Doped materials allow windows to be tuned to block targeted wavelengths

27 Sep 2023

Researchers at NCSU block wavelengths by doping glass with Li ions and e- in tungsten oxide hydrate.

Researchers at North Carolina State University (NCSU) have demonstrated a material for “next generation dynamic windows”, which would allow building occupants to switch their windows between three modes: transparent, or “normal” windows; windows that block infrared light, helping to keep a building cool; and tinted windows that control glare while maintaining a viewer’s outlook.

Dynamic windows based on electrochromism – meaning their opacity changes in response to electric stimulus – are not a new concept. But, to this point, most dynamic windows have been either translucent or dark.

“Our work demonstrates that there are more options available,” said Veronica Augustyn, co-corresponding author of a paper in ACS Photonics on the work and the Jake and Jennifer Hooks Distinguished Scholar in Materials Science and Engineering at NCSU. “Specifically, we’ve shown that you can allow light to pass through the windows while still helping to keep buildings cooler and thus more energy efficient.”

The researchers found that when water is bound within the crystalline structure of a tungsten oxide – forming tungsten oxide hydrate – the material exhibits a previously unknown behavior. Tungsten oxides have long been used in dynamic windows. Tungsten oxide is normally transparent but when lithium ions and electrons are injected into the material and an electric current is applied, the material darkens and blocks light.

Tunable light filter

The researchers have now shown that the wavelengths of light that are blocked can be tuned when lithium ions and electrons are injected into the related material tungsten oxide hydrate.

When lithium ions and electrons are injected into the hydrate material, it first transitions into a “heat blocking” phase, allowing visible wavelengths of light to pass through, but blocking infrared light. If more lithium ions and electrons are injected, the material then transitions into a dark phase, blocking both visible and infrared wavelengths of light.

“The presence of water in the crystalline structure makes the structure less dense, so the structure is more resistant to deformation when lithium ions and electrons are injected into the material,” said Jenelle Fortunato, first author of the paper and a postdoctoral fellow at NC State.

“Our hypothesis is that, because the tungsten oxide hydrate can accommodate more lithium ions than regular tungsten oxide before deforming, you get two modes. There’s a ‘cool’ mode – when injection of lithium ions and electrons affects the optical properties, but structural change hasn’t occurred yet – which absorbs infrared light. And then, after the structural change occurs, there’s a ‘dark’ mode that blocks both visible and infrared light.”

Delia Milliron, co-corresponding author of the paper and the Ernest Cockrell, Sr. Chair #1 in Engineering at the University of Texas at Austin, commented, “The discovery of dual-band [infrared and visible] light control in a single material that’s already well-known to the smart windows community may accelerate development of commercial products with enhanced features.

“More broadly speaking, the unforeseen role of structural water in producing distinctive electrochemical properties may inspire the research community beyond smart window developers, leading to innovation in energy storage and conversion materials.”