Several e-reader products use electrophoretic displays, in which each pixel consists of microscopic capsules that contain black and white particles moving in opposite directions under the influence of an electric field.
A drawback to this technology is that the screen image is closer to black-on-grey than black-on-white.
Also, the slow switching speed (approximately one second) due to the limited velocity of the particles prevents integration of other features such as touch commands, animation and video.
Researchers at the University of Cincinnati Nanoelectronics Laboratory are pursuing an alternative approach for low-power displays.
Prof Andrew J Steckl, director of the NanoLab at UC’s Department of Electrical and Computer Engineering, said: ‘Our approach is based on the concept of vertically stacking electrowetting devices. The electric field controls the “wetting” properties on a fluoropolymer surface, which results in rapid manipulation of liquid on a micrometer scale.
‘Electrowetting displays can operate in both reflective and transmissive modes, broadening their range of display applications. Improvements of the hydrophobic insulator material and the working liquids enable EW operation at fairly low driving voltages [approximately 15V].’
Steckl and Dr Han You, a research associate in the NanoLab, have demonstrated that the vertical stack electrowetting structure can produce multi-colour e-paper devices, with the potential for higher resolution than the conventional side-by-side pixel approach.
Furthermore, their device is claimed to have switching speeds that enable video content displays.
According to the Cincinnati team, this will lead to tablets and e-readers that are ‘more capable’ and more visually appealing.
Compared with other technologies, electrowetting reflective display screens have many advantages. The electrowetting displays are very thin, have a switching speed capable of video display, a wide viewing angle and are not power hungry.
An article entitled ’Three-Color Electrowetting Display Device for Electronic Paper’ by Han You and Andrew J Steckl will appear in the journal Applied Physics Letters.
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