Berkeley Lab researchers have unveiled a semiconductor nanocrystal coating material capable of controlling heat from the sun while remaining transparent. This heat passes through the film without affecting its visible transmittance, which could add a critical energy-saving dimension to “smart window” coatings. |
Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley
National Laboratory (Berkeley Lab) have unveiled a semiconductor nanocrystal
coating material capable of controlling heat from the sun while remaining
transparent. Based on electrochromic materials, which use a jolt of electric
charge to tint a clear window, this breakthrough technology is the first to
selectively control the amount of near infrared radiation. This radiation,
which leads to heating, passes through the film without affecting its visible
transmittance. Such a dynamic system could add a critical energy-saving
dimension to “smart window” coatings.
“To have a transparent electrochromic material that can change its
transmittance in the infrared portion of sunlight is completely unprecedented,”
says Delia Milliron, director of the Inorganic Nanostructures Facility with
Berkeley Lab’s Molecular Foundry, who led this research. “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.”
Dynamic window coatings could translate into significant energy savings in
buildings, which account for more than 40% of carbon emissions in the United States.
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% for air conditioning and 51% for lighting.
“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,”
says Guillermo Garcia, a graduate student researcher at the Foundry. “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.”
To generate this new coating, the team developed a nanocrystal film of
electrically doped indium tin oxide, 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,” Milliron adds. “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.”
“This work expands the versatility of electrochromic devices, opening up a
variety of new applications in solar and thermal control,” says coauthor Thomas
Richardson, a staff scientist in Berkeley Lab’s Environmental and Energy
Technologies Division. “The innovative mechanism with high coloration
efficiency offers hope for improved switching ranges and long term durability.”
