OLED displays have been the only flexible (bendable, foldable) displays, but this situation might change drastically due to flexible LCD displays under development.
The reliability of LCD displays is higher than that of OLED displays. At SID 2016, an academic conference on display technologies, which took place in San Francisco, the US, from May 22 to 27, 2016, a number of flexible LCD display technologies were exhibited.
Japan Display Inc (JDI) announced its flexible LCD display technologies and exhibited a prototype (announcement number: 9.2). The prototype was showed at the exhibition site and in the author's interview at a symposium.
The newly-developed flexible LCD display uses polyimide for its substrate, and an acrylic resin layer was formed on it to improve durability. When a plastic material such as polyimide is used for a substrate, it becomes necessary to prevent the leakage of backlight, which becomes a problem when the display is viewed obliquely. To solve this problem, JDI inserted an optical compensation film called "Positive C Plate" between the polyimide substrate and the polarizing plate.
The exhibited flexible LCD display can be bent to a curvature radius of 500mm. The thickness of the entire display is 400μm, and the thickness of the plastic substrate made of polyimide and acrylic is 10μm.
According to JDI, future challenges lie in the development of a backlight unit. Existing LED backlight units have a slim light-guiding plate and can be bent to a certain extent. However, to "fold" the flexible LCD display, the thickness of the backlight unit needs to be further reduced, the company said.
Tohoku University announced the results of the development of a foldable flexible LCD display at the symposium (announcement number: 9.1). To prevent liquid crystal from flowing and disturbing a displayed image at the time of bending the display, a wall made of a polymer material was formed in the LCD cell.
Also, to prevent the deterioration of an image when the display is viewed obliquely, the "Positive C Plate" optical compensation film was employed as in the case of the LCD display exhibited by JDI.
The thickness of the entire prototyped LCD cell is 100μm or less (excluding the backlight unit). The plastic substrate and the liquid crystal layer are 10μm and 5μm, respectively, in thickness.
As a future challenge, Takahiro Ishinabe, associate professor at the Department of Electronics, Graduate School of Engineering, Tohoku University, also mentioned the development of a backlight unit. The light-guiding plate of the current backlight unit can be bent to a curvature radius of 20mm, but, to further bend the plate, its thickness needs to be reduced.
Moreover, the current backlight unit concentrates light so that an image looks bright only when it is viewed at a right angle. Therefore, when the display is bent, the bent part of the image becomes dimmer. This is another future challenge, the university said.
At SID 2016, FlexEnable Ltd showed a prototype of its flexible LCD display. It was made by forming organic TFTs on a plastic substrate to drive the LCD. The company expects that the display will be used for wearable devices and plans to develop flexible sensors, etc for wearable devices. FlexEnable Ltd was established by former members of Plastic Logic, a company that was developing organic TFT-driven electronic paper devices.
AU Optronics (AUO) Corp announced its technologies to develop a flexible display with a curvature radius of 20mm at the symposium. Polyimide is used for its substrate. Its screen size and thickness are 3.5 inches and 25μ or less (excluding the backlight unit), respectively.