Researchers at the University of Strathclyde have demonstrated a flexible bistable reflective LCD suitable for mass production.

"One market we are looking at is advertising. We can do quite large displays," Professor Nigel Mottram told Electronics Weekly. "We are testing if it can be made roll-to-roll. So far we can do the electrodes, the liquid crystal, and the sub-structure."

Mottram is a mathematician, with a background in modelling LCDs on computers.

His LCD is unusual because the liquid crystal is retained in small separate wells rather than in a thin display-wide layer.

In most LCDs, the liquid crystal is given a fundamental orientation by brushing minute scratches into the inside of one of the sheets of glass it is made from.

In the Strathclyde LCD, this orientation is promoted by the shape of the polygonal wells, which are embossed into one internal surface of the display.

Bistability comes from the shape of the wells. "The liquid crystal has energy minima in two different positions," said Mottram.

Polygonal cell pattern

The structure consists of two plastic sheets, one embossed with the polygonal cell pattern. This is filled with standard liquid crystal, then the second sheet is bonded on top, creating the separate sealed wells.

"You don't need to brush, and the capping layer bonds without being interrupted by the liquid," said Mottram, "so we take out a few different production steps and only add the polygonal embossing."

ITO electrodes are deposited as parallel lines on the surface to form an X-Y addressing matrix. And switching between states can also be achieved using in-plane electrodes.

As it is a bistable technology, only requiring a passive matrix, there are no thin-film transistors. The back of the display then gets a vacuum-deposited mirror and the front gets a polariser.

The overall effect of mirror, LC wells, and polariser is that much like a watch display, the LCD appears either bright or black.

"The aim initially is to be as readable as newspaper," said Mottram. "We are close to that with a contrast ratio of six, seven or eight to one. I think we can get the reflectivity close to paper as well."

By using different size and shaped sub-pixels, a spatial grey-scale is possible, and colour is possible, said Mottram, trivially using colour-filtered sub-pixels, or for a good quality image by stacking three LCD planes.

Other points to note, include:

• Being a reflective, sun light viewabilty is not a problem.
• Flexibility, as usual with displays using ITO, is limited by cracking conductors.
• Reflective bistable LCDs only need power when a change of image is needed and they are often touted for e-books, shelf-edge labelling in shops, and for active posters.
• Established bistable LCD technologies include cholesteric - from US firm Kent Displays - and zenithal bistable displays from Worcestershire's ZBD.
• Cholesteric displays need no polarisers, but finger touches disrupt the image until it is refreshed. ZBD and the Strathclyde displays are immune from this issue.

E-books

E-book makers have so far spurned LCDs, even though ZBD demonstrated some good-looking high-resolution display prototypes, instead plumping for electrophorescent bistable structures using technology from E ink and others.

"Electrophorescent is quite slow. It takes a good second to refresh a page but it is perfect for the e-book market," said Mottram.

Passive addressing is not going to allow video speed update, but "if it is anywhere near the millisecond range we should be able to refresh a notice board", he said.

It is this speed, combined with large size, some grey scale, easy outdoor viewing, and enough flexibility to allow easy handling, that is pointing Mottram towards signage and advertising. He points out that low power consumption allows operation from solar power in off-grid applications.

In the far future, the Strathclyde team is imagining making the well walls and filling them with liquid crystal using ink-jet printing. "It is very much in the future," said Mottram, "we would need 5µm accuracy for the walls."

Funding

Cash to get the project off the ground came from Scottish Enterprise' innovative Proof-of-Concept fund.

"We only had ideas, nothing else," said Mottram. "Now we have proved pretty much everything and hope to spin out a company."

Intellectual property exploitation is being handled by www.university-technology.com the web-based initiative shared by Scotland's 13 universities.

The University of Strathclyde is also seeking a partner with expertise in interdigitive electrodes to help take this technology to market.