Graphene for Flexible Lighting and Displays

Tae Woo Lee

• Publisher: Woodhead Publishing
• Year: 2019
• Print ISBN: 9780081024829
• eBook ISBN: 9780081024836
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Graphene for Next Generation Lighting and Displays provides readers with a comprehensive overview of graphene, flexible graphene electrodes, and graphene-based next-generation display and lighting. The book covers a wide range of information, including the basic physics of graphene and recent trends in technical developments for graphene-based flexible and stretchable light-emitting devices. In addition, it discusses future prospects and suggests further directions for research on graphene-based next-generation displays and lightings. In addition, the book includes sections on the fundamental properties of graphene, synthetic methods of graphene, preparation of graphene electrodes and composite electrodes, and doping methods for graphene electrodes. Potential applications are also addressed including graphene-based flexible electrodes, buffer layer, emitters, and graphene-based stretchable electrodes. - Reviews the most promising applications, including OLEDs, graphene-based buffer layers for LEDs, quantum dot emitters, and stretchable graphene electrodes - Describes practical approaches in modifying the properties of graphene for the purpose of optoelectronic applications

1

Introduction

Tae-Woo Lee ¹ , and Sung-Joo Kwon ^{2 1} Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro Gwanak-gu, Seoul, Republic of Korea ² Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk, Republic of Korea

Abstract

This chapter briefly explains the contents of this book entitled “Graphene for Flexible Lighting and Displays.” Here, we first introduce the advance history about the various light-emitting diodes (LEDs) and prospect of use of graphene in LEDs. Graphene can be employed in various components of LEDs ranging from electrode to interfacial layers and emitting layers with proper modification. We briefly introduce the various modification approaches for graphene in LED technology. Finally, we provide an outlook for graphene to be used in stretchable electronics.

Keywords

Displays; Electrode; Graphene; Light-emitting diodes; Lightings; Properties

Humans have always had need for lighting and have recently developed a reliance on information displays (e.g., computer screens) based on lighting. The invention of the incandescent light bulb drove a revolutionary change in human life by freeing us from dependence on fire. However, this technology has poor luminous efficiency and severe heat generation, so it is about to be supplanted by light-emitting diodes (LEDs). Significant advance in LED research has achieved blue LEDs and white LEDs with luminous efficiency >100 lm/W [1]. Therefore, LEDs have diverse applications, including use in displays to visualize information for human interpretation. Advances in information technology have increased humans' need to share information, so emphasis on display technology has increased.

Organic light–emitting diodes (OLEDs) are also promising next-generation light sources. OLEDs have advantages such as light weight, easy color tunability, designable form, and suitability for large-area fabrication, so they also have applications in displays [2]. Alternative emitters (e.g., quantum dots (QDs), perovskite) are being evaluated for use in LEDs, and their emissive properties have been continuously improved. The luminous properties of display and lighting technology have been advanced remarkably, and the forms of lighting and display devices have been diversified in response to the demands of industry. To satisfy future requirements, lighting and displays should be bendable, foldable, and stretchable.

Conventional lighting and displays are mostly fabricated on a transparent electrode that is formed from a conducting oxide (e.g., indium tin oxide (ITO)), which is brittle. To achieve flexible lighting and displays, these brittle elements must be replaced with flexible components. Therefore, many flexible transparent conductors (e.g., graphene, carbon nanotubes, metal nanowires, and conducting polymers) have been evaluated as materials to replace ITO [3–6].

Graphene has remarkable electrical, optical, and mechanical properties and, therefore, has good potential as a flexible electrode to replace ITO in lighting and display devices (Fig. 1.1). However, pristine graphene has high sheet resistance (R _S ) and low work function (WF), so OLEDs with graphene electrode have poor luminous properties [3]. Chemical doping of graphene can modify Rs and the WF, and thereby substantially improve the charge injection from the graphene electrode to overlying layers. By this approach, the luminous properties of OLEDs based on graphene electrodes have been increased to levels comparable with those of OLEDs that use ITO electrodes [3,7,8].

Figure 1.1 Schematic drawings of future lighting and displays using graphene electrodes.

Further modifications in the properties of graphene are possible. Insertion of a graphene-based interfacial layer can improve the charge injection and increase the luminous properties of LEDs that use graphene. Chemical functionalization (e.g., oxidation, hydrogenation) can induce a bandgap in the electronic structure of graphene [9,10]; the bandgap can provide an intermediate step for charge injection, so the luminous properties of LEDs can be improved.

Graphene can also be used as an emitting layer. After chemical functionalization by surface passivation, a graphene QD emitter is less toxic, more chemically stable, and has higher carrier mobility than conventional inorganic QDs [11].

Ideally, flexible electronics should also be stretchable. Graphene has outstanding mechanical flexibility, but strong in-plane stiffness (340 N/m) and Young's modulus (0.5 TPa), which impede the use of graphene in stretchable electronics that requires stretchability above 10% [12]. Mechanical stress cannot be dissipated in the graphene lattice because of the strong bonding between the carbon atoms. For instance, CVD grown graphene on elastic substrate lose its electrical conductivity under 6% of mechanical strain [13]. Several structural modifications or combinations with other stretchable conducting materials have improved the stretchability of graphene and have been used in stretchable electronics [12,14–16].

In this book, we first introduce the electrical, optical, and mechanical properties of graphene and present the preparation of graphene and treatments to modify its electrical properties. Then we examine the use of flexible or stretchable electrodes with graphene in various lighting and displays (e.g., LEDs, OLEDs, QD-LEDs, perovskite LEDs). We also review the use of graphene in interfacial materials or emitting materials of LEDs.

References

[1] Narukawa Y, Narita J, Sakamoto T, Deguchi K, Yamada T, Mukai T. Ultra-high efficiency white light emitting diodes. Jpn. J. Appl. Phys. 2006;45(41):L1084–L1086.

[2] Park M.-H, Han T.-H, Kim Y.-H, Jeong S.-H, Lee Y, Seo H.-K, Cho H, Lee T.-W. Flexible organic light-emitting diodes for solid-state lighting. J. Photonics Energy . 2015;5(1):053599. .

[3] Han T.-H, Lee Y, Choi M.-R, Woo S.-H, Bae S.-H, Hong B.H, Ahn J.-H, Lee T.-W. Extremely efficient flexible organic light-emitting diodes with modified graphene anode. Nat. Photonics . 2012;6:105–110.

[4] Ou E.C.-W, Hu L, Raymond G.C.R, Soo O.K, Pan J, Zheng Z, Park Y, Hecht D, Irvin G, Drzaic P, Gruner G. Surface-modified nanotube anodes for high performance organic light-emitting diode. ACS Nano . 2009;3(8):2258–2264.

[5] Yu Z, Zhang Q, Li L, Chen Q, Niu X, Liu J, Pei Q. Highly flexible silver nanowire electrodes for shape-memory polymer light-emitting diodes. Adv. Mater. 2011;23(5):664–668.

[6] Cai M, Ye Z, Xiao T, Liu R, Chen Y, Mayer R.W, Biswas R, Ho K.-M, Shinar R, Shinar J. Extremely efficient indium–tin-oxide-free green phosphorescent organic light-emitting diodes. Adv. Mater. 2012;24(31):4337–4342.

[7] Han T.-H, Kwon S.-J, Li N, Seo H.-K, Xu W, Kim K.S, Lee T.-W. Versatile p-type chemical doping to achieve ideal flexible graphene electrodes. Angew. Chem. Int. Ed. 20...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• About the editor
• Preface
• Acknowledgments
• 1. Introduction
• 2. Structure and properties of graphene
• 3. Preparation of graphene electrode
• 4. Graphene doping for electrode application
• 5. Technical issues and integration scheme for graphene electrode OLED panels
• 6. Graphene-based buffer layers for light-emitting diodes
• 7. Graphene-based quantum dot emitters for light-emitting diodes
• 8. Graphene-based composite emitter
• 9. Stretchable graphene electrodes
• 10. Conclusions and outlook
• Index