Bistable flexible LCDs may be an advance on flexible electrophoretic displays for e-books, e-labels and other applications. For example, ability to use any LCD ink can mean that, compared with electrophoretic displays where the whole front plane must be purchased, printing flexible LCDs may lead to lower cost and choice of functions and greater security of supply. When we visited him on behalf of Printed Electronics World, we established that Professor Vladimir Chigrinov of Hong Kong University of Science and Technology sees the following opportunities:
- Electronic shelf label: 1x3'', very low power, color, robust, bar code within, no ghost, price is key
- E-Book publishing: 6-8", very low power, good reading performances including image transition, lightweight, price is key
- E-Newspaper: 10-13", communication, strong performance of layout, high resolution, bendable, price is key
- Education market: 8-10'', stable and durable, lower price, high resolution, strong interactive function, no glass material would be better
- E-Album: 8-10'', color, handsome device, lower price, high resolution
- Professional market: A4 & 10-14", communication and handwriting function, no ghost, bendable.
His process is a roll-to-roll technology using azo-dye photo aligning layers. It does not therefore call for the brushing of glass to make grooves to lock the LCD molecules as seen in today's glass television screens. His process can employ any commercial LCD, including bistable ones to save energy and color versions. Such a display is optically rewriteable by low cost LED. However, although the display is only 500 nanometers thick, the optical rewriting mechanism has yet to be miniaturised.
He points out that this arrangement is inherently low cost, likely to give much better colors than electrophoretics, be more robust and operate without need of a transistor active matrix backplane or ITO or alternative transparent electrodes with all their problems of cost and of cracking when bent.
Murayama et al demonstrated a plastic color twisted nematic liquid crystal display (TN-LCD) using a photo-alignment (PA) method. In 2005. A continuous coating with azo dye solution on a plastic substrate and continuous PA treatment was employed. It is confirmed that LC alignment on a plastic substrate is possible under continuous processes.
Color LCD by photo alignment
Source: Murayama et al
This is a non-contact method providing fine resolution.
Photo alignment of LCD
After the alignment of the azo-dye monomers by non-polarised UV, a rewritable procedure, the layer is stabilised by heat polymerisation. In the case of Professor Chidrikov's process, this is at 150ºC, so it employs a PES substrate. Lower cost PET may be possible later. The liquid crystal is then deposited on top. The advantages include:
- High order parameter with the saturation dependence with exposure time
- High azimuthal and polar anchoring energy (>10-4 J/m2)
- Photo- and thermal stability after polymerization (250ºC, 175 MJ/m2)
- Good adhesion to the glass and plastic substrates
- Easy pre-tilt angle generation for slantwise non-polarized light exposure (up to 5-100)
- Easy alignment on curved and 3D surface, super thin tubes and photonic holes
- Low dosage (150mJ/cm2 for a non-polarized light and 20 mJ/cm2 for a polarized light)
- Akita University has non-rewritable process using mercury lamps.
Optical image rewriting by photo alignment is carried out by NCKU Taiwan and Kent in the USA but only HKUST has a process tolerant of low cost LEDs we are told. Its azo-dye rotation is unique
The HKUST optical rewriting
The image can be hidden, only to be revealed by a polariser film, so this raises the possibility of anti-counterfeiting. Apparently, this is difficult to fake.
Work is proceeding on improving the color.
Color printable flexible LCD
Professor Chigrinov says, "The earliest commercialization of photo-aligned bistable LCDs will be in flexible displays for plastic cards. All the problems, like mechanical stabilization and electro-optical appearance, have been solved using photo-aligned azo-dye materials. The closest candidates for the application are π-BTN nematic LCD and ferroelectric LCD. The producers of plastic cards in Hong Kong (e.g., Octopus cards) can be approached for the commercialization of our research results.
After finishing the project, we are also going to approach LCD companies, that produce flexible displays for mobile phones, smart cards, PDAs, electronic books, etc., for which extremely low power operation is required due to the memory effect. Maintaining and updating the deliverables can be made at the Center of Display Research in HKUST."
For more see www.cdr.ust.hk 
. Professor Chigrinov will reveal his latest progress with this exciting technology at the IDTechEx event Printed Electronics Europe in Dresden Spring 2010.
. Professor Chigrinov will reveal his latest progress with this exciting technology at the IDTechEx event Printed Electronics Europe in Dresden Spring 2010.Top image: 9-digit reflective flexible TN LCD with SD-1/SDA-2 alignment layer.
To learn more attend Printed Electronics Asia 2009 or Printed Electronics USA 2009.
