Solar Windows Switch Between Tinted and Transparent

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©. Dennis Schroeder/NREL

A new addition into the solar window space is taking things in a different direction. Instead of solely focusing on transparency, new solar windows developed by scientists at NREL are made to be both efficient solar cells and windows, but not at the same time.

The solar switch windows darken when sunlight hits them, absorbing the light and producing electricity, but when the sunlight dims, they switch back into being normal transparent windows.

“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.”

When the windows are darkened, they only let in 3 percent of the solar spectrum, whereas when they're transparent, or in their so-called bleached state, they let in 68 percent. The efficiency of the solar technology at converting sunlight to electricity is 11.3 percent, which is comparable to products on the market today.

The new solar technology is made using perovskites and single-walled carbon nanotubes. It responds to the heat of sunlight by transforming into its tinted state. This change is thanks to methylamine molecules. When the device is heated, the molecules are driven out, leading to the darkening of the device. When the sun isn't shining, the device cools and the molecules are reabsorbed by the device, which switches back to being transparent.

In testing, the solar switch windows were able to go through repeated cycles of tinting and transparency, but over 20 cycles the efficiency began to decline. The team is now focusing on improving the stability of the device so that the switching happens without affecting performance.

The technology, if commercialized, could be integrated into buildings or vehicles. The energy produced could be used to charge electronics like smartphones or to power onboard electronic components like sensors or fans. If the windows were motorized, as in a vehicle, the electricity it produced could power the opening and closing of the windows.