The smart windows can control the tint of the glazing, saving up to 40 percent in an average building’s energy costs. Researchers put solar cells into windows that absorb near-ultraviolet (UV) light, making the windows self-powered. The technology is inexpensive and easy to apply to existing windows, they claim.
“Sunlight is a mixture of electromagnetic radiation made up of near-UV rays, visible light, and infrared energy, or heat,” explained 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,” she said.
The smart window controls the transmission of visible light and infrared heat into the building, while the new type of solar cell uses near-UV light to power the system. “This new technology is actually smart management of the entire spectrum of sunlight,” said Loo.
Because near-UV light is invisible to the human eye, the researchers set out to harness it for the electrical energy needed 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 explained. “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.”
Researchers used organic semiconductors – called contorted hexabenzocoronene (cHBC) derivatives – for making the solar cells. They chose the material because its chemical structure could be modified to absorb a narrow range of wavelengths – in this case, near-UV light.
These semiconductor molecules are deposited as thin films on glass with the same production methods used by organic light-emitting diode manufacturers. When the solar cell is operational, sunlight excites the semiconductors to produce electricity.
Researchers made 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 from the sun generates an electrical charge in the solar cell, the charge triggers a reaction in the electrochromic window, causing it to change from clear to dark blue. When darkened, the window can block more than 80 percent of light.
Nicholas Davy, a doctoral student in the chemical and biological engineering department at Princeton University said that the aim is to make 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 Davy. “Then you could control the sunlight passing into your home using an app on your phone, thereby instantly improving energy efficiency, comfort and privacy.”
Joseph Berry, senior research scientist at the National Renewable Energy Laboratory said that integrating the solar cells into the smart windows makes them more attractive for retrofits and “you don’t have to deal with wiring power”.
“And the voltage performance is quite good. The voltage they have been able to produce can drive electronic devices directly, which is technologically quite interesting,” he added.
Davy and Loo have started a new company, called Andluca Technologies, to explore other applications for the transparent solar cells. They explained that the near-UV solar cell technology can also power IoT sensors and other low-power consumer products.
“It does not generate enough power for a car, but it can provide auxiliary power for smaller devices, for example, a fan to cool the car while it’s parked in the hot sun,” Loo said.