A self-powered smart window features a novel type of solar cell that selectively absorbs near-UV light. The near-UV light is used to generate electricity, which powers chemical reactions that lighten or darken the glass as needed. Graduate student Nicholas Davy holds a sample of the special window glass. Courtesy of David Kelly Crow. Organic semiconductors — specifically, contorted hexabenzocoronene (cHBC) derivatives — were used for constructing the solar cells. The researchers chose this material because its chemical structure can be modified to absorb a narrow range of wavelengths — in this case, near-UV light. To create the cell, researchers deposited the semiconductor molecules as thin films on glass, following the same production methods as those used by OLED manufacturers. When the solar cell is operational, sunlight excites the cHBC semiconductors to produce electricity. At the same time, the team constructed a smart window consisting of electrochromic polymers, which control the tint, and can be operated solely by 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. The solar cells that selectively harvest near-UV photons can produce open-circuit voltages greater than 1.6 V and their power generation capabilities are scalable. Joseph Berry, senior research scientist at the National Renewable Energy Laboratory who was not involved in the project, said the research is interesting because the device scales well and targets a specific part of the solar spectrum. “Integrating the solar cells into the smart windows makes them more attractive for retrofits and you don't have to deal with wiring power,” said Berry. “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.” According to the researchers, other types of transparent solar cells that have been developed target IR energy, which carries heat. Using a cell that targets heat-producing energy to generate electricity could conflict with a smart window’s function of controlling the flow of heat in or out of a building. Although transparent near-UV solar cells don’t generate as much power as their IR counterparts, they complement the smart window’s task because they do not impede the transmission of IR radiation. “This new technology is actually smart management of the entire spectrum of sunlight,” said professor Yueh-Lin (Lynn) Loo. “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. 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,” said Loo. Princeton engineers invented a window system that could simultaneously generate electricity and lower heating and cooling costs. The team, led by Professor Yueh-Lin (Lynn) Loo, center, includes graduate students Nicholas Davy, left, and Melda Sezen-Edmonds, right. Courtesy of David Kelly Crow. The research team, from Princeton University, hopes 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. Then you could control the sunlight passing into your home using an app on your phone, thereby instantly improving energy efficiency, comfort and privacy,” said researcher Nicholas Davy. Smart windows equipped with controllable glazing could save up to 40 percent in an average building’s energy costs. Davy and Loo have started a new company, called Andluca Technologies, based on the technology and are exploring other applications for the transparent solar cells. They said that the near-UV solar cell technology could potentially power internet-of-things 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. The research was published in Nature Energy (doi:10.1038/nenergy.2017.104). A piece of the glass, which is shown in the video, harvests one portion of the light spectrum to control other parts of the spectrum. Specifically, it uses near-ultraviolet light to generate electricity, which powers chemical reactions that lighten or darken the glass as needed. When darkened, the window can block more than 80 percent of light. Courtesy of Princeton University.