Researchers add colour to electrochromic glass

The project was carried out by a team from the Fraunhofer Institute for Applied Polymer Research in collaboration with Tilse Formglas.

“Not only can we manufacture panes of glass in a wide range of colours, but we also achieve a much faster switching time than previous models,” said Dr. Volker Eberhardt, a scientist at Fraunhofer IAP.

According to Fraunhofer, manufacturers currently make electrochromic panes with glass that has been coated with a thin film of translucent indium tin oxide or fluorine-doped tin oxide to make the glass electrically conductive.

Two panes are required to make a smart window pane, with one receiving a second, vapour-deposited coating consisting of electrochromic tungsten oxide. The panes are then layered on top of each other with the coatings facing each other and a gel-like electrolyte in between.

When a voltage is applied to the glass, the tungsten oxide coating darkens. When the polarisation is reversed, the pane brightens. In the case of large windows of two to three square meters, it may take up to 15 to 20 minutes before the pane is completely darkened.

Fraunhofer IAP’s researchers are focusing on a different technology to darken the panes.

“We use organic monomers that have been mixed into specially developed resin,” Eberhardt said in a statement.

Although the researchers are using glass panes coated with tin oxide as an initial substrate, they are omitting the second coating. Instead, they layer the panes with the tin oxide coating facing inwards and fill the space between them with the resin and electrochromic molecule mix. The resin is then cured using heat or UV radiation. The researchers then apply direct current to ensure that the monomers on an electrode bond to form an electrochromic polymer.

Consequently, the pane can be switched at a lower voltage. Meanwhile, using an organic colorant offers various advantages; by selecting other monomers, it will be possible to install red or purple panes in the future. Furthermore, monomers react significantly faster.

“A 1.2-square-meter pane can darken in just 20 to 30 seconds; the standard tungsten-oxide-based electrochromic system would take at least ten minutes for that,” said Eberhardt.

According to Fraunhofer, the team tested the stability of the new electrochromic panes in accordance with applicable DIN standards.

“Even a pane comprising just two layers is sturdy enough for use as overhead glazing or in surfaces meant to be walked on. Previously you needed many more for that,” said Eberhardt.