Transparent solar panels for windows hit 8% efficiency

In a step closer to skyscrapers serving as power sources, a team led by researchers from the University of Michigan has set a new efficiency record for color-neutral, transparent solar cells.

The team achieved 8.1% efficiency and 43.3% transparency with an organic, as carbon-based, design instead of conventional silicon. While the cells have a light green tint, they are much more than the gray of sunglasses and car windows.

“Windows, which are on the face of every building, are an ideal location for organic solar cells because they cannot offer some silicon, 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 typically have a coating on them that reflects and absorbs some of the light, both in the visible and infrared parts of the spectrum, to reduce the brightness and heating in the building. Instead of wasting that energy, transparent solar panels could use it to take a bite out of the building’s electricity needs. The transparency of some existing windows is comparable to the transparency of Forrest’s solar cell group reports in the journal Procedures of the National Academy of Sciences.

“The new material we developed, and the structure of the device we built, had to balance multiple trade-offs to provide simultaneous absorption of sun, high voltage, high current, low resistance and color-neutral transparency,” said Yongxi Li, a Assistant Research Scientist in Electrical Engineering and Computer Science.

The new material is a combination of organic molecules made to be transparent in the visible and absorbing throughout the infrared, an invisible part of the spectrum that absorbs much of the energy in sunlight. In addition, the researchers developed optical coating to stimulate both the power generated by infrared light and transparency in the visible range – two qualities that are normally in competition with each other.

The color-neutral version of the device was made with an indium tin oxide electrode. A silver electrode improved the efficiency to 10.8%, with 45.8% transparency. The slightly greenish tint of that version may not be acceptable in some windows applications.

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

Both versions can be produced on a large scale, using materials that are less toxic than other transparent solar cells. The transparent organic solar cells can also be adapted to local latitudes, and benefit from the fact that they are most efficient when the sun’s rays attach them perpendicularly. They can be placed between the windows of double glazed windows.

Forrest and his team are working on several improvements to the technology, with the following goal of achieving a light efficiency of 7% and extending the life of the cell to about 10 years. They are also investigating the economics of installing transparent solar cells for windows in new and existing buildings.

The study is published in the Procedures of the National Academy of Sciences in the article, “Color Neutral, Semi-Transparent Organic Photovoltaics,” by Forrest, Li, and colleagues Xia Guo, Zhengxing Peng, Boning Qu, Hongping Yan, Harald Ade, and Maojie Zhang. The team includes researchers at North Carolina State University, Soochow University in China, and SLAC National Accelerator Laboratory.