New Solar Cell Uses Near-UV Light for Self-Powered Smart Windows

A sample of the special window glass that harvests near-UV light to generate electricity in windows that causes a chemical reaction to lighten or darken smart windows. Image credit: Princeton

Researchers at Princeton University have developed a new type of solar cell that uses near-ultraviolet light to power smart windows.

Smart windows are equipped with controllable glazing that augments lighting, cooling and heating systems saving up to 40 percent in an average building’s energy costs. However, most of these smart tinting windows require complicated power installations in existing buildings.

The Princeton technology from solar cells selectively absorbs near-ultraviolet light so that the windows are completely self-powered.

“Sunlight is a mixture of electromagnetic radiation made up of near-UV rays, visible light and infrared energy, or heat,” said Yueh-Lin (Lynn) Loo, director of the Andlinger Center for Energy and the Environment at Princeton. “We wanted the smart window to dynamically control the amount of natural light and heat that can come inside, saving on energy cost and making the space more comfortable.”

Because near-UV light is invisible to the human eye, researchers set out to use it as the source to activate the tinting technology.

“Using near-UV light to power these windows means that the solar cells can be transparent and occupy the same footprint of the window without competing for the same spectral range or imposing aesthetic and design constraints,” Loo said. “Typical solar cells made of silicon are black because they absorb all visible light and some infrared heat — so those would be unsuitable for this application.”

How They Did It

The researchers used organic semiconductors, known as hexabenzocoronene (cHBC) derivatives, for constructing the solar cells. Because of its chemical structure, the material was modified to absorb a narrow range of wavelengths (in this case near-UV light). The semiconductor molecules are deposited as thin films on glass with the same production methods used by organic light emitting diode (OLED) makers.

The team then went about creating a smart window consisting of electrochromic polymers, which control the tint and can be operated solely using power produced by the solar cell. When near-UV light hits the solar cell, the electrical charge triggers a chain reaction in the window causing it to change from clear to dark blue. When dark blue, it can block more than 80 percent of light.

Next, the Princeton researchers want to develop a flexible version of the solar-powered smart window system that can be applied to existing windows via lamination.

“Someone in their house or apartment could take these wireless smart window laminates — which could have a sticky backing that is peeled off — and install them on the interior of their windows,” said Nicholas Davy, a doctoral student in the chemical and biological engineering department at Princeton. “Then you could control the sunlight passing into your home using an app on your phone, thereby instantly improving energy efficiency, comfort and privacy.”

Other applications where this type of solar cell could be used are in the Internet of Things (IoT), other low-power consumer products or even to power a fan inside a car to cool it in the hot sun.

The full research can be found in the journal Nature Energy.