A “smart window” from Princeton University uses a transparent solar cell to selectively absorb and harvest near-ultraviolet light. The advanced 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.

The technology could simultaneously generate electricity and lower heating and cooling costs.

Tech Briefs spoke with researcher and Princeton professor Yueh-Lin (Lynn) Loo, director of the Andlinger Center for Energy and the Environment.

The team, led by Professor Yueh-Lin (Lynn) Loo, center, includes graduate students Nicholas Davy, left, and Melda Sezen-Edmonds, right.
Tech Briefs: How is your solar cell different from traditional solar cells?

Professor Yueh-Lin (Lynn) Loo: Our solar cells are transparent, and they can be built on glass or plastic. We envision a laminate that can be applied to existing windows. In this sense, they look very different from the solar panels on rooftops today.

Tech Briefs: How is your solar cell different from transparent solar cells?

Professor Loo: Existing transparent solar cells absorb infrared light. We have the only technology that absorbs and generates power with UV light. There is less UV light in the light spectrum so our cells are low current. UV light, however, is more energetic so our cells generate high voltage. We overcome the low current density by making larger cells.

Since the intent is window application, we have more than the area necessary to generate the current we need. What is unique to our technology is that we don’t compete with electrochromic windows for infrared light. Heat can be transmitted through our windows on a cold winter day to warm up a room instead of being used by the solar cell to generate power.

Tech Briefs: How is the window powered by near-UV light?

Professor Loo: There are two pieces to this technology: a solar cell that absorbs UV light and generates electricity, and an electrochromic window whose tint can be changed by an electrical pulse to regulate visible light and solar heat. The solar cell provides the electrical pulse needed to regulate the window. The two parts occupy the same footprint.

Today, near-UV light is typically reflected with a window coating. (UV light bleaches furniture and is bad for skin). Instead, we are harnessing this portion of light that is not typically useful to power the tinting of our window. And the tinting in turn reduces cooling and heating needs in buildings, and increases occupant comfort.

Tech Briefs: How does the technology manage the entire spectrum of sunlight?
Graduate student Nicholas Davy holds a sample of the special window glass. (Photos by David Kelly Crow)

Professor Loo: The spectrum of light consists of UV, visible light, and infrared (in the form of heat). We use the first part to generate electricity; that electricity is used to regulate the other two parts of the light spectrum. The active component in the solar cell only absorbs UV light whereas the tinting component of our window only transmits or absorbs visible light and infrared. By judicious selection of the active materials, we have designed a system that have uses the entire light spectrum and have complementary functions.

Tech Briefs: What's next?

Professor Loo: This technology is being further developed by Andluca Technologies, a company that a senior graduate student and I founded based on this promising demonstration. We’ve demonstrated scalability of the solar cells from a standard laboratory test size of 0.2 cm2 to 10 cm2 with no loss in performance. We therefore believe we can scale this nicely.

Tech Briefs: What else could be powered with this system?

Professor Loo: Our solar cells can provide wireless power to IoT-type devices to make homes and offices smarter and more energy efficient. These solar cells can be placed on walls or windows non-intrusively (because they are transparent and generate power locally) for small microphones or amplifiers that enable occupants to communicate with their smart devices and gadgets.

Tech Briefs: What is most exciting to you about this technology?

Professor Loo: The organic electronics community has been developing solar cells for about two decades now. And the focus has been to develop a broad absorber to maximize absorption of the light spectrum to generate as much power as possible. Our technology exploits the narrow absorption characteristics that are unique to organic absorbers and has allowed us to address a need that present technologies do not address. There is still much to do before our product will be on the market, but all demonstrations thus far indicate promise for an architecturally simple, visibly transparent source for wireless powering low-current applications.