Analysis of Flexible Printed Electronics and E-Paper Display Technology

E-paper display technology is a generic term for new flat panel display technologies that have a printed display effect similar to paper, mainly including the following technical solutions:

Electrophoretic Display Technology (EPD)
Cholesteric Liquid Crystal Display Technology (Ch-LCD)
Bi-stable Nematic Liquid Crystal Technology (Bi-TNLCD)
Electrowetting Display Technology (EWD)
Electrofluidic Display Technology (EFD)
Interference Modulation Technology (iMod)

Among these, electrophoretic display technology is the most representative, having been mass-produced for many years, with mature processes, low costs, and high performance, closely resembling traditional paper formats.

Electrophoretic display technology is one of the earliest developed display technologies resembling paper-like displays. The basic principle is to control the movement of charged particles within the liquid using an external electric field. When the particles move to a certain position, different colors are displayed.

Analysis of Flexible Printed Electronics and E-Paper Display Technology

◤ Microencapsulated Electrophoretic Display

The microencapsulated electrophoretic display material is a liquid material before coating, vividly referred to as “electronic ink water”.

Suspended within this liquid material are hundreds of thousands of microcapsules, each about the diameter of a human hair, composed of positively charged particles and negatively charged particles.

When a negative electric field is applied across the microcapsules, the positively charged white particles move to the negative electrode of the electric field, while the negatively charged particles move to the bottom of the microcapsule and “hide” there, resulting in a white display on the surface; conversely, it appears black, forming the images and text required for electronic ink display.

◤ Microcup Electrophoretic Display

The display principle of microcup electrophoresis is fundamentally the same as that of microencapsulated displays, differing in that the solution containing charged particles is sealed in specially designed microcups instead of capsules.

By applying and switching electric fields to this dispersed system, charged particles can achieve electrophoresis within the microcup to display images.

White charged particles are encapsulated with a dielectric liquid in microcups, effectively preventing leakage of electrophoretic liquid and displacement of particles.

The microcup structure allows for arbitrary shapes, structural integrity, and mechanical stability, demonstrating excellent display performance when bent, rolled, or compressed. It eliminates the need for sealing adhesives and can be cut into any required size and shape, preventing the electrophoretic liquids in adjacent areas from mixing or interfering with each other.

Additionally, the microcup structure provides a more uniform movement space for charged particles, and the heights of the microcups are relatively consistent, making it easier to control the display of three (or more) types of particles. Currently, products using three-color particles (red, black, white) have been mass-produced, and full-color prototypes have also been showcased.

Diagram of SiPix Imaging’s microcup electrophoretic display technology

◤ Cholesteric Liquid Crystal Display

The research institutions for this technology include Kent Display in the USA, Fujitsu in Japan, Fuji Xerox in Japan, and the Industrial Technology Research Institute in Taiwan, China.

Cholesteric liquid crystal is a special liquid crystal mode arranged in a helical structure. It achieves a unique arrangement by adding a chiral agent to nematic liquid crystals and utilizes the cholesteric liquid crystal molecules’ different reflective and transmissive states under varying potentials to achieve display effects.

Cholesteric liquid crystals are reflective displays that use external ambient light sources to display images without requiring a backlight, while also possessing bistable characteristics, making cholesteric liquid crystal display technology very energy-efficient.

This technology can also produce colors such as red, green, and blue by adding different chiral agents with varying helical pitches to meet the needs for color displays. The color is displayed through the reflection of external light on three stacked layers of cholesteric liquid crystal films.

The disadvantage of this technology is that it requires a higher driving voltage (30~45 V), making it challenging to create flexible products. Currently, it is widely used in writing tablet products, with the Boogie Board being a common example.

◤ Electrowetting Display (E-WET)

Electrowetting display technology utilizes the property of an insulating film that can change from hydrophobic to hydrophilic under voltage control to adjust the color of each panel by controlling the voltage underneath.

By laying a reflective film underneath the panel, it can serve as a reflective display similar to an electronic ink screen, with a reflectance of over 50% (E-ink carta has a reflectance of 44%).

Philips released the patent for this technology in 2002, with Liquavista Company developing it industrially. Samsung acquired this company in 2011, but sold it to Amazon two years later due to declining demand and unsatisfactory results.

According to reports, Amazon plans to use Liquavista technology to launch a color Kindle e-book, bringing the product to market.

Unlike electrophoretic ink screens, electrowetting displays can achieve refresh speeds suitable for video viewing. Their brightness is three to four times that of LCDs, yet they are more energy-efficient, providing longer usage times for products. The products include three types:

Liquavista Bright: black and white screen with high contrast, refresh speed sufficient for video viewing. Similar to ink screens, mainly used for e-books.
Liquavista Color: color screen suitable for video playback, reading, magazines, etc.
Liquavista Vivid: combines black and white and color screens, with a black and white mode (more suitable for reading) and a color mode (more suitable for video and image viewing).

◤ Printing Technology in E-Paper Production

In the current mainstream production of electrophoretic displays, coating is a very important process.

The image below shows the structure of a microencapsulated electrophoretic display, where the electronic ink layer is positioned between the front common electrode and the bottom pixel electrode.

Structure diagram of a microencapsulated electrophoretic display

The usual production method is to first coat the electronic ink onto the film. The coating methods include: slot coating, screen printing, inkjet printing, blade coating, spraying, etc.

To control production costs, based on the characteristics of electrophoretic display materials, the current microencapsulated electrophoretic display films use a roll-to-roll coating method, allowing for rapid production of display materials that meet product application needs. The image below shows rolled film materials.

Rolled electronic paper film material

Additionally, many supporting materials for electronic paper, such as protective films, also use roll-to-roll printing processes. Future flexible TFT substrates may also be produced using printing processes.

◤ Future Prospects

At this stage, the e-paper market is positioned to replace paper and printed materials, not to compete with multimedia, and does not aim to replace LCDs and other displays. However, future research trends will inevitably develop in multiple directions, including cost reduction, improved response speed, full-colorization, and flexibility.

Currently, the only companies capable of mass-producing e-paper globally are E-INK from the USA (merged with Taiwan’s Yuantai PVI in 1999, now renamed EIH) and Guangzhou Aoyi Electronics Technology Co., Ltd., both using electrophoretic display technology.

1. Cost Reduction

Electrophoretic display technology, especially microencapsulated display technology, has a simple manufacturing process and roll-to-roll coating method, similar to paper production, with yield rates expected to improve year by year.

As production volume and yield rates increase simultaneously, the cost of e-paper displays will inevitably decrease year by year. The price trend of e-paper displays will follow that of other electronic products, decreasing annually, leading to the emergence of various new applications as prices continue to decline.

2. Improved Response Speed

To meet the performance demands of bistability, electrophoretic display technology sacrifices response speed, resulting in long update times of several hundred milliseconds, which is insufficient for video applications.

With technological advancements, faster responding e-paper materials have emerged, currently achieving speeds of several tens of milliseconds, with potential for further improvement to meet customer demands in the future.

3. Full-Colorization

Currently, color electrophoretic display e-paper can be achieved in two ways: one uses color filters with black and white e-paper, and the other uses colored particles or dyes, both of which have produced samples.

Due to reliance on reflected light for imaging, e-paper screens appear somewhat dim compared to the brightness and color accuracy of LCD screens.

4. Flexibility

Just as general readers do not expect to roll up a book, the main purpose of using flexible e-paper displays is not for portability but for durability and impact resistance.

Flexible e-paper displays can choose plastic substrates as backplanes. E-paper with plastic substrates weighs about 80% less than glass materials and has a thickness of only 0.3 mm, meeting the demands for lightweight and impact resistance. However, the biggest challenge plastic substrates face is their heat and chemical resistance.

Source：International Film and Tape Exhibition

Analysis of Flexible Printed Electronics and E-Paper Display Technology

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Analysis of Flexible Printed Electronics and E-Paper Display Technology

◤ Microencapsulated Electrophoretic Display

◤ Electrowetting Display (E-WET)

◤ Printing Technology in E-Paper Production

◤ Future Prospects

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