The team, led by researchers at the University of Michigan, achieved 8.1% efficiency and 43.3% transparency with an organic design instead of conventional silicon. The cells reportedly have a slight green-grey tint, similar to sunglasses or car 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 Professor Stephen Forrest, 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 inside the building. Rather than wasting that energy, transparent solar panels could use it to reduce a building’s electricity needs. The transparency of some existing windows is similar to the transparency of the solar cells.
“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 colour-neutral transparency all at the same time,” said Yongxi Li, an assistant research scientist.
The new material is a combination of organic molecules engineered to be transparent in the visible spectrum and absorbing in the near infrared, which accounts for much of the energy in sunlight. The researchers also developed optical coatings to boost power generated from infrared light and transparency in the visible range – two qualities that are usually in competition with one another.
The colour-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. That version's slightly green tint might not be acceptable in some applications, however.
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 could be placed in between the panes of double glazing.
The team is working on several improvements to the technology, including reaching a light utilisation efficiency of 7% and cell lifetime of about 10 years. They are also investigating the economics of installing transparent solar cell windows into new and existing buildings.
The research was published in Proceedings of the National Academy of Sciences.
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