Flexible, Bistable LCDs Enter E-Paper Arena

Recently, it seems that as interest and sales in e-paper applications, most notably e-readers increases, so too has the number of new technologies that seek to offer the best solution to current challenges. Electrophoretic displays, specifically those using the technology developed by E Ink have dominated the market. However, another new technology is being developed at the Center for Display Research (CDR) at the Hong Kong University, which utilizes a photo-alignment process to enable using LCDs for flexible displays.

Color Printable LCD (Source: Hong Kong University of Science and Technology)

Color Printable LCD (Source: Hong Kong University of Science and Technology)

While Liquid Crystal Displays offer excellent picture quality with brilliant color and video, they require a brushing process on the inside of a glass sandwich to lock the twisted molecules. These twisted molecules are necessary for bistable displays, which offer low voltage advantages. As such, it is not possible to use flexible substrates, which are becoming a requirement for foldable/bendable e-readers. Recently, Professor Vladimir Chigrinov at Hong Kong University of Science and Technology has developed an experimental LCD electronic paper, which he claims is lightweight, flexible, thin, robust, durable and potentially low cost.

Photo-Alignment Process

Photo-of LCG (Source: Hong Kong University of Science and Technology)The process that researchers have developed is a roll-to-roll technology, which uses a new photo-alignable polymer-an  azo-dye, which has an anchoring energy that can be adjusted by changing the UV exposure time. Using a photo-alignment process eliminates the brushing of the glass, which is needed to lock LCD molecules in their twisted state.

The layer is stabilized by heat polymerization after the azo-dye monomers are photo-aligned. Professor Chigrinov used 150ºC on a PES substrate. Lower cost PET may be possible later. The liquid crystal is then deposited on top.

Cross section of flexible π-BTN
 

According to researchers this arrangement is inherently low cost, likely to give much better colors than electrophoretic technology, 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.
Specifically these advantages are outlined below:

  • High order parameter with the saturation dependent on 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)

While the display is also optically rewriteable by means of light emitting diodes (LEDs), there is a drawback, since the current prototype is only 500 nanometers thick, but the optical writing mechanism has not been shrunk proportionately. Other prototypes have been developed by Akita University using mercury lamps, however, this is a non-rewritable process. While optical image rewriting has been demonstrated, only the Honk Kong University of Science and Technology (HSUSR)-with its unique azo-dye rotation-has a process that is able to take advantage of LEDs. Moreover, it is claimed that the image can be hidden, and then revealed by a polarizer film. This would be advantageous for anti-counterfeiting applications.

Smart Cards

According to Professor Chigrinov, the initial target application for the process would be flexible displays for plastic cards. Prototype shown below.
Bistable LCD
“Problems, like mechanical stabilization and electro-optical appearance, have been solved using photo-aligned azo-dye materials,” he said. “The producers of plastic cards in Hong Kong (e.g., Octopus cards) can be approached for the commercialization of our research results.”  In addition, flexible displays for mobile phones and electronic paper in the form of e-readers and e-newspapers would be excellent markets for the product.  However, commercialization of the technology is still several years down the road.


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