Technology on Flexible Display
Technology on Flexible Display
  • Sungjoo Kwon (MSE, Ph. D. cand
  • 승인 2014.04.09 15:21
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A flexible display is literally a device that is flexible. It can be developed into a rolling or folding display. It is drawing attention as a future display that increases convenience sharply because it can be applied anything, including tables, windshields, clothes, skin, etc.
To make a display flexible, all elements have to be flexible. A display consists of a light emitting diode (LED), transistor, and many other components. At this time, flexibility of a LED is the main point. LED is classified into organic and inorganic according to whether it uses a non-organic emitting layer or an organic light emitting diode (OLED), which is more suitable for flexible displays. OLED maintains its characteristics when bent better than LED.
The basic principles of OLED follow. As voltage is applied to the positive and negative poles, electrons and holes are inserted into each pole. Each inserted electron and hole recombine in an emitting layer through an electron transport layer and hole transport layer, and they emit light. The light is emitted by transparent poles. The wavelength is different according to the kinds of organic elements in the emitting layer. All colors of nature can be produced using the organics emitting red, green and blue. But, transparent poles generally used are made of Indium tin oxide (ITO) and the substrate of OLED is made of glass. Both of these components are brittle, and it is impossible to make a flexible display with them.
To overcome the limit, carbon nano tube (CNT), metal nanowires, and graphene are gaining attention as replacements for ITO. Graphene, which has a honeycombed shaped plane structure, is one of main issues because it is applied to enormous fields such transistors, touch screens, and solar cells. It has greater electron mobility and electrical conductivity than silicon and a penetration ratio of 98 percent. Research is in progress to replace glass with a plastic substrate that can endure high temperature and is flexible. Processes used in most displays generate high and low temperatures so insufficient chemical or thermal resistance causes wrinkles and cracks. Now, Polyimide, Polyethersulfone, and Polyethylene terephthalate are researched as the matter of potential substrates.
Displays must also be water and oxygen tight to prevent degradation. For this, a method to deposit organic and inorganic by turns is necessary.
The technology needed to create flexible displays is being developed rapidly. Recently things are moving away from photolithography processes, which hinder productivity. Examples of alternatives are direct patterning printing like inkjet printing, imprinting, and roll-to-roll process, which makes continuous elements at room temperature and normal pressure (meaning high temperature and a high-degree vacuum aren’t necessary). It is urgent to apply and develop these processes because they increase productivity and decrease costs.
Various fields need to be developed for flexible displays to become a reality.


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