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Plastic solar cells: Now in a rainbow of colors

Dye molecule tailors the hue of organic solar cells without hindering performance

by Mitch Jacoby
June 15, 2017 | A version of this story appeared in Volume 95, Issue 25

This photograph shows a collection of vials containing colorful solutions.
Credit: Jaemin Kong/Yale U
Researchers added varying amounts of the dye ASSQ to a solar cell to tune its color.

Solar panels have increasingly been appearing on buildings and homes in the past few years. But their popularity could grow even more if these electricity-generating devices offered architects and designers a broader palette of color options than the standard monotone blue and black.

A rainbow of options for windows, facades, and other parts of buildings may soon be on the way thanks to an advance in polymer solar cells.

Yale University researchers report that by adding varying amounts of an organic dye to the two-component polymer blend typically used in these types of cells, they can finely tune the color of the devices from blue-green to purple. The dye not only controls a cell’s color but also transfers energy between the cell’s electron-donating and electron-accepting polymer components, playing an active role in device performance (Nano Energy 2017, DOI: 10.1016/j.nanoen.2017.05.032).

Researchers have previously devised strategies for coloring solar cells, for example, by including nanostructured layers that scatter light in a predetermined way or by chemically modifying cells’ photoactive, or light-absorbing, layer. But the former method is complex and costly, and the latter method tends to degrade cells’ power conversion efficiency (PCE)—the ratio of electricity produced to sunlight absorbed.

A team of Yale chemists and engineers, including Jaemin Kong and André D. Taylor, found that ASSQ, a type of dye called a squaraine that contains a four-carbon ring, has little effect on cells’ PCE but can tune the devices’ colors. In a proof-of-principle study, the team prepared a blue-green solar cell containing only an electron-donating polymer (called PCE10) and electron-accepting polymer (called N2200) in its photoactive layer. That cell had a PCE of 5%.

As they incrementally added ASSQ, the cell changed colors: 20% ASSQ by weight resulted in a blue cell with a slightly improved PCE of 5.5%. And a cell with 50 wt % ASSQ led to a reddish-purple cell with a PCE of just under 5%. The best organic solar cells have PCEs of about 11.5%.

“This work shows that solution-processed organic solar cells can be made to be very robust,” says Barry P. Rand, a photovoltaics specialist at Princeton University. “This approach points to the possibility of incorporating numerous absorbers to make organic solar cells that cover a large swath of the incoming solar spectrum,” he adds.



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