Thermochromic windows that are capable of converting sunlight to electricity at a high efficiency have been developed by scientists at the National Renewable Energy Laboratory (NREL). A proof-of-concept paper detailing their results was published in the prestigious journal Nature Communications, establishing a solar power conversion efficiency of 11.3 percent. This project is the first to validate dynamic photovoltaic glass.

Using advanced materials such as perovskites and single-walled carbon nanotubes, this new technology responds to heat from the sun by transforming from a transparent to tinted state. As the window darkens, it generates electricity. The color change is driven by molecules (methylamine) that are reversibly absorbed into the device. When solar energy heats up the device, the molecules are driven out, and the device is darkened. When the sun is not shining, the device is cooled back down, and the molecules reabsorb into the window device, which then appears transparent. This project has the potential to revolutionize buildings by transforming them into a source of clean renewable energy. Besides buildings, this technology has applications in vehicles and beyond.

In the paper, “Switchable Photovoltaic Windows Enabled by Reversible Photothermal Complex Dissociation from Methylammonium Lead Iodide,” the NREL team outlines the design of the window and the testing that was performed. Their demonstration device allows an average of 68 percent of light in the visible portion of the solar spectrum to pass through when it is in a transparent state. When the window changes color—a process that took about three minutes of illumination during testing—only 3 percent is allowed through the window. Existing solar window technologies are static, which means they are designed to harness a fraction of the sunlight without sacrificing too much visible light transmission needed for viewing or the comfort of the building occupants.

In testing under 1-sun illumination, the 1-square-centimeter demonstration device cycled through repeated transparent-tinted cycles, but the performance declined over the course of 20 cycles due to restructuring of the switchable layer. Ongoing research is focused on improving cycle stability.

This project participated in Energy I-Corps, where NREL’s team of researchers paired with industry mentors to learn about customer needs and develop a viable path to commercialization for their technology. During this program, funded by the Building Technologies Office (BTO), the team developed a market strategy for their product SwitchGlaze. Energy I-Corps is an intensive two-month training where national lab researchers define technology value propositions, conduct customer interviews, and develop viable market pathways for their technologies. Researchers return to the lab with a framework for industry engagement to guide future research and inform a culture of market awareness within the labs.