When Skyscrapers Catch the Light

Researcher Yongxi Li holds up a sample of a transparent solar cell. (Source: R. Coelius, U. Michigan)

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, 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 reports now.

“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 two to three percent, 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 ten years. They are also inves­tigating the economics of instal­ling transparent solar cell windows into new and existing buildings. (Source: U. Michigan)

Reference: Y. Li et al.: Color-neutral, semitransparent organic photovoltaics for power window applications, Proc. Nat. Ac. Sc., online 17 August 2020; DOI: 10.1073/pnas.2007799117

Link: Optoelectronic Components and Materials Group, University of Michigan, Ann Arbor, USA

Further reading: Smart Window with Transparent Solar Cells, PhotonicsViews, 5th July 2015

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