Transparent solar panels for windows hit record 8% efficiency
University of Michigan – August 17, 2020
-- In a step closer to skyscrapers that serve as power sources, a team led by
University of Michigan researchers 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, or carbon-based, design rather than
conventional silicon. While the cells have a slight green tint, they are much
more like the gray of sunglasses and automobile windows.
“Windows, which are on the face of every
building, are an ideal location for organic solar cells because they offer
something silicon can’t, 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.
Yongxi Li holds up vials containing the
polymers used to make the transparent solar cells. Image credit: Robert
Coelius, Michigan Engineering Communications & Marketing
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 inside the building. Rather than throwing that energy away, transparent
solar panels could use it to take a bite out of the building’s electricity
needs. The transparency of some existing windows is similar to the transparency
of the solar cells Forrest’s group reports in the journal Proceedings 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
good sunlight absorption, high voltage, high current, low resistance and
color-neutral transparency all at the same time,” said Yongxi Li, an assistant
research scientist in electrical engineering and computer science.
The new material is a combination of
organic molecules engineered to be transparent in the visible and absorbing in
the near infrared, an invisible part of the spectrum that accounts for much of
the energy in sunlight. In addition, the researchers developed optical coatings
to boost both power generated from infrared light and transparency in the
visible range—two qualities that are usually in competition with one another.
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. However, that version’s slightly
greenish tint may not be acceptable in some window applications.
Transparent solar cells are measured by
their light utilization efficiency, which describes how much energy from the
light hitting the window is available either as electricity or as transmitted
light on the interior side. Previous transparent solar cells have light
utilization efficiencies of roughly 2-3%, but the indium tin oxide cell is
rated at 3.5% and the silver version has a light utilization efficiency of 5%.
Both versions can be manufactured at
large scale, using materials that are less toxic than other transparent solar
cells. The 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 are hitting them at a perpendicular angle. They can be placed in
between the panes of double-glazed windows..
Forrest and his team are working on
several improvements to the technology, with the next goal being to reach a
light utilization efficiency of 7% and extending the cell lifetime to about 10
years. They are also investigating the economics of installing transparent
solar cell windows into new and existing buildings.
The research is published in the
Proceedings of the National Academy of Sciences in the article, “Color-Neutral,
Semitransparent 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.
This material is based upon work
supported by the U.S. Department of Energy Solar Energy Technologies Office as
well as the Office of Naval Research and Universal Display Corporation.
Forrest is also a professor of
electrical engineering and computer science, material science and engineering,
and physics.
https://news.umich.edu/transparent-solar-panels-for-windows-hit-record-8-efficiency/
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