A Paper-Thin, Flexible LCD

Prototype of a flexible optically re-writable color liquid crystal display. (Source: Zhang et al.)

Opto­electronic engineers in China and Hong Kong have manu­factured a special type of liquid crystal display (LCD) that is paper-thin, flexible, light and tough. With this, a daily news­paper could be uploaded onto a flexible paper­like display that could be updated as fast as the news cycles. It sounds like some­thing from the future, but scientists estimate it will be cheap to produce, perhaps only costing $5 for a 5-inch screen.

The team focused on two key inno­vations for achieving highly flexible designs. The first is the recent develop­ment of optically re­writable LCDs. Like conven­tional LCD displays, the display is struc­tured like a sandwich, with a liquid crystal filling between two plates. Unlike conven­tional liquid crystals where elec­trical connec­tions on the plates create the fields required to switch indi­vidual pixels from light to dark, opti­cally re­writable LCDs coat the plates with special molecules that realign in the presence of pola­rized light and switch the pixels. This removes the need for tradi­tional electrodes, reduces the structure’s bulk and allows more choices in the type and thick­ness of plates. Conse­quently, opti­cally re­writable LCDs are thinner than tradi­tional LCDs, at less than half a milli­meter thick, can be made from flexible plastic, and weigh only a few grams. “It’s only a little thicker than paper,” said Jiatong Sun from Donghua Univer­sity in China.

Optically rewritable LCDs are durable and cheap to manu­facture because of their simple structure. Moreover, like an elec­tronic paper screen in an e-book, energy is only required to switch display images or text. There­fore, running costs are low because these new LCDs don’t need power to sustain an image once it is written on the screen.

The second inno­vation involves the spacers that create the sepa­ration of the plastic or glass plates. “We put spacers between glass layers to keep the liquid crystal layer uniform,” Sun said. Spacers are used in all LCDs to deter­mine the thickness of the liquid crystal. A constant thick­ness is neces­sary for good contrast ratio, response time and viewing angle. However, when plates bend, it forces the liquid crystal away from the impact site and leaves sections of the screen blank and so altera­tions in spacer design are critical to prevent liquid crystal in flexible LCDs from moving exces­sively. Deve­loping a flexible design that overcomes this barrier has proven chal­lenging.

The researchers tried three different spacer designs and found that a mesh­like spacer pre­vented liquid crystal from flowing when their LCD was bent or hit. This inno­vation enabled them to create the first flexible opti­cally rewritable LCD.

An addi­tional inno­vation involved improved color rendering. The scientists report that until this study, optically re­writable LCDs had only been able to display two colors at a time. Now, their optically re­writable LCD simul­taneously displays the three primary colors. They achieved this by placing a special type of liquid crystal behind the LCD, which reflec­ted red, blue and green. To make this into a commer­cial product, Sun wants to improve the reso­lution of the flexible opti­cally rewritable LCD. “Now we have three colours but for full colour we need to make the pixels too small for human eyes to see,” Sun said. (Source: AIP)

Reference: Y. Zhang et al.: A flexible optically re-writable color liquid crystal display, Appl. Phys. Lett. 112, 131902 (2018); DOI: 10.1063/1.5021619

Link: Centre for Display Research, Hong Kong University of Science and Technology, Hong Kong

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