The Basics of Electrophoretic Displays

This tutorial gives a short introduction to the field of Electrophoretic Displays.

The tutorial is divided in 4 sections:

What are Electrophoretic Displays ?

The term electrophoresis is a composition of 'electro' and 'phoresis', two words that are derived from the Greek words for 'charge' and 'the act of carrying'. In that way, the name 'electrophoretic display (ED)' already gives a hint about its basic working principle. As shown on the picture below an ED is made of an ink layer, sandwiched between two layers that can be plastic, glass or even paper. The total thickness of the layer structure is between 0.5mm on glass and 0.1mm on plastic, which is in the order of a sheet of paper.

Principle of an electrophoretic display
Principle of an electrophoretic display

In the simplest case of a black and white display, the top substrate is covered with a single transparent electrode, while the bottom substrate contains a complex pattern of line-electrodes. Using active matrix driving, a single pixel can be addressed, meaning that the bottom electrode can be made either positive or negative compared to the top-electrode. The electrophoretic ink between these electrodes is a mixture of transparent liquid and microscopic charged pigment particles. The usual choice is negatively charged black particles (carbon black) and positively charged white particles (TiO2). In practice the ink is captured inside microcups, or microcapsules as in the figure. When a voltage is applied over the top- and bottom electrode, the charged pigments will move due to an electrostatic force to the attracting electrodes. For instance, when the bottom electrode is positive, it will attract black particles and repel white particles. These white particles gather at the top-electrode, where they reflect incident light in all directions. This is the white state. In the opposite case a negative bottom electrode pushes the black particles to the surface, where they absorb the light. This is the black state. This basic principle is different than most displays by the fact that it is reflective. So, an ED is a type of display that reflects or absorbs ambient light in contrast to transmissive displays such as the CRT or LCD. In practice this looks like:

Example of an electrophoretic display
Zoomed image of an electrophoretic display

Another alternative principle for electronic ink is based on spherical ink particles with opposite charges and colors at both sides. By applying an electric field, the particles rotate and hence change color.

Example of an electrophoretic display
Alternative construction of electrophoretic displays

What are the advantages of Electrophoretic Displays ?

The major advantage of the electrophoretic technology are:

  • High reflectivity of the white state and high contrast:

    Electrophoretic ink uses basically the same pigments as in regular ink for books, newspapers, etc. Therefore ED's have the same agreeable readability as printed paper.

  • High resolution:

    The current resolution is about 170 pixels per inch, which is similar to the current LCD monitors for instance.
    Excellent readability in direct sunlight and in dimmed light under all angles.

  • Very low energy consumption:

    There is no need of a backlight that is the main energy consumer in most displays. Energy consumption is 1/10 to 1/100 of a regular LCD display.

  • Possibility of thin, mobile and flexible displays:

    The technology of polymer transistors has advanced to the point that all components of an ED can be made flexible. This means that it is now possible to make a bendable or even rollable display.

Recent advances in the field of Electrophoretic Displays

Some recent new applications using the principle of electrophoretic ink.

Bendable clock
Bendable clock (E ink)

Prototypes of flexible displays by Fujitsu
Prototypes of flexible displays (Fujitsu)

Test-project for a newspaper in Belgium
Test-project for a newspaper in Belgium (Iliad)

Large size newspaper display
Large size newspaper display (E ink)

The Sony Reader
The Sony Reader

Rollable display (the 'Readius')
Rollable display, the "Readius" (Philips)

Flexible newspaper
Flexible newspaper (Philips)

Color display prototype Color display prototype
Color display prototype (E ink)

Some interesting links

The tutorial is divided in 3 sections:

Electrophoretic ink tutorial written by Filip Strubbe.