Flexible Displays: Still a Lot to Learn

The development of full-color video-capable flexible displays is well into its second decade. In order to bring products to market (and end-market requirements are not yet clear), display developers need to surmount several technology hurdles. Recent demonstrations show the diversity of approaches being used to create truly flexible displays.

WHILE the display industry has made many advances toward the goal of creating flexible displays, it is still deep in the process of selecting and refining the optimal combination of technologies. The three critical components are the display medium, the control mechanism, and the substrate. The display media that have been of greatest interest for flexible displays include all three of the main technologies: transmissive (LCD), emissive (OLED), and reflective (electrophoretic, as well as electrowetting, bistable reflective LCD, and cholesteric liquid crystal). Given the desirability of full-color and video capability in most applications, active-matrix backplanes are currently being used in the majority of cases, although some display types can utilize passive control. Many substrate types have been demonstrated, including various forms of plastic or acrylic materials and metal foils. These three key components impact each other; for example, high-temperature TFT processing cannot be performed on most plastics, but metal foils would preclude using transmissive liquid-crystal technology. These choices also relate to manufacturing processes, such as roll-to-roll, which in most cases are still in development.

While LCD technology increasingly dominates the display market as a whole, the fact that it normally operates in transmissive mode and [for twisted nematic (TN) and other standard modes] has a great sensitivity to the cell gap severely limits application of this dominant technology to flexible displays. However, it is possible to construct LCDs using rigid plastic, which can be lighter and more rugged than glass, but which do not allow the typical high-temperature processes used to fabricate TFT arrays on glass (Fig. 1).

Fig. 1: Samsung's plastic LCD uses a TFT backplane made with a low-temperature process (<220°C). The LC cell (not including the backlight) is only 0.44 mm thick and weighs 28 grams. Source: Samsung.

One way to utilize liquid-crystal technology for flexible displays is to use other LC modes. For example, Fujitsu has demonstrated flexible LCDs using a cholesteric liquid-crystal material (Fig. 2).

Fig. 2: Fujitsu's flexible LCD uses a cholesteric LC mixture. Source: Fujitsu.

Interestingly, this approach does not use active-matrix addressing; instead, it employs three passive-matrix monochrome LCDs in a stacked configuration.

The most mature medium for flexible displays is electrophoretic, which is typically made on a polyethylene terephthalate (PET) or other flexible sheet. However, the vast majority of electrophoretic products manufactured to date have been in the form of e-book readers, in which the electrophoretic material is attached to a glass substrate containing the TFT array. The crucial technology here will be in the backplane. Electrophoretic developers E Ink and AUO/SiPix have demonstrated flexible versions of their technologies using plastic substrates (Fig. 3), while others, such as LG Display, have built flexible electrophoretic displays using metal backplanes (Fig. 4).

Fig. 3: AUO's flexible display uses SiPix electrophoretic material on a polyethylene naphthalate (PEN) substrate, with an oxide TFT backplane deposited through a low-temperature (<180°C) process. Source: AUO.

Fig. 4: LG Display's 19-in. flexible electro-phoretic display uses a metal backplane. Source: LG Display.

Several other reflective display technologies have been demonstrated or proposed for flexible displays, including electrochromic (which to date has only been implemented in simple segmented forms) and electrowetting.

One of the areas of greatest interest for flexible displays is OLED technology. OLEDs have several attractive characteristics for flexible implementation, including full-color emissive operation (meaning no external lighting is required) and an inherently thin form factor with the potential for fabrication on a single substrate (both of which simplify the mechanics of bending). The visual quality of initial flexible OLED displays is better than any flexible display technology shown to date (Fig. 5).

Fig. 5: Samsung's flexible AMOLED displays demonstrate high-quality imagery. Source: Samsung Mobile Display.

The key challenges for flexible OLEDs are the requirement for a current-driven active matrix on a flexible substrate and the need for high levels of encapsulation, as the organic materials are susceptible to damage when exposed to oxygen and water vapor. The substrate must be able to withstand the high temperatures of the TFT manufacturing processes, which can be over 400?C for low-temperature polysilicon (LTPS) TFTs; research is ongoing with regard to the use of a-Si TFTs, as well as oxide and organic TFTs, but thus far these approaches do not appear to provide the required electron mobility. Polyimide film, while stable at high temperature, is typically yellowish. Taiwan's Industrial Technology Research Institute (ITRI) has demonstrated flexible AMOLEDs built on a polyimide film that is transparent and colorless, and also includes silica particles, increasing its temperature range. For more about this technology, see the article, "A Flexible Universal Plane for Displays," in this issue.

At the same time, ITRI has developed a bond/de-bond process using a polymer that is adhesive to a glass substrate that is used to hold the film during the deposition process, but not to the polyimide film itself, facilitating easy de-bonding. Another approach to the challenge of a flexible TFT backplane is to use organic materials for the TFT, which can be printed or deposited at low temperatures, allowing the use of plastic substrates (Fig. 6).

Fig. 6: Sony's rollable OLED uses an organic TFT. Source: Sony.

Other types of flexible emissive displays are also possible. Inorganic electroluminescent displays have been made in flexible forms, typically in low-information-content versions for clothing or other decorative uses. Inorganic LEDs can be strung or woven together to form a flexible sheet, but are inherently low resolution. By using individual tubes, plasma displays have been made in configurations that are flexible in one dimension.

The long-run potential for flexible displays is tremendous; DisplaySearch forecasts that by 2018, flexible-display revenues will exceed $8 billion, 100 times 2008 revenues. This includes flexible displays built on plastic, metal foil, or even ultra-thin glass, but does not include "formed" displays that are in a non-planar but inflexible form. Such displays could occupy a middle ground between flat and flexible displays, in applications where the display shape needs to conform with the overall system – for example, automobile dashboards – or to make a design statement, such as the contoured display in the Google Nexus S phone.

How to make flexible displays is one challenge, but the ultimate question about this technology – how it will be implemented – has drawn a great amount of speculation and conceptual thinking, from ideas for displays that roll up into a pen to those that fold up like a map. However, as with any significant innovation, these ideas may be too ambitious in the near term as well as too limiting in the long term. The reality is a sort of chicken-and-egg situation: flexible displays need to be built and commercialized in order for the industry to really understand their value and potential. Through the development of products using flexible displays, manufacturers will understand the operating requirements. Until then, it is not clear what matters most about flexible displays. While the goal may be the ability to operate in a curved form factor, or to bend actively during operation, other factors, such as light weight and the ability to operate in rugged settings, may be equally important. Flexible display developers will have to continue to search for the ideal combination of technologies in order to continue that learning process. ?