Now, researchers at Princeton University have developed a self-powered smart window that features solar cells that selectively absorb near-ultraviolet (near-UV) light, so the new 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. "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."
Loo is one of the authors of a paper published in Nature Energy that describes the technology developed in her lab.
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 in smart windows that augment lighting, cooling and heating systems.
"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."
In the paper the researchers described how they used organic semiconductors -- contorted hexabenzocoronene (cHBC) derivatives – to build the solar cells. The researchers chose the material because its chemical structure could be modified to absorb a narrow range of wavelengths, such as near-UV light.
To construct the solar cell, the 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 cHBC semiconductors to produce electricity.
The researchers also constructed a smart window consisting of electrochromic polymers that 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 over 80 per cent of light.
Nicholas Davy, a doctoral student in the chemical and biological engineering department and the paper's lead author, said other researchers have already developed transparent solar cells, but those target infrared energy.
However, infrared energy carries heat, so using it to generate electricity can conflict with a smart window's function of controlling the flow of heat in or out of a building. Transparent near-UV solar cells don't generate as much power as infrared equivalents but don't impede the transmission of infrared radiation, so they complement the smart window's task.
Davy said that the Princeton team's aim is to create 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."
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