Flexible Displays Come in Many Forms
by Ruiqing (Ray) Ma
Seven years ago, right around this time, I was busy preparing for my first Display Week Applications Tutorial on flexible displays. You may wonder why a display researcher was asked to talk about applications for one and a half hours. The reason was simple – no one knew much about flexible- display applications back then. Among the 65 slides I created, the most important one was titled “How Will Flexible Displays Develop?” On it, I described five conceptual phases of flexible-display development, with the first four phases being low power (efficient, thin, rigid), rugged (flat, unbreakable), bendable, and rollable. Let’s be clear: this was not an application roadmap, but a display roadmap based on the levels of difficulty in making such displays.
Looking to the market today, flexible displays are still in the “low power” or “efficient, thin, rigid,” phase. OLED displays and bi-stable reflective displays represent two of the best flexible-display technologies. However, both entered the market not for flexibility, but for the compelling selling points of low power and slim form factor. Two good product examples of these technologies are e-Readers with electrophoretic displays and smartphones with AMOLED screens. For smartphones that demand dimensions down to one-tenth of a millimeter, AMOLED displays clearly have an edge, with their very thin profile and power efficiency. In 2013, the first commercial products based on flexible AMOLED displays were introduced to the world. These displays were built on flexible substrates, but used in rigid designs in either flat or curved models. Even with the rigidity of the products, it is fascinating to see the variety of new forms being generated at the product level: uniform curves on the long or short dimensions, curved edges, circular displays, and displays with even slimmer bezels that take advantage of flexible substrates.
The theme of this special issue is “What’s next for flexible displays – applications and enabling technologies.” Three inspiring articles are featured to provide a clear picture of current R&D efforts in the field of flexible displays.
In the article entitled “Enabling Wearable and Other Novel Applications Through Flexible TFTs,” Dr. Facchetti and his colleagues at Polyera share their vision that flexible TFT technology is a key to enabling mechanical flexibility for the next round of flexible electronic products. Combining their organic TFT (OTFT) based flexible TFT with electrophoretic displays, the Polyera team has built a truly flexible wearable device: the WOVE Band. With an active area several times larger than that of most smartwatches, the display can be bent 50,000 times with a 15-mm radius.
While OTFTs are a good match for electrophoretic displays, LTPS and oxide TFTs are the dominant backplane technologies for AMOLED displays. Because of its amorphous nature, oxide TFTs have an advantage in flexibility. In the article titled “Bulk Accumulation Oxide-TFT Backplane Technology for Flexible and Rollable AMOLED Displays: Part I,” Professor Jin Jang from Kyung Hee University shares the latest results of bulk-accumulation oxide TFT for advanced flexible AMOLED displays. These TFTs can achieve an effective mobility up to 90 cm2/V-sec and show no obvious change in TFT characteristics, even when bent to a tight radius of 2 mm, showing great promise in driving ultra-flexible AMOLED displays.
To get a broad view of the current status of flexible AMOLED research, I would recommend the article titled “Flexible AMOLED Displays Make Progress,” in which Dr. Huang and her colleagues provide an overview of AU Optronics’ advancements in flexible AMOLED technology, from fixed curve to bendable, and now working toward a foldable display. A bendable phone with bending and touch sensors provides more intuitive operation and control, which is an excellent example of the advantages of flexible displays. You will also find a detailed discussion on key enabling technologies for foldable displays.
These three articles cover a wide range of topics in flexible displays. In terms of backplanes, three technologies – LTPS, oxide TFT, and organic TFT – are being explored for flexible displays. In terms of applications, researchers are working on bendable and wearable bands, bendable smartphones with bending as a sensor/control, and foldable devices. These are all truly flexible applications. Although rollable technology is not discussed in the articles, it is not necessarily more challenging than foldable technology, when bending-radius requirements are considered.
Reflecting back on my roadmap slide from 7 years ago, here are three things I have learned while preparing for this special issue. First, the “rugged,” or “flat unbreakable,” phase I predicted is not happening, at least not for mainstream applications. Second, researchers are actively working on all the following forms of flexible displays: rigid, bendable, foldable, and rollable. Third, in order to develop products using flexible displays, designers have to consider all the practical limitations such as the rigidity of electronic components. These limitations are unique to each specific application. As a result, for each form of flexible display, a variety of new forms will also be generated at the product level – flexible displays will come in many forms.
What’s the ultimate flexible display? Seven years ago, I gave my answer as the 5th phase of flexible-display development – “free form”– the display is extremely flexible in all directions, or simply put, paper-like. Have fun with that. •