The future of electronic paper: Fast-switching, full-color e-readers
By Dr. Harry Zervos, Technical Analyst, IDTechEx
The success of e-reader technology so far has only wetted the appetite for new devices with more functionality. The Kindle, for all its success, has its limitations and the technology companies behind it are fully aware of that. That is why E Ink, and its parent company PVI, are working intensely on the next generation of e-readers. Requirements include flexibility, the transition from black and white to full color displays, and switching speeds able to support video playback.
E Ink's electrophoretic technology is not the only company in the race though. Other companies are developing low-power, bi-stable, paper-like displays, with two different approaches – one MEMS based and one based on the phenomenon of electrowetting – described below.
Qualcomm's Mirasol display is a MEMs based technology using interferometric modulators. Liquavista's display on the other hand is based on the electrowetting properties of solid materials in order to construct its unit cells.
Qualcomm-Mirasol
The Interferometric Modulator (IMOD) element is a simple MEMS (micro-electro-mechanical system) device that is composed of two conductive plates. One is a thin film stack on a glass substrate, the other is a reflective membrane suspended over the substrate. There is a gap between the two that is filled with air. The IMOD element has two stable states. When no voltage is applied, the plates are separated, and light hitting the substrate is reflected as shown above. When a small voltage is applied, the plates are pulled together by electrostatic attraction and the light is absorbed, turning the element black. This is the fundamental building block from which Qualcomm's mirasol displays are made. The color of each element is determined by the size of the gap between the two plates, which leads to the creation of RGB pixels (the blue element has the smallest gap).
IMOD pixels are characterized by bistability (due to electromechanical memory also known as hysteresis), allowing for low power consumption. Finally, switching speeds in the order of microseconds allows for excellent video playback capabilities.
Liquavista
The electrowetting effect has been defined as “the change in solid/electrolyte contact angle due to an applied potential difference between the solid and the electrolyte” The image below demonstrates the principles behind an electrowetting display. A voltage is used to modify the wetting properties of a solid material. An example of such increased wettability is illustrated below. The top illustration shows a water droplet on a hydrophobic surface, with a minimized contact area.
In the illustration below, a voltage difference is applied between the electrode in the water and a sub-surface electrode present underneath the hydrophobic insulator material. As a result of the voltage, the droplet spreads, i.e. the wettability of the surface increases strongly.
In a display a reflecting electrode, a hydrophobic insulator, a colored
oil layer and water will be sandwiched between glass or polymeric substrates. In equilibrium the colored oil naturally forms a continuous film between the water and the hydrophobic insulator.
When a voltage difference is applied across the hydrophobic insulator, an electrostatic term is added to the energy balance and the stacked state is no longer energetically favorable. The system can lower its energy by moving the water into contact with the insulator, thereby displacing the oil and exposing the underlying reflecting surface.
Color demonstrators with reflectivity of about 20% have already been shown (with plans to optimize it to around 30% in the next few years), with switching speeds of less than 10 msec which are able to support video playback. Demonstrated below is the Liquavista Pebble, conceived to illustrate the values and benefits of electrowetting technology.
More information on the growth of the e-reader market and latest developments from companies such as E Ink, Liquavista, Kent Displays, Bridgestone and I-Rex will be presented at the IDTechEx Printed Electronics Europe conference and trade show, on the 13th and 14th of April in Dresden, Germany.
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