New controllable infrared coating for windows developed

Researchers from the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have unveiled a semiconductor nanocrystal coating material capable to control the amount of solar heat while remaining transparent. The technology is the first to selectively control the amount of near infrared radiation and it is based on electrochromic materials (which use electric charge to tint a clear window).

“Traditional electrochromic windows cannot selectively control the amount of visible and near infrared light that transmits through the film. When operated, these windows can either block both regions of light or let them in simultaneously”, said Guillermo Garcia, a graduate student researcher at the Berkeley Lab. “This work represents a stepping stone to the ideal smart window, which would be able to selectively choose which region of sunlight is needed to optimize the temperature inside a building.”

By filtering infrared radiation which passes through the film without affecting its visible transmittance, “smart window” coatings could enable substantial energy savings. According to studies conducted at the National Renewable Energy Laboratory, smart window coatings could offset the use of climate control and illumination systems by up to 49 percent for air conditioning and 51 percent for lighting.

“To have a transparent electrochromic material that can change its transmittance in the infrared portion of sunlight is completely unprecedented”, said Milliron. “What’s more, the coloration efficiency of our material – a figure of merit describing the amount of current needed to make this thing go – is substantially higher than standard electrochromic materials, which means it’s also very efficient.”

To generate this new coating, the team developed a nanocrystal film of electrically doped indium tin oxide (ITO), a transparent semiconductor typically used as a conductive coating for flat screen TVs.  By manipulating the electrons within this semiconducting film, they could tune the collective oscillations of these electrons (a phenomenon called plasmonics) across the near-infrared frequency range.

“Our ability to leverage plasmons in doped semiconductors with a very sensitive switching response in the near-infrared region also brings to mind applications in telecommunications”, said Delia Milliron, the lead researcher and director of the Inorganic Nanostructures Facility with Berkeley Lab’s Molecular Foundry. “We’ve also brought this synthesis into WANDA, our nanocrystal robot, which means we will be able to provide materials for a wide variety of user projects.”

The results of this research expand the versatility of electrochromic devices, and enable a variety of new applications in solar and thermal control. Aside high coloration efficiency, this newly developed coating offers hope for improved switching ranges and long term durability of electrochromic devices.

For more information, read the article published in the journal Nano Letters named: “Dynamically modulating the surface plasmon resonance of doped semiconductor nanocrystals”.