The thermochromic device dynamically responds to sunlight by switching from transparent to tinted while converting sunlight into electricity as it darkens. Such "smart" windows promise to improve energy efficiency in buildings by reducing heating, cooling, and ventilation costs.
Based on perovskites and single-walled carbon nanotubes, the window's color change is a result of methylamine molecules that are driven out of the device when it is heated by solar energy and then re-absorbed when the device is cooled back down (see video). Solar electricity production occurs during the heating process, while the window is shifting to opaque.
The prototype device allows an average of 68 percent of light in the visible portion of the solar spectrum to pass through when it's in a transparent state. When the window changes color - a process that took about 3 minutes of illumination during testing - only 3 percent of light is allowed through.
Existing solar window technologies are static, say the researchers, which means they harness a fraction of the sunlight without sacrificing too much of the visible light transmission needed by building occupants.
"There is a fundamental tradeoff between a good window and a good solar cell," says Lance Wheeler, a scientist at NREL and lead author of a paper on the research. "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."
The researchers' proof-of-concept paper established a solar power conversion efficiency of 11.3 percent. "There are thermochromic technologies out there but nothing that actually converts that energy into electricity," says Wheeler.
In testing, the prototype device cycled through repeated transparent-tinted cycles, but the performance declined over the course of 20 cycles due to restructuring of the switchable layer. The researchers are now focused on improving cycle stability.
Looking ahead at the technology's potential, Wheeler says it could be integrated into vehicles, buildings, and beyond. The