Electrochromism is defined as a reversible colour change of a material caused by the application of an electrical current or potential. It has potential applications in making smart windows for the modulation of incoming light, as well as displays, a field in which competition with liquid crystal displays (LCD) is possible.

 The top pictures shows an electrochromic window while the bottom pictures show an electrochromic display

  

In The Past 

There used to be only two types of electrochromic devices. In the first type, the electrodes are made of conducting glass and covered with an organic or inorganic polymer. The two materials would usually display complementary electrochromism and hence produce the



The diagram above shows the inorganic/electrode type. The power source produces an electric current which reduces the polymer giving it a blue colouration.

same color change when one is oxidized while the other is reduced. This type of construction is used in electrochromic windows. It is bistable, which means that once the color change has occurred, the state of the device remains even in absence of applied voltage. The limitation of this type of electrochromic devicesis the slowness of the color change, due to thelow migration rate of the counterions in the bulk polymer. It is also difficult to obtain strong color changes or bright colors.

In the second type, two complementary electrochromic  molecules are dissolved in a solvent. This type of system is very simple to build, reacts very fast and can produce dark or bright colors. The drawback of this type is that an electrical current is needed to maintain the colored state  because the two types of colored molecules diffuse through the

system and react with each other to restore the bleached states. It thereforecannot be used for large area devices or for battery-powered displays.


The diagram on the left shows the molecular/solution type. The particles can move rapidly this time giving high switching speeds

 

Problem and solution

We therefore need a system which is both bistable and changes colour rapidly. This can be done by attaching a suitable molecule that is colourless in the oxidized state and coloured in the reduced state onto the surface of a colourless semiconductor on conducting glass. When a sufficiently negative potential is applied, electrons are injected from the conducting glass into the conduction band of the semiconductor and reduce the adsorbed molecules. The reverse process occurs when a positive potential is applied and the molecules get bleached. The advantage of this system is that combines immobility of the electrochromic material with the rapidity and coloration efficiency of molecular systems.

However, a single molecular monolayer does not absorb a perceptible amount of light. This is where nanoparticles come in!


Nanocrystalline amplification

By using nanocrystalline semiconductor films, it was possible to amplify the light absorption of the molecular monolayer to obtain visible color changes. The nanocrystalline layer are highly porous and usually the real surface of a 5 µm thick layer is about 500 times the projected area. Therefore, as light passes through the layer, it crosses several hundreds of monolayers of coloured molecules giving a strong absorption.


Principle of the nanocrystalline amplification. The highly porous nanocrystalline layer of the semiconducting material allows more absorbed molecular monolayer to be present. Hence increasing the absorption of light.


Product

 Some commercial products has already been produced!

The most successful is the automatic dimming rear-view mirror sense ambient light and glare from behind, and reduce annoying glare from bright lights.

 
While the company NTera has produced paper quality displays. These displays are bi-stable, not requiring much power keep to a pixel coloured, and reflective, not needing power for backlighting, and therefore can operate with very little power. Coupled with the fast switching (i.e. changing of colour) due to amplification with nanoparticles, this technology could in most flat panel displays.

 The picture on the right and below show examples of the paper quality displays from NTera