This tutorial gives a short introduction to the field of Electrochromic Displays.
The tutorial is divided in 5 sections:
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.
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.
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.
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.
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 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:
|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.
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.