DAS BG CTO Knowledge Management Department presents
Article No. 607
1. Concept of Electronic Paper
Electronic paper, also known as digital paper, retains its original image even after power loss, exhibiting a certain memory capability and possessing most functions of traditional paper.
The substrate of electronic paper is mainly made of polyester compounds, coated with circuits on the surface, which control the movement of electronic capsules in the circuit through changes in the external electric field, enabling text and image transformations. Electronic paper features low power consumption, foldable and bendable capabilities, fine image display, wide viewing angles, and good visibility under sunlight without dead angles.
2. Applications of Electronic Paper
Media
Electronic paper can be used for flexible displays, especially in home decoration, cinema notices, mall labels, street banners, and medical electronic labels, compensating for the shortcomings of traditional paper displays, avoiding fading issues caused by direct sunlight, and meeting green environmental protection requirements. The content displayed on electronic paper can be remotely controlled and intelligently updated. This product is a carrier that combines digital information technology and static media technology.
In 2016, the first solar-powered electronic paper traffic sign was born in Sydney, using 3G technology to update content, consuming power only during updates.
Digital Publishing
With the rapid development of internet technology, people’s reading habits have shifted from traditional books to e-books, and electronic paper has entered the market with its lighter characteristics. Electronic paper can provide personalized services for newspaper groups and consumers, enabling on-demand publishing through online ordering and cloud delivery.
In recent years, as electronic reading becomes popular, many publications both domestically and internationally have stopped issuing paper versions, transitioning to provide electronic reading services, with paperless offices and electronic government increasingly favored by enterprises.
Consumer Electronics
The ultra-thin, ultra-light, and soft characteristics of electronic paper make electronic products aesthetically pleasing and visually impactful. In 2005, Seiko EPSON launched an electronic watch at “BaselWorld,” which was as thin as paper and could utilize near-field communication technology for short-range smart control, allowing the watch to closely adhere to the skin of the wrist and change patterns according to the owner’s preferences.
3. Display Principles of Electronic Paper
In the research of real-world technologies with electronic paper characteristics, various types of reflective display technologies with bistable properties are the main focus. Common types include electrophoretic displays, rotating ball displays, bistable liquid crystal displays, electrowetting displays, and fast-response electronic powder fluid displays, among which electrophoretic display technology has developed the fastest, and products developed from it have already been brought to market.
Electronic paper mainly consists of a surface layer, a bottom layer, and an intermediate layer, with tens of thousands of tiny ink particles in the intermediate layer, distributed in a transparent base liquid to form a suspension system. Each ink particle has a diameter of about 100ÎĽm and is easily charged. These charge-sensitive particles can move under the influence of an external electric field.
The core material of the microcapsule electronic ink technology developed by E Ink is electronic ink, which consists of two parts: black dye and charged titanium dioxide electrophoretic particles.
The electronic particles are suspended in the colorant, arranged uniformly and moving randomly, all encapsulated in a transparent capsule. Under the influence of an external electric field, the white particles can sense charges and move in different directions, allowing the side where the white particles gather to display white, while the opposite side displays the color of the dye, that is, black. Electronic paper thus utilizes this principle to achieve color conversion of text and images.
For a long time, electrophoretic electronic paper has been widely used in e-book display technology, but it could only present black and white displays. Worldwide, color electronic paper display technology has not yet left the laboratory, nor has it achieved mass production.
The technological challenges to progress from black and white to color increase geometrically, requiring significant breakthroughs in material combinations, color control, and hardware-software compatibility.
For color electronic paper display technology, many innovative entities both domestically and internationally are actively conducting research and development. Guangzhou Aoyi Electronic Technology, Samsung, and BOE have a larger number of patent layouts, followed by Tianma Microelectronics, E Ink Technology, Fanxin Technology, and Dali Technology. Additionally, the Korea Electronics and Telecommunications Research Institute, Epson, and Guohua Optoelectronics also have a certain amount of patent layouts.
Next, we will introduce how related innovative entities achieve color displays in electronic paper.
A. Guangzhou Aoyi Electronic Technology Co., Ltd.
The microcapsule electrophoretic electronic paper color display screen includes a transparent conductive layer, a driving substrate, and a microcapsule electrophoretic display layer located between the transparent conductive layer and the driving substrate.
The microcapsule electrophoretic electronic paper color display layer contains several uniformly and regularly arranged color pixel display units formed by microcapsules; each color pixel display unit is constructed in a “|||” arrangement of three color basic pixel display units; each of the three color basic pixel display units includes color microcapsules, which sequentially contain red, green, and blue charged pigment particles and electrophoretic liquid, achieving different color displays by driving the microcapsule electrophoretic display layer.
B. BOE Technology Group Co., Ltd.
The electronic paper includes a display layer, and the sub-pixel units of the display layer include:
A cavity structure, an electrophoretic liquid located within the cavity structure, and reflective particles distributed in the electrophoretic liquid.
For the reflective particles:
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The reflective particles of the red sub-pixel unit have half of their area as red and the other half as black, with the red area being the charged area;
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The reflective particles of the green sub-pixel unit have half of their area as green and the other half as black, with the green area being the charged area;
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The reflective particles of the blue sub-pixel unit have half of their area as blue and the other half as black, with the blue area being the charged area.
During display, since half of the reflective particles are red (or green, or blue), and half are black, and the red (or green, or blue) half is charged, an electric field is formed between the pixel electrode and the transparent electrode. According to the direction of the formed electric field, the reflective particles can orient their red (or green, or blue) side toward the display side, thus reflecting the external light incident on the reflective particles.
Moreover, depending on the strength of the formed electric field, the reflective particles can also move to different positions within the cavity structure, achieving different intensities of reflection of external light, thereby enabling color display on electronic paper.
C. Tianma Microelectronics Technology Co., Ltd.
The electronic paper display panel includes multiple pixel units arranged in a matrix, a first substrate and a second substrate that are oppositely set, and a first barrier layer located on the first substrate. The first barrier layer is provided with first and second plasma injection channels, both of which are comb-shaped, with the comb handle and multiple second comb teeth. The plasma injection channels contain plasma, which includes color-changing particles of different colors, allowing the pixel units corresponding to the first plasma injection channel to display the first color, and the pixel units corresponding to the second plasma injection channel to display the second color, enabling the electronic paper display device with this electronic paper display panel to achieve color display.
D. Guohua Optoelectronics Technology Co., Ltd.
A new type of reflective electrowetting display principle based on an oil/water two-phase dielectric electrowetting system.
The optical layered structure of this display device consists of a white reflective substrate, a hydrophobic dielectric layer, colored ink, and water. In the unpowered equilibrium state, the colored ink forms a continuous spreading film between the hydrophobic dielectric layer and water. Applying an electric field can break the energy balance of the original system, allowing water to replace the ink and form wet contact with the dielectric layer surface.
This optically promotes the contraction of the ink, revealing the white backing. The size of the external voltage determines the balance position between the corresponding electrostatic force and capillary force, thus achieving control over the display grayscale, and the expected color of the pixel can be displayed by regulating the ink color, resulting in a colorful display effect capable of showing color images and videos.
4. Technical Solutions
There are several main technical solutions: electrophoretic display technology (EPD), cholesteric liquid crystal display technology (Ch-LCD), bistable nematic liquid crystal technology (Bi-TNLCD), electrowetting display technology (EWD), electrofluidic display technology (EFD), and interference modulation technology (iMod). Among them, 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 forms.
1. Microcapsule Electrophoretic Display
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 inside a liquid using an external electric field. When the particles move to a certain position, they present a different color display effect.
The microcapsule electrophoretic display material is a liquid material before coating, colloquially known as electronic ink “water.” This liquid material suspends hundreds of thousands of microcapsules, each about the diameter of a human hair, consisting of positively charged particles and negatively charged particles. When a negative electric field is applied across the microcapsule, the positively charged white particles move toward the negative electrode, while the negatively charged particles move to the bottom of the microcapsule, “hiding” themselves. At this point, the surface will display white; conversely, it will appear black, allowing the required images and text to be displayed by electronic ink.
2. Microcup Electrophoretic Display
The display principle is fundamentally similar to that of the microcapsule type, with the difference being that the solution containing charged particles is encapsulated in specially designed microcups instead of capsules. By applying and switching the electric field to this dispersion system, charged particles can produce electrophoresis in the microcup to achieve image display. White charged particles are sealed within a dielectric liquid in the microcup, effectively preventing leakage of the electrophoretic liquid and displacement of particles. The microcup structure has arbitrary shaping, structural integrity, and mechanical stability advantages, showing excellent performance under bending, rolling, and pressure without the need for sealing glue, allowing it to be cut into any required size and shape. During electrophoretic display, the electrophoretic liquid in adjacent areas does not mix or interfere with each other. Additionally, the microcup structure provides more uniform movement space for charged particles, and the height of the microcups is relatively consistent, making it easier to achieve control 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 exhibited.
Chart: SiPix Imaging’s microcup electrophoretic display technology schematic
3. Cholesteric Liquid Crystal Display
This technology is developed by institutions such as 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 pattern, achieved by adding a chiral agent to nematic liquid crystal to attain a special arrangement structure. It utilizes the “reflection” and “transmission” of different polarized light rotation states presented by cholesteric liquid crystal molecules under different potentials to achieve a display effect. Cholesteric liquid crystal belongs to reflective displays, using external environmental light sources to display images without needing a backlight, while also possessing bistable properties, making cholesteric liquid crystal display technology very energy-efficient. Additionally, this technology can meet the demand for color display by adding chiral agents with different helical pitches, producing red, green, blue, and other colors. Stacked by red, green, and blue cholesteric liquid crystal films, it reflects external light to display color. The downside of this technology is that it requires a higher driving voltage (30 ~ 45 V), making it challenging to produce flexible products. Currently, this technology is widely used in writing board products, with Boogie Board being a common example.
4. Electrowetting Display (E-WET)
Electrowetting display technology utilizes the property of an insulating film that can switch from hydrophobic to hydrophilic under voltage control, adjusting the color by controlling the voltage under each panel. Laying a reflective film beneath the panel allows it to function as a reflective display similar to electronic ink screens, with a reflectivity of over 50% (E-ink carta reflectivity is 44%). Philips released a patent for this technology in 2002, which was industrially developed by Liquavista. Samsung acquired this company in 2011, but two years later, due to reduced demand and unsatisfactory results, it sold it to Amazon. It is reported that Amazon plans to use Liquavista technology to launch color Kindle e-books to market.
Unlike electrophoretic ink screens, electrowetting displays can achieve refresh rates suitable for video viewing. Its brightness is three to four times that of LCDs while being more energy-efficient, providing longer usage times for products. There are three types of products: Liquavista Bright: black and white screen with high contrast, refresh rate sufficient for video viewing, similar to ink screens, mainly used for e-books. Liquavista Color: color screen suitable for video playback, reading, magazines, and viewing images. Liquavista Vivid: combines black and white and color modes, with a black and white mode (more suitable for reading) and a color mode (more suitable for video and image viewing). High brightness, high contrast.
5. Printing Technology in Electronic Paper Production
In the current mainstream production of electrophoretic displays, coating is a very important process. Chart 94 is a structural diagram of microcapsule electrophoretic displays, where the electronic ink layer is between the front common electrode and the bottom pixel electrode. The usual production method is to first coat the electronic ink onto the film, with coating methods including: slot coating, screen printing, inkjet printing, blade coating, spraying, etc. Generally, the precision requirements for coating machines are quite high, needing to achieve +/- 1um coating precision.
Chart: Structure diagram of microcapsule electrophoretic displays
To control production costs, based on the characteristics of electrophoretic display materials, the microcapsule electrophoretic display film adopts a roll-to-roll coating method, quickly producing display materials that meet product application requirements. Chart 95 shows roll-type thin film materials.
Chart: Roll-type electronic paper thin film materials
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.
5. Future Outlook
Currently, the electronic paper market is positioned as a substitute for paper and printed materials, not competing with multimedia, and not replacing LCDs and other displays. However, future research trends will inevitably focus on reducing costs, improving response speed, achieving full color, and flexibility. Currently, only two manufacturers globally can mass-produce electronic paper: E-INK in the USA (merged with Taiwan’s E Ink PVI in 1999, now renamed EIH) and Guangzhou Aoyi Electronic Technology Co., Ltd., both using electrophoretic display technology.
a) Cost Reduction
Electrophoretic display technology, especially microcapsule display technology, has simple manufacturing processes and roll-to-roll coating methods similar to paper production, with yield expected to improve year by year. With simultaneous increases in output and yield, the cost of electronic paper displays will inevitably decrease annually. The price trend of electronic paper displays will follow that of other electronic products, decreasing year by year, leading to the emergence of various new applications.
b) Enhanced Response Speed
To meet bistable performance requirements, electronic paper display technology sacrifices response speed, with display update times being relatively long, lasting hundreds of milliseconds, which is insufficient for video applications. With technological advancements, increasingly faster response electronic paper materials have emerged, currently achieving tens of milliseconds, with opportunities for further enhancement in the future to meet customer demands.
c) Full Color
Currently, color electrophoretic display electronic paper can be realized through two methods: one uses color filters with black and white electronic paper, while the other uses colored particles or dyes, both of which have produced samples. Due to reliance on reflected light for imaging, electronic paper screens appear somewhat dim compared to the brightness and color accuracy of LCD screens. Therefore, achieving full color is a revolutionary breakthrough in electronic paper technology, with significant resources being invested in research and development, and it is believed that soon, we will see color electronic paper displays.
d) Flexibility
As general readers do not expect to roll up books, the main purpose of adopting flexible electronic paper displays is not to allow them to be rolled up but to enhance portability and impact resistance. Flexible electronic paper displays can choose plastic substrates as backing. Electronic paper with plastic substrates weighs about 80% less than glass materials, with a thickness of only 0.3 mm, meeting the demands for lightweight and impact resistance. However, the biggest challenge that plastic substrates face is the material’s heat and chemical resistance, requiring continuous improvement of substrate materials.
Conclusion
As a display technology widely used in e-books, electronic paper can only present black and white displays. Currently, color electronic paper display technology has not yet achieved mature industrialization.
From the perspective of patent layout, many innovative entities are actively conducting layouts for color electronic paper display technology. As research on color electronic paper technology matures, achieving mature industrialization of color electronic paper display technology is just around the corner.
Color electronic paper can continue to display for up to six months after power is cut, with a response speed of 0.1s, and its expected price is 1/6 that of an equivalent area OLED display. In the future, it can be widely used in electronic textbooks, electronic price tags, indoor and outdoor advertising, smart wearables, smart homes, etc., with enormous commercial application potential.
Source: Extraordinary Intellectual Property, Flexible Electronics and Smart Packaging
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