Polymers for flexible displays: From material selection to device applications

doi:10.1016/j.progpolymsci.2007.11.004

Progress in Polymer Science

Volume 33, Issue 6, June 2008, Pages 581-630

https://doi.org/10.1016/j.progpolymsci.2007.11.004 Get rights and content

Abstract

With digitalization, plenty of information is being exchanged through electronic media, and consumers are demanding high quality, convenient, and portable digital devices. Currently, flat panel displays, such as liquid crystal displays (LCDs) and plasma display panels (PDPs), satisfy them with regard to quality. Convenience and portability will be realized with flexible displays in the future. Polymers are very promising materials for flexible displays with many advantageous charateristics including transparency, light weight, flexibility, and robustness. They are also some of the least expensive materials and are suitable for mass production via roll-to-roll processes. In this review, we will discuss the kinds of polymers that are used, where and how polymer materials are used and the challenges to overcome in developing flexible displays.

Introduction

The topic of flexible displays has prompted many popular news stories. How do we define a flexible display? Flexible displays can be classified according to degree of flexibility: flat displays are made of plastic or another non-glass backplane, but only for the benefit of lightness or ruggedness; formed displays are bent once, such as a curved automobile dashboard, but do not flex further; flexible displays may be bent or flexed during use, but not over a range that includes folding or rolling; rollable displays are as flexible as fabric [1]. Recently, the literature on flexible displays has been expanding. It now includes a book on flexible flat panel displays written by Crawford [2] and a special edition of the Proceedings of the IEEE on flexible displays [3].

The prospects for flexible displays are promising, although the timing still depends on technical and manufacturing developments [4], [5]. Electrophoretic displays such as electronic papers using plastic substrates, which have a relatively simple structure, are just beginning to be produced in quantities approaching high volume. Displays that are intended to flex or roll during use may reach the market in several years, pending further developments in backplane and fabrication processes. The near-term revenue in dynamic signage and mobile phones will lead to the development of larger and more sophisticated displays with flexibility and rollability. Fig. 1, which has been adapted from the data of the iSuppli Flexible Display Report, shows the market prospects for flexible displays from 2007 to 2013 [1].

Polymers are very promising materials for flexible displays with many advantages. They are transparent, light in weight, flexible, and robust. Polymers are a good alternative to the glass substrates that have been actively used for flat panel displays such as liquid crystal displays (LCDs) and plasma discharge panels (PDPs). Glass is so rigid that it is very difficult to use in a flexible display. Polymers have mechanical properties that vary from strong rigidity, such as in engineering plastics, to softness, such as in rubber or polyethylene films. They are some of the least expensive materials and are suitable for mass production via roll-to-roll (RTR) processes. Therefore, polymers are being considered as the key materials for flexible displays in various application areas including transparent substrates, electrodes, active materials for organic light-emitting devices (OLEDs), LCDs and organic thin-film transistors (OTFTs), dielectric materials, and coating materials. All polymer-based flexible displays are being investigated.

In this review, we will discuss what, where, and how polymer materials are used and the challenges to overcome in the flexible display field.

Section snippets

Polymer substrates

In flexible displays, the flexibility depends on the substrate. Three kinds of substrates are considered to be flexible: thin glass, metal foil, and plastic. Thin glass films are bendable and have the highly desirable qualities of glass [6]. However, they are brittle. This property limits their application as flexible substrates. Metal foils can also handle high-process temperatures and provide a good barrier to moisture and oxygen, without the problems of breakability [7]. However, metal only

Barrier coatings

Inorganic transparent oxide films (e.g. silicon and aluminum oxide) on polymer films have been widely used as gas barrier materials for food and medical packaging. They provide at best only two to three orders of magnitude improvement over the oxygen transmission rates (OTR) of polymer substrates, whether deposited by plasma-enhanced chemical vapor deposition (PECVD), sputtering, or evaporation [42].

However, electronic devices, especially OLEDs, demand more stringent barrier confinement. For an

Transparent electrodes

Most electro-optic devices such as LCDs, OLEDs, and electronic papers (e-papers) require electrically conductive and transparent electrodes. Indium tin oxide (ITO) thin films deposited on glass substrates have been widely used as transparent conducting electrodes in many electro-optic devices because they possess attractive properties with respect to visible transparency and electrical conductivity [2]. However, ITO thin films have several drawbacks. They are very expensive because indium is

Liquid crystal displays (LCDs)

Flat panel LCDs have extended their boundaries from small size mobile phones to large-size televisions because they are thin and lightweight and have a low power consumption and excellent resolution. However, there are critical problems of stability when applying conventional LCD technologies to flexible LCDs [122]. Flexing the panel creates forces that will cause the liquid crystal to flow, resulting in cell-gap variations across the panel, visual distortions, and artifacts, which can easily

Thin-film transistors (TFTs)

Low-temperature process technologies in TFT fabrication are the most crucial for flexible displays [296]. The TFT processes developed for flat panel display using rigid glass substrates cannot readily be applied for use with flexible plastic substrates due to the limitations of process temperature, lack of dimensional stability, and thermal stresses between the TFT thin films and the substrate.

There are two main approaches for fabricating TFTs on plastic substrates. One is to transfer

Encapsulation

In the previous section, a multilayer barrier structure on plastic substrates was demonstrated, which protects devices, especially OLEDs, from moisture or oxygen [39]. A single layer of an inorganic compound such as SiO₂, Al₂O₃, SiN_x, or MgO can be adequate as a barrier layer for LCDs. A very dense and amorphous single layer also can be used as an encapsulant for an OLED on glass. But in a flexible display, it may be difficult to maintain the integrity of a single-layer barrier, especially if

Roll-to-roll (RTR) processes

The current method of producing display panels, circuit boards, and other electronic devices is a batch process using conventional vacuum deposition and lithography pattern technologies on silicon wafers or glass substrates. On the other hand, the RTR process is currently a well-known technology to the film manufacturer in diverse areas such as newspapers, labels, etc. [40].

RTR processing offers a significant advantage compared with the conventional batch process, as it increases throughput by

Conclusions

Flexible displays will be the ultimate choice in the future in the display industry because of their many advantages including convenience, portability, and large-size applications as well as low-cost production through RTR processes.

However, there are many challenges to surmount, as summarized in Fig. 25. The thermal stability, solvent resistance, thermal expansion coefficients, and gas absorption of polymer substrates need to be improved so that they are as good as glass substrates. OLEDs

Acknowledgments

The work was supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Laboratory Program funded by the Ministry of Science and Technology (MOST) (No. M10300000369-06J0000-36910), the SRC/ERC of MOST/KOSEF program (Grant #R11-2000-070-080020), and the Brain Korea 21 Project.

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Polymers for flexible displays: From material selection to device applications

Abstract

Introduction

Section snippets

Polymer substrates

Barrier coatings

Transparent electrodes

Liquid crystal displays (LCDs)

Thin-film transistors (TFTs)

Encapsulation

Roll-to-roll (RTR) processes

Conclusions

Acknowledgments

Syn Met

Mater Today

Thin Solid Films

Displays

Prog Mater Sci

J Membr Sci

Surf Coat Technol

Surf Coat Technol

Surf Coat Technol

Thin Solid Films

Thin Solid Films

Thin Solid Films

Surf Coat Technol

Thin Solid Films

Surf Coat Technol

Surf Coat Technol

Surf Coat Technol

Surf Coat Technol

Appl Surf Sci

Vacuum

Thin Solid Films

Appl Surf Sci

iSuppli flexible displays report

Veritas et Visus, Flex Substr

Flexible flat panel display technology

Special issue on: flexible electronics technology

Proc IEEE

OLED and paper-like display markets

Veritas et Visus, Flex Substr

Reel to real: prospects for flexible displays

Proc IEEE

Thin glass-polymer systems as flexible substrates for displays

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Plastic substrates for flexible displays

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Flexible organic electroluminescent devices based on fluorine-containing colorless polyimide substrates

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