Electrowetting Display

Electrowetting Display also known as EWD is a display technology being developed by Liquavista, a company spun out of Philips in 2006 and currently owned by Amazon. It utilizes all three optical modes (transmissive, reflective, and transflective).

Key facts

• All the assets of LCD, but with 2x, 3x, 4x the performance
• Uses electrowetting cell concepts
• Fundamentally brighter, more colorful displays
• Simplified LCD-like manufacturing process
• Patented by Liquavista
• Initially targeting eBook reader markets followed by mobile phones

Technology

Electrowetting involves modifying the surface tension of liquids on a solid surface using a voltage. By applying a voltage, the wetting properties of a hydrophobic surface can be modified and the surface becomes increasingly hydrophilic (wettable).

With Electrowetting displays, the surface tension of liquids on a solid surface is modified by apply a voltage to the surface to obtain a simple optical switch by contracting a colored oil film electrically. Without a voltage, the colored oil forms a continuous film and the color is visible to the consumer. When a voltage is applied to the display pixel the oil is displaced and the pixel becomes transparent. When different pixels are independently activated, the display can show content like an photograph or a video.

The materials used in Electrowetting displays are very simple: two pieces of glass or plastic and water and oil in between. In addition, an essential ingredient to complete the display is the dye that is dissolved in the oil. The choice of dye determines the color of the display, in particular in the off-state where the oil covers the entire pixel. This implies that a wide range of colors can be achieved with the electrowetting technology simply by varying the color of the dye.

Simplified explanation

If you place a drop of water on a dry surface it will form a small ball (the minimum physical size) as it doesn't like to touch the surface. However, if you touch the ball with a electric charge it will act like the surface has become receptive and the water will flatten its shape and spread over the whole surface. If the ball were colored water and the surface transparent then it would start with a large transparent area and then change into the color of the water as it covered the surface. This is the principle of electrowetting which has caused a switch, a transparent surface switched to a colored surface. If you make the surface really small (a pixel in size) then you can use a large number of these to form a display surface. Note that when the small physical ball is formed you can control where it will go into a corner of the pixel area.

Advantages

• Compared to LCD displays this display is brighter due to the lack of filters needed in LCD and the lack of the polarizing films.
• It can be switched just as fast as LCD but the lack of need for a bright backlight reduces power needs.
• The technology does not need a rigid surface so the screen can be flexible and rugged.

Different designs

Liquavista bright - A monochrome design similar to standard EPD eBook displays but with more brightness and can be backlit if needed. It has much higher performance and can also do video.

Color gamut of 3 layer EW architecture compared

Liquavista color - Color is available two different ways. The first is the same as LCD with a set of subpixels in RGB making up on pixel. This uses the known LCD manufacturing process. The second way is to stack the three colors on top of each other to make a higher density display possible. Since only a dye is needed for the oil you can see through the oil to view all of the layers.

Liquavista vivid - A vivid display can be accomplished using transreflective display technology. Reflective black and white can be combined with multi-color field sequential transmissive illumination.

Specifications

• Contrast 12:1 for single layer color, 18:1 for 3 layer color
• Reflectivity: 60
• pixel: 500uM down to 160 uM (micro meters)
• on/off response: 9 msec

Specifications

• Contrast 12:1 for single layer color, 18:1 for 3 layer color
• Reflectivity: 60
• pixel: 500uM down to 160 uM (micro meters)
• on/off response: 9 msec

Low Power mode

The inherent persistence of the display in reflective mode can be used to reduce refresh time to several seconds resulting in a quasi-bistable display that is competitive to a number of gray-scale bistable displays. This lowers the power requirement to nearly an EPD display.

For more information

Technical paper from Liquavista PDF