One step closer to skyscrapers are great sources of energy, a team led by University of Michigan researchers set new efficiency record for transparent solar cells in neutral color.

The team made a 8.1% efficiency and 43.3% transparency with an organic design instead of conventional silicon. While the cells have a slight green tint, they look much more like the gray in sunglasses and car windows.

Windows are a great place for organic solar cells because they offer something that silicon cannot, which is a combination of very high efficiency and very high visible transparency.

Stephen Forrest.

Buildings with glass facades often have a coating that reflects and absorbs some of the light, both in the visible and the infrared part of the spectrum, to reduce glare and heat inside the building. Instead of wasting this energy, transparent solar panels could use it to reduce the building’s electricity needs.

The transparency of some existing windows is similar to the transparency of solar cells that Forrest’s group described 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 be balanced to provide good sunlight absorption, high voltage, high current, low resistance and neutral color transparency.

Yongxi Li, assistant researcher in electrical and computer engineering.

The new material is a combination of organic molecules designed to be transparent in the visible and absorb in the near infrared, an invisible part of the spectrum that represents a large part of the sun’s energy.

In addition, researchers have developed Optical coatings to increase both the energy generated by infrared light and transparency in the visible range, two qualities that normally compete with each other.

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

Transparent solar cells are measured by their light use efficiency, which describes the amount of energy available from the light hitting the window, in the form of electricity or light transmitted inside.

The above transparent solar cells have a light utilization efficiency of about 2-3%, but the indium tin oxide cell has a light utilization efficiency of 3.5 % and the silver version has a light efficiency of 5%.

Both versions can be manufactured on a large scale, using materials that are less toxic than other transparent solar cells. Transparent organic solar cells can also be adapted to local latitudes, taking advantage of the fact that they are most efficient when the sun’s rays hit them at a perpendicular angle. They can be placed on double glazed windows.

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

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