LG talks about e-paper

 January 25, 2011, Photonics West, San Francisco–Cheng-Dong Kim, vice president of R&D at the LG Research Center talked about “technology challenges in e-paper to flexible display application”. The universe of displays is changing dramatically, and the old standby CRTs are rapidly becoming obsolete.

User needs and requirements are driving technology changes in displays. CRTs are being replaced by flat-panel displays which has numerous advantages over CRTs. They’re flat and they range in size from small to very large. The LCD-based displays are also evolving and moving towards flexible, smart, and even transparent substrates.

A flexible display is inherently portable because it is thin and light, rugged and foldable, and relatively unbreakable. These flexible displays are easier to design into products then traditional LCD-based displays. The LCD displays are heavy and fragile, have fixed frames that are very rigid and use a lot of power to operate the backlight.

The challenges for flexible displays are that the substrates need to be flexible and have good thermal and chemical resistance. The manufacturing process has to be compatible with the existing LCDs manufacturing process and ideally can migrate to a room temperature printing operation. The thin-film transistors need to have high-performance over temperature and the semiconductor materials must be insensitive to bending. Finally, the electronics for the circuitry needs to be integrated into the display.

Flexible displays must be able to operate in any configuration and the display mode must be irrelevant to substrate bending. Candidate materials for substrates need to be rugged, bendable, and rollable.

Displays come in three basic types. Flexible LCDs have limited flexibility, cell gaps, and need a flexible backlight. Other limits are due to the actual LCD crystals and cells. Flexible OLED have issues in scaling to large size and the TFT’s in AMOLED need more work to become fully production ready. An e-paper display (EPD) has a simple structure and manufacturing process without the challenges of the active displays.

EPD can replace paper in many applications. Currently, the world uses about 5.7 million tons of newsprint which amounts to 67 million trees and 86 billion tons of waste water. Electronic paper equivalent addresses user requirements for more information and greater portability coupled with faster information access through wireless connections. This is a reflective display technology that uses no power except to change the display. A paper can be integrated with touch controls and sound.

The technology for flexible EPD depends on a flexible substrate and electrophoretic material that changes orientation with an applied field. When the field is reversed, the material resumes its original orientation. Applications for e-paper include books and by tiling, large displays. In the future, materials will support color which will broaden the range of applications available for e-paper.

Technical issues for these displays include the substrate materials. Of the possible materials; thin glass, metal foil, or plastic, each has advantages and disadvantages. Adding color increases the challenges, since current approaches apply a color filter on top of the e-paper resulting in lower reflectance. Light is absorbed by the filters and also by the alignment of the filters with the underlying display structures.

Color ink could solve the problems with much higher reflectivity and better color ratios as well as very low color crosstalk. The problem is that current technologies are not able to make a color electronic ink. Color would require three electrodes per pixel and needs much more complex electronics to compensate for her the different drive thresholds in inks.

Although EPD is usable in sunlight, it’s reflectance is only about 20 percent compared to paper at greater than 60 percent. Contrast ratios are fairly low. Integrating touch controls adds additional requirements for an overlay on the surface. Scaling EPD to large size is a great manufacturing challenge because static visual defects are highly noticeable. Tiling smaller displays is an interim solution but still requires gaps for the bezels. Large size displays could be used as digital interiors, information walls, and signage.

Other companies are also working on EPD’s and are looking at different technologies. Everyone is trying to overcome the inherent difficulties of the encapsulated microcapsules which are slow and only allow black-and-white representations. So far, all of the alternatives have significant implementation challenges.

LG’s work on flexible EPD’s has focused on the e-ink and OLED. They’re doing a lot of work on substrates and in color chemistries to eliminate a color substrate. Manufacturing and EPD design have enabled 11.5 inch color EPD which can be increased to 19 inches in a tabloid format. There are plans to integrate touch and driver ICs into the displays in the near future. LG plans to advance their work in plastic color displays with integrated functions. They want to increase the size and performance while reducing costs. The technology roadmap calls for them to address the existing shortcomings of the e-ink, such as needing to flashy race and area before writing a new page.