Function-oriented Knowledge Base \ Electronic paper \ Decrease power consumtion 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.
