New smart windows darken in the sun—and generate electricity at the same time

Solar power has undergone a revolution in recent years, thanks to an upstart family of crystalline materials called perovskites. Now, perovskites are transforming windows, keeping them clear on cold days, but turning them dark in the hot summer sun. Two research groups report that they’ve created perovskite-tinted windows that not only transition based on the temperature, but also harvest power like solar cells. The new technology could one day help cool buildings by shading out sunlight and generating power to boot.

“Smart windows” that switch between transparent and opaque have been around for decades. For example, the Boeing 787 Dreamliner airplane employs so-called electrochromic windows, which require an outside power source to darken. But such windows have failed to make a broad impact in the building market, because of their higher cost, inconsistencies in their ability to block outside light, and, for some, their demand for external electrical power.

But perovskites offer a possible route to smart windows and solar windows at the same time. Perovskites are materials made of a mix of elements with a particular crystalline structure, and solar cells made from them are nearly as efficient at converting sunlight to electricity as state-of-the-art silicon solar panels: The best ones convert more than 22% of the energy in sunlight to electricity, compared with 25% for silicon. By changing the perovskites’ elemental components, researchers can also control their transparency. What’s more, the starting materials for perovskites are far cheaper than existing solar cells.

The first advance in combining all these features came in November 2017, when researchers reported they had created a lead-based perovskite solar window that switched from transparent to opaque when the temperature hit 60°C. At cooler temperatures, the perovskite in the center of the solar cell forms a complex with an organic compound called methylamine. When warmed, the methylamine vaporizes and breaks away from the perovskite, causing the latter to darken and absorb sunlight. When the heat dissipates, either when the sun goes down or during the winter, the methylamine vapor moves back into the perovskite, turning it transparent once again. The warmed perovskite transforms up to 11.3% of its energy into electricity.

Still, the windows have their drawbacks. The biggest is that the efficiency of the solar cells drops after only a few times switching back and forth, probably because the methylamine fails to fully return the perovskite to its starting crystalline arrangement. So for nearly a year, the window’s inventors—led by chemist Nathan Neale and mechanical engineer Lance Wheeler of the National Renewable Energy Laboratory in Golden, Colorado—have been working to come up with a different perovskite recipe that could switch without the methylamine chemical reaction.

Peidong Yang, a chemist at the University of California, Berkeley, beat them to the punch. Today, Yang reports in Nature Materials that his team has created a cesium-based perovskite solar window that turns opaque and produces electricity when heated, but without methylamine. That allows the windows to switch back and forth repeatedly without a drop in performance. “It’s an attractive idea that you would have the solar cell capability and the smart window at the same time,” says Michael McGehee, a materials scientist at Stanford University in Palo Alto, California, who studies both perovskite solar cells and smart windows.

The newest solar windows still have their downsides. For starters, they don’t switch from transparent to opaque unless heated to more than 100°C. Plus, their efficiency is only about 7%, well below conventional solar cells. McGehee notes that in addition to their stability and efficiency issues, all the perovskite windows have a reddish tint to them when opaque. “That’s crucial,” says McGehee, as most builders shy away from using colored windows.

However, Yang says he and his colleagues have already come up with a variation that switches between 50°C–60°C; they’re just waiting until they can improve its solar conversion efficiency. The hunt continues for new perovskites that satisfy all the demands for windows of the future.