Lance Wheeler (front) developed a switchable photovoltaic window along with (from left) Nathan Neale, Robert Tenent, Jeffrey Blackburn, Elisa Miller, and David Moore. Source: Dennis Schroeder/NRELScientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have developed thermochromic windows that can convert sunlight into electricity. The highly efficient technology uses perovskites and single-walled carbon nanotubes to transform the windows from transparent to tinted, generating electricity as they darken.

The color change is driven by molecules (methylamine) that are reversibly absorbed into the device. When heated by solar energy, the molecules are driven out and the device darkens. When the sun is not shining, the device cools back down, the molecules re-absorb and the device appears transparent.

In its transparent state, the NREL-developed demonstration device allows an average of 68% of light in the visible portion of the solar spectrum to pass through. When the window changes color, only 3% is allowed through. The process took about 3 minutes of illumination during testing.

Because existing solar window technologies are static, they harness only a fraction of the available sunlight in order to preserve the visible light transmission needed for viewing as well as for the comfort of building occupants.

“There is a fundamental tradeoff between a good window and a good solar cell,” said Lance Wheeler, a scientist at NREL. “This technology bypasses that. We have a good solar cell when there’s lots of sunshine and we have a good window when there’s not. There are thermochromic technologies out there but nothing that actually converts that energy into electricity.”

This technology, Wheeler says, could be integrated into vehicles, buildings and beyond. The electricity generated by the solar cell window could charge batteries to power smartphones or on-board electronics such as fans, rain sensors and motors that would open or close the windows as programmed.

The paper, Switchable Photovoltaic Windows Enabled by Reversible Photothermal Complex Dissociation from Methylammonium Lead Iodide, is published in Nature Communications. Co-authors David Moore, Rachelle Ihly, Noah Stanton, Elisa Miller, Robert Tenent, Jeffrey Blackburn and Nathan Neale are all from NREL.