Scientists at Massachusetts Institute of Technology (MIT) have developed a new way to make a smart window that can switch rapidly from transparent to opaque. While the technology already exists, this new one offers significant advantages compared to other systems as it has faster response time and requires a little to no power to work.

As the team reported in the journal Chem, this self-shading window is made from electrochromic materials, which can change color and transparency in response to an applied electric voltage. Unlike photochromic materials, such as those found in eyeglasses that become darker when exposed to bright light, electrochromic materials have a much faster response time to undergo change in the levels of opacity.

Existing electrochromic materials have similar limitations as that of photochromic ones. For example, electromic materials used in windows of the Boeing 787 aircraft take a few minutes to turn dark. The reason for the slow reaction time is that the changes within the material depend on a movement of electrons, and this gives the window a negative charge. Then, in order to restore the electrical balance within the material, positive ions rush in through it, creating the color-changing effect. But as ions move much more slowly compared to electrons which flow rapidly through materials, the overall response time slows down.

MIT's Self-shading Smart Window

Luckily, the team were able to overcome that by using sponge-like materials called metal-organic frameworks (MOFs), which can conduct electrons and ions at very high speeds, increasing the overall reaction speed. MOFs have been used for about 20 years for their ability to store gases within their structure, but the MIT team was the first to harness them for their electrical and optical properties. The team had actually worked on MOFS in their earlier studies and had made material that could turn from clear to shades of blue or green. But this time, they have succeeded their long-sought goal of producing a coating that can turn from perfectly clear to almost black, by blending two complementary colors, green and red.

The team reported that the material is made by combining two chemical compounds – an organic material and a metal salt, which when once mixed, self-assemble into a thin-film of the switchable material.

“It’s this combination of these two, of a relatively fast switching time and a nearly black color, that has really got people excited,” said Mircea Dinca, the MIT professor of chemistry, in a statement. “The new windows have the potential to do much more than just preventing glare.”

“These could lead to pretty significant energy savings by drastically reducing the need for air conditioning in buildings with many windows in hot climates. You could just flip a switch when the sun shines through the window, and turn it dark, or even automatically make that whole side of the building go dark all at once,” he added.

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