In a step closer to turning skyscrapers into power stations, researchers at the University of Michigan researchers have set a new efficiency record for transparent solar cells.

The team achieved 8.1 percent efficiency and 43.3 percent 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 grey of sunglasses and automobile windows.

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 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 colour-neutral version of the device was made with an indium tin oxide electrode. A silver electrode improved the efficiency to 10.8 percent, with 45.8 percent transparency. However, that version's slightly greenish tint may not be acceptable in some window applications.

Transparent solar cells are measured by their light utilisation 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 utilisation efficiencies of roughly 2-3 percent, but the indium tin oxide cell is rated at 3.5 percent and the silver version has a light utilisation efficiency of 5 percent.

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.

Text from the University of Michigan / Image: