A team of researchers at China’s Harbin Institute of Technology (HIT) has created transparent films that can change color on demand to block out sunlight and heat.

According to its developers, the electrochromic coatings — which function as so-called sunglasses for windows — change color on demand and subsequently prevent light from entering a space, thereby keeping it cool.

Source: ACS Energy Letters 2024Source: ACS Energy Letters 2024

The researchers added that the electrochromic films also work in reverse, switching to their transparent form, thus allowing light to come in when needed.

To create the coatings, the team used a class of porous polymers called metal-organic frameworks (MOFs). “MOF is a combination of porous organic and inorganic materials,” the team added.

Further, the structure is also composed of organic linkers that bind to metal ions, and the pattern reportedly repeats itself to create two and even three-dimensional structures, the researchers explained.

The team selected MOFs to manufacture their electrochromic films because they offer the flexibility necessary to make thin films. With MOF pore sizes ranging from about 0.5 nm to 2 nm, the structures can be further adjusted by altering the length of the organic ligand that binds to the metal ion.

This reportedly enabled the team to improve the current flow while also achieving more precise control over the colors that the film could transform into.

Specifically, when a voltage of 0.8 V was applied, the film went from colorless to green within two seconds; meanwhile when a voltage of 1.6 V was applied, the film turned a dark red in the same amount of time.

Additionally, unless a reverse voltage was applied to turn the film transparent again, it was able to retain the different colors for more than 40 hours at a time.

The new color-changing coatings are detailed in the article, “Biphenyl Dicarboxylic-Based Ni-IRMOF-74 Film for Fast-Switching and High-Stability Electrochromism,” which appears in the journal ACS Energy Letters.

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