Flexible Flat-Panel Displays

Flexible flat-panel displays need a lot of work, but they offer exciting possibilities for entirely new displays and display-related products.

by Gregory P. Crawford

IS THE DISPLAY INDUSTRY on the cusp of a revolution? Industrious and creative researchers are striving to realize the successful development and mass production of truly flexible flat-panel displays (FPDs), but they face an industry that typically chooses economics over technology. Optimistic display scientists must therefore overcome not only technological barriers, but also the economic challenges of producing financially viable products. Will the pioneers overcome the difficulties they face, or will the desired revolution remain a dream? No one can predict what the future holds, but it is possible for researchers to realize truly flexible displays and bring them to market, potentially enabling a wealth of new product categories.

Flexible-FPD technologies offer many potential advantages, including light weight and robust thin profiles; the ability to flex, curve, conform, roll, and fold for extreme portability; high throughput in manufacturing; the ability to be integrated into garments and textiles; and remarkable engineering design freedom, including oddly shaped displays (Fig. 1).

But for flexible FPDs to become a reality, extensive industrywide transformations must occur. The most obvious transformation will be the replacement of rigid glass substrates by flexible organic or inorganic substrates, which would have major implications for the entire display industry. In particular, the infrastructures for manufacturing, processing, and assembly will take on a completely new look, especially if roll-to-roll methods are implemented. If continuous roll-to-roll web processing can replace conventional batch processing, significant economies of scale will accrue, as will the potential for extremely large-area displays.

Definition

The flexible-display concept has been in the minds of display designers and engineers for many years, but that does not mean they agree on what it means. A random survey of the display community asking for the definition of a flexible display would almost certainly produce many different answers, depending on how each of the polled individuals envision the potential application. Many commonalities would, however, exist in all of these definitions. The broad definition of a flexible FPD is "an FPD constructed of thin (flexible) substrates that can be bent, flexed, conformed, or rolled to a radius of curvature of a few centimeters without losing functionality" [P. J. Slikkerveer, "Bending the Rules," Information Display 19, No. 3, 20–24 (2003)].

Slikkerveer argues that defining a flexible display is akin to defining modern art, with both exhibiting thought-provoking design freedom. Because of the many possible applications of flexible-display technology, it is difficult to propose an all-encompassing definition. Some flexible displays may only be flexed once during their lifetime – during manufacturing to create a permanently conformed display, for example – while a rollable display may require more than 100 rollings and unrollings per day.

Convergence

The pursuit of flexible FPDs is not a new undertaking; plastic liquid-crystal displays (LCDs) were evaluated in the 1980s and 1990s but fell short of commercial success. Some of the problems that plagued early researchers still persist. But something is different today. The convergence of many technologies in recent years, including substrates, conducting layers, barrier layers, electro-optic materials, thin-film-transistor (TFT) technologies, and manufacturing processes, is providing a rationale for the rejuvenation of the flexible-FPD concept.

Display companies around the world have developed flexible-FPD prototypes. Many smaller entrepreneurial firms that focus on display components are "growing up" around the flexible-display concept and are betting on a paradigm shift to the flexible market in the near future.

The field combines principles from nearly all of the engineering and science disciplines. The pursuit of flexible displays has led researchers to draw on the expertise and technologies of seemingly unrelated industries, such as printing, packaging, and manufacturing, and, in doing so, push the envelope of technological innovation and uncover countless new opportunities.

Of course, there are many obstacles and challenges preventing flexible FPDs from realizing their full commercial potential. These obstacles and challenges represent great opportunities for researchers and high-technology companies.

Enabling Technologies

Anticipating a new market opportunity, the display industry has been developing materials targeted specifically at the requirements of flexible FPDs. The necessary technologies include robust flexible substrates, transparent conducting oxides and conducting polymers, electro-optic and reflecting materials, inorganic and organic electronics, and packaging technologies. Processes, including roll-to-roll manufacturing, coating, and printing technologies, must be developed and optimized in concert with the materials development.

Flexible Substrates and Barrier Layers

Substrates present one of the most formidable challenges for flexible displays because they must not only be of high optical quality but must also have dimensional stability when subjected to a variety of manufacturing-process conditions including chemicals and temperature.

Two choices for flexible substrates currently exist: polymer and thin glass. William A. MacDonald and his co-workers at DuPont Teijin Film have been developing polymer films engineered for flexible-display technologies. Limitations on manufacturing-process temperature must be taken into account when replacing a glass substrate with a plastic one, and these limitations may vary with polymer type, optical properties, thermal properties, and surface smoothness.

Fig. 1: Concepts for several flexible-display products have been illustrated by artists. (Renditions courtesy of Suraj Gorkhali, Brown University; reproduced by permission of John Wiley & Sons, Ltd.)

 (a)  (b)  (c)

Fig. 2: (a) and (b) ITO can crack under bending and tensile loads, resulting in a jump in resistance. (Courtesy of Peter J. Slikkerveer, Philips Research Laboratories; reproduced by permission of John Wiley & Sons, Ltd.) (c) Conducting polymer performs better under fatigue testing (rolling and unrolling of the substrate) than the more-brittle ITO conductive coating. (Courtesy of Darren R. Cairns, 3M TouchScreens; reproduced by permission of John Wiley & Sons, Ltd.)

The manufacturing-process temperature required by the deposition of subsequent layers is one of the greatest challenges for polymericsubstrates. In the foreseeable future, it is highly unlikely that flexible displays will be completely organic. Instead, they will consist of a hybrid of inorganic and organic layers and components.

The manufacturing-process temperatures for many inorganic layers have been decreasing, and the thermal stability of polymer substrates has greatly improved. These changes further enable flexible displays. The anticipated move to roll-to-roll processing will impose a new set of requirements on the films associated with roll-to-roll handling, and it will encourage simplification of the substrate structure.

When a polymeric substrate is employed in a flexible-display application, a barrier layer is required to protect the enclosed functional materials and layers from oxygen and water permeation through the substrate. Oxygen and water permeation is of particular importance to organic light-emitting-diode (OLED) devices since the diffusion of oxygen and moisture through the polymer substrates degrades the performance and lifetime of the OLED device. Although single barrier layers provide the active materials with some protection, it is clear that multiple layers are necessary if OLED applications are to have long-term stability.

Gordon Graff and his colleagues at Pacific Northwest Laboratories and Robert Praino and co-workers at Vitex Systems have cooperated on many factors associated with barrier layers, including packaging requirements of flexible displays, various barrier-layer solutions, permeability models, and measurement techniques. The Vitex solution, consisting of multiple inorganic/organic hybrid barrier layers, demonstrates great promise in satisfying the requirements for an OLED material.

Another flexible-substrate candidate is inorganic-based. Glass has ideal barrier properties and is resistant to display-manufacturing-process temperatures and chemicals, but it lacks the flexibility and ease of handling found in polymeric substrates. Armin Plichta and co-workers at Schott AG are developing a glass-manufacturing process that can produce glass substrates as thin as 30 μm. To improve the mechanical stability of the glass substrate, a polymeric layer is deposited upon it. This hybrid solution makes it possible to capitalize on the positive attributes of glass while making it more flexible and easier to process.

Transparent Conducting Oxides

Indium tin oxide (ITO) is a conducting layer frequently used in displays. But to obtain ITO on glass with a sufficiently low sheet resistance and high optical throughput, it is necessary to use manufacturing-process temperatures that are generally higher than those that should be used for plastic substrates. David Paine and his co-workers at Brown University have been investigating lower-temperature transparent conducting oxides (TCOs) as coating materials. Their work has greatly contributed to the general understanding of TCO performance in flexible FPDs.

Roll-to-roll vacuum web coating of flexible polymer substrates is already an established industry, with touch screens being the application that is most familiar to the display community. The technology for producing complex multilayer structures in a single pass through a web-coater is already available, allowing the FPD industry to leverage the technology for flexible applications.

Although ITO has excellent sheet resistance and optical properties for display applications, it does have one shortcoming in the flexible-display realm [E. Lueder, "Plastic Substrates for Flat-Panel Displays," Proc. 7th Asian Symposium on Information Display, 13–14 (2002)]. When ITO is deposited on a polymeric substrate, it can crack under tensile strain or buckle under compression (Fig. 2). In a flexible-display application, ITO cracking or buckling can cause catastrophic failure. Therefore, significant effort is being devoted to the mechanics of ITO deposition on plastic substrates.

Peter J. Slikkerveer and Piet Bouten at Philips and Yves Leterrier at EPFL Lausanne in Switzerland have been cooperating to better understand the nature of these cracking and buckling mechanisms. They have reported on the conductive failure of ITO on polymer substrates and have shown that the incidence of compressive failure may exceed tensile failure. Much of the literature to date has focused on conductive failure in tension, so these new studies are significant.

Jeong-In Han at the Korea Electronics Technology Institute has also been investigating the stability of deformed ITO films and has developed a number of theoretical models. Han has reported on the interesting phenom-enon of a buffer layer with an engineered Young's modulus lower than the critical value relieving the mechanical stress in a film. The models and fundamental scientific findings derived from studying ITO on polymeric substrates will certainly be used to design flexible displays in the future. The basic science is equally applicable to other components, such as inorganic TFTs on plastic.

Organic Conducting Layers

Conducting polymers are also being considered for flexible-display applications, possibly to compete with TCO films. Although their sheet resistances and optical properties are not as attractive as those of ITOs to date, they have exceptional mechanical properties and can be processed at low temperatures. Bert Groenendaal at AGFA has been at the forefront of developing the underlying chemistry of conducting polymers for commercial applications. As demonstrated by Darren R. Cairns at 3M TouchSystems, conducting polymers have mechanical characteristics that are superior to those of their TCO counterparts [Fig. 2(c)].

Fig. 4: Low-temperature-polysilicon technology has been used to produce a flexible full-color TFT-LCD. (Courtesy of Akihiko Asano, Sony Corp.; reproduced by permission of John Wiley & Sons, Ltd.)

More recently, new conducting substrates based on nanotechnology have entered the flexible-FPD arena. Eikos is developing flexible and transparent electrode solutions using carbon-nanotube dispersions in combination with wet-coating processes and printing techniques [D. Arthur et al., "Flexible Transparent Circuits from Carbon Nanotubes," SID Symposium Digest Tech. Papers 35, 582–585 (2004)].

For now, ITO will probably remain the TCO conducting layer of choice until alternative technology improves or until ambitious display designers exceed ITO's mechanical limits.

Thin-Film Transistors

Most of the electro-optic materials being considered for flexible displays require an active-matrix backplane for high resolution. Significant effort is being dedicated to developing processes for printing and patterning organic electronics on polymeric substrates. John Rogers at the University of Illinois and Graciela Blanchet at DuPont Central Research have worked extensively in microcontact printing and patterning techniques and have successfully deposited TFTs over large areas (Fig. 3).

Raj Apte and co-workers at the Palo Alto Research Center and the Xerox Research Center in Canada have been developing high-throughput ink-jet-printing techniques that minimize material waste and achieve easy registration in order to target cost-sensitive markets. Edzer Huitema and co-workers at Philips have demonstrated rollable active-matrix displays with organic electronics using a solution-based process. Operational organic TFTs, simple logic gates, large-area circuits, and flexible displays have all been demonstrated.

There is also significant research and development in processes for inorganic TFTs on foil and polymer substrates. Sigurd Wagner and co-workers at Princeton University and Zhigang Suo at Harvard University have studied the failure mechanisms of inorganic TFTs on flexible substrates in detail.

The development of successful TFTs for plastic substrates has enabled high-resolution flexible-FPD prototypes. Jin Jang and co-workers at Kyunghee University and Bo Sung Kim and co-workers at Samsung Electronics have developed a high-resolution full-color flexible TFT-LCD based on a low-temperature a-Si active-matrix technology. Akihiko Asano at Sony Corp. has demonstrated the utility of a low-temperature-polysilicon (LTPS) process for flexible full-color TFT-LCDs (Fig. 4).

Sumio Utsunomiya and co-workers at Seiko-Epson Corp. have developed a TFT-transfer process to circumvent the problems associated with low-temperature fabrication, namely, decreased TFT performance and the dimensional instability of the plastic substrates. By using this process, called surface-free technology by laser ablation/annealing (SUFTA), the group at Seiko-Epson has successfully demonstrated both TFT-LCD and OLED prototypes.

The solutions for TFTs on plastic are extremely varied. At this point, it is difficult to determine which approach holds the most promise because many of them have significant positive attributes for certain applications. In the end, performance requirements and cost targets will dictate the choice of TFT technology to be used in each flexible-display application. The interdisciplinary nature of TFT research, the battle between organic and inorganic solutions, and the challenging scientific issues surrounding each of these technologies will make it a fertile and highly competitive area for display researchers for years to come.

Electro-Optic Materials

The various types of electro-optic materials for flexible-display applications fall into three major categories: emissive, reflective, and transmissive. For emissive applications, small-molecule- and polymeric-OLED materials are being developed. Pioneer, DuPont Displays, and Universal Display Corp. are three of the many companies developing OLED materials. Mark Hildner and co-workers at DuPont Displays have been developing polymer LEDs for both flexible passive- and active-matrix displays. OLEDs benefit from the widely held perception that they are a natural choice for flexible displays because of their thin structure, solid-state construction, and active material composition.

To develop a truly low-power display, a reflective and bistable display mode will have to be implemented on flexible substrates. Three contenders exist, all of which are compatible with polymer substrates without a stringent demand for sophisticated oxygen and moisture barriers. J. William Doane and Asad Khan at Kent Displays, Inc., have developed polymer-encapsulation techniques for cholesteric liquid crystals for flexible-display applications (Fig. 5). The ability to vertically integrate red, green, and blue reflecting panels makes the cholesteric technology attractive for full-color applications.

Fig. 6: A prototype of the Philips paintable display can be bent to a rather small radius of curvature. (Courtesy of Dirk J. Broer, Philips Research Laboratories; reproduced by permission of John Wiley & Sons, Ltd.)

Karl Amundson and co-workers at E Ink Corp. are optimizing micro-encapsulated electrophoretic materials for flexible-display applications. The paper-white appearance is attractive for paper-surrogate and electronic-book applications [Fig. 5(c)]. Nicolas Sheridon and co-workers at Gyricon, LLC, are developing a bi-chromal ball technology. The white-on-black appearance of Gyricon displays makes them particularly appealing for electric-signage applications [Fig. 5(d)].

Dirk J. Broer and co-workers at Philips Research Laboratories, The Netherlands, are developing one of the most far-reaching display concepts, a paintable display technology (Fig. 6). A light-induced photo-enforced stratification of liquid crystal and polymer shows great promise for flexible displays because of its coatability and ease of fabrication. The device is further simplified because the stratification process creates a top polymer substrate during the fabrication process.

Manufacturing

Compared to the batch processing of one or more components at a time, roll-to-roll processing represents a marked deviation from current display-manufacturing practices. If and when roll-to-roll manufacturing tech-nology matures for display fabrication, it promises to reduce capital-equipment costs, reduce display-component costs, and significantly increase throughput, perhaps even eliminating component supply-chain issues if all processes are performed by roll-to-roll techniques. Display scientists at Abbie Gregg, Inc., have extensively studied the roll-to-roll manufacturing process, including the custom-tools requirement, product-design requirements, and cost models. Although batch processing can be used to manufacture flexible FPDs, many researchers and technologists believe roll-to-roll manufacturing will ultimately be implemented.

Conclusions

The recent development of many components and supporting technologies for flexible-FPD applications is rapidly bringing the flexible-FPD concept closer to a reality. Several display groups around the world have manufactured impressive flexible-FPD prototypes.

Although intense research in flexible-display technology is under way, efforts have just begun. Strategic relationships and cooperative ventures have already coalesced from the existing vast collections of resources, investment, and manufacturing infrastructure provided by the display industry. Despite the considerable intellectual and monetary investments being made in flexible-display technology, many technological hurdles remain.

In time, these hurdles will be overcome, and an answer will emerge to the critical question of what the breakthrough application will be. Despite the difficulty of obtaining market intelligence on flexible displays, Kimberly Allen at iSuppli Corp. has performed an extensive market analysis of flexible FPDs. These displays would be suitable for many of today's applications, but we still do not know what clever applications the industry will devise once flexible flat panels enter the marketplace – or how early adopters will react to the new technology.•