Scientists at Philips Research are using a technique called ‘electrowetting’ to build reflective, paper-like displays that are at least twice as bright as displays based on any other technology.
In the electrowetting display, the shape of a confined water/oil interface is controlled by an applied voltage. This shown schematically for one pixel <a href=’http://www.e4engineering.com/content_images/film23dr.gif’>here</a>.
When no voltage is applied, the (coloured) oil forms a flat film between the water and a hydrophobic (water-repellent), insulating coating of an electrode, resulting in a coloured pixel. When a voltage is applied between the electrode and the water, the tension between the water and the coating changes. As a result, the water moves the oil aside. This results in a partly transparent pixel, or, if a reflective white surface is used under the switchable element, a white pixel.
Displays based on electrowetting have several attractive features. For one, the switching speed is fast enough to allow video content to be displayed.
Furthermore, electrowetting is a low-power and low-voltage technology, and displays based on the effect can be made flat and thin. The reflectivity and contrast are better or equal to those of other reflective display types and are approaching those of paper. In addition, the technology can be used to deliver high-brightness full-colour displays that are four times brighter than reflective LCDs and twice as bright as other emerging technologies.
Instead of using red, green and blue (RGB) filters or alternating segments of the three primary colours, which effectively result in only one third of the display reflecting light in the desired colour, electrowetting allows for a system in which one sub-pixel is able to switch two different colours independently. This results in the availability of two thirds of the display area to reflect light in any desired colour.
This is achieved by building up a pixel with a stack of two independently controllable coloured oil films plus a colour filter. The colours used are cyan, magenta and yellow – a so-called subtractive system that is comparable to the principle used in inkjet printing for example.
The researchers say that the technology is particularly appealing for electronic-paper like applications, for which high-brightness and contrast-rich reflective displays are needed.