Transparent solar panels, that could one day be used to generate renewable energy from buildings, have reached a record-breaking efficiency of 8 per cent, researchers have said.
A team from the University of Michigan said they had also achieved 43.3 per cent transparency with their organic, carbon-based design rather than using conventional silicon.
Although the cells have a slight green tint, they are much more like the grey of sunglasses and car windows, the researchers said. While 8 per cent may not sound like an impressive figure on paper, even the best commercially available solar panels today only achieve just over 22 per cent efficiency – and that’s without the loss of light spectrum incurred from transparency.
“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 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 have developed.
“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 assistant research scientist Yongxi Li.
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.
The researchers also 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 colour-neutral version of the device was made with an indium tin oxide electrode. A silver electrode improved the efficiency to 10.8 per cent, with 45.8 per cent transparency. However, that version’s slightly greenish tint may not be acceptable in some window applications.
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 customised 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 also be placed in between the panes of double-glazed windows.