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Function-oriented Knowledge Base \ Electronic paper \ Improve image quality of paper-like display

Reflective mesh electrode reduces switching time of electrophoretic display

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Introduction
In electrophoretic visual displays, the color of a pixel is changed due to the movement of a suspension of pigment particles of a dark contrasting color dye in a dielectric suspending liquid. Under the influence of a corresponding potential difference between portions of the front and rear electrodes, the dark pigment particles are driven to one of the electrodes. The pigment particles located at the transparent front electrode are visible to a viewer, while those deposited on the rear electrode are invisible due to a relatively thick liquid layer. Such electrophoretic displays require a substantial amount of switching time. The switching time is proportional to the second power of the distance between the electrodes and reversely proportional to the voltage between the electrodes. However, the distance between the electrodes is not reduced below some threshold because of practical reasons. With a predetermined distance between the electrodes, the upper voltage threshold is limited as well. Nevertheless, it is necessary to reduce the switching time and voltage of an electrophoretic display.
 
Description
Using an auxiliary reflective mesh electrode to reduce the switching time is proposed. The auxiliary mesh electrode is positioned on a transparent insulator layer, which separates the rear electrode from the auxiliary electrode and from an electrophoretic suspension. The rear electrode is blackened. The mesh electrode is reflective and made (for example) of aluminum. The mesh electrode holes are barely visible to the naked eye. The dark pigment particles are charged negatively. When voltage applied to the front and rear electrodes is negative with respect to the mesh electrode, the negatively charged pigment particles settle on and cover the surface between the mesh holes of the mesh electrode. The observer can see only the black color visible through the transparent electrode and the liquid, because the spaces within the mesh are also black. When the voltage on the mesh and rear electrodes is reversed, the pigment particles are deposited in the mesh holes and the observer sees the exposed upper reflective metal of the mesh electrode between the holes. For switching the display color (changing the reflectivity of the mesh electrode), the pigment particles must travel only the distance between the upper surface of the mesh metal and the mesh holes. This distance is substantially shorter than the distance between the rear and upper electrodes. The particles travel this shorter distance more rapidly. This greatly reduces the switching time of the display. The switching time is thus reduced.
 
Additional information
The thickness of the suspension layer is 50 micrometers. The mesh has holes of 8 micrometers in diameter and the distance between the centers of the holes is 19 micrometers. Lower operating voltages correspond to smaller distances that the pigment particles have to travel for the switching to occur. The operating voltage of -100 Volts may be reduced to -20 Volts, while at the same time maintaining the reduced switching time.
 
Reference
US Patent 4071430; Link >>
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