Enabling Wearable and Other Novel Applications through Flexible TFTs
Mechanical flexibility is a key feature for the next generation of display-based electronic products. An essential component of this capability is flexible TFT technology, which requires a materials set specifically designed to perform optimally under mechanical stress. Progress to this end has been made by several companies, including Polyera Corp.
by Antonio Facchetti, Chung-Chin Hsiao, Edzer Huitema, and Philippe Inagaki
ORGANIC and flexible electronics are revolutionary technologies that enable the fabrication of unconventional electronic devices (including displays) where mechanical flexibility and light weight are essential characteristics.1-4 The unique properties of organic materials enable them to be used to fabricate TFT backplanes using either standard fab processes (spin-coating, slot dye coating plus photolithography) or processes borrowed from the graphic-arts industry, such as printing. These are all low-temperature solution-based processes that are potentially compatible with the plastic substrates that enable flexible lightweight displays.
By using the above-mentioned materials and processes, single electronic elements such as transistors (Fig. 1) and capacitors can be made, as well as networks of devices forming circuits10such as memories or display driver arrays. In turn, entire devices including flexible displays,11 radio-frequency identification (RFID) tags, disposable diagnostic devices, rollable solar cells, and batteries could be fabricated with this new materials set. This article will describe how organic TFTs can be used to create flexible backplanes for displays and other devices, and ends with a discussion of a wearable flexible-display product, the Wove Band, which is now under development.
Fig. 1: The structure of a (bottom-gate top-contact) organic thin-film transistor (OTFT) with operation in p-channel (hole transport) and n-channel (electron transport) modes is shown. For driving a display, only one type of TFT (either p- or n-) is necessary, but for other devices such as RFID and sensors, complementary circuits based on both polarity OTFTs provide a far better platform in terms of stability and reliability.
Options and Obstacles for Organic Electronics
It is important to explain that the aim of organic electronics is not to replace conventional silicon-based electronics. Rather, it offers several opportunities to reduce the cost of certain devices by circumventing production limitations of the conventional semi-conductor industry and, specifically for flexible electronics, enabling completely new products impossible to fabricate using silicon technologies because of intrinsic and/or processing limitations.
The conventional obstacle for the realization of this technology has been on the materials side, since solution-processed electronic materials, particularly the gate dielectric and the semiconductor, exhibit poor electrical performance for current standards. Furthermore, it has been challenging to scale up this new material set, formulate it in fab-acceptable solvents, and identify quality-control parameters for solid materials and formulations. Modification and optimization on the production equipment side have been proposed to cope with the different processing parameters of organic materials; however, the electronic industry is reluctant to follow this approach. Finally, new device and circuit design as well as tools to investigate and qualify the performance of devices fabricated for flexibility, on plastic substrates, may be necessary.
Solution Processing and Flexible Electronic Materials
As in the case of conventional electronics, organic electronic devices need a core materials set for charge accumulation, injection, and transport, as well as specific materials to enable particular device functions. For instance, in every type of electronic device there is the need for a certain control of the current flow as well as memory.
For thin-film transistors (TFTs), the key device-enabling multiple electronic technologies (Fig. 1), three fundamental materials are needed: the conductor, the gate dielectric, and the semiconductor (Fig. 2).
Fig. 2: Electronic materials classes for organic thin-film transistors include those for semiconductors, conductors, and dielectrics.
Then, depending on the specific device application, additional active materials may be necessary. For instance, for OLEDs, an emissive material is necessary for efficient conversion of electricity to light. For organic photovoltaics (OPVs), besides the materials needed for efficient charge transport, it is necessary to have a photosensitizer and/or an efficient light absorber for photon absorption and dissociation. Displays may be based on different technologies for pixel fabrication including OLED, electrophoretic inks (proper dyes are necessary), liquid-crystal (LC molecules are used), and electrochromic compounds. Many other types of chemicals/ materials may also be necessary for device fabrication, including small molecules as interfacial layers for efficient charge injection or surface-energy match, additives used as dopants or stabilizers and polymers for encapsulation.
Tremendous progress has been made during the past 5 years in enhancing the performance of solution-processable electronic materials.5-8 Particularly, organic semiconductors in transistors have achieved exceptionally large field-effect mobilities, approaching those of poly-Si. However, these performances have not been demonstrated with a scalable and fab-compatible materials set. For instance, mobilities of > 10 and > 40 cm2/V-sec have been reported for organic TFTs based on solution-processed polymeric and molecular semiconductors; however, the gate dielectric was an oxide and/or the solvent used in formulation was unacceptable (Table 1). In addition, most processes were carried out at high temperatures during film deposition or post-deposition, which is unacceptable in standard equipment, and the TFT array area was very small.
The authors’ specific work in this area is to provide a total materials set to enable OTFT fabrication via wet process of the key materials components on mechanically flexible plastic foils. Other companies, such as Merck, are also pursuing a similar goal. Our materials set includes a polymeric photocurable buffer to coat the TFT substrate, the organic semiconductor, a photolithography material compatible with our semiconductor channel, a photo-curable gate dielectric, a passivation layer, and a planarization layer. The combination of this material enables, in a conventional fab, excellent transistor I–V characteristics and monolithic integration into a TFT array over Gen 2.5 substrates (Fig. 3).
Fig. 3: The authors’ organic TFT technology is represented in terms of electrical, morphological, and mechanical characteristics.
This set of electronic materials has satisfied chemical scalability (they can all be scaled to commercial kg-scale volumes using safe and environmentally acceptable chemical processes) and stringent electrical performance parameters (acceptable performance variance and negligible bias stress). In addition, the corresponding films, combined into multiple layers that are integrated into a TFT architecture, are mechanically robust and flexible, as demonstrated by our mechanical tests. Therefore, we have made great efforts to develop a materials set with performance equal to or greater than that for amorphous-silicon and processed in a conventional fab infrastructure without any modifications.
For specific applications in flexible displays and wearable electronics, we demonstrated that this material stack is compatible with plastic substrates and the fabrication of flexible displays via bond-debond processes (Fig. 4). The bond-debond processes are the most advanced, and here the plastic substrate is glued to a glass plate that is then processed using standard tools for the fabrication of the TFT backplane and then the front plane.9 Certain plastic substrates such as PET and PEN impose temperature restrictions on the process because they start to deform at temperatures >200°C. Recently, developed PI can withstand far higher temperatures, although it is more expensive. At the end of the process, the display is debonded from the glass plate. If the display is sufficiently bendable, it can be rolled off the glass plate at moderate temperature. The glass plate can then be cleaned and reused.
Fig. 4: The process for the fabrication of the display module begins with a plastic substrate and TFT materials.
Flexible Displays
A typical display structure comprises at least a substrate, a TFT backplane driving the pixel, a frontplane enabling the image, and passivation/encapsulation films. Touch is used in nearly every advanced portable display product. Frontplane technologies suitable for flexible displays must be very thin because the display itself must be very thin. Probably the most advanced (and complementary in several aspects) frontplane technologies are electrophoretic12 and OLED.13 Figure 5 shows examples of displays fabricated with these technologies.
Fig. 5: Examples of flexible displays from different companies include those based on OLED and electrophoretic technology.
Electrophoretic displays have been on the market in e-Readers since 2004. The electro-phoretic layer is 20–40 µm, which is relatively thick, but does not require any optical films, a backlight, or extreme oxygen barriers. This is a bistable technology, in which the image remains without the need of refresh. Thus, power consumption is minimal and is ideal for wearable displays. Furthermore, this technology does not need very high-performance transistors to drive the pixel, and the performance of amorphous-silicon is sufficient. This display medium is therefore very well suited for flexible displays where low power is required. Polymer Vision, Seiko-Epson, Sony, AUO, Plastic Logic, and Polyera (among others) have demonstrated flexible electrophoretic displays.9
OLED displays do not require a backlighting unit; however, they do rely on an optical stack at the front side that typically includes a polarizing and a retarding film to increase the daylight contrast. The severe sensitivity of the OLED material stack to air during operation requires the use of barrier films with very low permeation to oxygen and water, something that glass and steel films can provide but that plastic substrates cannot unless using organic-inorganic multi-layered coatings. Furthermore, the transistors needed to drive OLEDs have higher stability and uniformity requirements compared to that for electrophoretic displays and LCDs.
Currently, mainly LTPS transistors are used to drive the OLED pixels. For OTFTs to be able to drive OLED pixels, two main requirements need to be fulfilled. The first is a carrier mobility that is high enough to supply the current to the OLED pixels using small enough transistors. To achieve this goal, a mobility higher than 1 cm2/V-sec, preferably higher than 3 cm2/V-sec, is required. The second is TFT stability under electrical stress that is so good that no image artifacts appear on the display, even after years of use. This requires not only a good semiconductor, but also an excellent dielectric material. In general, the state of the art for OTFTs is such that EPDs and LCDs can already be driven, but in order to drive OLED displays, further advances, especially in the area of stability under electrical stress, are needed in the coming years. Once these advances have been achieved, OTFTs will be the prime candidate to realize the full potential of flexible and rollable OLED, which requires a robust TFT backplane solution that is currently missing.
Since OLEDs are current driven, power consumption is a concern for wearable flexible displays, though LG, Samsung, AUO, and Sony have demonstrated flexible OLED displays. For the above reasons, electro-phoretic display technology is ideal for products such as Polyera’s Wove Band, which is described in the next section.
Wearing the Flexible Display
The vision behind the Wove Band is to enable a display that forms a natural fabric-based interface with the digital world, summarized in articles like “Digital Goes Material,” which appeared in a recent issue of Wired.14 We believe such a display must be flexible, reflective like fabric, always on, and extremely low power. Polyera has developed a technology that meets these requirements – called Polyera Digital Fabric (see https://www. youtube.com/watch?v=TsxJMkvtQ98 for an example video).
In Fig. 6, the organic transistor stack (top left) incorporated into the flexible-display module stack (bottom left) is shown. The display module stack benefits from the OTFT stack by giving it a much higher degree of flexibility and robustness than possible when using conventional silicon-based transistors. The display module also incorporates a flexible multi-touch sensor that at the same time is used as the hard-coated top layer of the display. The display module is integrated with a unique mechanical support structure (as shown at right in Fig. 6) that limits the bending range of the display while at the same time minimizing mechanical stress during bending. It consists of an assembly of individual links that are connected in a unique way such that the display does not stretch or compress when flexed in any position.
Fig. 6: The backplane and stack at left enable flexible-display-based products such as the Wove Band shown at right.
The specifications of the Wove Band are shown in Table 2. The product uses state-of-the-art electronic components based on the ARM A7 architecture and Bluetooth LE to connect to a phone and runs the Android operating system. This enables receiving messages, alerts, and, in general, content from the Internet, as do other smartwatches.
The display area of the Wove Band is six times larger than that of the Apple Watch, as indicated in Fig. 7. This is enabled by, for the first time, using a flexible display in a real flexible product where the digital fabric is wrapped around the wrist of the user. Curved displays have been used in past years by Samsung, LG, and Sony in phones and wearables, but always in a rigid curved configuration. Although this allows the use of a formed cover glass over the display, it greatly limits the use of flexible displays in new product categories. By using a much more robust OTFT stack that uses organic (“plastic”) materials that can handle much more mechanical deformation, we have achieved for the first time the goal of using a flexible display in a dynamically flexible product.
Fig. 7: A typical screen shot includes a variety of information that can be displayed on the Wove Band. Indicated in blue are the relative size of the small (38 mm) and large (42 mm) Apple Watch displays that have a 6.5 and 5.4 times smaller display area than the Wove Band, respectively.
The much larger touch-sensitive display improves on the small screens on smartwatches and gives the user the capability of displaying multiple applications simultaneously on their wrist. This essentially solves two very important problems of current smartwatches. The first is the inefficient methodology in which the user has to swipe through a large number of screens to reach the desired function on the watch. The second problem is the absence of “at-a-glance” information. Current smartwatches are typically not capable of displaying the information that a user needs at any specific moment, due to the small display size and the fact that the display has to be turned off most of the time to conserve power.
The Wove Band solves the above problems by offering a much larger display that can show multiple pieces of relevant information in parallel and has a display that is always on. A complete menu can be displayed at once, which enables quick navigation through settings and application menus, very similar to current smartphones. This is demonstrated in Fig. 7, where a typical content mix appears on the Wove Band. Multiple applications are showing real-time status. Users can change the layout and applications to their needs, such that the most relevant information is shown.
Applications that will be enabled on the Wove Band are the types that are typically found on current smartwatches, such as notifications and messages including full text, music player control, fitness trackers, calendars, and watch faces. On the Wove Band, however, multiple applications typically will be active simultaneously and showing information on different portions of the display.
A Fabric for the Future
The digital fabric created by Polyera to create the Wove Band can also be incorporated into a wide range of products in the near future, such as wristbands, smartphone cases, headphones, bags, and much more. It will also enable software companies (apps, existing social systems, etc.) to re-imagine how their content and services might operate on this alternative surface. The work carried out by the authors at Polyera Corp. has merged expertise spanning from chemistry, materials science, and applied physics to mechanical, electrical, and software engineering to enable a complete materials solution and product possibilities for unconventional devices – particularly mechanically flexible displays and circuits – that have yet to be imagined.
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