods, while various modifications of processes are possible. These methods utilize roll-type flexible substrates, which are ultra-thin glasses, stainless steel foils, flexible films, etc. On roll-type flexible substrates, bottom electrodes are fabricated by R2R processes. In one case, OLED devices are fabricated in R2R processes, followed by cutting to individual flexible OLED devices [13–15]. In another case, roll-type substrates are cut to individual substrates after fabricating barrier layers and bottom electrodes [16, 17]. The cut substrates are bonded to dummy substrates, followed by fabrications of OLED devices.

2.4 Required Properties for Flexible OLEDs One of the most important properties of flexible OLEDs is gas barrier property. Although the clear requirement of gas barrier property has not been authorized, it is usually believed that flexible OLEDs require WVTR (Water Vapor Transmission Rate) values of the order of 10−5 g/m2 /day or lower [18]. WVTR is the most commonly used specification in the evaluations of gas barrier. Figure 2.17 shows required WVTR values in various applications. The required WVTR values for OLEDs are very severe, being much lower than other applications. Therefore, high gas barrier technologies are essentially required in flexible OLED devices. In addition, flexible OLEDs require several properties adding to normal rigid OLEDs. They are thin thickness, flexibility including bendability, rollability, foldability, etc., compatibility with R2R processes, mechanical stability, etc.

References

17

Fig. 2.17 Required WVTR values in various applications

References 1. M. Koden, OLED Displays and Lighting (Wiley, IEEE Press, Chichester, 2017) 2. C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51, 913–915 (1987) 3. M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 75, 4–6 (1999) 4. A. Endo, M. Ogasawara, T. Takahashi, D. Yokoyama, Y. Kato, C. Adachi, Adv. Mater. 21, 4802 (2009) 5. L. Zhang, S. Logunov, K. Becken, M. Donovan, B. Vaddi, in SID 10 Digest, P-197L (2010) 6. H. Yamaguchi, T. Ueda, K. Miura, N. Saito, S. Nakano, T. Sakano, K. Sugi, I. Amemiya, M. Hiramatsu, A. Ishida, in SID 2012 Digest, 74.2L (2012), pp. 1002–1005 7. H. Yamaguchi, T. Ueda, K. Miura, N. Saito, S. Nakano, T. Sakano, K. Sugi, I. Amemiya, in Proceedings of IDW/AD’12, AMD8/FLX7-1 (2012), pp. 851–854 8. K. Teramoto, E. Fukumoto, T. Fukuda, K. Shimokawa, T. Saito, T. Tanikawa, M. Suzuki, G. Izumi, M. Noda, S. Kumon, T. Arai, T. Kamei, M. Kodate, S. No, T. Sasaoka, K. Nomoto, in Proceedings of IDW/AD’12, AMD8/FLX7-2 (2012), pp. 855–858 9. S. Hong, C. Jeon, S. Song, J. Kim, J. Lee, D. Kim, S. Jeong, H. Nam, J. Lee, W. Yang, S. Park, Y. Tak, J. Ryu, C. Kim, B. Ahn, S. Yeo, in SID 2014 Digest, 25.4 (2014), pp. 334–337 10. Y.-L. Lin, T.-Y. Ke, C.-J. Liu, C.-S. Huang, P.-Y. Lin, C.-H. Tsai, C.-H. Tu, P.-F. Wang, H.H. Lu, M.-T. Lee, K.-L. Hwu, C.-S. Chuang, Y.-H. Lin, in SID 2014 Digest, 10.4 (2014), pp. 114–117 11. F. Li, E. Smits, L. van Leuken, G. de Haas, T. Ellis, J.-L. van der Steen, A. Tripathi, K. Myny, M. Ameys, S. Schols, P. Heremans, G. Gelinck, in SID 2014 Digest, 32.2 (2014), pp. 431–434 12. A. Chida, K. Hatano, T. Inoue, N. Senda, T. Sakuishi, H. Ikeda, S. Seo, Y. Hirakata, S. Yamazaki, S. Yasumoto, M. Sato, Y. Yasuda, S. Okazaki, W. Nakamura, S. Mitsui, in SID 2013 Digest, 18.2 (2013), pp. 196–198 13. J. Hast, M. Tuomikoski, R. Suhonen, K.-L. Väisänen, M. Välimäki, T. Maaninen, P. Apilo, A. Alastalo, A. Maaninen, in SID 2013 Digest, 18.1 (2013), pp. 192–195 14. T. Tsujimura, J. Fukawa, K. Endoh, Y. Suzuki, K. Hirabayashi, T. Mori, in SID 2014 Digest, 10.1 (2014), pp. 104–107 15. T. Minakata, M. Tanamura, Y. Mitamura, M. Imashiro, A. Horiguchi, A. Sugimoto, M. Yamashita, K. Ujiiye, S. Sunahiro, Y. Yada, N. Ibaraki, H. Tomiyasu, in SID 2015 Digest, 16.4 (2015), pp. 219–222

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16. T. Furukawa, K. Mitsugi, H. Itoh, D. Kobayashi, T. Suzuki, H. Kuroiwa, M. Sakakibara, K. Tanaka, N. Kawamura, M. Koden, in Proceedings of IDW’14, FLX3-4L (2014), pp. 1428–1429 17. T. Furukawa, M. Koden, IEICE Trans. Electron E100-C, 949–954 (2017) 18. P.E. Burrows, G.L. Graff, M.E. Gross, P.M. Martin, M.K. Shi, M. Hall, E. Mast, C. Bonham, W. Bennett, M.B. Sullivan, Displays 22, 65–69 (2001)

Chapter 3

History of Flexible OLEDs

Abstract A flexible OLED has already been reported in 1992, though the technological level was still primitive. Research and development on flexible OLEDs have been activated around 2010. In such development, various prototypes of flexible OLED displays and lighting were reported. The first products of flexible OLED display and lighting were commercialized in 2013, respectively. At present (in 2022), flexible OLEDs are mainly applied to smartphones and will increase their applications to various displays such as TV and PC, lighting, wearable devices, healthcare devices, robots, etc., in future. This chapter overviews such active developments and commercialization of flexible OLEDs. Keywords Flexible · OLED · Display · Lighting · Smartphone · TV · History

3.1 Early Research and Development of Flexible OLEDs In addition to research and development of OLEDs substantially started by the impact from a paper of Tang and VanSlyke of Eastman Kodak Company (USA) in 1987 [1], a flexible OLED was firstly reported by Gustafsson, Heeger et al. of Uniax Corporation (USA) in 1992 [2]. They fabricated a flexible OLED on PET (polyethylene terephthalate) substrate, using soluble organic materials. They reported that the fabricated flexible OLED device is mechanically robust and may be sharply bent without failure, being easily visible under room lighting, while the external quantum efficiency was about 1%. In 1997, a primitive active-drive flexible OLED device was reported by Wu, Forrest et al. of Princeton University (USA) [3]. In this device, a-Si-TFT and a top-emitting OLED device were fabricated on a stainless steel foil. In 2003, a 3-inch full-color passive-matrix flexible OLED display was reported with a strong impact by Pioneer Corporation (Japan), which has already commercialized the world’s first OLED product in 1997 [4, 5]. The prototype had 160 × 120 dots and showed a luminance of 70 cd/m2 . An emitting picture of the prototype is shown in Fig. 3.1. They reported that the thickness is 0.2 mm and the weight is only 3 g including ICs. The device structure with barrier technologies is shown in

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Koden, Flexible OLEDs, Display Science and Technology, https://doi.org/10.1007/978-981-19-3544-2_3

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Fig. 3.1 An emitting picture of a 3-inch full-color flexible OLED display reported by Pioneer Corporation in 2003 [4, 5] (Reprinted with permission from John Wiley and Sons, Ref. [4] Copyright 2003.)

Fig. 3.2 [6, 7]. They reported that SiON barrier layers were deposited by sputtering on flexible substrates and SiN encapsulating barrier layers were deposited on the OLED by PE-CVD (Plasma Enhanced Chemical Vapor Deposition).

Fig. 3.2 The fundamental device structure of a 3-inch full-color flexible OLED display reported by Pioneer Corporation in 2003 [6, 7]

3.2 Development and Commercialization of Active-Matrix …

21

3.2 Development and Commercialization of Active-Matrix Flexible OLED Displays In 2012, Sony (Japan) and Toshiba (Japan) reported impressive prototypes of flexible active-matrix full-color OLED displays, respectively. These prototypes were fabricated by “coating-debonding” method (see Fig. 2.13). The flexible active-matrix OLED display reported by Sony was a 9.9-inch qHD (960 × 540 dots) full-color display driven by IZGO (In-Ga-Zn-O) TFTs [8]. The device structure is top-emission type. An example of the display is shown in Fig. 3.3. The resolution is 111ppi and the thickness is 110 µm. The flexible active-matrix OLED display developed by Toshiba Corporation (Japan) and Toshiba Mobile Display Co., Ltd. (Japan) was an 11.7-inch qHD (960 × 540 dots) full-color display driven by IZGO (In-Ga-Zn-O) TFTs [9]. The device structure is bottom-emission type. An example of the display is shown in Fig. 3.4. The resolution is 94ppi. In 2013, the world’s first products of flexible OLED displays were commercialized by Samsung Display Co., Ltd. (Korea) and LG Display Co., Ltd. (Korea), respectively. These flexible OLED displays were curved displays, being applied to smartphones. Samsung’s one is a 5.7 full HD flexible OLED display with a curved shape [10]. LG’s one is a 5.98 flexible OLED display with a curved shape, a thickness of 0.44 mm, and a weight of 7.2 g [11]. The flexible OLED display of LG is shown in Fig. 3.5. Semiconductor Energy Laboratory Co., Ltd. (SEL) (Japan) has actively developed various types of flexible active-matrix full-color OLED displays. In 2014, SEL reported side-roll and top-roll flexible OLED displays shown in Fig. 3.6 by the collaboration with Nikia Corporation group [12]. The specifications of the panel are diagonal size of 5.2-inch, 960 × 1280 dots (quad VGA), 302ppi, etc. In 2014, SEL also demonstrated foldable and tri-fold flexible OLED displays shown in Fig. 3.7 by the collaboration with Nikia Corporation group [13, 14].

Fig. 3.3 9-inch flexible active-matrix full-color flexible OLED displays developed by Sony Corporation in 2012 [8] (Reprinted with permission from John Wiley and Sons, Ref. [8] Copyright 2012.)

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Fig. 3.4 11.7-inch flexible active-matrix full-color flexible OLED displays developed by Toshiba Corporation and Toshiba Mobile Display Co., Ltd. in 2012 [9] (Reprinted with permission from John Wiley and Sons, Ref. [9] Copyright 2012.)

In 2015, SEL demonstrated a 13.3-inch 8K4K foldable OLED display with 664ppi as shown in Fig. 3.8 [15]. In 2015, SEL also developed tiling displays, which they called Kawara-type multi-displays, using flexible OLED displays and seamless tiling technologies. They fabricated an 81-inch 8K4K OLED display with 36 unit panels as shown in Fig. 3.9. [16] In 2015, LG Display Co., Ltd. (Korea) developed a large-size 18-inch flexible OLED display shown in Fig. 3.10 [17]. The specifications of this display are 810 × 1200 dots (WXGA), luminance of 300 cd/m2 , top-emitting OLED structure, oxide TFT drive, etc. In 2018, LG Display Co., Ltd. (Korea) reported the world’s first 77-inch transparent flexible OLED display with ultra-high-definition (UHD) resolution, which can be rolled up to a radius of 80 mm with a transmittance of 40% [18]. Figure 3.11 shows pictures of this display. In 2019, LG Display Co., Ltd. (Korea) demonstrated a 65 rollable OLED TV shown in Fig. 3.12. In 2017, Samsung Display Co., Ltd. (Korea) reported a 9.1-inch stretchable AMOLED display based on LTPS technology [19]. Figure 3.13 shows the examples.

3.2 Development and Commercialization of Active-Matrix …

23

Fig. 3.5 One of the world’s first products of flexible OLED display commercialized in 2014 by LG Display Co., Ltd. [11] (Reprinted with permission from John Wiley and Sons, Ref. [11] Copyright 2014.)

Fig. 3.6 Side-roll and top-roll flexible OLED displays developed by Semiconductor Energy Laboratory Co., Ltd. (SEL) [12] (Reprinted with permission from John Wiley and Sons, Ref. [12] Copyright 2014.)

The world’s first foldable OLED display was commercialized by Royole (China) in 2018. This was a 7.8-inch outward folding display and was applied to their smartphone “FlexPai” [20] At present (in 2022), flexible active-matrix full-color OLED displays are widely applied to smartphones by various manufacturers. Figure 3.14 shows some examples of smartphones with a flexible OLED display. The world’s first inward foldable OLED display was commercialized by Samsung (Korea) in 2019. The display is a 7.3-inch QXGA and applied to their smartphone “Galaxy Fold” shown in Fig. 3.15.

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Fig. 3.7 Foldable and tri-fold flexible OLED displays developed by Semiconductor Energy Laboratory Co., Ltd. (SEL) [13, 14] (Reprinted with permission from John Wiley and Sons, Ref. [13, 14] Copyright 2014.)

In addition, various flexible OLED displays are actively developed. Some examples are shown in Fig. 3.16.

3.3 Development and Commercialization of Flexible OLED Lighting After white OLED devices were experimentally fabricated and reported [21, 22], OLED lighting has been one of the promising applications of OLEDs. While the

3.3 Development and Commercialization of Flexible OLED Lighting

25

Fig. 3.8 13.3-inch 8K4K foldable OLED display with 664ppi developed by Semiconductor Energy Laboratory Co., Ltd. (SEL) [15] (Reprinted with permission from John Wiley and Sons, Ref. [15] Copyright 2015.)

world’s first OLED lighting was substantially commercialized by Lumiotec (Japan) in 2011 [23], the world’s first flexible OLED lighting was commercialized in 2013 by LG Chem (Korea) [24]. This is a bendable thin-glass type panel with the size of 210 × 50 mm, the thickness of 0.33 mm, the power efficiency of 55 lm/W, and the lifetime of LT70 = 18,000 h. An example is shown in Fig. 3.17. In 2014, Konica Minolta Inc. (Japan) commercialized flexible OLED lighting, fabricated by the world’s first roll-to-roll (R2R) OLED manufacturing line using plastic barrier substrate [25]. The R2R OLED manufacturing line and their flexible OLED lighting are shown in Fig. 3.18 and Fig. 3.19, respectively. Recently, applications of red flexible OLED lighting to rear light of automotive cars attract attention. Figure 3.20 shows some demonstrating examples in Tokyo Motor Show in 2017. Another promising application of flexible OLED lighting seems to be design lighting for signs, packages, advertisements, souvenirs, etc. For example, in a special marketing campaign, Coca-Cola Singapore adopted illuminated bottles with flexible OLED lighting manufactured by Inuru (Germany)

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Fig. 3.9 Kawara-type 81-inch 8K4K OLED display with 36 flexible OLED panels developed by Semiconductor Energy Laboratory Co., Ltd. (SEL) [16] (Reprinted with permission from John Wiley and Sons, Ref. [16] Copyright 2015.)

Fig. 3.10 18-inch large-size flexible OLED display with a bending radius of 30 mm developed by LG Display Co., Ltd. [17] (Reprinted with permission from John Wiley and Sons, Ref. [17] Copyright 2015.)

3.3 Development and Commercialization of Flexible OLED Lighting

27

Fig. 3.11 77-inch transparent flexible OLED display with ultra-high-definition (UHD) resolution developed by LG Display Co., Ltd. [18] (Reprinted with permission from John Wiley and Sons, Ref. [18] Copyright 2018.) Fig. 3.12 65” rollable OLED TV developed by LG Display Co., Ltd

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Fig. 3.13 9.1-inch stretchable AMOLED display developed by Samsung Display Co., Ltd. [19] (Reprinted with permission from John Wiley and Sons, Ref. [18] Copyright 2017.)

Fig. 3.14 Smartphones with a flexible OLED display

3.3 Development and Commercialization of Flexible OLED Lighting

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Fig. 3.15 Samsung’s smartphone “Galaxy Fold” with an inward foldable OLED display

in 2019 [26]. When touching the labels, the OLED with the lightsaber design lights up. In 2020, as applications to souvenirs, Mr. Furukawa et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) applied flexible OLED lighting with a special design to a notepad, a bookmark, and an info, which is Japanese unique pillbox with the heraldry of Tokugawa shogun families. The examples are shown in Fig. 3.21. When touching the certain position, the flexible OLED lights up.

30

Fig. 3.16 Recent developments in flexible OLED displays Fig. 3.17 An OLED lighting with the world’s first bendable OLED lighting fabricated by LG Chem

3 History of Flexible OLEDs

3.3 Development and Commercialization of Flexible OLED Lighting

31

Fig. 3.18 World’s first roll-to-roll OLED manufacturing line of Konica Minolta Inc. [25] (Reprinted with permission from John Wiley and Sons, Ref. [25] Copyright 2014.)

Fig. 3.19 Flexible OLED lighting of Konica Minolta Inc. (Reprinted in part with permission from John Wiley and Sons, Ref. [25] Copyright 2014.)

Fig. 3.20 Some rear light demonstrated in Tokyo Motor Show in 2017

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Fig. 3.21 Applications of flexible OLED lighting to souvenirs developed by T. Furukawa et al. of Yamagata University

References 1. C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51, 913–915 (1987) 2. G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357, 477–479 (1992) 3. C.C. Wu, S.D. Theiss, G. Gu, M.H. Lu, J.C. Sturm, S. Wagner, S.R. Forest, in SID 97 Digest, 7.2 (1997), pp. 67–70 4. A. Yoshida, S. Fujimura, T. Miyake, T. Yoshizawa, H. Ochi, A. Sugimoto, H. Kubota, T. Miyadera, S. Ishizuka, M. Tsuchida, Hitoshi Nakada, in SID 03 Digest, 21.1 (2003), pp. 856– 859 5. T. Miyake, A. Yoshida, T. Yoshizawa, A. Sugimoto, H. Kubota, T. Miyadera, M. Tsuchida, H. Nakada, in Proceedings of IDW’03, OEL2-2 (2003), pp. 1289–1292 6. A. Sugimoto, A. Yoshida, T. Miyadera, Technical Report of Pioneer R&D 11(3), 48–56 7. T. Nagashima, H. Yamada, M. Hanaoka, T. Ichikawa, T. Ishida, K. Oda, Pioneer R&D 13(3), 65–73 8. M. Noda, K. Teramoto, E. Fukumoto, T. Fukuda, K. Shimokawa, T. Saito, T. Tanikawa, M. Suzuki, G. Izumi, S. Kumon, T. Arai, T. Kamei, M. Kodate, S. No, T. Sasaoka, K. Nomoto, in SID 2012 Digest, 74.1L (2012), pp. 998–1001 9. H. Yamaguchi, T. Ueda, K. Miura, N. Saito, S. Nakano, T. Sakano, K. Sugi, I. Amemiya, M. Hiramatsu, A. Ishida, in SID 2012 Digest, 74.2L (2012), pp. 1002–1005 10. News release of Samsung Electronics, https://www.samsungmobilepress.com/pressreleases/ samsung-galaxy-round-pioneers-the-curved-display-smartphone-experience. Accessed 9 Oct 2013 11. S. Hong, C. Jeon, S. Song, J. Kim, J. Lee, D. Kim, S. Jeong, H. Nam, J. Lee, W. Yang, S. Park, Y. Tak, J. Ryu, C. Kim, B. Ahn, S. Yeo, in SID 2014 Digest, 25.4 (2014), pp. 334–337 12. R. Kataish, T. Sasaki, K. Toyotaka, H. Miyake, Y. Yanagisawa, H. Ikeda, H. Nakashima, N. Ohsawa, S. Eguchi, S. Seo, Y. Hirakata, S. Yamazaki, C. Bower, D. Cotton, A. Matthews, P. Andrew, C. Gheorghiu, J. Bergquist, in SID 2014 Digest, 15.3 (2014), pp. 187–190 13. R. Komatsu, R. Nakazato, T. Sasaki, A. Suzuki, N. Senda, T. Kawata, H. Ikeda, S. Eguchi, Y. Hirakata, S. Yamazaki, T. Shiraishi, S. Yasumoto, C. Bower, D. Cotton, A. Matthews, P. Andrew, C. Gheorghiu, J. Bergquist, in SID 2014 Digest, 25.2 (2014), pp. 326–329

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14. Y. Jimbo, T. Aoyama, N. Ohno, S. Eguchi, S. Kawashima, H. Ikeda, Y. Hirakata, S. Yamazaki, M. Nakada, M. Sato, S. Yasumoto, C. Bower, D. Cotton, A. Matthews, P. Andrew, C. Gheorghiu, J. Bergquist„ in SID 2014 Digest, 25.1 (2014), pp.322–325 15. K. Takahashi, T. Sato, R. Yamamoto, H. Shishido, T. Isa, S. Eguchi, H. Miyake, Y. Hirakata, S. Yamazaki, R. Sato, H. Matsumoto, N. Yazaki, in SID 2015 Digest, 18.4 (2015), pp. 250–253 16. D. Nakamura, H. Ikeda, N. Sugisawa, Y. Yanagisawa, S. Eguchi, S. Kawashima, M. Shiokawa, H. Miyake, Y. Hirakata, S. Yamazaki, S. Idojiri, A. Ishii, M. Yokoyama, in SID 2015 Digest, 70.2 (2015), pp. 1031–1034 17. J. Yoon, H. Kwon, M. Lee, Y.-Y. Yu, N. Cheong, S. Min, J. Choi, H. Im, K. Lee, J. Jo, H. Kim, H. Choi, Y. Lee, C. Yoo, S. Kuk, M. Cho, S. Kwon, W. Park, S. Yoon, I. Kang, S. Yeo, in SID 2015 Digest, 65.1 (2015), pp. 962–965 18. C.I. Park, M. Seong, M.A. Kim, D. Kim, H. Jung, M. Cho, S.H. Lee, H. Lee, S. Min, J. Kim, M. Kim, J.-H. Park, S. Kwon, B. Kim, S. J. Kim, W. Park, J.-Y. Yang, S. Yoon, I. Kang, in SID 2018 Digest, 54.1 (2018), pp. 710–713 19. J.-H. Hong, J. Mi. Shin, G.M. Kim, H. Joo, G.S. Park, I.B. Hwang, M.W. Kim, W.-S. Park, H. Y. Chu, S. Kim, in SID 2017 Digest, 5.5 (2017), pp. 47–50 20. Press release of Royole. https://global.royole.com/us/Dynamics?id=578. Accessed 1 Nov 2018 21. T. Ogura, T. Yamashita, M. Yoshida, K. Emoto, S. Nakajima, Japanese Patent 2661804 (disclosed in 1992) 22. J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Appl. Phys. Lett. 64(7), 815–817 (1994) 23. Lumiotec, News Release, 27 July 2011 24. LG Chem, News Release, 30 September 2013 25. T. Tsujimura, J. Fukawa, K. Endoh, Y. Suzuki, K. Hirabayashi, T. Mori, in SID 2014 Digest, 10.1 (2014), pp. 104–107 26. Inuru, News Release, 6 December 2019

Chapter 4

Flexible Substrates

Abstract Flexible substrate is one of the important key components for flexible OLED devise. Sect. 4.1 describes requirements for flexible substrates in flexible OLED devices because flexible substrates need to possess not only flexibility but also various features such as gas barrier property, temperature stability, chemical stability, size stability, surface smoothness, etc. Later, Sects. 4.2, 4.3, 4.4, and 4.5 review three actual flexible substrates for flexible OLED devices, describing ultrathin glass, stainless steel foil, and barrier film, accompanied by novel research and development on these technologies in our research group (Research Group for Flexible Technologies) of Yamagata University (Japan). These sections describe not only features about these flexible substrates but also applications to flexible OLED devices. Keywords Flexible substrate · Ultra-thin glass · Stainless steel foil · Barrier film · Gas barrier

4.1 Requirements for Flexible Substrates Flexible substrates for flexible OLED devices are required to possess various properties. First, gas barrier is necessarily required for keeping the reliability of OLED devices, because OLED devices are very sensitive to moisture and oxygen. Gas barrier ability is often represented by water vapor transmission rate (WVTR). In general, the required WVTR for OLED devices is said to be lower than the order of 10−5 g/m2 /day (see Fig. 2.17). In addition, flexible OLED devices are required to achieve thin thickness, mechanical flexibility, etc. Moreover, for compatibility with device fabrications, temperature resistance, chemical resistance, surface smoothness, size precision, handling properties, etc., are also required.

4.2 Candidates of Flexible Substrates Candidates of flexible substrates for flexible OLED devices are ultra-thin glasses, stainless steel foils, flexible films (barrier films), etc. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Koden, Flexible OLEDs, Display Science and Technology, https://doi.org/10.1007/978-981-19-3544-2_4

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Table 4.1 Comparisons of flexible substrates for flexible OLED devices

Ultra-thin glass

Stainless steel Barrier film foil

Specific gravity

2.4–2.6

7.7–7.9

0.9–2.2

Temperature resistance

Excellent >600 °C

Excellent >1,400 °C

Low PET: ~110 °C PEN: ~180 °C PI: ~350 °C

Size stability

Excellent CTE (10–6 /K): ~9

Good CTE (10–6 /K): 10–18

Not good CTE (10–6 /K): >30

Surface smoothness

Excellent

Poor

Poor

Handling issue

Breakable

Conductivity

Not rigid

Gas barrier

Excellent

Excellent

Poor

CTE: Coefficient of Thermal Expansion

Table 4.1 shows comparisons of these flexible substrates. At present (in 2022), major flexible substrates for flexible OLED devices are polyimide films, which are utilized as shown in Fig. 2.13. However, ultra-thin glasses, stainless steel foils, and other flexible films also have a positive potential for actual uses because they possess several advantages that cover the disadvantages of polyimide films. In the following Sects. 4.3, 4.4, and 4.5, these flexible substrates are reviewed with the applications to flexible OLED devices.

4.3 Ultra-Thin Glass In a sense, glass is the most suitable substrate for flat panel displays such as LCDs, OLEDs, etc., and for OLED lighting, because glass has such superior advantages as surface smoothness, chemical stability, temperature stability, size stability, etc. Especially surface smoothness of glass is excellent, being much better than other substrates. Since organic layers of OLED devices are very thin (around 100 nm), surface roughness is essentially undesirable, causing electrical shorts between anode and cathode. While thick glass substates with the thickness of 0.5, 0.7 mm, etc., are rigid, glass substrates change to flexible with reducing thickness. Such flexible glass is commonly called ultra-thin glass. The relationship between the thickness of glass substrate and flexibility is shown in Fig. 4.1 [1]. It is said that the average breaking stress of glass is around 100–200 MPa. However, the safety limit of the breaking

4.3 Ultra-Thin Glass

37

Fig. 4.1 The relationship between the thickness of glass substrate and the flexibility [1]. (provided by Nippon Electric Glass Co., Ltd.)

stress is assumed to be about 50 MPa, since the breaking stress is reduced by the edge condition. Therefore, for example, the acceptable bending radius of glass substrates with a thickness of 50 µm is estimated to be about 40 mm. Ultra-thin glasses can be produced as roll-type substrates. For example, Nippon Electric Glass Co., Ltd. (Japan) developed and commercialized ultra-thin glass rolls, which were fabricated for the overflow down draw process as shown in Fig. 4.2 [1]. Figure 4.3 shows an example of ultra-thin glass with a thickness of 50 µm and a width of 800 mm. They reported that the ultra-thin glass with a length of over 100 m was successfully rolled. The minimum radius of bending was 40 mm. The surface roughness (Ra) of unpolished ultra-thin glass is around 0.1–0.2 nm, which is excellent for fabricating OLED devices. It was also reported that the WVTR (Water Fig. 4.2 Overflow down draw process [1]. (provided by Nippon Electric Glass Co., Ltd.)

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Fig. 4.3 Examples of ultra-thin glasses [1]. (provided by Nippon Electric Glass Co., Ltd.)

Vapor Transmission Rate) of ultra-thin glass with a thickness of 30 µm was lower than 7 × 10−7 g/m2 /day (85 °C, 85% RH), which was the detection limit of their evaluation experiment using the API-MS (Atmospheric Pressure Ionization Mass Spectrometer) method. As commercial products, in 2013, LG Chem (Korea) commercialized the world’s first flexible OLED lighting with ultra-thin glass. The panel specifications released in 2013 were the size of 210 × 50 mm, the thickness of 330 µm, 55 lm/W, etc. [2]. Some examples are shown in Fig. 4.4. Kuo et al. of Chunghwa Picture Tubes (CPT) (Taiwan) developed a 6-inch flexible AMOLED display with 480 × 640 dots on 0.1-mm ultra-thin glass in 2013 [3]. After laminating a G4 size ultra-thin glass on a carrier glass with a thickness of 0.5 mm, a-Si-TFT is fabricated, followed by depositions of OLED layers. Next, a passivating inorganic layer such as SiOx and SiNx is deposited by reactive plasma deposition (RPD) and then a barrier film is combined with sheet-type adhesive as an encapsulation layer. Finally, the carrier glass is separated by a mechanical de-bonding method and is cut to individual devices by a laser cutting. The module thickness of the AMOLED display is about 0.25 mm. Matsuyama et al. of Asahi Glass Co., Ltd. (Japan) reported carrier laminated ultrathin glass [4]. In this method, ultra-thin glass is laminated on carrier glass substrate with media layer. They reported that ultra-thin glass can be easily separated at the

Fig. 4.4 Flexible OLED lighting of LG Chem

4.3 Ultra-Thin Glass

39

Fig. 4.5 Carrier laminated ultra-thin glass technology developed by Asahi Glass Co., Ltd. [4]. (provided by AGC Inc.)

boundary between the ultra-thin glass and the media layer. Therefore, this technology can be applied to device fabrications on ultra-thin glasses as shown in Fig. 4.5. Ikari and Tamagaki of Kobe Steel, Ltd. (Japan) developed a roll-to-roll (R2R) sputtering deposition of ITO on ultra-thin glass by using an R2R vacuum deposition equipment as shown in Fig. 4.6 [5, 6]. Using an ultra-thin-glass roll with a thickness of 50 µm, a width of 300 mm, and a length of 10 m, they successfully deposited ITO layers with a thickness of 190 nm and a sheet resistance of 7.5 /sq. Junghaehnel et al. of Fraunhofer Institute of Organic Electronics (Germany) have developed R2R processes for flexible OLED devices on ultra-thin glass by using R2R vacuum deposition equipment [7–9]. An example of R2R-processed OLEDs on flexible ultra-thin glass un-winded from a coil is shown in Fig. 4.7. One of the serious issues of ultra-thin glass is mechanical fragility. In particular, glass cutting induces microcracks at the cutting edge, reducing the mechanical strength of ultra-thin glass. While such conventional cutting methods as the “scribe and break” method and the “bending stress” method make edge defects such as chips [10]. Such mechanical fragility seriously increases the handling difficulty in manufacturing processes and reduces the reliability of products. Therefore, applications of ultra-thin glass to flexible OLED devices are closely related to counter technologies on mechanical fragility. Inayama and Fujii of Nippon Electric Glass Co., Ltd. (Japan) reported that CO2 laser fusing cutting gave a smooth fire-polished surface, fusing glass edge and forming rounded edge shapes [10]. The image of CO2 laser-cutting equipment is shown in Fig. 4.8.

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Fig. 4.6 R2R vacuum deposition equipment of Kobe Steel, Ltd. [5, 6]. The equipment is set in the Innovation Center of Organic Electronics (INOEL) of Yamagata University

Fig. 4.7 Roll-to-roll processed OLEDs on flexible ultra-thin glass un-winded from a coil [8]. (provided by Fraunhofer Institute of Organic Electronics)

Li et al. of ITRI (Taiwan) reported that edge cracks were removed by laser peeling [11]. They reported that the minimum bending radius after laser peeling of ultra-thin glass with the thickness of 100 µm is less than 1 cm, though the minimum bending radius before laser peeling is larger than 3.5 cm, suggesting that the bending strength is enhanced by about 100–500 MPa. He et al. of SCHOTT Glass (China) and SCHOTT AG (Germany) developed ultrathin glass with tight dimensional tolerance and high strength by using ion-exchange technologies. They reported to achieve a safe bending radius as low as 3 mm for 70 µm thick glass [12]. It is known that their ultra-thin glass is applied to Galaxy

4.3 Ultra-Thin Glass

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Fig. 4.8 Image of CO2 laser cutting equipment [10]. (provided by Nippon Electric Glass Co., Ltd.)

Z Flip and foldable Galaxy Z Fold 2, both of which are Samsung’s smartphones [13, 14]. Koden et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) have also developed flexible OLED devices using ultra-thin glass, in the collaboration with Nippon Electric Glass Co., Ltd. (Japan), NEC Lighting (Japan) (currently HotaluX, Ltd.), etc. [15, 16]. The prototype samples and the device structure are shown in Fig. 4.9. The white emitting OLED device is fabricated on an ultra-thin glass with a thickness of 50 µm, followed by a dam-fill encapsulation with stainless steel foil with a thickness of 50 µm. The device size and emitting area are 92 × 92 mm and 75 × 75 mm. The ultra-thin glass and the encapsulating stainless steel foil are supplied from Nippon Electric Glass and Nippon Steel and Sumitomo Metal Corporation Group (currently Nippon Steel Corporation

Fig. 4.9 Flexible OLED lighting prototypes on ultra-thin glass fabricated by NEC Lighting

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Fig. 4.10 Flexible OLED devices on an ultra-thin glass with a protecting film developed by T. Yuki et al. of Yamagata University

Group), respectively. The flexible OLED devices are thin, lightweight, and bendable as shown in Fig. 4.9. Yuki et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) developed flexible OLED devices on an ultra-thin glass with a protecting film. Figure 4.10 shows some prototype samples. In these cases, a PEN substrate is used as a protecting film for preventing the break of glass. The PEN substrate is laminated on an ultra-thin glass by using adhesive resin. By applying a protecting film, the mechanical stability is drastically improved and the trouble in fabrication processes of OLED devices is remarkably reduced. Furukawa et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) developed, novel R2R processes and novel cutting technologies using ultra-thin glass [17–23]. In addition, they developed cutting technologies for flexible OLED devices on ultra-thin glass [23, 24] and protecting technologies with an unique gel material for improving the mechanical strength of flexible OLED devices on ultra-thin glass [25]. Several examples of developed flexible OLED devices on ultra-thin glass are shown in Fig. 4.11. The details of technologies are described in the Sect. 7.5.

4.4 Stainless Steel Foil Stainless steel foil has attracted attention as flexible substrates for flexible devices. The advantages of stainless steel foil are excellent gas barrier ability, thermal and chemical stabilities, size stability, mechanical stability, etc. On the other hand, the issues of stainless steel foil are surface roughness and intrinsic conductivity.

4.4 Stainless Steel Foil

43

Fig. 4.11 Flexible OLED devices on ultra-thin glass developed by the collaboration of Furukawa et at. of Yamagata University and Nippon Electric Glass, etc.

In 1997, Wu et al. of Princeton University (USA) reported an OLED device with aSi-TFT on stainless steel foil [26]. They reported higher luminance than 10,000 cd/m2 and the durability of the devices. In 2003, Xie et al. of the City University of Hong Kong reported a flexible topemitting OLED device on stainless steel foil with a 1-µm-thick coated spin-on-glass (SOG) planarization/insulating layer [27]. In 2006, Troccoli et al. of Lehigh University (USA) reported poly-Si-TFT (Thin Film Transistor), which can be applied to OLEDs, etc. [28]. In 2006, Jin et al. of Samsung SDI reported a 5.6-inch flexible full-color activematrix OLED display with low-temperature poly-Si (LTPS)-TFT and top-emitting OLED structure on stainless steel foil as shown in Fig. 4.12a [29]. The thickness of stainless steel foil is 140 µm. For planarizing the surface, a super mirror technique was applied. In 2006, Chwang et al. of the Universal Display Corporation (UDC) (USA) also reported a 4-inch full-color 100-dpi active-matrix OLED display with LTPS-TFT on flexible stainless steel foil, collaborating with Palo Alto Research Center (USA),

Fig. 4.12 Examples of flexible active-matrix OLED display on stainless steel foil. (Reprinted with permission from John Wiley and Sons, Ref. [29, 30] Copyright 2006.)

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Vitex Systems (USA), and L3 Communications (USA) [30]. The thickness of stainless steel foil is 100 µm. Since the surface roughness is in order of 100 nm rms, a polymer material with a temperature resistance of about 350 °C is coated for insulating and planarization. The emitting picture is shown in Fig. 4.12b. While the potential of stainless steel foil has been reported by the papers mentioned above, Yamada et al. of Nippon Steel Corporation Group (Japan) have developed practical R2R processes for stainless steel foil [31, 32]. For planarizing the surface of stainless steel foil, they developed planarization technologies of stainless steel foil by coating films of organically modified silicate materials by the sol-gel process. The coating solution for the planarization layer was synthesized from organoalkoxysilane and alkoxysilane by using a sol-gel method. The thermal desorption spectroscopy (TDS) of the planarized stainless steel foil showed that outgas deriving from the organic component began above 400 °C. It is reported that the coating films show excellent insulating resistance as high as 1 × 109  cm2 and good resistance to repeated bending. Figure 4.13 shows AFM images of stainless steel foils with or without the planarization layer, which was coated by spin-coating. The surface smoothness is remarkably improved by the planarization layer. In addition, they developed R2R fabrication technologies of stainless steel foil with a planarization layer, which also plays a role of insulation [32]. Figure 4.14 shows the R2R coating process of the planarization layer and the resultant R2R planarized stainless steel foil [32]. The planarization layer was coated by a kind of gravure coater. The thickness and width of the stainless steel foil are 50 µm and 300 mm, respectively. Koden et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) have reported flexible OLED devices using stainless steel foils in the collaboration with Nippon Steel Corporation Group (Japan) [15, 16]. The device structure is a top-emitting type because stainless steel foils are not

Fig. 4.13 AFM images of stainless steel foil with or without the planarization layer [32]. (provided by Nippon Steel Corporation Group)

4.4 Stainless Steel Foil

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Fig. 4.14 R2R coating process of the planarization layer and the resultant R2R planarized stainless steel foil [32]. (provided by Nippon Steel Corporation Group)

transparent. Figure 4.15 shows the OLED device structure fabricated on the stainless steel foil and the I-V characteristics of OLED device. The stainless steel foil has a coating insulating layer with a thickness of 3 µm. The anode is Al. It may be believed that Al is not suitable as anode because the work function of Al is not so high (about 4.2). On the other hand, Matsushima et al. of the Japan Advanced Institute of Science and Technology (Japan) have reported that MoO3 can generate ohmic contact of anode/HTL interface [33]. Although they did not report on Al anode, their report implies that a hole can be injected from Al anode when MoO3 is applied. The semi-transparent cathode is a stacking electrode of Ag and Al. The thicknesses of Ag and Al are 20 nm and 1.5 nm, respectively, according to the previous report about the effect of the thickness of Ag and Al on the OLED characteristics by Okamoto et al. of Sharp Corporation (Japan) [34]. In the I-V characteristic shown in Fig. 4.15, it should be noted that the current under the turn-on voltage (about 2 V) is low enough. It clearly indicates that the planarization layer on stainless steel foil effectively plays a role in the prevention of the leakage between anode and cathode.

Fig. 4.15 The device structure and the I-V characteristics of a flexible OLED on stainless steel foil supplied by Nippon Steel Corporation Group [15]

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Fig. 4.16 Flexible OLED devices with the stainless steel foil supplied from Nippon Steel Corporation Group [16, 35]

Using the stainless steel foils with the planarization layer, several prototype OLED devices were fabricated as shown in Fig. 4.16 [15, 16, 35]. Furukawa et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) have developed R2R fabrication processes of bottom electrodes on stainless steel foil with the planarization layer in the collaboration with Nippon Steel Corporation Group (Japan) [16, 36, 37]. The developed process technologies are shown in Fig. 4.17. In these processes, after wet cleaning of stainless steel foil by an R2R wet-cleaning equipment, Ag alloy and ITO are deposited by continuous sputtering deposition using an R2R vacuum deposition equipment, followed by a second R2R wet-cleaning. The thickness of Ag alloy and ITO are 100 nm and 15 nm, respectively. In the next step, for patterning Ag alloy and ITO, the etching paste is printed using R2R screen printing equipment. By heating to 170 °C, Ag alloy and ITO under the printed etching paste are etched without no form change. The etched Ag alloy, the etched ITO, and the etching paste are washed off by third R2R wet-cleaning, giving patterned Ag alloy and ITO. In the next step, an insulator is printed by R2R screen printing equipment, followed by a fourth R2R wet-cleaning. After cutting the substrates, flexible OLED devices are fabricated. The device structure and some demonstration samples of the developed flexible OLED devices on the stainless steel foils are shown in Fig. 4.18.

4.4 Stainless Steel Foil

47

Fig. 4.17 Flexible OLED fabrication on stainless steel foil with patterned electrodes fabricated by R2R processes

Fig. 4.18 Device structure and a demonstration sample of the developed flexible OLED devices on the stainless steel foils with patterned electrodes fabricated by R2R processes

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4.5 Barrier Film Barrier films are the most common flexible substrates in flexible OLEDs. Indeed, at present (in 2022), all or most of the commercial products of flexible OLED displays use barrier films (polyimide films with a barrier layer) by the coating-debonding method shown in Fig. 2.13. Barrier films are flexible plastic films with a gas barrier layer. Plastic film is a thin polymeric material. The candidates of plastic films for flexible OLED devices are shown in Fig. 4.19. The gas barrier ability of plastic films is essentially poor. The WVTR (Water Vapor Transmission Rate) values of most plastic films are higher than 10 g/m2 /day, which is quite unsuitable for OLED devices. Therefore, for applying plastic films to OLED devices, the gas barrier layer is necessarily required as shown in Fig. 4.20a. However, it is almost impossible to fabricate perfect gas barrier layers because of the presence of defects, pinholes, etc., as shown in Fig. 4.20b. Practical technologies for reducing such defects and pinholes and/or reducing the influence of such defects and pinholes are essentially required. The details of gas barrier technologies are described in Chap. 5. Barrier films are applied to flexible OLED devices that are fabricated by bondingdebonding method and/or R2R methods, which are shown in Figs. 2.14 and 2.16, respectively. For example, Li et al. of Holst Centre (Netherlands) and IMEC (Belgium) developed flexible active-matrix OLED displays on PEN substrates, using bondingdebonding method [38]. The barrier layers consist of inorganic-organic-inorganic

Fig. 4.19 Candidates for plastic films for flexible OLED devices

4.5 Barrier Film

49

Fig. 4.20 Gas barrier film

Fig. 4.21 Examples of rollable top-emitting AMOLED displays fabricated by Holst Centre and IMEC [38]. The flexible substrate is 25 µm PEN film. (Reprinted with permission from John Wiley and Sons, Ref. [38] Copyright 2006.)

multilayer stack. The inorganic layers are SiN layers deposited by plasma-enhanced chemical vapor deposition (PE-CVD). The fabricated samples are shown in Fig. 4.21.

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Our research group (Research Group for Flexible Technologies) at Yamagata University (Japan) has developed various types of barrier films, which are applied to flexible OLED devices [35, 39–42]. The details of technologies are described in Sects. 5.3, 5.4, and 5.5.

References 1. T. Murata, T. Yanase, S. Miwa, H. Yamazaki, Proc. IDW’11, FMC-8 (2011), pp. 1735–1737 2. LG Chem, News release (2013, Sept 30) 3. C.C. Kuo, J.-Y. Chiou, S.-F. Liu, C.-H. Chiu, C.-H. Lin, Y.-C. Sun, M.-C. Chen, Y.-W. Chiu, Proc. IDW’13, OLED3-3 (2013), pp. 886–888 4. Y. Matsuyama, K. Ebata, D. Uchida, T. Higuchi, S. Kondo, Proc. IDW’13, FLX2-1 (2013), pp. 1518–1521 5. Y. Ikari, H. Tamagaki, Proc. IDW 2012, FLX5/FMC5-1 (2012), pp. 1493–1496 6. H. Tamagaki, Y. Ikari, N. Ohba, Surf. Coat. Technol. 241, 138–141 (2014) 7. M. Junghaehnel, S. Weller, T. Gebel, W. Skorupa, T. Schumann, Proc. IDW’15, FLX4-3 (2015), pp. 1376–1379 8. M. Junghaehnel, T. Preussner, J. Westphalen, S. Mogck, Proc. IDW’16, FMC5/FLX2-2 (2016), pp. 508–510 9. M. Junghaehnel, M. Fahland, T. Preussner, Proc. IDW’18, FLX4-3 (2018), pp. 1517–1519 10. N. Inayama, T. Fujii, Proc. IDW’13, FLX4-4 (2013), pp. 1552–1554 11. C.-H. Li, C.-J. Huang, K.-T. Chen, M.-C. Lin, Proc. IDW’14, FLXp1-12 (2012), pp. 1502–1505 12. F. He, M. Jotz, H. Wegener, M. Hahn, N. Da, J. Zimmer, SID 2017 Digest, 17.3 (2017), pp. 218–221 13. Press Release of SCHOTT (2020, May 13) 14. Wikipedia; Samsung Galaxy Z Fold 2 15. M. Koden, H. Kobayashi, T. Moriya, N. Kawamura, T. Furukawa, H. Nakada, Proc. IDW’14, FLX6/FMC6-1 (2014), pp. 1454–1457 16. M. Koden, Proc. AM-FPD’15 (The Twenty-second International Workshop on Active-matrix Flatpanel Displays and Devices), 2-1 (2015), pp. 13–16 17. T. Furukawa, M. Koden, IEICE Trans. Electron, E100-C (2017), pp. 949–954 18. T. Furukawa, K. Mitsugi, H. Itoh, D. Kobayashi, T. Suzuki, H. Kuroiwa, M. Sakakibara, K. Tanaka, N. Kawamura, M. Koden, Proc. IDW’14, FLX3-4L (2014), pp. 1428–1429 19. T. Furukawa, N. Kawamura, M. Sakakibara, M. Koden, Proc. IDMC’15 (International Display Manufacturing Conference), S4-4 (2015) 20. T. Furukawa, N. Kawamura, H. Nakada, M. Koden, Proc. ICFPE2016 (The International Conference on Flexible and Printed Electronics), O15-6 (2016) 21. T. Furukawa, M. Sakakibara, N. Kawamura, M. Koden, Proc. IDW/AD’16, FLX3-3 (2016), pp. 1360–1363 22. T. Furukawa, N. Kawamura, T. Noda, Y. Hasegawa, D. Kobayashi, M. Koden, IDW’17, FLX6-2 (2017), pp. 1535–1538 23. T. Furukawa, J. Hauptmann, T. Nakagaki, R. Ikeuchi, M. Sagawa, D. Nagata, J. Nakatsuka, IDW’21, FLX5/FMC6-1 (2021), pp. 948–951 24. T. Nakagaki, T. Kawabata, H. Takimoto, T. Furukawa, Proc. IDW’19, FLXp1-9L (2019), pp. 1638–1641 25. M. Natsuka, Y. Ono, H. Mataki, S. Usui, H. Suzuki, M. Abe, T. Furukawa, IDW’21, FLX5/FMC6-2 (2021), pp. 952–955 26. C.C. Wu, S.D. Theisis, G. Gu, M.H. Lu, J.C. Sturm, S. Wagner, S.R. Forest, SID 97 Digest, 7.2 (1997), pp. 67–70 27. Z. Xie, L.S. Hung, F. Zhu, Chem. Phys. Lett. 381, 691–696 (2003)

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28. M.N. Troccoli, A.J. Roudbari, T.-K. Chuang, M.K. Hatalis, Solid-State Electron. 50, 1080–1087 (2006) 29. D.U. Jin, J.K. Jeong, H.S. Shin, M.K. Kim, T.K. Ahn, S.Y. Kwon, J.H. Kwack, T.W. Kim, Y.G. Mo, H.K. Chung, SID 06 Digest, 64.1 (2006), pp. 1855–1857 30. A. Chwang, R. Hewitt, K. Urbanik, J. Silvernail, K. Rajan, M. Hack, J. Brown, J.P. Lu, C. Shih, J. Ho, R. Street, T. Ramos, L. Moro, N. Rutherford, K. Tognoni, B. Anderson, D. Huffman, SID 06 Digest, 64.2 (2006), pp. 1858–1861 31. N. Yamada, T. Ogura, Y. Kubo, SID 08 Digest, 12.1 (2008), pp. 136–139 32. N. Yamada, S. Yamaguchi, J. Nakatsuka, Y. Hagiwara, K. Uemura, Proc IDW’15, FMC3-1 (2015), pp. 400–403 33. T. Matsushima, Y. Kinoshita, H. Murata, Appl. Phys. Lett. 91, 253504 (2007) 34. K. Okamoto, Y. Fujita, Y. Ohnishi, S. Kawato, M. Koden, 7th Japan OLED Forum, S9-2 (2008) 35. M. Koden, T. Furukawa, T. Yuki, H. Kobayashi, H. Nakada, Proc. IDW/AD’16, FLX3-1 (2016), pp. 1352–1355 36. Y. Hagiwara, H. Itoh, T. Furukawa, H. Kobayashi, S. Yamaguchi, N. Yamada, J. Nakatsuka, M. Koden, H. Nakada, Proc. IDW/AD’16, FLXp1-5 (2016), pp. 1408–1411 37. Y. Hagiwara, T. Furukawa, T. Yuki, S. Yamaguchi, N. Yamada, J. Nakatsuka, M. Koden, H. Nakada, Proc. IDW’17, FLXp1-9L (2017), pp. 1572–1574 38. F. Li, E. Smits, L. van Leuken, G. de Haas, T. Ellis, J.-L. van der Steen, A. Tripathi, K. Myny, M. Ameys, S. Schols, P. Heremans, G. Gelinck, SID 2014 Digest, 32.2 (2006), pp. 431–434 39. K. Taira, T. Furukawa, N. Kawamura, M. Koden, T. Takahashi, IDW’17, FLXp1-8L (2017), pp. 1568–1571 40. A. Suzuki, H. Takahagi, A. Uehigashi, Shigeki Hara, SID 2014 DIGEST, 10.2 (2014), pp. 108– 110 41. K. Taira, T. Suzuki, W. Konno, H Chiba, H. Itoh, M. Koden, T. Takahashi, T. Furukawa, IDW’18, FLX2-4L (2018), pp. 1492–1595 42. T. Yuki, T. Nishikawa, M. Sugimoto, H. Nakada, M. Koden, ITE Trans. MTA 4(9), 216–221 (2021)

Chapter 5

Gas Barrier Technologies

Abstract Gas barrier technologies are key technologies in flexile OLED devices, being essentially related with the storage reliability. Indeed, if a gas barrier technology applied to flexible OLED device is poor, the storage lifetime of the device is extremely short by penetration of gases such as H2 O. In flexible OLED devices, gas barrier technologies are applied to flexible substrates and encapsulations. This chapter describes evaluation methods for gas barrier properties and various gas barrier technologies that are applied to flexible substrates and encapsulations, while the encapsulating technologies are also described in Chap. 6 in more detail. Novel gas barrier technologies developed by our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) are also described. Keywords Gas barrier · Ca corrosion method · WVTR · Multi-gas barrier layer · Roll to roll · ALD

5.1 Overview of Gas Barrier Properties Gas barrier technologies are essentially required for OLED devices because OLED devices are easily degraded by moisture and other gasses [1–3]. The main role of gas barrier technologies is to protect gas penetration to OLED devices. Such gas barrier technologies are mainly applied to substrates and encapsulations. As is described in Fig. 2.17, the required gas barrier technologies for flexible OLED devices are very severe. Gas barrier properties are often evaluated by WVTR (water vapor transmission rate) values with the unit of g/m2 /day. Section 5.2 describes evaluation methods for gas barrier properties and WVTR, describing three technologies that are OLED device, Ca corrosion method, and WVTR measuring equipment. Sections 5.3, 5.4 and 5.5 review several gas barrier technologies that were studied by our research group (Research Group for Flexible Technologies) in Yamagata University (Japan).

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Koden, Flexible OLEDs, Display Science and Technology, https://doi.org/10.1007/978-981-19-3544-2_5

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5.2 Evaluation Methods for Gas Barrier Properties For developing gas barrier technologies, evaluation methods are necessarily required. In general, three technologies are used as evaluation methods of gas barrier properties. They are summarized in Fig. 5.1. In a sense, the storage test of OLED devices is a direct method for evaluating gas barrier properties. In this method, the obtained results are directly related with the reliability of the OLED devices. In this method, when the gas barrier is poor, non-emission areas, which are socalled dark spot, dark area, black spot, etc., appear in OLED devices stored under high-temperature and high-humidity conditions such as 40 °C/90% RH. An example is shown in Fig. 5.2. However, this method has several disadvantages. It should be noticed that degradations such as dark spot, etc. can be caused by not only gas penetrations but also other reasons such as out-gas from materials and components used in the OLED device. In addition, it is difficult to calculate WVTR values. Ca corrosion method is an useful conventional method [4]. Figure 5.3 shows Ca corrosion method. In this method, Ca is deposited on a sample and encapsulated. Ca is damaged by a reaction with H2 O, accompanying with a change of optical reflection as is shown in Fig. 5.3. The change in the optical reflection is detected and modified by a binarization treatment with a certain threshold of the reflectivity for calculations of the damaged areas. Such binarization is useful for clarifying the ratio of damaged and undamaged areas. When the increasing rate in the damaged area under a certain

Fig. 5.1 Evaluation methods for gas barrier properties

5.2 Evaluation Methods for Gas Barrier Properties

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Fig. 5.2 Degradation of OLED device under storage test [6]

Fig. 5.3 Ca corrosion method

period is almost constant, WVTR values are calculated from the increasing rate of the damaged area. Ca corrosion method has several advantages. One significant advantage of Ca corrosion method is visual images of damaged samples. As is found from the comparison of Figs. 5.2 and 5.3, damaged images of Ca corrosion devices are similar to those of OLED devices. Such visual images are very useful to discuss about the cause of damages and to make counter plans for improving gas barrier. In addition, one practical advantage of Ca corrosion method is simultaneous evaluations of many samples because this method requires only an environmental stress chamber under storage tests with high temperature and high humidity.

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On the other hand, Ca corrosion method has several disadvantages. One is a requirement of equipment and know-how for fabricating Ca corrosion devices. In addition, it is difficult to evaluate barrier properties of samples with low light transparency. Moreover, Ca corrosion method can detect degradations induced by H2 O but cannot detect degradations caused by other gases. On the other hand, WVTR (water vapor transmission rate) is directly measured by using WVTR measurement equipment. Various evaluation equipment have been developed and applied to gas barrier evaluations. Typical methods are equal pressure method and differential pressure method. In equal pressure method, pressures on both sides of an evaluated sample are equal. Water vapor passing the evaluated sample is analyzed by a detector. One of the merits of this method is to prevent any damage by pressure difference. However, a special sensitive detector or device is necessarily required because the concentration of water vapor passing the sample is extremely low. On the other hand, in differential pressure methods, pressures on both sides of an evaluated sample are different. This method can achieve to evaluate high barrier samples, thought samples might be damaged due to the difference pressure. By setting samples to such dedicated equipment for WVTR, WVTR values are measured with no device fabrication. Samples with low transparency can also be evaluated. However, this method has several disadvantages. First, accidental errors such as serious pinholes, scratches, etc. can largely influence on evaluation results, giving no correct information about barrier properties. Second, optical observation is impossible. Therefore, when the evaluation results are not good, it is difficult to discuss about the causes from the evaluated samples. Third, in most equipment, only one sample is evaluated. Therefore, the required time for evaluation tends to be elongated, reducing the developmental efficiency. Figure 5.4 shows an example of WVTR measurement equipment, accompanying with examples of evaluation data. The method of this equipment is modified differential pressure method with an attached support (MA method) developed

Fig. 5.4 Gas and water vapor transmittance measurement equipment of MORESCO Corporation and sample data

5.3 Dry Gas Barrier Layers Deposited by Roll-to-Roll (R2R) PE-CVD

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by MORESCO Corporation (Japan) and National Institute of Advanced Industrial Science and Technology (AIST) (Japan). The MA method is characterized by the use of a support layer between the space connected to the water vapor/gas quadrupole mass spectrometer (QMS) and the measurement sample. The support layer has the effect of delaying the gas flow into the space on the QMS side, making it possible to prevent contamination of the space on the detector side when replacing samples. As a result, a high degree of vacuum is always maintained in the space on the QMS side, making it possible to shorten required time for preprocessing or omit this step. This equipment with the MA method can evaluate wide ranges of WVTR such as 101 –10− 7 g/m2 /day. In addition, this equipment can warrant the WVTR value by the attached compensating unit developed by Suzuki et al. of Chemical Materials Evaluation and Research Base (CEREBA) (Japan) and AIST [5]. Moreover, this equipment is able to evaluate the transmission rate of not only water vapor but also various gases.

5.3 Dry Gas Barrier Layers Deposited by Roll-to-Roll (R2R) PE-CVD PE-CVD (plasma-enhanced chemical vapor deposition) is an often used technology for fabricating barrier layers in flexible OLED devices because PE-CVD can deposit SiNx, SiOx, etc. with high density. On the other hand, the process speed of PE-CVD is not so fast and PE-CVD equipment are expensive. These are issues of PE-CVD because these induce expensive cost. Furukawa et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) have developed roll-to-roll (R2R) PE-CVD deposition technologies on flexible substrates in the collaboration with Teijin Limited (Japan) and Tosoh Corporation (Japan), aiming at novel R2R technologies for fabricating barrier films [6–8]. In addition, in this study, transparent electrodes were fabricated on the barrier films for investigating the influence of electrodes on barrier properties, since the influence of electrodes on barrier ability has not been investigated in detail. The substrates for this study were two PEN substrates supplied from Teijin Limited (Japan). The one is Q65H, which is normal PEN with the thickness of 125 µm. The other is PQDA5, which is surface-planarized PEN with the thickness of 100 µm. The fundamental structure of PEN is shown in Fig. 4.19. The barrier layers and transparent electrodes were deposited by roll-to-roll (R2R) vacuum equipment of Kobe Steel, Ltd. (Japan). This equipment has functions of PE-CVD and sputtering [9, 10]. The fabrication process of barrier layer and transparent electrode on PEN substrate is shown in Fig. 5.5.

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Fig. 5.5 Roll-to-roll fabrication process of SiOx barrier and IZO layers on PEN substrate

After a PEN substrate is washed by a R2R wet cleaning equipment of FEBACS CO., LTD. (Japan), SiOx barrier layer is deposited by the R2R PE-CVD, using HMDSO (hexamethyldisiloxane) as a precursor. The molecular structure of HMDSO is shown in Fig. 5.6. The introduced gas to the CVD chamber is a mixture of O2 and HMDSO with the ratio of 10:1. The process pressure was maintained at 2 Pa. The plasma power is 1.2 kW and the typical conveying speed is 1.0 m/min. The substrate with the deposited SiOx barrier layer is washed by the R2R wet cleaning equipment again and then IZO (indium zinc oxide) is deposited by the R2R sputtering. The introduced gas to sputtering chamber is a mixture of O2 and Ar with the ratio of 1:100. The plasma power is 1.0 kW and the typical conveying speed is 0.1 m/min. The deposited IZO is patterned by using an etching paste printed by a R2R screen printing equipment of SERIA ENGINEERING, INC. (Japan), followed by the final Fig. 5.6 Molecular structure of HMDSO

5.3 Dry Gas Barrier Layers Deposited by Roll-to-Roll (R2R) PE-CVD

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Fig. 5.7 Ca corrosion test (40 °C/90% RH) of single SiOx barrier layers with different thickness

R2R wet cleaning. The detail of R2R patterning technology using an etching paste is described in Sect. 7.5. The barrier properties were evaluated by a Ca corrosion method under the storage of 40 °C/90% RH, which is described in Sect. 5.2. Figure 5.7 shows the evaluation results of Ca corrosion test of single SiOx barrier layers with different thickness on normal PEN substrates (Q65H). The thinnest sample (thickness of 100 nm) shows heavy damages after only 118 hours. The damages in the samples with 200- and 300-nm-thick SiOx are still observed, while the damage levels are reduced. The sample with 600-nm-thick SiOx shows no damage. These results clearly indicate that thin SiOx layers possess many pinholes, defects, etc. In order to evaluate the influence of IZO layer on barrier ability, Ca corrosion samples with and without an IZO layer were prepared on PEN substrates (Q65H) with 800-nm-thick SiOx layer. The evaluation results under 40 °C/90% RH are shown in Fig. 5.8. As is obvious in Fig. 5.8, the degradations of the samples with IZO layer are larger than that without IZO. Indeed, while the calculated WVTR of a sample without IZO is 1.5 × 10−5 g/m2 /day, the calculated WVTR values of samples with IZO are 1.6 × 10−4 and 2.2 × 10−4 g/m2 /day. Such difference with about one order is estimated to be caused by defect creations in SiOx layer induced by mechanical stress of IZO. In order to confirm the hypothesis described above and to reduce the mechanical stress induced by IZO, Ca corrosion samples with an IZO layer with different thickness were fabricated on PEN substrates (Q65H) with 600-nm-thick SiOx layer. The evaluation results under 40 °C/90% RH are shown in Fig. 5.9. As shown in

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Fig. 5.8 Ca corrosion test (40 °C/90% RH) of SiOx barrier layers (thickness: 800 nm) with or without IZO electrode (thickness: 100 nm). Ca corrosion images are obtained after 595 h. The WVTR values are calculated from the temporal change of Ca corrosion area

Fig. 5.9 Ca corrosion test (40 °C/90% RH) of SiOx barrier layers (thickness: 600 nm) with IZO electrode (thickness: 100 or 30 nm). Ca corrosion images are obtained after 666 h. The WVTR values are calculated from the temporal change of Ca corrosion area

Fig. 5.9, the damaged area of the sample with 30-nm-thick ITO is smaller than that with 100-nm-thick IZO. Such difference is also recognized in the comparison of the calculated WVTR values of both samples. This result implies that the reduced mechanical stress induced by thinner IZO layer gives rise to improve barrier ability, reducing defects in SiOx layer induced by mechanical stress of IZO.

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Fig. 5.10 Ca corrosion test (40 °C/90% RH) of SiOx barrier layers with IZO electrode on normal PEN (Q65H) and planarized PEN (PQDA5) films. Ca corrosion images are obtained after about 600 h. The WVTR values are calculated from the temporal change of Ca corrosion area

Looking at the results shown in Figs. 5.7, 5.8 and 5.9, defects in SiOx barrier layers largely influence gas barrier ability. Based on such results, it is also estimated that surface condition of flexible film before SiOx deposition is also important for obtaining good barrier ability. In order to investigate the effect of surface condition on barrier ability, two types of PEN substrates were prepared. The one is normal PEN film (Q65H) with the thickness of 125 µm. The other is surface-planarized PEN film (PQDA5) with the thickness of 100 µm. The images of surfaces of these two PEN films are shown in Fig. 5.10. Although the normal PEN film shows many surface defects, the planarized PEN film shows smooth surface. On these PEN films, SiOx layer and IZO layer were deposited. The IZO layers were patterned as is described above. The results of Ca corrosion test using two types of PEN films under 40 °C/90% RH are shown in Fig. 5.10. As given obviously in Fig. 5.10, the planarized PEN gives improved barrier ability, indicating that surface defects induce defects in barrier layer and reduce barrier ability. Based on the developed technologies, flexible OLED devices were successfully fabricated. Examples are shown in Fig. 5.11.

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Fig. 5.11 Developed flexible OLED devices

5.4 Multi-Layer Barrier Using Wet and Dry Layers It is commonly known that wet processes can provide layers with excellent coverage of defects, pinholes, etc. Therefore, such wet layers can be expected to contribute to barrier ability because gas barrier ability largely depends upon defects, pinholes, etc. of barrier layers. In the applications of wet layers to gas barrier layers, several approaches have been investigated and reported. Burrows et al. of Pacific Northwest National Laboratory (USA) and Vitex Systems Inc. (USA) reported an alternatingly stacked structure with dry and wet layers [1]. Such type of technology is also used for TFE (thin film encapsulation) technology, which is described in Sect. 6.3. Suzuki et al. of LINTEC Corporation (Japan) have developed a wet coating barrier layer with plasma-assisted surface modification [11]. Using this technology, Yeh et al. of Industrial Technology Research Institute) (ITRI) (Taiwan) developed a 7-inch foldable AM-OLED display [12]. Satake et al. of AZ Electronic Materials Manufacturing Japan K.K. (Japan) developed polysilazane materials for barrier layers [13]. They reported that the developed polysilazane AZ NL 120A layer on PEN film achieved the WXTR of 7.9 × 10−3 g/m2 /day. Using the polysilazane material, our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) has developed gas barrier layers with a wet coating polysilazane layer and a dry SiOx layer deposited by roll-to-roll (R2R) CVD [14]. The barrier layer structure consists of two layers with a dry SiOx barrier layer and a wet polysilazane barrier layer. On a PEN film (thickness: 100 µm) supplied from Teijin DuPont Films (Japan), a SiOx layer with the thickness of 340 nm was deposited by a R2R CVD equipment of Kobe Steel, Ltd. (Japan). On the SiOx layer, a polysilazane layer AZ NL 120A was coated by a wet process. The WVTR of the developed barrier layer is 1.6 × 10−5 g/m2 /day as shown in Fig. 5.12. It is

5.4 Multi-Layer Barrier Using Wet and Dry Layers

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Fig. 5.12 Ca corrosion test (40 °C/90% RH) of stacked barrier layer with a SiOx layer (thickness: 340 nm) deposited by R2R CVD and a coated polysilazane AZ NL 120A layer

suggested that defects and pinholes in the SiOx layer are covered by the polysilazane materials. For investigating the barrier performance, developed barrier films were applied to an encapsulating substrate in OLED devices as shown in Fig. 5.13. As a reference, a normal glass substrate is also applied to the encapsulating substrate. In these OLED devices, non-emission regions spread under high-temperature and high-humidity acceleration test (60°C/90% RH) due to the penetration of water through the sealing

Fig. 5.13 OLED storage test

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Fig. 5.14 Flexible OLED device with the developed two-layer stacked barrier with a dry SiOx layer and a wet polysilazane layer

resin and the barrier film. The data of the reference OLED indicates that water vapor penetrates to OLED devices through the sealant after about 100 hours. The curve of the developed barrier layer is similar to the normal glass, implying that the developed barrier layer has considerably high barrier property. A flexible OLED device was also fabricated on the developed barrier layer as shown in Fig. 5.14, achieving uniform emission.

5.5 Multi-Layer Barrier Using Sputtering and ALD It is well known that the most common methods for achieving high gas barrier properties seems to be PE-CVD (plasma-enhanced chemical vapor deposition), since PECVD can deposit high-dense barrier layers such as SiOx, SiNx, etc. [6–8]. However, the process costs of PE-CVD tend to be expensive because of high price of equipment, slow deposition rates, etc. On the other hand, sputtering has excellent productivity. As a pioneering work, Yoshida et al. of Pioneer Corporation (Japan) reported an alternatingly stacked barrier layers with sputtering SiON layers and an organic resin layer for fabricating a flexible 3-inch full-color OLED display, though the WVTR values are not described [15]. In general, sputtering technique is a high productive method but it is well known that the deposited layers tend to possess many defects and pinholes. Therefore, counter methods for covering such defects and pinholes are necessarily required. While one method is a combination with coating layers [15], Yuki et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) have been interested in a combination with depositing layers by ALD (atomic layer deposition) [16, 17].

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It is well known that ALD gives excellent defect coverage [18]. Therefore, if ALD layers can cover defects and pinholes of layers deposited by sputtering, the combination of sputtering and ALD can be an alternative candidate instead of PECVDs. In this study, plural stacking barrier layers deposited by sputtering and ALD on flexible films were investigated, achieving high gas barrier properties with WVTR of the order of 10−5 g/m2 /day or better. In addition, using the developed barrier films, flexible OLED devices were fabricated. As flexible films, PEN (polyethylene naphthalate) films with the thickness of 125 µm and EXPEEK® films with the thickness of 50 µm were utilized. PEN films were supplied from Teijin Limited (Japan). EXPEEK films, which were supplied from KURABO INDUSTRIES LTD. (Japan), are simultaneously biaxially stretched PEEK (polyetheretherketone) films with high temperature tolerance with Tg of about 300 °C [19]. The fundamental molecular structures of PEN and PEEK are shown in Fig. 4.19. The sputtering equipment is Tokki’s model SPK-503, which is a typical flat platetype magnetron sputtering system. The depositing direction is facedown. The target is Si3 N4 . The RF power is 500 W. The vacuum level is 0.2–0.3 Pa. The flow gas is Ar with the flow rate of 1–3 sccm. The substrate temperature under sputtering is not controlled. The ALD equipment is made by SUGA CO., Ltd. (Japan), in which the depositing direction is facedown. The precursor is trimethylaluminum (TMA). The oxidizing agent is ozone (O3 ). The carrier gas is N2 . The substrate temperature is 100 °C. For depositing Al2 O3 layer with the thickness of 45 nm, 550 cycles with the unit cycle of 20 s are required. The barrier properties were evaluated by a Ca corrosion method [4], which are described in Sect. 5.2. The Ca corrosion devices were evaluated under 40 °C/90% RH condition. First, single barrier layers deposited by sputtering or ALD were evaluated. Figure 5.15 shows the calculated WVTR values of single barrier layers deposited by sputtering or ALD on EXPEEK. The Si3 N4 layers with the thickness of 100 nm

Fig. 5.15 WVTR values calculated by Ca corrosion tests of EXPEEK films with a single Si3 N4 layer (100 nm) by sputtering or a single Al2 O3 layer (90 nm) by ALD. The results from four samples are plotted in each case

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Fig. 5.16 WVTR values calculated by Ca corrosion tests of EXPEEK films with binary layer architectures of Si3 N4 (sputtering) and Al2 O3 (ALD). The results from four samples are plotted in each case

show poor barrier properties with the WVTR of the order of 10−3 g/m2 /day. This result suggests that such thin Si3 N4 layer deposited by sputtering would possess many defects and pinholes as passes of gas penetration. The WVTR values of single Al2 O3 layers with the thickness of 90 nm deposited by ALD are in order of 10−2 g/m2 /day, being worser than Si3 N4 layers with the thickness of 100 nm. This result implies that the barrier ability of single Al2 O3 layer with the thickness of 90 nm deposited by ALD is not so excellent by itself. In order to improve barrier properties, various stacking barrier layers with Si3 N4 and Al2 O3 were fabricated and investigated. Figure 5.16 shows the calculated WVTR values of binary layer structure of Si3 N4 and Al2 O3 . While the Si3 N4 (50 nm)/Al2 O3 (45 nm) architecture still shows poor barrier properties with WVTR of the order of 10−3 g/m2 /day, the Si3 N4 (100 nm)/Al2 O3 (90 nm) architecture shows obvious improvement, comparing with mono-Si3 N4 (100 nm) layer. The Si3 N4 (100 nm)/Al2 O3 (90 nm) architecture achieves WVTR of the order of 10−4 g/m2 /day. This result implies that defects and pinholes of the Si3 N4 layer are effectively covered by Al2 O3 layer deposited by ALD. Figure 5.17 shows WVTR values of triply stacking barrier layers with Si3 N4 layers (200 nm) deposited by sputtering and Al2 O3 layers (30, 60, or 90 nm) deposited by ALD on PEN films for investigating the effect of the thickness of an Al2 O3 layer upon the barrier properties. With increasing the thickness of Al2 O3 layer, WVTR values are improved, indicating that Al2 O3 layers deposited by ALD effectively cover defects and pinholes in Si3 N4 deposited by sputtering. The best sample shows WVTR of about 1 × 10−6 g/m2 /day. The barrier properties of another triply stacking layers with Si3 N4 layers deposited by sputtering and an Al2 O3 deposited ALD on EXPEEK films are shown in Fig. 5.18. Both barrier layers of Si3 N4 (100 nm)/Al2 O3 (90 nm)/Si3 N4 (100 nm) and Si3 N4 (50 nm)/Al2 O3 (45 nm)/Si3 N4 (50 nm) show WVTRs with the order of 10−5 g/m2 /day, which are applicable levels for flexible OLEDs. Further stacking barrier layers with five or six layers were also investigated as shown in Fig. 5.19, aiming at further improvement of barrier properties. However, the effect of the increase in the number of layers on barrier properties is not obvious, though one sample shows WVTR of about 1 × 10−6 g/m2 /day. Rather, two samples

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Fig. 5.17 WVTR values calculated by Ca corrosion tests of PEN films with triply stacking barrier layers with Si3 N4 layer by sputtering and Al2 O3 layer by ALD

Fig. 5.18 WVTR values calculated by Ca corrosion tests of EXPEEK films with triply stacking layer architectures of Si3 N4 (sputtering) and Al2 O3 (ALD). The results from four samples are plotted in each case

Fig. 5.19 WVTR values calculated by Ca corrosion tests of EXPEEK films with five or six layer architectures of Si3 N4 (sputtering) and Al2 O3 (ALD). The results from four samples are plotted in each case

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Fig. 5.20 Device structure and an emission picture of flexible OLED devices fabricated on the developed high gas barrier EXPEEK film with high temperature tolerance. The substrate size is 50 × 50 mm. The emission area is 32 × 32 mm

show WVTRs with the order of 10−4 g/m2 /day. From practical point of view, triply stacking layers would make compatibility of barrier property and productivity. The developed high gas barrier EXPEEK films with a Si3 N4 (100 nm)/Al2 O3 (90 nm)/Si3 N4 (100 nm) stacking barrier layer were applied to flexible OLED devices. The device structure and the emitting picture are shown in Fig. 5.20. As an encapsulating film, the developed high gas barrier EXPEEK films were also applied. Flexible OLEDs were successfully fabricated with no serious problem.

References 1. P.E. Burrows, G.L. Graff, M.E. Gross, P.M. Martin, M.K. Shi, M. Hall, E. Mast, C. Bonham, W. Bennett, M.B. Sullivan, Displays 22, 65–69 (2001) 2. M. Schaer, F. Nüesch, D. Berner, W. Leo, L. Zuppiroli, Adv. Funct. Mater. 11, 116–121 (2001) 3. S.F. Lim, W. Wang, S.J. Chua, Mater. Sci. Eng., B 85, 154–159 (2001) 4. G. Nisato, M. Kuilder, P, Bouten, L. Moro, O. Philips, N. Rutherford, SID 03 DIGEST, P-88 (2003), pp. 550–553 5. A. Suzuki, H. Takahagi, A. Uehigashi, Shigeki Hara, SID 2014 DIGEST, 10.2 (2014), pp. 108– 110 6. K. Taira, T. Furukawa, N. Kawamura, M. Koden, T. Takahashi, IDW’17, FLXp1-8L (2017), pp. 1568–1571 7. K. Taira, T. Suzuki, W. Konno, H Chiba, H. Itoh, M. Koden, T. Takahashi, T. Furukawa, IDW’18, FLX2-4L (2018), pp. 1492–1595 8. T. Suzuki, W. Konno, K. Taira, H Chiba, H. Itoh, M. Koden, T. Takahashi, T. Furukawa, IDW’18, FLXp1-10L (2018), pp. 1561–1564 9. T. Okimoto, Y. Kurokawa, T. Segawa, H. Tamagaki, Proc. of IDW’14, FLX5-2 (2014), p. 1448– 1451 10. H. Tamagaki, Y. Ikari, N. Ohba, Surf. Coat. Technol. 241, 138–141 (2014) 11. Y. Suzuki, K. Nishijima, S. Naganawa, K. Nagamoto, T. Kondo, SID 2014 DIGEST, 6.4 (2014), pp. 56–58

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12. Y.-H. Yeh, S.-T. Yeh, K.-T. Chen, G. Chen, J.-C. Ho, J. Chen, C-C. Lee, SID 2018 DIGEST, 21.4 (2018), p. 299–301 13. N. Satake, S. Kawato, Y. Ozaki, M. Kobayashi, Proc. of IDW’13, FLX4-3 (2013), pp. 1549– 1551 14. M. Koden, T. Furukawa, T. Yuki, H. Kobayashi, H. Nakada, Proc. of IDW’16, FLX3-1 (2016), p. 1352–1355 15. A. Yoshida, S. Fujimura, T. Miyake, T. Yoshizawa, H. Ochi, A. Sugimoto, H. Kubota, T. Miyadera, S. Ishizuka, M. Tsuchida. H. Nakada, SID 03 DIGEST, 21.1 (2003), pp. 856–859 16. T. Yuki, T. Nishikawa, M. Sugimoto, H. Nakada, M. Koden, IDW’20, FLX2-3 (2020), pp. 891– 894 17. T. Yuki, T. Nishikawa, M. Sugimoto, H. Nakada, M. Koden, ITE Trans. MTA 9(4), 216–221 (2021) 18. M.D. Groner, S.M. George, R.S. McLean, P.F. Carcia, Appl. Phys. Lett. 88, 051907 (2006) 19. T. Nishikawa, T. Odagawa, H. Yasuda, Japanese Patent No. JP-5847522B

Chapter 6

Encapsulating Technologies

Abstract Flexible OLEDs require flexible encapsulation technologies that are essentially different from encapsulation technologies for rigid OLED devices. Flexible encapsulation technologies need to have not only sufficient gas barrier ability but also thin thickness, flexibility, etc. Typical flexible encapsulation technologies are dam-fill, TFE (thin film encapsulation) and laminating. This chapter describes these three technologies, accompanying with novel research and development on these technologies in our research group (Research Group for Flexible Technologies) of Yamagata University (Japan). Keywords Encapsulation · Dam-fill · Thin film encapsulation · TFE · Laminating

6.1 Fundamentals of Encapsulating Technologies for Flexible OLEDs An encapsulating technology is one of important key technologies in OLED devices. In addition, flexible OLED devices require flexible encapsulations with thin thickness, flexibility, etc. Therefore, encapsulating technologies for flexible OLED devices are essentially different from those applied to rigid OLED devices. While typical encapsulating technologies for rigid OLED devices are described in Fig. 2.9, the encapsulating thickness of around 1 mm can be accepted in rigid OLEDs. Therefore, counter-glass substrates and metal cans can be applied. In addition, desiccant can also be applied, especially in bottom emitting OLED devices. On the other hand, in flexible OLED devices, the rigid encapsulating technologies shown in Fig. 2.9 cannot be applied because of thick thickness and lack of flexibility. Typical flexible encapsulating technologies for flexible OLED devices are dam-fill encapsulation, TFE (thin film encapsulation), and laminating encapsulation as shown in Fig. 6.1. Sections. 6.2, 6.3, and 6.4 describe these encapsulation technologies.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Koden, Flexible OLEDs, Display Science and Technology, https://doi.org/10.1007/978-981-19-3544-2_6

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Fig. 6.1 Typical encapsulating structures of flexible OLED devices

6.2 Dam-Fill Encapsulation Dam-fill encapsulation uses counter-flexible substrate, sealant, and fill material. Figure 6.2 shows a typical process of dam-fill encapsulation. First, a resin material for dam is dispensed with a certain pattern enclosing an emission area on a counter encapsulating substrate. On this substrate, a fill material is filled into the enclosed area by dispensing or dropping process. Finally, the counter-substrate is combined with an OLED device and then cured for adhesion of both substrates. For example, Lin et al. of AU Optronics (Taiwan) fabricated a prototype of 65-inch OLED TV by using a dam-fill encapsulation [1]. In dam-fill encapsulations, key materials are dam and fill materials. For dam materials, low gas penetration rate is essentially required. However, since dam materials Fig. 6.2 Typical process of dam-fill encapsulation

6.2 Dam-Fill Encapsulation

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Fig. 6.3 Chemical reaction of OleDry and moisture [2]

are usually organic resins, perfect gas barrier cannot be expected. Therefore, fill materials are often required to have desiccant ability. Futaba Corporation (Japan) has developed fill-in-type transparent desiccant “OleDry” for dam-fill encapsulation [2]. OleDry is composed of aluminum complex which dissolves in hydrocarbon solvent. The chemical reaction between fill material and moisture is shown in Fig. 6.3. They also developed a modified transparent liquid descant “OleDry-F” [3, 4]. By using “OleDry-F” they developed and demonstrated 3.5- and 1.3-inch multi-color passive-matrix OLED displays [3]. The 1.3-inch OLED display is see-through type with the top-emitting OLED device structure. JSR Corporation (Japan) has also developed coatable or printable UV-activated transparent desiccant for fill materials [5–7]. The material consists of organic metal compounds and binder polymer, being non-solvent-type material and changing to solid state by UV-cure. In our collaborative consortium “Yamagata University Organic Thin Film Device consortium” (FY2013-FY2015) led by Koden of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan), NEC lighting Ltd. (Japan) fabricated flexible OLED lighting prototypes using dam-fill encapsulations [8, 9]. The prototype samples and their device structures are shown in Fig. 6.4. The flexible substrates used in the prototypes are ultra-thin glass, stainless steel foil, and barrier film. The size of flexible OLED devices is 92 × 92 mm. The emission area is 75 × 75 mm.

Fig. 6.4 Flexible OLED lighting prototypes fabricated by NEC lighting in “Yamagata University Organic Thin Film Device consortium” (FY2013-FY2-15)

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6.3 Thin Film Encapsulation (TFE) Thin film encapsulation (TFE) means encapsulations using thin films in rigid and flexible OLED devices. As TFE technologies, two methods are well known. The one is single- or multi-layer structures with inorganic gas barrier layers. The other is multi-layer structures with alternating stacking of inorganic and organic layers. In the application of inorganic gas barrier layers to TFE, the required points are not only high gas barrier but also low-temperature deposition such as lower than 100 °C, low film stress, excellent step coverage, etc. From these points of view, PE-CVD (plasma-enhanced chemical vapor deposition) is often used. As TFE technologies with only inorganic gas barrier layer, Pioneer Corporation (Japan) applied a SiN passivation layer deposited by PE-CVD to 5.2-inch rigid and 3-inch flexible full-color passive-matrix OLED displays [10–12]. On the other hand, the concept of multi-layer structure is very useful for encapsulation of OLED devices because mono-layer structures with an inorganic gas barrier layer tend to have defects and pinholes. The concept of the alternating stacked multilayer structure is called “tortuous path model” shown in Fig. 6.5. Though barrier abilities of organic layers are low, organic layers on inorganic gas barrier layers play several roles, which are covering of defects and pinholes, planarization, and elongations of gas penetration path. Burrows et al. of Pacific Northwest National Laboratory (USA) and Vitex Systems Inc. (USA) have reported stacking barrier layers with vacuum deposited polyacrylate and vapor-barrier material such as Al2 O3 [13]. At present (in 2022), all or most of active-matrix mobile OLED displays for smartphones utilize multi-layer TFE encapsulations with alternatingly stacked inorganic and organic layers. While the ideal number of such TFE layers is three, consisting of two inorganic layers sandwiching one organic layer, multi-layer structures with five, seven, or more layers are also utilized for achieving requested reliability of OLED devices. Our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) reported an improvement of barrier property of TFE by applying a novel organic resin that was developed by Toyo Ink SC Holdings Co., Ltd (Japan). The organic resin has several unique advantages that are non-solvent type, UVcurable, applicability to ink-jet processes, excellent adhesion with inorganic SiNx layer, etc.

Fig. 6.5 Tortuous path model of multi-layer TFE structure with inorganic and organic layers

6.4 Laminating Encapsulation

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Fig. 6.6 Barrier properties of TFE structures

As fundamental evaluations, TEF structures with the organic resin layer sandwiched by two SiNx layers deposited by sputtering were fabricated on PEN substrates. The samples were evaluated by a Ca corrosion method. The device structure and typical results are shown in Fig. 6.6. As organic resins, the novel developed material and a reference material were applied. Under storage tests with 40 °C/90% RH, the sample with the reference resin showed obvious corrosion after about 1,000 h. On the contrary, the sample with the novel resin showed no serious corrosion under the same storage test. The WVTR values of the developed and referenced samples are in the order of 10−6 and 10−5 g/m2 /day, showing one order difference. This result cannot be simply explained by the tortuous path model shown in Fig. 6.5, clearly indicating another important role of organic resin sandwiched by inorganic barrier layers. Probably, defect coverage ability and planarization ability of organic resin would be important in TFE technologies. By covering defects and pinholes of under inorganic barrier layer and by making planar surface, the barrier quality of upper inorganic barrier layer is supposed to be improved. Using the developed TFE technologies with the novel organic resin, several prototype OLED devices were fabricated and demonstrated as shown in Fig. 6.7.

6.4 Laminating Encapsulation Laminating encapsulation is a simple encapsulating technology, in which a laminating substrate with barrier resin is laminated to an OLED device. The process flow is shown in Fig. 6.8. Barrier resins for laminating encapsulation are thermal cure (hot melt) adhesive, UV-cure adhesive, pressure-sensitive adhesive (PSA), etc. In some cases, inorganic barrier layers are deposited on OLED devices before lamination for improving barrier ability. The required properties for the barrier resins are adhesive ability, gas barrier property, high light transparency, low haze, temperature stability, adhesive strength,

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Fig. 6.7 OLED device samples with the TFE structure using the novel organic resin of Toyo Ink SC Holdings Co., Ltd

Fig. 6.8 Typical process of laminating encapsulation

flexibility, etc. In actual applications to flexible OLED devices, one of significant key points is the protection of gas penetration from side of device (so-called “side leak”). The gas permittivity of resin is depending upon both gas solubility and gas diffusion. The techniques for achieving low solubility of gas are use of hydrophobic resin, use of inorganic filler, high hydrophobicity of resin surface, etc. [14]. The techniques for achieving low gas diffusion are crosslink density of polymer, improvement of thermal stability, tortuous path effect by anisotropic fillers, doping of desiccant materials, etc. [14]. Nishijima et al. of LINTEC Corporation (Japan) reported thermal curing encapsulation adhesive sheets for flexible OLED devices [15]. The material for the adhesive formulation is polyolefin with epoxy resin. The reliability of OLED devices with the developed adhesive sheet is reported to achieve 360 h under 60 °C/90% RH. In addition, the developed adhesive sheet was applied to a 7-inch top-emitting AM-OLED display fabricated by Industrial Technology Research Institute (ITRI) (Taiwan) as shown in Fig. 6.9.

6.4 Laminating Encapsulation

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Fig. 6.9 ITRI’s 7-inch top-emitting AM-OLED prototype encapsulated with LINTEC’s adhesive sheet. (Reprinted with permission from John Wiley and Sons, Ref. [15] Copyright 2017.)

Ohashi et al. of Ajinomoto Fine-Techno Co., Inc. (Japan) developed laminating encapsulation films with pressure-sensitive adhesive (PSA) for OLED devices without inorganic passivation [16]. The base polymers are hydrophobic polymers grafted with a cross-linkable group (e.g., maleic anhydride). They investigated several types of PSA designs, achieving a storage lifetime of over 500 h under 85 °C/85% RH in the formulation with hygroscopic nano-sized particles. The fabricated samples are shown in Fig. 6.10. Our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) has developed flexible OLED devices using laminating encapsulation films developed by Ajinomoto Fine-Techno Co., Inc. (Japan). Figure 6.11 shows fundamental evaluation results of encapsulating ability by a Ca corrosion method. While the distance between the device edge and the emission area is 9 mm,

Fig. 6.10 The barrier PSA films developed by Ajinomoto Fine-Techno and a flexible OLED device encapsulated with Al foil and PSA [16]. (Reprinted with permission from John Wiley and Sons, Ref. [16] Copyright 2020.)

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Fig. 6.11 The evaluation of barrier ability of the laminating encapsulation of Ajinomoto FineTechno

Fig. 6.12 Flexible OLED devices with laminating encapsulation of Ajinomoto Fine-Techno

no serious damage of Ca was observed under the storage test of 60 °C/90% RH with about 8,000 h. The calculated WVTR values are around 1 × 10−6 g/m2 /day. Using the encapsulating films, various prototypes of flexible OLED devices were developed as shown in Fig. 6.12. We also developed flexible OLED devices with PSA-type encapsulating films by the collaboration with MORESCO Corporation (Japan). Some of them are shown in Fig. 6.13.

References

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Fig. 6.13 Flexible OLED devices encapsulated with a PSA film developed by MORESCO

References 1. L.-F. Lin, T.-H. Shih, J.-Y. Lee, W.-H. Wu, S.-C. Wang, Y.-H. Chen, C.-C. Chen, C.-L. Chen, P.P. Lin, Y.-H. Chen, S.-J. Yu, C.-H. Liu, H.-C. Ting, H.-H. Lu, L. Tsai, H.-S. Lin, C.-Y. Chen, L.-H. Chang, Y.-H. Lin, Proc. IDW’13, AMD2-1 (2013), pp. 264–267 2. Y. Tsuruoka, S. Hieda, S. Tanaka, H. Takahashi, SID’03 Digest, 21.2 (2003), pp. 860–863 3. T. Niiyama, S. Tanaka, Y. Hoshina, M. Sisikura, R. Kajiyama, SID 2013 Digest, 55.3 (2013), pp. 673–766 4. Y. Hoshina, T. Niyama, S. Tanaka, M. Miyagawa, Proc. IDW’13, OLED4-5L (2013), pp. 900– 901 5. K. Konno, T. Arai, M. Takahashi, T. Kajita, 13th Japanese OLED Symposium, S3-2 (2011) 6. T. Arai, K. Konno, T. Miyasako, M. Takahashi, M, Nishikawa, K. Azuma, T. Ueno, M. Hasuta, 15th Japanese OLED Symposium, S7-4 (2012) 7. H. Katsui, T. Miyasako, T. Arai, M. Takahashi, N. Onimaru, N. Takamatsu, T. Yamamura, K. Konno, K. Kuriyama, Proc. IDW’14, OLED3-3 (2014), pp. 659–662 8. M. Koden, H. Kobayashi, T. Moriya, N. Kawamura, T. Furukawa, H. Nakada, IDW’14, FLX6/FMC6-1 (2014), pp. 1454–1457 9. M. Koden, Proc. AM-FPD’15 (The Twenty-second International Workshop on Active-matrix Flatpanel Displays and Devices), vol. 2-1 (2015), pp. 13–16 10. H. Kubota, S. Miyaguchi, S. Ishizuka, T. Wakimoto, J. Funaki, Y. Fukuda, T. Watanabe, H. Ochi, T. Sakamoto, T. Miyake, M. Tsuchida, I. Ohshita, T. Tohma, J. Lumin. 87–89, 56–60 (2000) 11. A. Yoshida, S. Fujimura, T. Miyake, T. Yoshizawa, H. Ochi, A. Sugimoto, H. Kubota, T. Miyadera, S. Ishizuka, M. Tsuchida, H. Nakada, SID 03 Digest, 21.1 (2003), pp. 856–859 12. T. Miyake, A. Yoshida, T. Yoshizawa, A. Sugimoto, H. Kubota, T. Miyadera, M. Tsuchida, H. Nakada, Proc. of IDW’03, OEL2-2 (2003), pp. 1289–1292 13. P.E. Burrows, G.L. Graff, M.E. Gross, P.M. Martin, M.K. Shi, M. Hall, E. Mast, C. Bonham, W. Bennett, M.B. Sullivan, Displays 22, 65–69 (2001) 14. Y. Kageyama, Japan Society for the Promotion of Science, Committee No. 142 of organic materials for information science, Abstract of seminar on 7 July 2017 (2017), pp. 25–31 15. K. Nishijima, T. Hasegawa, M. Kashio, SID 2017 Digest, 31-3 (2017), pp. 441–444 16. S. Ohashi, E. Baba, M. Okuno, M. Hosoi, Y. Shindo, M. Takada, SID 2020 Digest, 69-4 (2020), pp. 1036–1039

Chapter 7

Novel Electrode Technologies

Abstract Current common transparent electrodes for OLED devices are ITO (indium tin oxide) and IZO (indium zinc oxide) which are usually fabricated by sheet-to-sheet sputtering and photolithography. However, these electrodes have issues in cost and the compatibility with flexible devices. Based on such background, this chapter describes several non-ITO electrodes, which are transparent conducting polymer, silver nanowire (AgNW) and implanted metal-mesh electrodes, and roll-to-roll (R2R) fabrications of ITO and IZO. Keywords Non-ITO electrode · Conducting polymer · Silver nanowire · Metal mesh · Roll to roll

7.1 Background In OLED devices, one of commonly used transparent bottom electrodes is ITO (indium tin oxide), while IZO (indium zinc oxide) is also often used. However, ITO and IZO have several issues. The one is cost issue due to several reasons. The fabrication processes of ITO and IZO are expensive, including vacuum depositions and photolithography. In addition, the price of indium (In) is high because indium is a rare metal and the reserve of indium is small. The second issue is the compatibility with flexible devices. The fabrication processes of ITO and IZO layers generally include high-temperature (over 200 °C) processes for obtaining low resistance because the resistance of ITO and IZO is related with the crystallization induced in high-temperature processes. Such crystallization also induces mechanical stress of ITO and IZO layers. In flexible devices, such mechanical stress often induces several problems such as undesirable curl of substrate, fracture of ITO and IZO layers, etc. Based on such background, non-ITO transparent electrodes have actively been researched and developed [1]. The candidates of non-ITO transparent electrodes are conducting polymer, metal-mesh, silver nanowire (AgNW), carbon nanotube, graphene, etc. In addition, inexpensive fabrication processes of ITO and IZO are also demanded for reducing fabrication costs of devices.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Koden, Flexible OLEDs, Display Science and Technology, https://doi.org/10.1007/978-981-19-3544-2_7

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The following sections describe our research and developments of novel technologies on conducting polymers, silver nanowires, metal-mesh electrodes, and R2R fabrications of ITO and IZO.

7.2 Transparent Conducting Polymer Transparent conducting polymers are known as solution-type non-ITO transparent electrode materials. Typical conducting polymers are polyaniline (PANI) and poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS). Their fundamental structures are shown in Fig. 7.1. Gustafsson et al. of Uniax Corporation (USA) reported flexible OLED devices with polyaniline (PANI) hole-injecting electrode (200 /sq, transmittance of 70% at visible wavelength) on PET (polyethylene terephthalate) substrates in 1992 [2]. On the other hand, the intrinsic conductivity of PEDOT:PSS is not so high, typically being lower than 10 S/cm. A typical method for enhancing conductivity of PEDOT:PSS is doping of poly-alcohols [3], high-dielectric solvents such as dimethyl sulfoxide (DMSO) [4], N,N-dimethylformamide (DMAc) [5], etc. OLED devices with such modified PEDOT:PSS electrodes were reported [3–5]. Our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) has developed OLED devices using practical transparent conducting polymers [6, 7]. Figure 7.2 shows characteristics of OLED devices with Fig. 7.1 Fundamental molecular structures of polyaniline and PEDOT:PSS

7.2 Transparent Conducting Polymer

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Fig. 7.2 Characteristics of OLED devices with the developed conducting polymer or ITO [6, 7]

the developed conducting polymer or with ITO, accompanying with the device structures. The conductivity of the conducting polymer is about 100 S/cm. The I-V characteristics, L-I characteristics, and lifetime curves of the OLED device with the developed conducting polymer are almost same as those with ITO. These results indicate that the developed conducting polymer can be used in practical applications. One of the problems of conducting polymers is low conductivity. Indeed, the conductivity of around 100 S/cm is acceptable for small pixels of displays but is too low in OLED devices with larger emission area such as OLED lighting. Therefore, for applying OLED devices with lager emission area, assisting electrodes are necessarily required. One example is a combination with assisting metal electrode as shown in Fig. 7.3 [6]. The size and emission area of the OLED device are 5 × 5 cm and 3.2 ×

Fig. 7.3 OLED device with the developed conducting polymer [6]. The size and emission area of the OLED device are 5 × 5 cm and 3.2 × 3.2 cm, respectively

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Fig. 7.4 Flexible OLED device with printed non-ITO electrode

3.2 cm, respectively. The assisting electrode is the stacking electrode of Mo/Al/Mo with the total thickness of 400 nm. The width and pitch of the assisting electrode are 30 µm and 1.5 mm, respectively. As shown in Fig. 7.3b, uniform emission is obtained with no serious problem such as non-uniform emission at the edge of the assisting electrode. This prototype was fabricated in 2014 and demonstrated in the lobby of our laboratory from 2014. At present (in 2022), no degradation is observed, indicating no serious problem in the reliability of the developed conducting polymer. Furukawa et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) have developed printing assisting electrodes for combining with the conducting polymer by using roll-to-roll (R2R) processes [8– 10]. The structure and the emitting picture of the developed flexible OLED device are shown in Fig. 7.4. As a flexible substrate, an ultra-thin glass with the thickness of 50 µm supplied from Nippon Electric Glass Co., Ltd. (Japan) was used. On the substrate, silver assisting electrodes were printed by using a R2R gravure off-set printing. The width, height, and pitch of the Ag electrode are 10–30 µm, 2–4 µm, and 1.5 mm, respectively. The conducting polymer was printed by R2R flexography printing and the insulating resin was printing on the position of the printed assisting electrode by using a R2R screen printing. The width and height of the insulating pattern are 80–150 µm and 6–10 µm, respectively. On this substrate, an OLED device was fabricated and encapsulated by using a laminating encapsulation as shown in Fig. 7.4. Uniform emission was obtained as shown in Fig. 7.4. As another assisting electrodes, we have also developed silver nanowire (AgNW) and metal-mesh electrode. The combination of the conducting polymer and silver nanowire is described in Sect. 7.3. The combination of the conducting polymer and silver nanowire is described in Sect. 7.4.

7.3 Silver Nanowire (AgNW) Silver nanowire (AgNW) is a printable transparent electrode with low resistivity of lower than 10 /sq and can be applied to OLED devices [11, 12].

7.3 Silver Nanowire (AgNW)

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Our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) has developed OLED devices with an AgNW electrode [9, 13–15]. In our preliminary investigations, we found that it was not so easy to obtain an uniform emission in OLED devices with an AgNW electrode. Figures 7.5 and 7.6 show the experimental results of the comparison of ITO and AgNW electrodes. While the OLED device with an ITO electrode shows a uniform emission, the emission of

Fig. 7.5 Device structures and emission pictures of OLED devices with an AgNW layer or normal ITO. The emission area is 2.0 × 2.8 mm

Fig. 7.6 I-V characteristic, L-I characteristics, and emission spectra of OLED devices shown in Fig. 7.5

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Fig. 7.7 Device structures and emission pictures of OLED devices with an AgNW layer or the stacking electrode of AgNW and conducting polymer. The emission area is 2.0 × 2.8 mm

the OLED device with an AgNW electrode is not uniform. This would be caused by non-uniform hole injection from the AgNW electrode to the adjacent organic layer. In addition, the I-V characteristic of the OLED device with an AgNW electrode is inferior to the case of ITO, though the L-I characteristics of both OLED devices are almost comparable. This phenomenon was also reported by the previous paper [11]. The increased voltage in an OLED device with an AgNW electrode seems to suggest the issue of hole injection from an AgNW electrode. To solve these problems, we investigated a stacking electrode of AgNW and conducting polymer. In this stacking electrode, AgNW and conducting polymer paly individual roles, respectively. The role of AgNW is low resistivity and the role of conducting polymer is hole injection. Figure 7.7 shows typical emission of OLED devices with the AgNW or the stacking electrode of AgNW and conducting polymer. Being different from the AgNW electrode shown in Fig. 7.7a, the OLED device with the stacking electrode of AgNW and conducting polymer shows uniform emission as shown in Fig. 7.7b. This result suggests that uniform hole injection to the adjacent organic layer is achieved by the introduction of conducting polymer. Figure 7.8 shows the I-V characteristics and the lifetime curves of OLED devices with a normal ITO electrode or the stacking electrode of AgNW and conducting polymer. The OLED device with the stacking electrode of AgNW and conducting polymer shows comparable I-V characteristics and better lifetime curve, comparing with the OLED device with an ITO electrode.

7.3 Silver Nanowire (AgNW)

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Fig. 7.8 I-V characteristic and lifetime curves of OLED devices with an ITO layer or the stacking electrode of AgNW and conducting polymer

Fig. 7.9 Flexible OLED devices with the stacking electrode of AgNW and conducting polymer [9]

The developed stacking electrode with AgNW and conducting polymer was applied to several OLED prototype devices. Figure 7.9 shows a flexible OLED device with the stacking electrode of AgNW and conducting polymer [9]. Figure 7.10 shows an OLED device with the stacking electrode of AgNW and conducting polymer, both of which are printed by flexography printing [13].

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Fig. 7.10 OLED device with the stacking electrode of AgNW and conducting polymer, both of which are printed by flexography printing [13]

7.4 Implanted Metal-Mesh Electrode Metal-mesh electrode is also a candidate for non-ITO electrodes. As a novel metalmesh electrode, Waguri et al. of Toyo Aluminium K.K. (Japan) have developed implanted metal-mesh electrode as shown in Fig. 7.11 [16, 17]. The materials of the metal-mesh electrodes are Al, Cu, etc. The typical thickness and width of the metal-mesh electrode are 15 µm and 75 µm, respectively. The implanted metal-mesh electrodes can be applied to not only rigid substrates but also flexible substrates. Such implanted structure can realize a flat surface, which is an advantage in the fabrication of OLED devices. In addition, due to the implanted structure, thick metal electrodes can be fabricated, giving low resistivity. Our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) has developed OLED devices with the implanted metal-mesh electrodes by the collaboration with Toyo Aluminium K.K. (Japan) [18]. The fabrication process of the implanted metal-mesh electrodes is shown in Fig. 7.12. As a first step, a metal sheet is laminated on a protecting film at 100 °C by a roll-type laminator. In the second step, an etching resist ink with the thickness of 3 µm is printed with a certain pattern and then the metal is etched, followed by wet

Fig. 7.11 Schematic structure and demonstrated samples of implanted metal-mesh electrodes developed by Toyo Aluminium K.K.

7.4 Implanted Metal-Mesh Electrode

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Fig. 7.12 Fabrication process of implanted metal-mesh electrodes developed by Toyo Aluminium K.K.

cleaning and drying. In the third step, a resin is printed on the patterned metal-mesh electrode by screen printing and cured by UV exposure. Dry-laminating-type adhesive with the thickness of 5 µm is coated on the resin and dried. Then, a substrate is laminated. In the fourth step, the protecting film is de-bonded. It should be noted that this process is compatible with roll-to-roll (R2R) processes with high productivity because it contains no complicated processes such as vacuum deposition, polishing, photolithography, etc. Figure 7.13 shows SEM images of implanted Cu mesh electrodes. As is shown in Fig. 7.13, flat surface and sharp pattern are achieved.

Fig. 7.13 SEM images of implanted Cu mesh electrodes developed by Toyo Aluminium K.K. (provided by Toyo Aluminium K.K.)

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Fig. 7.14 Device structure of OLED devices with the implanted Cu mesh electrode

Using the substrates with the implanted Cu mesh electrode developed by Waguri et al. of Toyo Aluminium, OLED devices were fabricated. The device structure of OLED devices with the implanted Cu mesh electrode is shown in Fig. 7.14. For hole injection from electrode to the organic material of OLED device, PEDOT:PSS is coated as a solution-type conducting electrode. The OLED device shows uniform emission as shown in Fig. 7.15, in which the emission of OLED device with normal ITO is also shown as reference. Figure 7.16 shows the I-V and L-I characteristics of

Fig. 7.15 Emitting pictures of OLED devices with the implanted Cu mesh electrode or ITO

Fig. 7.16 I-V and L-I characteristics of OLED devices with the implanted Cu mesh electrode or ITO

7.5 Roll-to-Roll (R2R) Fabrication of Transparent Electrodes

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Fig. 7.17 Prototype OLED devices with the implanted Cu mesh electrode. The substrate size is 60 × 100 mm

OLED devices with the implanted Cu mesh electrode or ITO. The characteristics of the OLED devices with the implanted Cu mesh is comparable with those with ITO. Rigid and flexible prototype OLED devices were successfully fabricated as shown in Fig. 7.17. These results indicate that the implanted Cu mesh electrodes can be applied to OLED devices. In particular, the implanted metal-mesh electrodes are useful for large-size OLED lighting devices which require low resistive electrodes for sufficient current supply. Since this type of implanted metal-mesh electrode is compatible with high productive processes such as roll to roll (R2R), it has promising potential to not only OLED but also other applications.

7.5 Roll-to-Roll (R2R) Fabrication of Transparent Electrodes The most common transparent electrodes in OLEDs, LCDs, etc. are ITO (indium tin oxide) and IZO (indium zinc oxide), which are included in the category of transparent conducting oxide (TCO). Indeed, ITO and IZO have such several advantages as reasonable conductivity, good transparency, chemical and temperature stabilities, etc. However, the process cost of these electrodes still has issues due to the costs of In, complicated photolithography process, expensive equipment, etc. Based on such background, Furukawa et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) have developed novel rollto-roll (R2R) fabrication technologies of transparent electrodes such as ITO, IZO, etc. on flexible substrates for flexible OLED devices by using no photolithography [7–9, 19–21]. The fundamental process flow of the developed technologies is shown in Fig. 7.18. As flexible substrates, ultra-thin glasses, stainless steel foils, and barrier films were applied. The width of flexible substrates is 300 mm. After the first wet cleaning of a flexible substrate by a R2R wet cleaning equipment of FEBACS CO., LTD. (Japan), a transparent conducting oxide (TCO) such as ITO, IZO, etc. is deposited by a R2R sputtering equipment of Kobe Steel, Ltd. (Japan) [22, 23], followed by the second wet cleaning. On the deposited TCO layer, an etching paste is printed by a R2R screen printing equipment of SERIA ENGINEERING, INC. (Japan) [24].

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Fig. 7.18 Fundamental process flow of R2R fabrication of transparent electrode on flexible substrate

By heating the substrate up to 150–170 °C, TCO is etched with no flow. After the substrate is cooled to room temperature, the etched TCO and the residual etching paste are removed by the third wet cleaning, giving a certain TCO pattern. These processes use no photolithography that is long and expensive process. In the sputtering deposition of TCOs, process temperature is a key parameter. Figure 7.19 shows some experimental results on the ITO layers fabricated on ultrathin glasses by the R2R sputtering [20]. When the process temperature is high (250 °C), the surface roughness Ra (arithmetical mean height) and the curl are large. Such rough surface is disadvantage in OLED device fabrication and such large curl is a serious issue in R2R processes. These phenomena seem to be attributed to crystallization of ITO deposited at high temperature. Indeed, the AFM image of ITO deposited at 250 °C shows good crystallinity, which can reduce resistivity of ITO. On the contrary, when the process temperature is −20 °C, the Ra and the curl are small. Such small or no curl is important because large curl tends to induce various issues in R2R processes. The AFM images of ITO deposited at −20 °C show poor crystallinity. It is supposed that such poor crystallinity does not induce large mechanical stress, giving small curl. One of the problems of ITO deposited at − 20 °C is high resistivity. Indeed, the as-deposited ITO layers deposited at −20 °C show higher resistivity than 30 /. However, post-annealing treatment can realize comparable resistivity with 250 °C, inducing no change in surface roughness and curl. On the deposited TCO layers, an etching paste is printed by a R2R screen printing equipment of SERIA ENGINEERING, INC. (Japan). This screen printing equipment

7.5 Roll-to-Roll (R2R) Fabrication of Transparent Electrodes

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Fig. 7.19 Characteristics of ITO layer deposited with different condition on ultra-thin glass by R2R sputtering [20]

is unique, consisting of a printing roller, a flat stencil mask, a suction type roller, etc. as shown in Fig. 7.20 [24]. Classical screen equipment use a flat stage and a flat stencil mask, which is deformed by a press of squeegee at screen printing process. On the contrary, the R2R equipment in this study has a print roller instead of a flat stage. A flexible substrate is sandwiched between a print roller and a flat stencil mask. In this equipment, the gap between the stencil mask and the flexible substrate

Fig. 7.20 Fundamental mechanism of roll-to-roll (R2R) screen printing equipment of SERIA ENGINEERING INC. [26]

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Fig. 7.21 Etching mechanism of TCO (transparent conducting oxide) by etching paste

Fig. 7.22 Deveoped flexible OLED devices using roll-to-roll (R2R) fabrication os transparent electrode

is zero because of no deformation of the stencil mask, while the gap in the classical screen printing is not zero. The zero gap induces small error of accuracy of position and size of patterns. The process mechanism of the TCO etching by etching paste is shown in Fig. 7.21. By heating the flexible substrate up to 150–170 °C, TCO is etched with no flow. After cooling to room temperature and wet cleaning, the etched TCO and the residual etching paste are removed, giving certain TCO patterns. These processes use no photolithography process which is long and expensive process. In addition, this etching process can also be applied to stacking electrode consisting of Ag-alloy and TCO [25, 26] Based on the fundamental processes mentioned above, assisting electrodes and insulators can be printed on the TCO patterns by roll-to-roll (R2R) printings. After cutting the flexible substrates with TCO patterns, flexible OLED devices were fabricated as shown in Figs. 3.21, 4.11, 4.18, 5.11, and 7.22.

References

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References 1. J. Colegrove, Inf. Disp. 30(4), 24–27 (2014) 2. G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357, 477–479 (1992) 3. W.H. Kim, A.J. Makinen, N. Nikolov, R. Shashidhar, H. Kim, Z.H. Kafafi, Appl. Phys. Lett. 80(20), 3844–3846 (2002) 4. J. Ouyang, C.-W. Chu, F.-W. Chen, Q. Xu, Y. Yang, Adv. Funct. Mater. 15(2), 203–208 (2005) 5. K. Fehse, K. Walzer, K. Leo, W. Lövenich, A. Elschner, Adv. Mater. 19, 441–444 (2007) 6. M. Koden, H. Kobayashi, T. Moriya, N. Kawamura, T. Furukawa, H. Nakada, IDW’14, FLX6/FMC6-1 (2014), pp. 1454–1457 7. M. Koden, Proc. AM-FPD’15 (The Twenty-second International Workshop on Active-matrix Flatpanel Displays and Devices), 2-1 (2015), pp. 13–16 8. T. Furukawa, N. Kawamura, M. Sakakibara, M. Koden, International Display Manufacturing Conference (IDMC’15), S4-4 (2015) 9. M. Koden, T. Furukawa, T. Yuki, H. Kobayashi, H. Nakada, Proc. IDW/AD’16, FLX3-1 (2016), pp. 1352–1355 10. T. Furukawa, M. Sakakibara, N. Kawamura, M. Koden, Proc. IDW/AD’16, FLX3-3 (2016), pp. 1360–1363 11. Z. Yu, Q. Zhang, L. Li, Q. Chen, X. Niu, J. Liu, Q. Pei, Adv. Mater. 23, 664–668 (2011) 12. F. Pschenitzka, SID 2013 DIGEST, 61.4 (2013), pp. 852–855 13. T. Furukawa, N. Kawamura, J. Inoue, H. Nakada, M. Koden, SID 2015 Digest, P-57 (2015), p. 1355 14. H. Nakada, N. Kawamura, M. Koden, Proc. of 20th Japan OLED forum, S6-3 (2015) 15. T. Yuki, N. Kawamura, H. Nakada, M. Koden, Proc. of 21th Japan OLED forum, S4-9 (2015) 16. R. Waguri, K. Den, R. Nakao, H. Minamiyama, Proc. of MES2020, 1B1-2 (2020) 17. R. Waguri, Toyal Technical Report (2019) 18. M. Koden, N. Kawamura, T. Yuki, H. Nakada, R. Waguri, K. Den, R. Nakao, H. Minamiyama, The 31th Meeting of Japan OLED Forum, S7-2 (2020) 19. T. Furukawa, K. Mitsugi, H. Itoh, D. Kobayashi, T. Suzuki, H. Kuroiwa, M. Sakakibara, K. Tanaka, N. Kawamura, M. Koden, IDW’14, FLX3-4L (2014), pp. 1428–1429 20. T. Furukawa, M. Koden, IEICE Trans. Electron, E100-C (2017), pp. 949–954 21. T. Furukawa, N. Kawamura, T. Noda, Y. Hasegawa, D. Kobayashi, M. Koden, IDW’17, FLX6-2 (2017), pp. 1535–1538 22. Y. Ikari, H. Tamagaki, Proc. IDW 2012, FLX5/FMC5-1 (2012), pp. 1493–1496 23. H. Tamagaki, Y. Ikari, N. Ohba, Surf. Coat. Technol. 241, 138–141 (2014) 24. D. Kobayashi, N. Naoi, T. Suzuki, T. Sasaki, T. Furukawa, IDW’14, FLX3-1 (2014), pp. 1417– 1420 25. Y. Hagiwara, H. Itoh, T. Furukawa, H. Kobayashi, S. Yamaguchi, N. Yamada, J. Nakatsuka, M. Koden, H. Nakada, Proc. IDW/AD’16, FLXp1-5 (2016), pp. 1408–1411 26. Y. Hagiwara, T. Furukawa, T. Yuki, S. Yamaguchi, N. Yamada, J. Nakatsuka, M. Koden, H. Nakada, Proc. IDW’17, FLXp1-9L, pp. 1572–1574

Chapter 8

OLEDs with On-Demand Patterns by Ink-Jet Printing

Abstract In addition to current major OLED applications such as mobile phones, large-size TVs, etc., one of other promising application fields might be OLED lighting devices with certain design patterns. Such OLED devices can be applied to various products such as direction indicators, emergency signs, labels, packaging, advertisements, souvenirs, name plates, name tags, etc. For fabricating such OLEDs, ondemand patterning of OLEDs was developed by patterning of insulators fabricated by ink-jet printing by Sugimoto et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan). The developed OLEDs with ondemand patterns show comparable performances with OLEDs without the patterned insulators. By preventing ink spreading, we achieved high-resolution patterns with L/S = 61/62 μm, which is almost comparable with 400 dpi. Keywords On-demand · OLED · Ink jet · Design lighting · Signage · Advertisement

8.1 Background While OLED display technologies have induced huge markets in mobile uses, largesize TV, etc., one of other interesting application fields might be OLED lighting devices with certain design patterns. Doust of Cambridge Display Technology Ltd. (CDT) (UK), which belongs to Sumitomo Chemical Group (Japan), presented that OLED lighting devices with certain designs can be fabricated by wet processes and can be applied to various applications such as promotional signage, etc. [1]. INURU (Germany) developed flexible OLED devices with the lightsaber pattern of a seen of “Star Wars”. The flexible OLED devices were applied to Coca Cola bottles [2]. The lightsaber part lights up by naturally touching. YU-FIC (Yamagata University Flexible Electronics Japan-Germany International Collaborative Practical Utilization Consortium), which is led by Furukawa of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) demonstrated several flexible OLED samples that are applied to a notepad, an inro (Japanese unique medicine case with coat of arms) and a bookmark, all of © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Koden, Flexible OLEDs, Display Science and Technology, https://doi.org/10.1007/978-981-19-3544-2_8

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which can light up with the coat of arms of Tokugawa family who produced Japanese emperors from seventeenth to nineteenth centuries. Figure 8.1 shows their prototype samples. In such OLEDs with certain patterns, electrodes or insulators on electrode patterns are usually patterned with a certain design. For patterning electrodes or insulators, photolithography process is usually used. Figure 8.2 shows a typical fabrication process of photolithographic patterning of insulators on bottom electrode (normally anode). However, photolithography is an expensive technology. This method requires photomasks for exposure. When the production amount is small, photolithography is unreasonable because each design requires each photomask.

Fig. 8.1 Prototype samples with a flexible OLED with a special design

Fig. 8.2 Typical photolithographic process for fabricating a certain pattern of insulators on bottom electrodes

8.2 Experimental

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Fig. 8.3 Ink-jet on-demand patterning of insulators on bottom electrode (normally anode)

As one of the counter-approaches, Sugimoto et al. of our research group (Research Group for Flexible Technologies) in Yamagata University (Japan) developed an ondemand patterning of insulators on electrodes, by using on-demand ink-jet printing [3–5]. The schematic process of the developed technologies is shown in Fig. 8.3. On-demand patterns of insulators are printed by ink jet on a substrate with anode, followed by curing with UV exposure and thermal baking. Using these substrates, OLED devices with on-demand patterns are easily fabricated. In this technology, ink-jet printing technologies on anodes of OLEDs were developed using organic resin for TFE (thin film encapsulation) of OLEDs. Sections. 8.2, 8.3, 8.4, and 8.5 describe the ink-jet fabrication processes of insulators, the OLED performances (I-L-V, lifetime, etc.), prototype OLED samples etc., indicating that the developed technologies can realize fine patterns with L/S = 61/62 μm and comparable performance with normal OLEDs and can be applied to practical applications.

8.2 Experimental The insulating materials used in this study are LIORESISTTM NSP 800 series of TOYOCHEM CO., LTD. (Japan). These materials have been developed for the resin sandwiched by inorganic barrier layer in TFE (thin film encapsulation) structure for OLED devices. These materials are non-solvent and UV-cure type. The applications of this type of materials to TFE technologies are described in Sect. 6.3. This study uses three types of materials that are A, B, and C with different contact angle. After wet cleaning and baking of glass substrate with ITO electrodes, the insulator material is printed on the substrates by ink-jet machine. The ink-jet machine used in this study is DMP-2850 of FUJIFILM Corporation (Japan). After ink-jet printing of the insulator material and baking, OLED devices are fabricated on the substrates, followed by encapsulation.

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8.3 On-Demand Patterns of Insulators Printed by Ink Jet On-demand OLEDs need fine patterns of required designs. For achieving fine patterns, pre-treatment before ink-jet printing and insulating materials should be optimized. Figure 8.4 shows resultant patterns of insulator material A printed by ink jet on various substrates with different pre-treatment. When the pre-treatment is wet cleaning only, the resultant pattern is soggy as shown in Fig. 8.4a. This result indicates that the spreading of ink is remarkably influenced by the un-uniform surface condition. On the other hand, when the surface is cleaned by additional UV/O3 treatment, resultant patterns are drastically improved as shown in Fig. 8.3b–g. Comparing Fig. 8.3b–g, it is found that the sharper pattern is obtained by the stronger UV/O3 treatment, which means higher treatment temperature and longer treatment time. These results clearly indicate that surface cleanness is absolutely required for obtaining fine patterns. For obtaining further fine patterns, insulator materials are optimized. Figure 8.5 shows resultant patterns of insulator materials A, B, and C printed by ink jet. The materials A, B, and C show different contact angle. The surface treatment is 10 min. UV/O3 under 300 °C, which is the best condition obtained in Fig. 8.5. In the samples A and B, in which the contact angles are 14 and 16°, respectively, the resultant patterns spread about 190 μm. On the other hand, in the sample C with the contact angle of 24°, the resultant width of line is 62 μm. Figure 8.6 shows the resultant line and space patterns of the insulator material C printed by ink jet on the glass substrate treated by 10 min. UV/O3 under 300 °C. The obtained line and space are 61/62 μm, which is almost comparable with the resolution of 400 dpi.

Fig. 8.4 Resultant patterns of insulator material A printed by ink jet on various substrates with different pre-treatment

8.4 OLEDs Fabricated on Insulators Patterns by Ink-Jet Printing

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Fig. 8.5 Resultant patterns of insulator materials A, B, and C printed by ink jet after the surface treatment of 10 min. UV/O3 under 300 °C

Fig. 8.6 Resultant line and space pattern of insulator materials C printed by ink jet after the surface treatment of 10 min. UV/O3 under 300 °C

8.4 OLEDs Fabricated on Insulators Patterns by Ink-Jet Printing On the glass substrates with insulator patterns by ink-jet printing, OLED devices were fabricated. The OLED architecture is ITO (150 nm)/HATCN (5 nm)/α-NPD (80 nm)/Alq3 (35 nm)/ETM (24 nm)/LiF (1.5 nm)/Al (100 nm). Figure 8.7 shows the emitting pictures of OLEDs with or without insulator. The OLED with the insulator printed by ink jet shows clear uniform emission with no difference from the OLED without the insulator. Figure 8.8 shows the I-L-V characteristics of the OLED devices with the on-demand pattern of the insulator or without it. Figure 8.9 shows the lifetime curves of the OLED devices with the on-demand

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Fig. 8.7 Emitting pictures of OLEDs with or without insulator

Fig. 8.8 I-L-V characteristics of OLED devices with the on-demand pattern of the insulator or without it

Fig. 8.9 Lifetime curves of OLED devices with the on-demand pattern of the insulator or without it

pattern of the insulator or without it. The I-L-V characteristics and lifetime curve of the OLED device with the on-demand pattern of the insulator are almost comparable with that without the insulator. These results indicate that the insulator printed by ink jet gives no negative damage on OLED performances.

8.5 OLEDs with On-Demand Patterns by Ink-Jet Printing

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8.5 OLEDs with On-Demand Patterns by Ink-Jet Printing On-demand OLED devices were fabricated using the developed technologies mentioned above. The emission pictures are shown in Fig. 8.10. Fine patterns are successfully fabricated. Since the developed technology achieves fine patterns with L/S = 61/62 μm, which is almost comparable with 400 dpi, it can be applied to various OLED devices with on-demand designs. Being much different from the common photolithography process, the developed technology can significantly reduce the production cost because of no photomask, short fabrication term, drastically reduced human resources, etc. This technology can contribute to novel applications of OLEDs. Figure 8.11 shows some prototype applications of the OLEDs with an on-demand pattern. Since these are only-one designs in the world, the developed on-demand technology is suitable for such only-one uses as greeting card to a certain person, message board for special event, today’s menu, etc.

Fig. 8.10 Prototype on-demand OLED sample

Fig. 8.11 Prototype applications of an OLED with an on-demand pattern

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References 1. A. Doust, Printed-Electronics-2016 (IDTechEx) (2016) 2. News release of INURU Coca-Cola launches illuminated Star Wars Limited Edition (2019) 3. M. Sugimoto, Y. Fukuchi, H. Tsuruta, M. Koden, H. Nakada, T. Yuki, The 31st Meeting of Japan OLED Forum, S7-3 (2020) 4. M. Sugimoto, Y. Fukuchi, H. Tsuruta, M. Koden, H. Nakada, T. Yuki, The 32nd Meeting of Japan OLED Forum, S5-1 (2021) 5. M. Sugimoto, Y. Fukuchi, H. Tsuruta, M. Koden, H. Nakada, T. Yuki, A-COE2021 (The 13th Asian Conference on Organic Electronics), PA-17 (2021)