Electrochromic displays

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

The tutorial is divided in 5 sections:

Introduction

Since time immemorial, mankind has been fascinated by light and how to manipulate it. Some big inventions, like the breaking of light on a prism, the photograph, and the television, have only sharpened the curiosity of scientists. That is why up till now, a lot of research happens in the field of display technology and integration of colours in displays. Nowadays, we have already tens or maybe hundreds of different sorts of displays. Some of the most famous are: cathode ray tube (CRT: television or computer screen), liquid crystal displays (LCD), electrophoretic displays (E-INK), (organic) light-emitting diodes ((O)LED), electrochromic displays (ECD), …

In this section, we present you a survey of what chromic, and in particular, electrochromic materials are and what they are used for.

The chromic effect

Chromic materials have the possibility of changing their colour reversibly when they are placed in a different environment. Every day, new materials are discovered with chromic properties. To classify all these chromic materials, we put them in groups along their stimuli. For instance: thermochromic materials are materials that change their colour when the temperature of the environment is raised or lowered. This effect is called thermochromism and examples of such materials are bianthrones and cobalt hexacyanoferrate. Besides this chromic effect, you have photochromism (exposure to electromagnetic radiation changes the colour of the material), halochromism (a change in pH of the solution), solvatochromism (a reversible change of colour induced by the present of solvents), … .

One of the most useful forms of chromism is electrochromism. In this case, a material is able to reversibly change its colour when it is placed in a different electronic state. So by absorbing an electron (the materials is reduced) or by ejecting one (the material is oxidised), the material is able to change its colour.

Principle of oxidation and reduction
Principle of oxidation and reduction

Electrochromism was discovered in 1968 by S.K. Deb and J.A. Chopoorian and has a broad range of commercial applications. Some of those applications are smart windows and mirrors (e.g. darkening a window to control the inlet of sun light), active optical filters (e.g. sunglasses), displays and computer data storage.

Example of electrochromic display
A flexible, electrochromic display

Electrochromes (electrochromic materials) can be classified in different groups depending on their physical state at room temperature. This way, three different types of electrochromes can by distinguished. Type I electrochromic materials are soluble and remain in the solution during usage. Type II electrochromic materials are soluble in their neutral state and form a solid on the electrode after electron transfer, whereas type III electrochromic materials are solid and remain solid during usage. In reality, three big groups of electrochromes are popular in making electrochromic devices (ECD’s): metal oxide films (inorganic type III), conducting polymers (organic type III) and molecular dyes (type I). The research in our group is mainly on all-solution systems,
better known as the molecular dyes.

Molecular dyes

The technology for making a working electrochromic cell is very similar to the technology used in LCD displays. One way of making a working cell is by placing the electrochromic material between two transparent electrodes (preferentially Indium Tin Oxide, better known as ITO). The colouring of the EC-material results from changing the potential of the cell by charging the electrodes.

Structure of ECD Structure of ECD
Structure of ECD in off-state Structure of ECD in on-state

An example of an electrochromic material that is able to turn blue is EV (Ethyl Viologen) and is one of the many electrochromic dyes, originating from the bipyridinium group. In most cases, the viologen molecules are symmetrical, so R' is equal to R''. For EV for instance, R' is equal to the ethylene group:

Example of electrochromic molecule Example of diethyl viologen
Molecular structure of the bipyridinium group Molecular structure of the ethylene group

The colouring of EV from completely transparent to intense blue happens by absorption of an electron. This process is reversible. If the EV-molecule absorbs a second electron, EV turns pale blue. This reduction however is irreversible and definitely not wanted.

Colouring of ev
Schematic representation of the colouring mechanism in EV

A typical property of all-solution electrochromic devices is the big difference between the transparent state and the dark state. On top of this, one can obtain a complete set of tones of the same colour just by varying the applied voltage.

Transmission measurement
Transmission measurement of EV at different potentials

Advantages of electrochromic devices

  • Low power consumption:
    Recent EC displays are bistable and are able to work in reflection. This means that when you switch off the power, the colour remains. You can even lower the power consumption by the addition of high reflective molecules, like Titania. This way, you don't need a backlight, so you are able to use the display as an electronic book.
  • Cheap:
    The raw materials used in EC displays are cheap.
  • Switching is quite fast:
    One of the main issues of the first generation EC devices was the long switching time. This is the time needed to change colour, starting from the transparent state (about 5 seconds). This however can be solved by using porous electrodes, like is done now with the EC displays of the new generation. Typical switching times are now of the order of 200 milliseconds.
  • Integration of colours without colour filters:
    Researchers have developed a molecular dye that can display red, green or blue, depending upon the applied voltage. This implies that you wouldn't need to use colour filters, which diminish the brightness of the display.
  • Easy transformation to make them in large amounts:
    Recent studies show that existing LCD-manufacturers could easily transform their assembling machines to assemble EC displays.

Some interesting links





Electrochromic tutorial written by Matthias Marescaux.