1.1 Introduction

For over a thousand years, before the world of electronics, paper was the dominant medium for people to share written and later printed information. People become familiar with paper at an early age, and there is an enormous worldwide infrastructure for the production and distribution of printed material. Despite this huge built‐in advantage, paper and print now fall short in providing for many demands of modern life. The past few decades have seen the emergence of electronic networks that transmit vast amounts of information, on‐demand, for use in various ways. Electronic displays are a necessary part of this infrastructure, converting bits to photons and serving as the final stage of transmitting information to people.

Over the years, several electronic display technologies have waxed and waned; cathode ray tubes, plasma displays, and super twisted nematic (STN) displays come to mind. A few technologies are dominant; backlit active‐matrix liquid crystal displays, active matrix organic light‐emitting diode (OLED) displays, and inexpensive, passively addressed liquid crystal displays. Along the way, tens of different types of display technologies have been invented and explored, but ultimately have failed to catch on. A few displays have found a home in niche products and promise greater future application. Reflective displays, particularly electronic paper, are examples that have managed to find a place in the display ecosystem, with unique applications best served by these technologies.

This book aims to update on some of the most exciting new areas in electronic paper technology. This introductory chapter focuses primarily on electrophoretic displays (EPDs) and how they became synonymous with electronic paper. The story starts in the early 1970’s, with the proposal and first demonstration of the electrophoretic movement of charged particles to make an optical effect. After intense effort, the technology was mostly abandoned, only to be resurrected by a start‐up company, E Ink. Several key, and rather improbable inventions had to be made to develop a technology competitive to the dominant liquid crystal technology. Finally, the right application and ecosystem, the Amazon Kindle electronic book, was necessary to cement commercial success.

The field of reflective displays is very rich. The many other chapters in this book and recent reviews [1, 2] provide a wealth of resources for understanding the many technologies that have been developed in the quest to achieve a paper‐like display. In this chapter, we will examine the following:

• A description of print‐on‐paper and how the optics of real paper compare with potential electronic paper competitors.
• A hierarchical summary of the different technical approaches for reflective displays.
• A detailed look at the historical development of EPDs, starting with the invention of the technology and ending with the introduction of the Amazon Kindle. Looking at the various developments in the context of its times, the EPD story offers some lessons in what it takes for a technology to transition from the laboratory to commercial success.

1.2 Why Electronic Paper?

Electronic paper has undoubtedly caught the imagination of the world. A Google search for electronic paper in September 2021 returns over 12 billion hits. This interest reflects people's love affair with paper as a medium for transmitting information. Yet, it is easy to recognise that printing ink onto dead trees is not easily compatible with today's networked world. What are the attributes of print‐on‐paper that make it so important?

• Paper is a reflective medium that automatically adapts to changing lighting
• Unlike most emissive displays, paper can be easily read in bright sunlight.
• The appearance of paper is relatively constant over different viewing angles, without significant shifts in luminance or color.
• Paper can be lightweight and flexible. The user can easily annotate it. with a pen or pencil.
• Paper is inexpensive
• Paper can be archived.

Nevertheless, physical paper cannot be instantly updated with information from electronic networks or easily serve as an interface with electronic devices. Today's backlit LCDs and OLED displays are ubiquitous as a means of transmitting information, but with the limitations that emissive displays possess, including eyestrain and low visibility in sunlight. Electronic paper can combine the power of electronic devices and networks with all the attributes of paper.

So what strategies can be taken to enable electronic paper? It is instructive to understand the composition and design of print‐on‐paper and see how many of these properties can be converted to something under electronic control to compete with printed media.

1.3 Brightness, Color, and Resolution

Conventional, non‐electronic paper consists of a mat of tightly pressed fibers, most commonly derived from parchment or wood pulp. The combination of fibers and embedded air pockets scatter light and provide the reflective characteristics of paper. Historically, additives to the paper pulp during fabrication have also provided glossiness, color, aid in manufacture, or other desirable characteristics (Figure 1.1).

Compared to a white optical standard, the perceived reflectivity of paper often ranges from 50–80%, but can be even higher. The whiteness or brightness of paper depends on several factors, including the density of fibers, the paper thickness, the presence of additives such as titanium dioxide, clays, or fluorescent agents, and whether the viewing surface is made glossy through calendaring and coatings. The color of light reflected from white paper may differ somewhat from a perfect reflector due to the fluorescing whiteners' presence, or some underlying color absorbance from the paper. The human eye readily accommodates for these changes, though, so the perception of consistent color and lightness of a page relative to its surroundings is easily achieved (Figure 1.2).

Print‐on‐paper consists of drops of colored ink impregnated into the paper fiber. It is straightforward to devise dyes and pigments that absorb red, green, or blue. To generate the color characteristics of print, the CMYK subtractive color system can be used (Figure 1.3). The colors in print are usually comprised of cyan (absorb red), magenta (absorb green), and yellow (absorb blue) (Figure 1.4). Black pigment (the K in CMYK) is also commonly used, as it is challenging to achieve a neutral black color by mixing cyan, magenta, and yellow.

To achieve a wider color gamut, inkjet printing may use six or more colors to print. Additionally, spot color printing can deposit specialty inks (such as fluorescent pigments) that currently have no analog in emissive displays.

Depending on the industry, a variety of different color spaces and metrics have been developed for printed and reflected color. For example, the CIE 1976 (L*a*b*) color space is widely used to measure reflective colors and print. The human perception of lightness is measured by L*, which roughly scales as the cube root as the reflected luminance level.

For color reproduction in the print industries, the SNAP standard (Specifications for Newspaper Advertising Production) and SWOP standard (Specifications for Web Offset Publications) are widely used [1]. These print standards are rarely applied to electronic displays, though the advent of colored reflective displays approaching print‐like appearance could change this situation.

Grayscale in printing is achieved using halftones (Figure 1.5). Each dot on a printed page defines an area where smaller halftone dots are printed. The more halftone dots, the deeper the color or darker the black while white is the absence of halftone dots. The smaller the dots, the higher the resolution. Print is often defined in lines per inch.″ Some examples of everyday printed objects include:

• Newspaper (monochrome) – 65–100 LPI
• Books and magazines (color) – 120–150 LPI
• Art books (color) – 175–250 LPI
• Photorealistic inkjet printer (color) – 250–300 LPI

Likewise, the resolution‐defined dot for color printing consists of multiple prints of smaller dots. The combination of different colors and black and the underlying brightness and color of the paper provide the specific color and lightness of that dot.

With inkjet printing, commercial printers can also control the size of the drop, such as achieving four different sizes (Figure 1.6). The number of achievable gray levels is a combination of the number and size of the printed dots within the equivalent printed pixel [6, 7].

1.4 Reflectivity and Viewing Angle

An important aspect of paper is that the image printed on the page appears to have constant lightness and color irrespective of the viewing angle and lighting conditions. If the paper is glossy, there may be some glare from the surface, but that reflectance rarely interferes with the user interacting with the page. The near‐constant appearance of the printed page is representative of Lambertian reflectance. The underlying paper scatters light, impinging onto the surface from many angles and then reflected uniformly into all spatial angles. Whether the page is illuminated by a collimated source such as a light bulb in a dark room or a more uniform source such as a cloudy sky, the distribution of reflected light from the paper does not change that much. Likewise, the amount of light scattered into the viewer's eyes for a given spot on the paper is also constant with viewing angle [8].

1.5 Translating Print‐on‐Paper into Electronic Paper

With this background, we can examine why designing an electronic display with the appearance of high‐quality print‐on‐paper is challenging.

1.5.3 An Overview of Approaches to Color Electronic Paper

Many inventive approaches have been proposed to achieve electronic color paper. Some designs take advantage of an electro‐optical medium that can change color at the subpixel level intrinsically. Other approaches use color filters with an otherwise colorless black and white effect. Accurate and uniform grayscale is essential for high‐quality images and is often very difficult to achieve.

Figure 1.7 illustrates an EPD in combination with color filters [15]. The function of a color filter is to absorb portions of the visible spectrum so that reflected light is colored. RGB or CMY primary colors can blend the primaries to provide an extended color range. Grayscale is adjusted by controlling the state of the reflective medium, similar to gray in black and white displays. These displays inevitably must make trade‐offs between color saturation, color space coverage, and brightness. Reasonable brightness is usually achieved at the cost of color purity and a restriction of available colors.

Another approach is the adoption of multiple color particles as electrophoretic elements [16, 17]. Different colors can be generated in each sub‐pixel by mixing C, M, Y translucent particles with scattering white particles, as shown in Figure 1.8. This approach was firstly demonstrated by Fujifilm in 2012 [16], and later on by E ink [17], which named it Advanced Color ePaper (ACeP). The colored pigments have different electrophoretic mobilities and responsivities, which facilitate to shuffle the color particles with driving waveforms of different voltage and pulse widths. As shown in Figure 1.8, different primary colors can be presented by positioning the white particle layer above or underneath the translucent C, M, Y particles. Integrating the CMYW particles into a display unit, ACeP can present different colors and gray levels in each subpixel by controlling the driving waveforms. These driving waveforms are complex, and frame rates are currently slower than other EPD approaches. Nevertheless, the multiple color particle design dramatically increases the color saturation and reflective luminance achievable compared to the color filter approach. Figure 1.9 compares the color performance of these two types of color EPD prototypes [16, 17].

Many other inventive approaches have tackled the problem of making the electro‐optical medium capable of switching color. Controlled lateral migration of colored fluid and particles enables a bi‐primary color system. This design allows a single pixel to be changed through mixtures of color states [19].

A variety of other physical phenomena can be used to modulate reflected light electrically. The following table describes several varieties of reflective displays that have been developed over the past several decades. The fact that so many technologies have been pursued shows both the interest in the electronic paper and the difficulty in achieving a paper‐like display. We compare the various reflective display technologies in Table 1.1, showing the strengths and weaknesses of the different approaches. More information on these different technologies can be found in reviews [1, 2, 20] and other chapters in this book.

	Electrophoretic (commercial)	Electrophoretic (eTIR)	In‐plane EPD	Electro kinetic	Liquid powder	Electrochromic	Electrowelting	Electro fluidic	MEMs (IMOD)	Cholesteric LC	PDLC	Reflect LCD w polarizer	Flip dot	Zenithal bistable device
White, color %R	44% W [21]	60% [10]	75% [23]	∼62% [15]	25 ∼ 30% [28]	70% [30]	70% W [34]	72% W [37]	61% (Indoor) 80% (Outdoor) [40]	40% [35]	50% W	42% W [43]	80% [45]	39% [48]
Black %R	3% [21]	4% [10]	3% [23]	2%	3 ∼ 4%[28]	–	3–5%	<3% [37]	>3% [35]	5% [35]	5%	2%	6%	2% [48]
Contrast ratio	∼15 : 1 [21]	15 : 1 [10]	23 : 1 [23]	30 : 1	8 : 1	38.1 : 1 [31]	>35 : 1 [35]	>30 : 1 [37]	15 : 1	8 : 1 [35]	10 : 1	>20 : 1 [43]	13.3 : 1	20 : 1 [48]
Lambertian	Yes	Partial [10]	Yes	Yes	Yes	Yes	Partial	Yes	No	Partial	No	Partial	Yes	Partial[48]
mono SNAP/SWOP	–	Maybe SWOP [10]	Maybe SWOP [23]	SNAP	–	Maybe SWOP [30]	SNAP [34]	SNAP	SNAP	–	Maybe SNAP	Maybe SNAP [43]	–	–
Driving voltage	15	2–4 [11]	<10	∼5 [26]	40–70	1.4 [31]	15 ∼ 20	10–20	5–10	(lab) < 4 (product) 25–40 [41]	5	∼3 [43]	4 ∼ 125[46]	20
Bistable	Yes	Yes [10]	No	No	Yes	Yes [32]	Yes [34]	Yes	Yes	Yes	No (Yes with matrix)	Yes	Yes	yes
Switching speed (msec)	100's	(33 fps) 33 [10]	15 (100 V) 30 (40 V) [24]	<300 [27]	<0.2 [29]	<10 [33]	10	∼4 [38]	0.01's	300 (vertical) 5 (in‐plane) [41]	51 [42]	48 [43]	∼66 [47]	20 ms
Matrix drive	AM	AM [11]	AM, PM	AM	PM	PM	AM	AM, PM	PM	PM	AM	PM	–	AM [49]
Greyscale approach	Pulse	Pulse [10]	Analog/Pulse	Pulse	Multi‐write	–	Analog/Partial filling/Spatial dithering [36]	Pulse	Spatial/Temporal	Pulse	Analog	Halftone	Pulse	Analog [49]
Grayscale bit level	4	4 [10]	5 (∼30) [25]	>4 [27]	2	6 [30]	4	4 [39]	6	4	8 (60 Hz) 1 (6 Hz) [42]	2 [44]	1	4 [49]
Neutral white point	Yes	–	Yes	Yes	Yes	Possible	Yes	Yes	Yes	No	Yes	Possible	–	Yes [48]
Lifespan	Good	–	Unproven	Unproven	Good/ltd.	Ltd./unproven	Unproven	Unproven	Good	Good	Good	Great	Commercial	Good
Power (Static)	None	None [10]	Very low	Very low	None	Very low	None [34]	None	Very low	None	Low	Moderate	None	Very low
Power (Video)	Low	Low [11]	n/a	n/a	Low	Very high	High	High	High	Moderate	Moderate	Low	High	Very low
Years of research	Since 1969 [22]	Reported 2018 [10]	Since 2000 [1]	Since 2009 [1]	Since 2003 [1]	Since 1989 [1]	Since 2003 [1]	Since 2009 [1]	Since 1997 [1]	Since 1994 [1]	In mid 1980s [1]	Since 2001 [1]	–	Since 1995 [48]
Maturity	Many products	Prototype [10]	AM & PM demo	AM demo	ESL product/large PM	Smart card products	AM demo	Segment product	PM products	PM products	AM prototypes	Products	Products	Products

There are several interesting aspects of Table 1.1 worth noting:

• Several e‐paper technologies reach the reflective luminance of monochromatic SNAP (R ~ 60%), and a few of them move toward SWOP (R ~ 76%).
• The required driving voltages for many e‐paper technologies are below 15 V, the maximum voltage provided by mainstream active matrix backplanes. As we will describe in a later section of this chapter, active‐matrix backplane compatibility is a significant practical advantage.
• Many e‐paper technologies have demonstrated a video speed response. However, it is important to note that high‐quality video reproduction requires accurate grayscale performance, and accurate color if present. Not all technologies report this metric.

Later chapters in this book provide an up‐to‐date review of several essential e‐paper technologies.

• In Chapters 2 and 3, the fundamental mechanisms, physical models, driving waveforms, and image processing of EPDs will be reviewed.
• Chapter 4 describes the Clear Ink EPD with a total internal reflective structure, enhancing the display's brightness.
• Chapters 5 through 8 describe several different categories of reflective liquid crystal displays. Chapter 5 introduces reflective displays based on cholesteric liquid crystals that rely on Bragg reflection from helical liquid crystal structures. Chapter 6 introduces the zenithal bistable display (ZBD), which utilizes a grating‐type surface alignment structure to achieve bistable reflective states. Chapter 7 introduces the memory‐in‐pixel (MIP) liquid crystal display technology, which realizes a bistable display by building a static memory circuit behind the reflective electrode for each pixel. Chapter 8 introduces optically rewritable (ORW) technology, which uses a photoalignment layer to control the orientation of the liquid crystals and spatially generate bright and dark reflective states.
• Chapter 9 reviews electrowetting display technology, in which an applied voltage causes the physical translation of a dyed fluid within the pixel, with a resulting optical appearance change.
• Chapter 10 discusses electrochromic technology, generating reversible color change through an electrochemical redox reaction.
• Chapter 11 introduces the phase transition material technology. Under optical or electrical energy stimulation, a phase change material cycles between amorphous and crystalline states with a resulting change in reflectivity.
• Chapter 12 reviews the metrology of reflective displays, providing the means to characterize the performance of different technologies.

It is fair to say that the electronic paper technology that has seen the greatest success in transitioning to high volume products is the microencapsulated EPD technology from E Ink Corporation. The next section of this chapter will review the historical development of EPDs, including the many inventions and milestones needed to achieve a competitive device. It took over 35 years to go from the initial concept, proposed by Xerox and Matsushita, to a successful high‐volume product, the Amazon Kindle. During this development period, many engineers and scientists' insights and hard work developed a combination of technical achievements and manufacturing scale that could enable competitive display products. Even with the baseline technology resolved, it still took the right product concept ready at the right time to lead to the commercial success of the Amazon Kindle. There are lessons to learn from observing how a laboratory curiosity was transformed into a technology and product that opened up a new industry.

1.6 The Allure of Electronic Paper vs. the Practicality of LCDs

The idea of a personal electronic device allowing user input and a display output screen is widely attributed to work done in the late 1960's by Alan Kay of Xerox PARC. Kay coined the term DynaBook for the apparatus [50]. He also laid out many of the characteristics of the device, including that the visual output should be, at the very least, of higher quality than what can be obtained from newsprint. While electronic paper was not explicitly called out as a display medium, that analogy to newsprint is a natural precursor for an electronic book. Nevertheless, until the late 2000's, liquid crystal displays dominated all implementations of personal electronic devices.

Why did it take so long to develop attractive electronic paper products? One overwhelming factor is the success of the liquid crystal display. LCD technologies started development about the same time as EPDs, and one successful variant (the super twisted nematic, STN) LCD did not appear until over ten years later. Reflective LCDs' suffered from poor brightness, poor contrast, and poor viewing angle characteristics. Nevertheless, LCDs can be matrix addressed to enable high pixel count, have reasonable response time, and be used in transmissive mode. LCDs were seen as “good enough” for many early applications. With volume production came incremental improvements in performance, reliability, and cost year‐by‐year. For a new electronic paper technology to take hold, it must outperform LCDs in multiple aspects and have an application important enough to attract investment in manufacturing at scale. For decades, no reflective display technology seriously challenged LCDs.

1.7 The Evolution of Electrophoretic Display‐Based Electronic Paper

1.8 Initial Wave of Electrophoretic Display Development

The 1970's and early 1980's were a period of intense development in EPDs. Multiple companies developed the technology, including Matsushita, Xerox, Philips, Plessey, EPID Inc., Thomson‐CSF, Seiko Epson, Exxon, and Copytele. It is instructive to describe what was and what was not accomplished by these pioneering companies.

During this period, the development focus was primarily on single color electrophoretic pigments suspended in a contrasting fluid. Many papers described progress on the electrophoretic suspension and in controlling the switching and memory characteristics of the panel. Some highlights in the technology development are described as follows.

1.8.3 Optics

While the white TiO₂ pigment is similar to the pigments used in white paint, there are multiple aspects of the EPD that significantly reduce the reflectivity of a display compared to white paint. The reflectivity of a white pigment below a clear, dielectric surface will be reduced due to refraction and total internal reflection within the device and the reduction of illuminating light reflected from the display surface. An EPD device containing a dyed fluid, there will be additional losses due to residual colored dye in the fluid above the electrophoretic particles, even in the bright state. Reducing the dye concentration improved brightness, but at the cost of contrast due to reduced hiding of the white pigments in the dark state (Figures 1.10 and 1.11).

One of the first analyses of this interplay between brightness and contrast was provided by Vance [66]. Vance modeled the reflectivity of TiO₂ pigment suspended in a dielectric fluid medium without dye to estimate the upper limit to reflectivity. He then analyzed experimentally the potential contrast for different dye concentrations and cell thickness and the impact on bright state reflectivity. (Figure 1.12) shows the relationship predicted between device reflectance and contrast ratio at different cell thicknesses and dye concentrations. Realizing that significant cell gaps will result in unacceptably high voltages and slow response times, for a 25 μm cell thickness, he estimated that an ideal device could achieve brightness between 35% and40% for a contrast ratio between 5 and 10. However, this estimate assumed that the electrophoretic medium contained a particle concentration of 10% or higher, which was considered unrealistic for a stable device.

Fitzhenry‐Ritz published a similar analysis in 1981, examining the combinations of yellow pigment/red dye and green pigment/black dye from both a simulation and experimental perspective [67]. While her results were complicated by using color‐contrasting states, the results were similar to Vance in that it did not seem possible to achieve true paper‐like brightness and contrast with EPD systems.

Vance states, “It thus appears unlikely that displays with the contrast (10:1) and reflectivity (>70%) of black and white printed pages can be produced by these systems. Rather, as has been universally observed, practical ‘black and white' EP displays will operate between two shades of gray.” Interestingly, he notes that “devices employing undyed suspending fluids state (sic) such as those disclosed in the patent literature will be capable of much higher reflectance than the dye cells described here.” Despite this prediction, the appearance of two‐particle black and white EPDs will need to wait until the introduction of the microencapsulated EPDs more than 10 years later.

Analysis by Shu et al. determines that the relative reflectivity of white pigment in a dielectric fluid under glass is limited to <65% due to total internal reflection and other incoming and outcoupling losses [9]. Additional optical elements can be deployed to raise the effective brightness under some lighting conditions, as will be described in Chapter 4.

1.9 The Revival of EPDs

By the late 1990's several pieces of the puzzle started coming together.

• There was little progress in improving the optical appearance of reflective LCDs. If people wanted high‐quality, bright images on a portable electronic device, then backlit LCDs were required, with the usual limitations for emissive displays compared to paper.
• While several other technologies were investigated for reflective displays that could potentially be considered a rival for paper, none met with any commercial success.
• The steady growth of the LCD display marketplace led to a significant investment in TFT panel manufacturing plants. While TFT panels for LCDs were not optimized for EPDs, they potentially could solve the pixel addressing a problem that plagued early versions of EPDs.

EPDs did see a revival in the 1990's, with the introduction of microencapsulated EPDs. In a paper published in the journal Nature, Comiskey et al. from MIT demonstrated that it is possible to create an EPD by microencapsulating the electrophoretic medium in urea‐formaldehyde microcapsules [22]. The microcapsules were subsequently coated between two electrodes with an in‐situ polymerized polyurethane binder. Many of the positive features of EPDs were preserved, including near‐Lambertian viewing characteristics and bistability. Moreover, the microencapsulation seemed to provide a means of solving the agglomeration and pigment migration problems that plagued conventional EPD displays. Since the pigments and fluid were confined within a small microcapsule, migration of particles across the panel could be avoided.

Since the microcapsules that contained the electrophoretic fluid were dispersed in a polymer film, the display could be flexible as well. A 1998 SID Digest paper by Drzaic et al. demonstrated a fixed (bistable) image microencapsulated EPD display wrapped around a pencil in Figure 1.13 [80]. The display material comprised titanium dioxide particles and a blue dielectric fluid and was reported to exhibit ±90 V switching voltage, 12% luminance, and a 6:1 contrast ratio.

As an interesting footnote, the company Nippon Mektron had filed for several Japanese patents in the area of microencapsulated EPD, the earliest filed in 1987 and published in 1996 [81]. There is no evidence that these patents were filed outside of Japan or publications on this technology until 1998. That year, Nakamura et al. from the Japanese company NOK published a paper on microencapsulated EPDs at the 1998 SID symposium [82]. NOK showed an actual working display images, with a 5 × 7 matrix design‐driven with external switches connected by through‐film vias to reflective electrodes. Display characteristics included:

• TiO₂ in blue dye dielectric
• 50 μm capsules made from gelatin/gum arabic
• Mixing of microcapsules into silicone binder, coating into a single layer, and laminating onto flexible film
• +/50 V switching voltage
• 50 nA/cm2 driving current
• Contrast ratio 4:1
• 1000‐hour stability (memory function)

Despite this early entry, further activity at NOK on the technology seems limited. There appears to be only one additional publication by that company, in 1999, using a flexible sheet of microencapsulated electrophoretic material with the photoconductive drum of a laser printer to form a rewritable flexible paper.

There are also several Japanese patents on microencapsulated EPDs filed in the late 1980's and early 1990's by Nippon Mektron Corporation, with the earliest appearing in 1987 [83]. Nevertheless, there are no technical publications in this area until the NOK 1998 paper, and no technical papers afterward. For the balance of this chapter, we will focus on developing EPDs at E Ink Corporation and collaborators.

1.10 Developing a Commercial Display

The startup company E Ink Corporation was founded in 1997 to commercialize “electronic ink” based on the MIT technology, and set up shop in Cambridge, MA. Professor Joseph Jacobson first drew attention to the concept in a 1997 publication aptly titled “The Last Book.” [84]. In 1998, the company received significant publicity, including being named one of Fortune Magazines 30 Cool Companies of 1998. The company also secured US$15.8 M in venture and industrial funding by mid‐1998, backed by the Hearst Corporation, Motorola, and others [85]. The venture capital model expected that E Ink would develop products that would enable the company to scale rapidly, supplying products in high‐volume markets such as publishing and advertising.

Much of the interest was driven by the vision of radio paper. In a 2000 interview, Jacobson described radio paper as “electronic paper, coated with e‐ink, with tiny transistors in it to act as electrodes, solar cells, and radio receivers.” The paper “would be able to upload pretty much anything, even video, using no more energy than it draws from ambient light.” “I fundamentally think five years is the right time scale for the static paper to be phased out and replaced by radio paper’ Jacobson says.” [86].

While the microencapsulated technology seemed promising, there was a long way to go to achieve any commercial product, let alone “radio paper.” Issues to overcome included:

• Contrast and brightness were not better than reflective LCDs and certainly inferior to backlit LCDs.
• The operating voltage was high. ±90 V switching is not compatible with high‐resolution devices
• The displays could not be passively addressed. Options were either direct drive (for low pixel count displays) or, in principle, active‐matrix backplanes (for high pixel count/high resolution) displays. At that time, TFT panels were expensive, hard to source, and not designed for the switching characteristics needed for EPDs.
• No scalable manufacturing method was demonstrated.
• Reproducibility and reliability not demonstrated

The vision of “radio paper” was compelling but incredibly challenging. Remarkably, the engineers at E Ink and their collaborators managed to make multiple inventions, overcome these limitations, and develop a competitive display technology that, while not quite “radio paper,” still met a market need in a compelling way. The following sections describe many of the key achievements to developing a practical display suitable for use in high‐volume electronic paper products, up to the introduction of the Amazon Kindle.

1.11 Enhancing Brightness and Contrast

The expectation that electronic paper would look like print on paper requires a display with reasonable brightness and contrast. As mentioned previously in this chapter, for EPDs using titanium dioxide dispersed in a dyed fluid, Fitzhenry‐Ritz pointed out a tradeoff in dye concentration, display brightness, and display contrast [67]. Using titanium dioxide in a dyed fluid, the microencapsulated EPD E Ink display exhibited a brightness of 12% and a contrast ratio of 6 : 1, well under the theoretical maxima predicted for EPD displays. While brightness and contrast ratio could be traded off by adjusting dye concentration, there was no expectation that a brightness >40% with good contrast could be achieved in this system.

A way out of this dilemma would be implementing a two‐particle system with white and black pigments. If one color of the colloid could be brought to cover the top surface of the microcapsule, with the other color hiding behind this layer, then higher brightness and contrast could be achieved. The challenge is that while previous papers and patents had disclosed the concept of such two‐particle systems, there is little indication that such a system was reduced to practice. There are significant challenges to such an implementation:

• There are two types of particles needed to possess opposite charges to respond to the applied field properly. Such opposite charges would attract, so that irreversible particle flocculation was a serious issue.
• The electrophoretic mobilities of the particles would need to be made stable, and any charging agents within the fluid compatible with both types of particles.
• This two‐particle system would need to be compatible with the microencapsulation process.

Honeyman and coworkers developed a means of grafting polymers onto the colloid surface, which achieved these goals [87]. They first attached a polymerizable or polymerization‐initiating group to the particle, then reacted with one or more polymerizable species. These secondary polymers could comprise branched polymer chains, which would present a barrier retarding the flocculation of oppositely‐charged particles. These polymers could also incorporate charged or chargeable groups. Since the polymers were chemically bonded to the particle surface, they would be much more stable over time previous methods of merely physabsorbing polymers onto a surface.

Achieving a two‐particle system resulted in a dramatic increase in display brightness compared to the single‐particle/dye system. As described in the following section, carefully engineered microcapsules were equally important in improving device optics.

1.12 Microencapsulation Breakthrough

While the microcapsules used by E Ink promised to resolve many of the instabilities found in conventional EPD displays, they introduced another problem regarding reduced contrast and high switching voltages. The following Figures 1.14–1.16 can be used to understand the source of these issues [80]:

• There is a wide distribution of microcapsule sizes. Since the particles are rigid, they tend to layer on top of each other, increasing the cell gap across the film. This increased cell gap raises the total voltage required to switch.
• The rigid particles do not close pack. There are large gaps between microcapsules in which no switching occurs. These optically “dead” areas do not switch between light or dark states and degrade brightness and contrast.
• The distribution of the electrophoretic particles within the microcapsule is not uniform. Particularly in the dark state, the centers of the microcapsules showed larger optical contrast than the edges, which did not switch to the same degree.

Loxley and colleagues deftly resolved many of these issues by introducing microencapsulation chemistries to make the microcapsule walls compliant and developing a coating formulation to deposit a monolayer of capsules in an aqueous binder [89]. As the coating dried, surface tension pulled the microcapsules together. The compliant sidewalls enable the microcapsules to close pack, with minimal gaps between capsules. Additionally, the sidewalls between neighboring capsules take on a more vertical profile, which minimizes any curvature‐induced packing issues in the microcapsules.

Figure 1.17 shows a schematic‐view of the close packed capsules in a complete display. Figure 1.18 shows a top view of the microcapsules, with each square spaced at 200 ppi (127 μm on a side). Switching voltages of ±15 V became much more compatible with active‐matrix backplanes [90].

The photograph also illustrates another key feature of this formulation. The size of the microcapsules does not directly define the achievable resolution of the display. It is seen that with a given microcapsule, part of the microcapsule can be switched dark, and the remaining part light, depending on the field applied by adjacent electrodes. As such, high‐resolution images can be rendered.

1.13 Image Retention

One of the potential advantages of microencapsulated EPD systems is that the display's image can be retained for long periods. While this phenomenon had been noted in the early EPD work, the controlling factors were not fully understood. Sedimentation due to gravity was recognized as a cause of images fading over time, particularly severe in high‐density particles such as titanium dioxide. Coating the particles with polymer could reduce their density somewhat.

As mentioned previously, Ota and Novotny and Hopper's publications attributed memory effects to van der Waals and electrostatic attractive force between the pigment particles and the electrode. In principle, the particle surfaces could be modified to enable sufficient attractive forces to counteract sedimentation forces. Still, the reality is that it is challenging to alter the particles to weakly and reversibly flocculate this way. In a two‐particle system, the problems become even more serious.

Webber developed stable and reversible image stability for two particle systems [91]. Webber discovered that adding an oil‐soluble high molecular weight polymer (>20 000) to the electrophoretic fluid can dramatically increase image stability without increasing the fluid viscosity. If the polymer is non‐absorbing onto the particle surfaces, the well‐known phenomenon of depletion flocculation will create osmostic pressure between the particles and induce soft flocculation [92]. Webber demonstrated image stability of 10⁶ seconds in a black and white system (Figure 1.19).

1.14 Active‐Matrix Compatibility

High‐resolution images for EPDs require an active matrix backplane for addressing. Moreover, to fulfill the vision of “radio paper,” it seemed important that the active matrix backplane was scalable, flexible, and ultimately printable. Fortunately, silicon‐based TFT arrays were becoming available, and new flexible and printable technologies were on the horizon.

The first achievement of a silicon TFT, matrix‐addressed EPD, was made in late 1999. Bob Wisnieff of IBM provided to E Ink some active‐matrix panels designed for use in LCDs that would likely be compatible with the EPD material. This initial demonstration led to a successful collaboration with IBM, resulting in a 2001 report of active‐matrix driven EPDs with high pixel counts.ⁱⁱ Just as importantly, these prototypes demonstrated compatibility with technology already being deployed in the massive liquid crystal display industry [93].

In 2001 the engineers at E Ink further demonstrated microencapsulated EPD display driven by two other flexible active‐matrix panels.

• Using facilities at Princeton University, Chen designed and fabricated an a‐Si TFT array on a flexible stainless steel backplane. The resulting display is the first example of a flexible a‐Si microencapsulated EPD array [94].
• In collaboration with Lucent/Bell Labs, a 16 × 16 TFT array was built using organic transistors fabricated using a stamp‐transfer methods [95,96]. The resulting array was the first example of using organic semiconductors to fabricate a flexible display. This achievement gained international attention, winning the American Chemical Society Team Innovation award in 2002 and is designated as an “Editor's Choice Best‐of‐the‐Best” R&D100 award from R&D Magazine in 2001.

After this basic active‐matrix compatibility was demonstrated, custom electronic controllers and backplane designs enabled low power driving of high‐resolution panels. Early work leveraged existing AMLCD TFT backplane architectures, using specialized controllers to interface with the panel drivers, and providing specialized pixel‐level voltage pulses unique to the E Ink EPD material [97]. Gates et al. described an improved controller for EPD that elegantly repurposed the 16 bit wide LCD interface used for driving color panels to two 8‐bit representations of the current and to‐be‐updated EPD pixel gray levels. The display subsystem was also shut off between updates, allowing for ultra‐low power operation due to the EPD image stability [97,98].

The collaboration with Philips Corporation was critical for the successful evolution of the E Ink displays. Many such improvements are described in the paper by Henzen et al. [98]. EPD‐specific backplane modifications included two series transistors per pixel to enable large swing voltages, an enlarged storage capacitor to suppress kickback effects, and the adoption of a thick acrylate coating as an inner‐layer dielectric to create a field‐shielded pixel design. The Philips collaboration included a team from the Netherlands (Alex Henzen, Jan van de Kamer, Neculai Aileni, Roger Delnoij, Guofu Zhou, Mark Johnson), and Japan (Michael Pitt, Masaru Yasui, Tadao Nakamura, Tomohiro Tsuji).

1.15 Electronic Book Products, and E Ink Merger

The first EPD‐based electronic reader, The Sony Librie EBR 1000, was introduced for sale in Japan in April 2004 [104]. The device was developed through a collaboration between Sony, E Ink, Philips, and Toppan Printing.

The device used a proprietary Sony eBook format. Content could only be purchased from content providers like Publishing Link via a PC and then transferred to the Libre by USB cable or memory stick. Digital rights management built into the system allowed for content to be accessible for 60 days, after which it expired and became unreadable. No support for pdf or other ebook formats was enabled. The device opened to middling reviews, in large part due to these content‐based restrictions. While the screen appearance was well‐received, many complained about the slow device interface, limited book offering, and the forced erasure of content [105].

Over the next decade, Sony released a series of upgraded readers and started marketing them outside of Japan in 2006. The devices never reached the volumes expected for such a product, and Sony exited the e‐reader market entirely in 2014 [106,107].

The Amazon Kindle was released in November 2007 and set the stage for a flourishing ebook ecosystem. This first device was a 6″ diagonal display, 800 × 600 @167 pixels per inch, and four gray levels per pixel. E Ink provided the microencapsulated electrophoretic material, the packaging design, and the driving scheme. Toppan printing mass‐produced the coating and conversion into sheet form the moisture‐barrier protective sheet. Philips developed the backplane, timing controller, and other ICs, and assembled the final module. Amazon sold the Kindle reader provided a large library of content for sale, and made it easy for customers to access that content. Year‐by‐year improvements led to greater adoption and increasing sales of e‐paper displays [107].

Despite the technical and product success of the Kindle, E Ink, the company was burdened by significant debt. The company had invested in many years of technology and business development. Its large area sign business only achieved modest success, and the Libre did not become a breakout product. Rather than maintain status as a standalone company, E Ink was sold to the Taiwanese Company Prime View International (PVI) in 2009 to enable further commercialization of the EPD technology [108]. Since then, PVI has renamed itself E Ink Holdings and continues to develop electronic ink for ebooks, signage, and other products. Later on, SiPix was merged with E Ink, with the microcup embossing technology enhancing the realization of high‐performance color e‐paper [109,110].

1.16 Summary

As is true for many technologies, the path for success for the electronic paper was not predictable. Periods of rapid technical progress were followed by periods of little interest due to combinations of application status, competitive technologies, and the commercial and financial landscape. Still, the rise, fall, and rise of electronic ink provides a nice success story for a technology that was envisioned to overcome a limitation of electronic displays and eventually managed to do just that.

The development of electronic paper did not end with the introduction of the Kindle. The remaining chapters of this book describe many exciting areas of development in this important field.

AcknowledgmentsPSD is grateful for the assistance of Rob Zehner, Guy Danner, Howie Honeyman, Rich Webber, Glen Crossley, Andrew Loxely, and Gregg Duthaler for useful discussions, and in proofreading the chapter.BRY is grateful for the assistance of Guangyou Liu, Mingyang Yang, Zheng Zeng, Linyu Song, Hao Shu, Yifan Gu, Yunhe Liu, Jie Liu, Xinzao Wu for manuscript preparation and in proofreading the chapters.AC would like to acknowledge Joel Pollack for oral accounts of early development activities on Electrophoretic Display Technology at various Xerox R&D Centers up to mid‐1970; and Warren Jackson for facilitating fact‐checks from Xerox document archives.

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