Growth in mobile data consumption has pushed mobile-display sizes beyond 4 in., creating a potential market for foldable displays that enable further enlargement of the display without increasing the mobile-device size and weight. The applications, alternatives, and status of the technology for foldable displays will be discussed.

by Edzer Huitema

MOBILE data consumption has grown enormously over the past 5 years, with a worldwide growth rate of 159% in 2010 and a predicted annual growth rate of 92% until 2015, according to the Cisco Visual Networking Index. This growth was enabled on the one hand by the increasing availability of faster 3G networks, but also by the evolution of mobile-phone hardware, for which battery capacity, processing power, and graphics capabilities have all improved. A crucial aspect of this evolution is the display, which has increased in size dramatically with the introduction of the iPhone in 2007, with an on-screen touch interface replacing all of the buttons that were present 10 years ago. The display is now the dominant face of most mobile phones.

With the start of the roll-out of faster 4G networks, the pressure on display sizes is increasing, while the form factor of the mobile phone is limited; users have to be able to carry the devices all day. With the latest announcements of, for example, the HTC Titan with a 4.7-in. display and the Samsung Galaxy Note with a 5.3-in. display, the limits of mobile-device size have been reached and even exceeded for most consumers.

On the other side of the spectrum, tablets have emerged as the portable alternative for laptops, netbooks, and eReaders as a new portable dedicated reading device. Tablets and eReaders offer a display size of up to 10 in. and can be carried around. The fact that they are about five times the size and weight of smartphones and do not fit into a pocket limits the current sales volumes of tablets and eReaders to about 5% of smartphone sales.

A solution for enabling even larger displays in truly mobile devices would be to replace the glass display with a larger, lightweight display that can be bent to a shape that fits the pocket when the device is not in use. Although the first application areas of foldable displays will be pocket-sized eReaders and mobile phones, other markets, such as tablets, laptops, and eventually monitors and TVs, might also benefit from the use of these displays.

Flexible and Foldable Displays

The term flexible display is used to cover all displays that are intrinsically flexible, as opposed to glass-based displays that are rigid in nature. A more quantitative definition that is commonly used is that a flexible display is able to be bent at least once to a radius smaller than 50 cm (20 in.). Flexible displays mainly use plastic substrates instead of glass, but thin steel or thin glass substrates can be used. The latter two have the advantage of being able to withstand higher processing temperatures, while plastic substrates are more flexible, lower in weight, and more robust. The use of plastic substrates restricts the temperature range during processing to a maximum of 130–200°C, depending on the type of plastic used. The typical thickness of a flexible display is 0.2–1.0 mm.

Additional potential advantages for flexible displays include weight and thickness reduction and a reduced chance in breakage, which is a problem for larger-sized glass displays. They also enable products in which the display is used in a curved position, such as for advertisement or in-car displays.

Companies that have demonstrated flexible-display prototypes recently include Samsung,⁶ LG,⁷ Plastic Logic,⁸ Bridgestone,⁹ E Ink,¹⁰ Fujitsu,¹¹ Sharp,¹² and others. Companies that have introduced flexible displays on the market include Fujitsu, with the Flepia eReader based on a stacked red, green, and blue cholesteric liquid-crystal (CLC) display using plastic substrates, introduced in Japan in 2009, and recently by Plastic Logic, which introduced eReaders based on plastic substrates and electrophoretic technology in Russia.

A foldable display has different mechanical states in the product, so the flexibility of the display is really used. It must be able to be bent many times. Also, the bending radius has to be small enough to fit in a compact device. A more quantitative definition is that a foldable display should be able to be bent at least 1000 times to a radius smaller than 7.5 mm (0.3 in.), although for typical consumer products a specification of at least 25,000 times bending is required.

The first report on foldable displays dates back to 2005.¹ A number of companies have demonstrated advanced foldable-display prototypes^2–5 that enable thin, light-weight products with displays as large as 6 in. and a form factor not larger than an average smartphone.

The Use Case and Competing Solutions for Foldable Displays

Foldable displays are only one solution to the problem of having a small device with a large perceived display area. In Fig. 1, a foldable display device is shown along with other competing solutions: pico-projectors and near-to-eye (NTE) displays.

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Fig. 1: Three different ways to create a large perceived image from a small device include (a) a foldable display device, (b) a pico-projector, and (c) a head-mounted near-to-eye display.

Pico-projectors have already found their way into mobile phones, with the introduction of the Samsung Galaxy Beam in 2010, while LG and NTT introduced models with add-on pico-projectors. They are all based on Texas Instruments' DLP technology and are particularly useful for presenting data or photos to groups. For private and mobile use, it has its limitations due to the projection surface that is required. Also, battery life is limited to a few hours of projection.

NTE displays, also referred to as head-mounted displays (HMDs), are already on the market and are particularly useful for watching video or playing games individually. There are also see-through HMDs designed for augmented-reality applications. The recently introduced Sony HMZ-T1 is the first HMD with support for 3-D viewing and surround-sound aimed at the mass consumer market. Because it is immersive and requires AC power, it is not intended for mobile use.

Foldable display devices retain the advantages of current mobile phones, such as small form factor, low weight, and long battery life, while extending the display size. The key innovations needed for such a device are the display and the mechanics to support the display in all configurations. Prototypes of foldable displays have been shown by Polymer Vision since 2005¹ and later also by Seiko-Epson⁵ and Sony.^3,4

Current State of the Art of Foldable Displays

Some of the latest foldable displays are shown in Fig. 2. Polymer Vision has demonstrated foldable displays that are only 0.1 mm thick and can be bent more than 100,000 times to a radius as small as 5 mm (0.2 in.).² These displays are based on plastic substrates of 25 μm combined with organic transistors processed at low temperature and an electrophoretic-display medium. The latest display module hasa 6-in. diagonal and SVGA resolution (170 ppi). Finger touch is included in the panel by use of a novel integrated capacitive-sensing solution.

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Fig. 2: The current state of the art of foldable displays is represented by (a) a Polymer Vision foldable electrophoretic display, (b) a Seiko-Epson foldable electrophoretic display, (c) a Sony foldable electrophoretic display, (d) a Sony foldable OLED display, and (e) Samsung OLED glass panels with foldable stitching.

Epson demonstrated a foldable display with a thickness of only 0.1mm in 2006.⁵ The display was based on plastic substrates combined with polycrystalline transistors and an electro-phoretic-display medium. There have not been any recent disclosures by Epson on this topic.

Sony demonstrated a foldable display both by using electrophoretic material⁴ and OLED material³ in 2010 and 2011. Plastic substrates are used combined with organic transistors processed at low temperature. Sony is still actively working on this technology.

Samsung demonstrated in 2009 and 2010 a very different type of foldable display construction,¹³ where only the bending zone is non-rigid. Two glass-based OLED panels were stitched together by a very narrow foldable stitching zone in such a way that in the flat position the stitching was almost not visible. Of course, this approach does not have the low-weight and thickness advantage of the other approaches where truly foldable displays are used.

At the moment, foldable displays are still in research and development like most flexible-display technologies. The main reason is that the process flow for making these displays is not the same as the flow for glass displays. The temperature budget is lower due to novel plastic materials and, in some cases, novel patterning methods are used as well. This requires substantial changes to the standard manufacturing flow used for glass displays and also the development of new manufacturing equipment. For foldable displays, the form factor of new products needs to be changed: they are not a simple drop-in replacement for glass displays.

Technologies Used to Produce Foldable Displays

The main technologies used to produce foldable or flexible displays are shown in Fig. 3. The general issue to solve is that a flexible substrate is not suited for thin-film processing without attaching it to a flat and rigid support carrier somehow. Although plastic substrates are supplied on rolls, processing takes place sheet-to-sheet as the resolution and overlay requirements for displays are in the micron range, which is beyond the current capabilities of roll-to-roll processing.

Fig. 3: The main process methods used to make flexible and foldable displays are (a) bond–debond process, (b) laser release process, and (c) etch-back process.

With bond–debond processing, the plastic substrate is glued to a glass plate. The plate is then processed on standard tooling for processing displays on glass plates. The plastic substrate imposes temperature restrictions on the processing, as it starts to deform and melt at temperatures above 200°C, although recently high-temperature plastics have been developed that are less foldable but can withstand temperatures up to 300°C. At the end of the processing, the display is debonded from the glass plate. If the display is sufficiently foldable, it can be rolled off the glass plate at moderate temperature. The glass plate can then be cleaned and reused. This technology is used by Polymer Vision to produce foldable displays. Sony is using a similar technology to produce foldable displays and AUO, LG, and Plastic Logic have been using such technology to produce flexible displays.

Another technology is so-called transfer processing. Here, the display is produced on rigid glass plates and subsequently transferred to a foldable substrate. Key to this process is a (partly) sacrificial layer, typically a-Si or polyimide, that is used to separate the stack from the glass plate by a laser release process. This layer is one of the first layers that is put down on the glass. After the release step, the final plastic substrate is glued to the backside of the stack. Seiko-Epson has been using this process to make foldable electrophoretic displays. In this process flow, an additional temporary top substrate was used to protect the stack during the release process. E Ink also uses a similar process for flexible electrophoretic displays.

With yet another technology, so-called etch-back processing, displays are produced on a rigid glass or steel substrate. At the end of the processing the substrate is etched back so far that the display becomes flexible. LG has used this process route to make flexible electrophoretic and OLED displays. Foldable displays have not been made using this route because the etched back substrates are, in general, not flexible enough to allow bending.

Display Media Suitable for Foldable Displays

Display media suitable for foldable displays must be very thin, as the display in total should be at most 0.2 mm thick. Besides the switching material itself, display media might also require certain optical layers, such as polarizer films or retarders. These layers add significant thickness to the stack. For example, polarizer films are typically 100–300-μm thick, which is too thick for a foldable display. There have been efforts to make very thin polarizing films, but the quality is too low to be competitive.

Liquid-crystal displays are by far the most dominant type of rigid flat-panel displays on the market. Since LCDs require two polarizer films, flexible LCDs are quite thick and have never been prototyped at the extremely low thickness needed for bendability. The backlighting unit needed for a foldable transmissive LCD is an additional complication.

OLEDs have the advantage that they do not require a backlighting unit, but they do rely on an optical stack at the front side that typically includes a polarizer and retarding film to increase the daylight contrast. The extreme sensitivity of the OLED material stack to oxygen and water puts high barrier requirements on the substrates that are used, something that glass and steel substrates do provide but that plastic substrates cannot provide without multi-layered barrier coatings. The transistors in OLEDs have higher stability and uniformity requirements compared to that for LCDs, which is more difficult to achieve on a restricted temperature budget. LG,⁷ Samsung,⁶ and recently AUO have demonstrated flexible OLED displays. Sony has demonstrated a foldable OLED display³ and, recently, Polymer Vision also made a foldable OLED panel together with the Holst Centre (to be discussed shortly).

Electrophoretic displays have been in eReaders since 2004. The electrophoretic layer is 20–40 μm, which is relatively thick, but does not require any optical films, a backlight, extreme oxygen barriers, or high transistor performance. Therefore, this display medium is very well suited for flexible and foldable displays. Polymer Vision,² Seiko-Epson,⁵ and Sony,⁴ have shown foldable electrophoretic displays, while a large number of companies have shown flexible versions.

Electrowetting displays have been prototyped since 2003. They rely on switching by manipulation of the contact angle between a dielectric layer and a fluid system by a voltage. Although electrowetting displays are not bistable, they have all the other advantages of electrophoretic displays, combined with video-speed switching. Liquavista is well known for its work on electrowetting displays.¹⁴(The company was acquired by Samsung in early 2011.) Its technology is best suited for rigid displays, as the fluid layers are more than 50 μm thick. In 2009, Gamma Dynamics, a spin-off from the Novel Device Laboratory of the University of Cincinnati, launched a new electrowetting concept that is better suited for flexible and foldable displays.¹⁵ In this so-called electro-fluidic display concept, the fluid layers are only 3–5 μm thick, thus enabling robust flexible and foldable electrowetting displays. Gamma Dynamics is working with Polymer Vision on foldable displays.

Product Concepts for Foldable Displays

Although no products using foldable displays are on the market yet, a number of interesting concepts have been revealed (Fig. 4). Polymer Vision has shown a family of concept devices over the years. The first was a foldable eReader concept and prototype in which the display was wrapped around the body of the device. The size of the eReader was as small as a mobile phone, while the display was 5–6 in. The company has also shown a foldable eReader phone concept, in which the foldable eReader display is stored inside the device. The bend radius is controlled by a hinge construction that creates a controlled loop inside the device. An extension of this is a phone-tablet concept in which a foldable full-color video display is stored inside the device. After the device is opened, the foldable display can be further extended by pulling the two device halves apart, thereby unwinding the display from a roll at the far end of the device.

Fig. 4: Product concepts using a foldable display include (top row left to right) a Polymer Vision foldable eReader, a foldable eReader phone, and a foldable tablet phone and (bottom row, left) a Samsung foldable phone and (bottom row, right) a Sony foldable laptop.

Samsung recently published the Samsung Galaxy Skin phone concept using a foldable OLED display. It is not clear what the mechanics are that support the display except that graphene is mentioned as the material to be used for the flexible casing. Sony recently published a foldable laptop concept in which one continuous foldable OLED display is used.

When Will Foldable Display Devices Come to Market?

Over the past several years, the technology and concepts for foldable display devices have been maturing. Polymer Vision and Sony are leading in this capacity. The final steps toward commercialization will be early market acceptance of these novel form-factor devices, allowing the final investment into production capacity. The market is more ready than ever with the increasing pressure on the display size in mobile devices. Investment into production capacity can be kept relatively low by having a technology that is compatible with the already existing LCD manufacturing infrastructure. Polymer Vision's technology is fully compatible and only requires lamination and delamination equipment to be added for the bond–debond process.

The first type of devices using foldable displays will be pocket-sized electrophoretic eReaders with 5–8-in. displays. The displays have been demonstrated, and the foldable electrophoretic display technology is more mature than foldable OLED and electro-wetting technologies.

Foldable OLED or electrowetting devices will be the next wave, as they offer full-color video performance. As there are still technical hurdles to overcome for both technologies, such as improved switching speed and optical performance for electrowetting and proven operational and shelf lifetime for both OLED and electrowetting technologies, it will probably take 3–5 years before these devices will be on the market. When they finally arrive, they will truly offer a tablet in your pocket.

References

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¹⁵J. Heikenfeld, "Electrofluidic displays using Young–Laplace transposition of brilliant pigment dispersions," Nature Photonics 3, 292 (2009). •