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Optically Rewritable LCD Could Enable Flexible Displays

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Engineers from Donghua University have manufactured a flexible, optically rewritable LCD (ORWLCD), about as thin as a piece of paper, that would allow information to be uploaded onto a flexible paperlike display, where it could be updated quickly.

A flexible blue ORWLCD, Donghua University.
Combined flexible blue optically rewritable LCD. Courtesy of Zhang et al.

As with conventional LCDs, the ORWLCD is structured like a sandwich, with a liquid crystal filling between two plates. However, unlike conventional LCDs, where electrical connections on the plates create the fields needed to switch pixels from light to dark, the plates of ORWLCDs are coated with 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. ORWLCDs are <.5-mm thick, can be made from flexible plastic and weigh only a few grams.

Spacers are used in all LCDs to determine the thickness of the liquid crystal. A constant thickness is necessary for a good contrast ratio, response time and viewing angle. When plates bend, it can force the liquid crystal away from the impact site, leaving sections of the screen blank. Spacer design is therefore critical to the design of flexible LCDs.

To achieve flexibility in ORWLCDs, only the spacers and the substrates need to be flexible, because the driving unit and the display unit are separate, and there are no electronics in the display part of the ORWLCD.

The researchers investigated three spacer methods and found that a nonadhesive, mesh spacer prevented liquid crystal from flowing when the LCD was bent or hit, enabling the ORWLCD to be flexible. Polyethersulfone substrates and flexible spacers were used to make the optically rewritable cell insensitive to mechanical force.

“We put spacers between glass layers to keep the liquid crystal layer uniform,” said researcher Jiatong Sun.

Researchers were also able to improve color rendering of the ORWLCD. A cholesteric liquid crystal colored mirror with a polarizer behind it was used as the colored reflective backboard of the ORWLCD. This enabled the ORWLCD to simultaneously display three primary colors — red, blue and green. Previous ORWLCDs have only been able to display two colors at a time.

The flexible display technology could benefit printed media. For example, a daily newspaper could be uploaded onto the display and updated almost as quickly as news breaks. ORWLCDs 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 researchers hope to commercialize the flexible ORWLCD but first plan to improve the resolution.

“Now we have three colors but for full color we need to make the pixels too small for human eyes to see,” said Sun.

The research was published in Applied Physics Letters (doi:10.1063/1.5021619).

Photonics Spectra
Jul 2018
A sub-field of photonics that pertains to an electronic device that responds to optical power, emits or modifies optical radiation, or utilizes optical radiation for its internal operation. Any device that functions as an electrical-to-optical or optical-to-electrical transducer. Electro-optic often is used erroneously as a synonym.
Research & TechnologyeducationAsia-PacificDisplaysLCDlight sourcesopticsoptoelectronicsflexible displaysoptically rewritable LCDflexible LCDTech Pulse

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