What is e-paper

this page is widely based on a paper from epaper.org.uk

• Introduction
• Electrophoretic frontplane
• Bichromal frontplane  
• Electrowetting frontplane
• Electrochromic frontplane
• LCD frontplane
• OLED frontplane
• Organic semiconductor backplane
• Flexible silicon backplane
• Electromechanical backplanes
• Touch screen

Introduction

It is a term that has been used rather loosely for a long time, but broadly speaking it is a display technology that has all the attributes of paper but can be written to and erased electronically. We can list some of these basic attributes as follows:

• High resolution (150dpi or better).
• High contrast, equal to that of print on paper (about 10:1 or better).
• Readable in any ambient light conditions
• Readable at any viewing angle
• Excellent ergonomic features, easy to hold, carry, and use.
• Light weight, at most comparable to an equal sized sheet of card.
• Robust, will withstand being dropped, hit, etc.
• Flexible, or at least bendable.
• Bistable, once a display is written it will stay displayed even when power is switched off.
• Cheap, maybe not as cheap as paper, but easily affordable by everyone.
• Reasonable large area, preferably A4 (298x212mm)

• Low contrast ratio and low resolution lead to eyestrain in long periods of continuous reading off a computer screen.
• The size, and weight of a computer screen means that the reader cannot easily position himself/herself at a proper viewing distance, leading to further eyestrain.
• Computer displays are light generative, or backlit, and often not viewable in a wide range of ambient light conditions or viewing angles, leading to further eyestrain.
• Lack of portability, even with a laptop, limits the times and places in which a document can be read off screen.
• The landscape format of a computer display contrasts to the portrait format of most printed paper documents, resulting in the need for page scrolling of documents that are formatted for print.

Although some people, especially younger computer users, are happy to read from screen for long periods, most users find that the above reasons limit the time that they can comfortably spend reading off screen. Indeed, the problem is sufficiently serious to be recognised by health authorities, and in the UK, the normal fee for eye tests can be waived for computer users.
This means that reading from a screen is usually confined to quick scan reading and searching for information, rather than careful in depth reading. Consequently most will opt for printing out a page that they wish to read carefully.

Why E-paper offers improved readability.
In all computer displays, including e-paper, the display is made up from a number of very small picture points, or pixels, the image on the screen being formed by the pattern in which they are turned off and on. Most conventional computer displays in use today have a resolution of between 70 and 100ppi (pixels per inch). A standard laser or ink jet printer will print using a resolution of between 300 and 600ppi.
At an average viewing distance of about 60cms a screen resolution of 100ppi gives a fairly sharp image, however, at a closer distance, such as the 30cms average viewing distance when reading a printed sheet of paper, the digitisation becomes noticeable, thus reducing both the quality of the typography and the readability of small and/or serif fonts.
This means that paper replacement displays which will be viewed at a closer distance will need to have a resolution of at least 150ppi and preferably 200ppi for a monochrome display if it is to equal the quality of standard newsprint, and 300ppi or better if it is to equal the quality of magazine and book printing. Most e-paper technologies are well able to deliver resolutions up to 300ppi and many have already been demonstrated at 150-180ppi.
The clarity of printed text also depends upon the contrast ratio between the respective reflectivity of the paper and the ink. In newsprint the contrast ratio is typically around 10:1, though in higher quality magazines and books it can be much higher. A typical LCD display, however, will only have a contrast ratio of about 5:1. In general the higher the contrast ratio of a display the easier it is to read text based information, and the aim of any text display technology should be to aim for a contrast ratio at least equal to that of printed paper. Most e-paper technologies achieve a better than 10:1 ratio, which is about the same as that of a printed newspaper.
With colour computer displays the problem is more complex since each pixel consists of a triad of different coloured pixels, one red, one blue, and one green, the combination of these three colours together with the intensity of each will determine the resultant colour of the pixel triad. The use of such pixel triads means that the overall resolution of colour displays is usually much lower than that of monochrome displays - so, since they require three times as many pixels, a 150ppi display will require 450 colour points per inch.
However, in print, a four colour combination is used: cyan, magenta, yellow and black, which gives the high contrast black that is necessary for text, whilst at the same time offering the colour triad to generate a full colour palette. Although full colour e-paper is not yet in production, it will probably follow the four colour system used in print rather than the three colour system used in computer displays if it is to have the necessary contrast for displaying good quality text.
The readability of text, in particular the contrast between ink and paper, is also very dependent upon the ambient light conditions and the viewing angle. In a conventional CRT display, which is light generative, or a LCD display which is backlit and transmissive the display is easily 'washed out' in very bright ambient light.
However, in bright light a sheet of printed paper becomes easier to read because it is being read by reflected light. In general the human eye finds it far easier to read using reflected light than any form of light generative/backlit display. Most e-paper technologies use reflective displays, and this will probably be a major factor in their popularity since this type of display will generate considerably less eye strain.
Another readability factor where e-paper technologies will have an advantage is the viewing angle. Both CRT and LCD screens need to be viewed almost straight on, look at them from an angle and in the case of a CRT one sees reflections of the room, or in the case of LCD the contrast simply disappears. A sheet of paper can, however, be viewed at virtually any angle, the same applies to most of the e-paper technologies.

Why E-paper offers improved usability.
A thin lightweight display has considerable ergonomic advantages over the conventional LCD and CRT displays available today. The projected weight of an A4 e-paper display based document reader, including battery, will be under 200gms, about the same weight as a magazine like the Economist. This means that it can be comfortably held in one hand and read in any position or location that the user wishes.
The light weight of an e-paper display based reader device, coupled with the fact that it will probably be keyboardless (relying instead on a touch screen and virtual keys) also means that like a sheet of paper it can be easily used in either landscape or portrait mode. Indeed e-paper displays have another advantage in that they can be more easily manufactured in a wide range of sizes and shapes for specialist display applications.
Another ergonomic advantage of e-paper displays is that because they are reflective and offer a high contrast they can be read in any ambient light condition that will allow a paper document to be read.
The low power consumption and bistability of an e-paper display means that they can be used for long periods without recharging or replacing batteries. Manufacturers of first generation e-paper display based readers are quoting figures of 10,000 page displays on  two AA batteries, or about three months of average use.
When an e-paper display is combined with a touch screen overlay the combination offers the capability of becoming an exact electronic analog of a pad of paper and a pencil. It will be possible to draw handwritten notes or diagrams onto the display, thus allowing manual annotation of printed material, note taking etc.

A more in depth look at some of the enabling technologies that are being used to build e-paper systems.

Electrophoretic frontplane  

Electrophoresis is a process originally developed for medical/biological purposes, allowing the separation of molecules, according to their size and electrical charge, by applying electric current to them.
In an electrophoretic frontplane small, sub-micron, ink particles are given an electric charge and then suspended in a dielectric fluid medium that is encapsulated into a sub-pixel size cell or microcapsule. When an electric field is applied across this cell or capsule the ink particles will move towards the electrode with the opposite charge.
If the electrode is transparent then that surface of the cell or capsule will assume the colour of the ink when the field is applied.
The contrast can be enhanced by taking ink particles with opposite colours - black and white - and charging them with opposite polarities. Mixed together in a capsule, when an electric field is applied, all the black particles will migrate to one side, and all the white to the other. Switch the field, and the capsule will change colour.
This makes it possible to switch between all black particles and all white particles on the transparent front electrode of the cell or microcapsule and thus assure the high contrast ratio that is a feature of electrophoretic displays.
Another important feature of electrophoretic frontplane displays is that they are bistable because once the particles have migrated towards an electrode they will stay there, even when the power is switched off. This means that power is only required to change a cell from white to black or vice versa, not to maintain it, hence the term bistable. This is a very important feature since it allows large displays to be used in portable battery powered devices, without need for constant battery recharging.
The difference between the various different electrophoretic frontplane technologies lies simply in how the charged ink particles and fluid medium are encapsulated to give a sufficiently fine pixel resolution.
The big drawbacks with the current generation of electrophoretic displays are firstly that they have slow refresh times, since the pigment particles take time to move, and secondly that they are monochrome. Both problems are the subject of considerable research effort and look set to be overcome in the not too distant future.

Bichromal frontplane

A bichromal frontplane consists of a very thin sheet of flexible plastic containing a layer of microscopic plastic beads, each one encapsulated in a little pocket of oil and thus able to freely rotate within the plastic sheet. The two hemispheres of each bead are coloured differently (hence the term bichromal) and each hemisphere is given a different charge.
When an electric field is applied across the sheet all the beads will rotate so that the hemisphere with the negative charge on each bead is next to the electrode with the positive charge, and vice versa. If the electrodes used to create the field are transparent then the colour of the sheet will reflect the polarity of the electrode. Reverse that polarity and the colour of the sheet under the electrode will change to the other hemisphere colour.
An important feature of bichromal frontplane displays is that they are bistable. This means that power is only required to change the display, not to maintain it. This is a very important feature since it allows large displays to be used in portable battery powered devices, without need for constant battery recharging.
The disadvantages of bichromal displays is firstly that they are monochrome, and secondly that the display resolution is limited by the bead size. However, both these problems are currently being addressed by ongoing research and should, according to the researchers, be solved in the not too distant future.

LCD frontplane 

The LCD screen is over thirty years old and is now a well understood technology. However, developing the technology for the frontplane of a flexible display brings considerable challenges. As we indicated in the introduction, the frontplane in a conventional LCD unit is made of a rigid piece of glass, in order to ensure that the cell gap between it and the backplane is precisely maintained. Even slight variations in this gap will produce image distortions.
Maintaining such a precise gap in a rollable or bendable display is extremely difficult, and requires the use of spacers to maintain the gap. Creating such spacers is primarily a manufacturing technology problem, and is one that several companies around the world, including IBM, and Philips are understood to be now addressing. Two companies, HP and Fujitsu, have demonstrated actual devices.

Many industry analysts, the authors of this report included, believe that flexible LCD technology will take a major part of the flexible/bendable display market sometime during the period 2010 to 2020. The technology behind LCDs is well understood and the manufacturing techniques needed to solve the 'gap maintenance' problem are already being developed. In the medium term flexible LCD technology will give high speed, full colour, high resolution displays, and thanks to recent developments like cholesteric LCDs, these displays will have fairly low power consumption and feature both bistability and reflectivity.
With cholesteric LCDs the liquid crystal molecules form spirals, which means that incoming light is reflected or transmitted depending on the spiral's axial direction, which changes in line with the height and length of voltage pulses. Light is reflected, if the spiral's axes align in "the planar state," which is a direction of the paper's thickness, while light is transmitted, if the axes align in "the focal conic state," which is a direction perpendicular to the planar state.
The use of a cholesteric liquid crystal means that the display has a far better readability than a display using conventional nematic liquid crystals and can be made thinner, since it reflects 50% of certain wavelengths, removing the need for colour filters and polarizing layers. This in turn means that the colour is more vivid, and the contrast much better, than conventional reflective-type LCDs.

Organic semiconductor backplane  

Developments during the 1980s in conductive polymer technology, primarily at the Cavendish Laboratory in Cambridge, have led to the discovery of a range of different polymers, in other words plastics, with conductive and semi-conducting properties. Like their inorganic equivalents these special materials can be used to build working active electronic circuits, in particular transistors, memory cells and logic gates, all the basic building blocks of digital circuitry.
This technology was spun out of Cambridge University in 2000 as Plastic Logic, a company given the task of turning this basic research into commercial products. Plastic Logic may be one of the leading companies and a technology pioneer in plastic electronics but they are not alone. Other companies who are working in this area include Philips, which recently spun out its research into Polymer Vision, and a number of other high tech companies including Xerox.
All the companies working on organic semiconductors have identified flexible active matrix display backplanes as a potential big market for their technology, and both Plastic Logic and Polymer Vision now have prototype production lines and together with partner companies will be in commercial production of flexible displays by 2007.
The main companies involved have demonstrate flexible active-matrix monochrome electrophoretic displays based on solution-processed organic transistors on 25- m-thick flexible plastic substrates. The displays can be bent to a radius of 1 cm without significant loss in performance.

The great advantage of organic semiconductors is that compared to conventional silicon semiconductors they are relatively easy to make, and therefore relatively cheap to manufacture. This is because the circuits can be fabricated using well understood and conventional printing technologies at room temperature and using cheap lightweight plastic substrates, as opposed to the high temperature furnaces, vacuum chambers, and expensive silicon wafers used in silicon semiconductor manufacture.
One way to further reduce costs is to integrate (part of) the display drive circuitry, such as row shift registers, directly on the display substrate. Using the same process flow Philips have developed row shift registers. With 1,888 transistors, these are the largest organic integrated circuits reported to date. More importantly, the operating frequency of 5 kHz is sufficiently high to allow integration with the display operating at video speed. This work therefore represents a major step towards 'system-on-plastic'.
Although silicon chips will initially be used to drive organic electronic backplanes, in the longer term we will see integrated row and column drivers, shift registers and decoders constructed using organic electronics. For most display applications, organic electronics can be efficiently used as drivers. At a clock rate of one kilohertz, it is possible to change all of the information in a 1,000-by-1,000 element display in one second. With clever electronics this can be reduced to a fraction of a second. (note this is much quicker than loading a Web page using a conventional computer)
Although organic semiconductors, at this moment, have a low component density, and are very slow. However, neither of these limitations poses any problem in building active matrix backplanes for flexible displays. It is an application tailor made for the technology.

Flexible silicon backplane

Of all the backplane technologies the best understood, and capable of offering the most advanced features, is silicon. Silicon thin film on a glass substrate has been used for TFT active matrix backplanes in LCD screens for many years. They can be manufactured with high reliability using processes that are well known and generically similar to those used in IC manufacture.
Glass substrate silicon backplanes are used in most of the first generation e-paper displays now currently in use. They are used in most signage products and will work with most frontplane technologies. A typical example is the Sony LIBRIe e-book reader with its 800x600 monochrome E-Ink frontplane display.
The problem with silicon thin film on a glass substrate as a display backplane is that it is rigid, heavy, and being made of glass very fragile. This largely precludes this type of backplane being used in e-paper displays since they do not live up to many of the points in our definition.
However, recent research has shown that it is possible to put silicon thin-film circuits, including display backplanes, onto a lightweight, flexible plastic substrate. The resulting backplanes certainly live up to the definition of e-paper, and although commercially they are in the more distant future the technology needs to be considered within the scope of this report.

Electrochromic frontplane

Electrochromic materials are materials that change colour upon application of an electrical potential. Electrochromic systems have been used successfully in the past in mirrors and windows for anti-glare and anti-reflective applications. However, other potential applications have yet to reach the market, largely due to the limitations of existing electrochromic materials.
To date, most commercial electrochromic technologies have used solution based systems, which rely on organic electrochromic species dissolved in the electrolyte compartment of an electrochemical cell which has at least one transparent electrode.

The most commonly used electrochromophores, salts of 4,4'-bipyridines, also called viologens, are synthetically tunable which allows for different colours, and have intrinsically high extinction coefficients, yielding excellent colouration intensities. The switching speed depends on the diffusion of these and other redox active species in the electrolyte to the electrodes and is typically in the order of seconds.
Because the redox active species are dissolved in an electrolyte these mobile molecules will diffuse to both electrodes once an appropriate electrical potential has been applied to the circuit. Once the potential has been removed, the charged species mix, transfer their charges, and the colour dissipates from the system. Therefore there is no open circuit memory in these devices and power must be applied continuously to maintain colouration.

OLED frontplane

The term OLED stands for Organic Light Emitting Diode and as this implies the technology is based upon the use of special organic compounds, light emitting polymers, that emit light when electricity is passed through them. The technology was developed back in the 1980s at the Cavendish laboratory at the University of Cambridge.
The technology of Light Emitting Polymers or LEPs allows the construction of full colour displays that are much cheaper to make and run than CRTs because the active material is plastic. Like the CRT, LEP is an emissive technology, meaning that it emits light as a function of its electrical operation. An LEP display consists solely of the polymer material manufactured on a substrate of glass or plastic and does not require additional elements such as the backlights, filters and polarizers typical of LCDs.
LEP is a platform technology that will scale from tiny devices literally millimeters in dimension to large high-definition devices that could be up to a couple of metres.

The only real drawback to OLED technology for use in portable battery powered devices is that because it is an emissive technology it uses a lot more power than most reflective technology displays, neither does it have the power saving feature of bistability shown by many other technologies covered in this report.
It is also limited in its appeal to devices designed for use in dim ambient light; since it is almost impossible to see what is displayed on an OLED display when viewed in bright sunlight.
Whilst in theory OLED displays can be constructed on thin flexible plastic substrates, all of today's commercial OLED displays use rigid glass substrates. This is partly because it has been easier to construct TFT active matrix backplanes on glass substrates using standard LCD manufacturing equipment, but it is also because of the fact that the polymers used in OLEDS are quickly destroyed by exposure to air and moisture.
This means that means that they need to be very well encapsulated to protect them, and to date a reliable encapsulation technique for flexible substrate OLED displays has not been developed to a commercial stage. However, research into ways of solving this problem is ongoing and researchers have expressed the belief that it will probably be solved within the next year. In the meantime the only reliable way of encapsulating OLED displays is with a rigid substrate like glass.

Electrowetting frontplane

The phenomenon of electrowetting offers the possibility of creating flexible displays with a far faster refresh rate than is possible with electrophoretic displays, making such displays suitable for displaying video content. The colour displays will also be much brighter than conventional LCD displays since they will not rely upon filters.
This technology could be regarded as very similar in principle to electrophoretic display technology, but without several of its drawbacks. It is still in the development stage, but should be in commercial production by 2010.
Each pixel of the display consists of a micro cell that contains a drop of coloured, oily ink suspended in an aqueous medium. At the bottom of the cell is a reflective white background that has been coated first with a transparent material that conducts electricity - permitting electrical control of the pixel colour - and then with a transparent film of a water-repellent plastic.

Normally the ink droplet will spread across the entire pixel thus obliterating the white background. However, if a voltage is applied, the oil droplet retracts, much like a bead of water in a Teflon pan, thereby exposing the white area below.
If the microcell is small enough, these white and ink covered regions are not individually visible. Instead, the effect is that the pixel acquires just an average brightness level, so that when the droplet is fully spread, the pixel looks dark, and when it retracts, the pixel looks much lighter.
A very significant feature of this technology is the fact that the larger the applied voltage, the more the ink droplet retracts. This means that the ink capable of a continuous grey scale, not just of the bichromal contrast found in most electrophoretic technologies. This feature will remove much of the "jaggies" or roughness due to digitisation, and will produce images that look very smooth.
The key to the system's success is its switching voltage. It is low enough that controlling the electronic ink requires only a small power source. Switching between dark and bright states takes only about ten milliseconds - fast enough to produce sharp video images.
In principle full-colour images might be produced with this technology by using four sub-pixels inked with the standard yellow-cyan-magenta-black  quadruplet system. However, this technology is not bistable, requiring an applied voltage to maintain the image.

Electromechanical backplanes

Until quite recently many in the technology industry were uncertain whether organic electronic backplane technology would become a commercial reality in the near future. At least one company has taken a bet that it would not, and developed an ingenious micro-electromechanical alternative.
Whilst it looks as if the bet did not pay off, the technology may nevertheless prove to be quite successful in some applications, in particular large e-paper signs.

Touch screen 

A touchscreen is technology that accepts direct onscreen input. This can be done by either an external light pen or an internal device, a touch overlay and controller, that relays the X,Y coordinates of the fingertip or pen to the computer. For bendable e-paper displays the most likely form of touchscreen input is the resistive touchscreen.
Resistive touchscreens are a well developed technology and are widely used on CRT and LCD screens where they form a transparent overlay that is attached to the surface of the display and to the control electronics. This technology should work on all semi-rigid/flexible e-paper displays, and since the transparent touch film transmits 85% of the light it should not significantly degrade display quality and readability.

Resistive touchscreen displays rely on a touch overlay, which is composed of a thin flexible top layer and a more rigid bottom layer separated by insulating dots. The inside surface of each of the two layers is coated with a transparent conductive coating of Indium Tin Oxide that creates a resistance gradient across each layer when voltage is applied.
Pressing the flexible top sheet with a fingertip or a pen creates an electrical contact between the resistive layers, producing a switch closing in the circuit. The control electronics alternate voltage between the layers and pass the resulting X and Y touch coordinates to the touchscreen controller. The touchscreen controller data is then passed on to the computer operating system for processing.
Resistive touchscreen technology possesses many advantages over other alternative touchscreen technologies (acoustic wave, capacitive, Near Field Imaging, infrared). It is highly durable, and less susceptible to contaminants that easily infect acoustic wave touchscreens. In addition, resistive touchscreens are less sensitive to the effects of severe scratches that would incapacitate capacitive touchscreens, they are also more cost-effective than Near Field Imaging touchscreens.
Although resistive touchscreen technology exists in 4-wire, 5-wire, or 8-wire forms, 8-wire resistive technology is the preferred form because of its benefits over its counterparts. Whereas 8-wire touchscreens are available in all sizes, 4-wire resistive technology is restricted to small flatpanels (<10.4"). In contrast to 5-wire resistive touchscreens, 8-wire touchscreens do not experience spacer dots and Newton rings. Additionally, 8-wire resistive touchscreens are not susceptible to problems caused by high-level short-term variances and axis linearity and drift.
At the moment Philips Polymer Vision has said that it is actively developing a touch screen version of the flexible organic electronic/e-ink display. No details have been given by the company about input resolution, but it was implied that both virtual keyboard and handwritten input would be possible.