Stretchable display is a fundamentally different technology to foldable and rollable displays and will be more difficult to realize, a South Korea professor says.
Stretchable display will require elastic substrates and circuits, while it will also have to endure pressure from three axes, Soonchunhyang University professor Moon Dae-gyu said at a local seminar on form factor innovation and next-generation display material and production processes.
Displays that can stretch without distorting the screen is considered superior to flexible, foldable and rollable displays.
Foldable and rollable displays, where they are folded or rolled, receives pressure only from one-axis, but stretchable will have to endure pressures from x-axis, y-axis and z-axis, Moon said. It will have to ensure both expansion and compression, the professor said.
The stress on the display will proportional to the how long it stretches, or the strain. The stress will be inversely proportionate to the curvature radius, the professor said. Smaller the curvature radius, the higher the stress. Curved displays have a curvature radius between 200 to 300R. Large rollable displays have 50R. Smaller rollable and foldable displays have 1.4 to 5R.
Stains will cause creases and cracks on the screen. For a stretchable technology to secure flexibility, reliability, durability and usability, problems in substrate and drivers as well as pixel must be resolved together, the professor said.
Thin-film transistors and organic light emitting diode (OLED), which are needed for a stretchable display, uses ceramic materials to be made. Ceramic is a hard material. TFT and OLED will be cut, even from 1 to 2% of strain. MicroLED and quantum dot nanorod LED can be potential candidates. Currently, for foldable and rollable displays, a flexible substrate is used with OLED deposited on them.
It is impossible to stretch ceramic materials, the professor said. A new material that will offer a stretchable substrate will be needed. TFT and OLED will be layered on top of that, then adding stretchable circuits will allow for a stretchable display, the professor said. OLED will still need encapsulation.
Samsung Display introduced a stretchable display at SID 2019, Moon said. This display used an elastic substrate. Low-temperature pollycrystalline silicon TFT, OLED and encapsulation was attached on it. It had a hinge for stretching. The display could be stretched by 5% for 10,000 times, the professor said.
At last year’s SID. BOE introduced its stretchable display called Kirigami. The display had 100ppi resolution and had green shades shown in BOE’s published materials, the professor said. BOE was still behind in Samsung Display in related technology, Moon added.
Last year in June, LG Display was chosen as the lead in a national project to develop stretchable display. The company plans to unveil a display that can stretch 20% by 2024.