Transparent solar panels for Windows reach new efficiency record – could help enable skyscrapers to serve as power sources

In a further step towards skyscrapers that serve as energy sources, a team led by researchers at the University of Michigan has set a new efficiency record for transparent, neutral-colored solar cells.

The team achieved 8.1% efficiency and 43.3% transparency with an organic or carbon-based design rather than conventional silicon. While the cells have a slight green tint, they look much more like the gray in sunglasses and auto glass.

“The windows, which are on the facade of every building, are an ideal location for organic solar cells because they offer something that silicon cannot, which is a combination of very high efficiency and very high visible transparency,” said Stephen Forrest, the Peter A. Franken Distinguished University Professor of Engineering and Paul G. Goebel Professor of Engineering, who led the research.

Buildings with glass facades are usually covered with a coating that reflects and absorbs some of the light, both in the visible and infrared parts of the spectrum, to reduce the brightness and heating inside the building. Rather than throwing away this energy, transparent solar panels could use it to reduce the building’s electricity needs. The transparency of some existing windows is similar to the transparency of solar cells reported by the Forrest group in the newspaper Proceedings of the National Academy of Sciences.

“The new material that we developed, and the structure of the device that we built, had to balance several compromises to provide good sunlight absorption, high voltage, high current, low resistance and transparency. neutral in color, all at the same time, ”said Yongxi Li, assistant researcher in electrical and computer engineering.

The new material is a combination of organic molecules designed to be transparent in the visible and absorbent in the near infrared, an invisible part of the spectrum that makes up much of the sun’s energy. In addition, researchers have developed optical coatings to increase both the power generated by infrared light and transparency in the visible range – two qualities that generally compete with each other.

The neutral colored version of the device was made with an indium tin oxide electrode. A silver electrode improved the efficiency to 10.8%, with a transparency of 45.8%. However, the slightly greenish tint of this version may not be acceptable in some window applications.

Transparent solar cells are measured by their light-use efficiency, which describes how much energy from the light hitting the window is available as electricity or light transmitted from the indoor side. Previous transparent solar cells have a light utilization efficiency of around 2-3%, but the indium tin oxide cell is rated at 3.5% and the silver version has an efficiency 5% light usage.

Both versions can be manufactured on a large scale, using materials that are less toxic than other transparent solar cells. Transparent organic solar cells can also be customized for local latitudes, taking advantage of the fact that they are most efficient when the sun’s rays hit them at a perpendicular angle. They can be placed between the panes of double-glazed windows.

Forrest and his team are working on several technology improvements, with the next goal being to achieve a light use efficiency of 7% and extend the life of the airframe to around 10 years. They are also studying the economics of installing transparent solar cell windows in new and existing buildings.

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Reference: “Neutral Color Semi-Transparent Organic Photovoltaic” by Forrest, Li and colleagues Xia Guo, Zhengxing Peng, Boning Qu, Hongping Yan, Harald Ade and Maojie Zhang, August 17, 2020, Proceedings of the National Academy of Sciences.

The team includes researchers from North Carolina State University, The University of Soochow in China and the SLAC National Accelerator Laboratory.

This material is based on work supported by the Office of Solar Technology, US Department of Energy as well as the Office of Naval Research and Universal Display Corporation.

Forrest is also a professor of electrical and computer engineering, materials science and engineering, and physics.