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Solar cells capture the sunlight and turning it into energy, much like a leaf does with a tree.

That idea makes it more than ironic now that new and efficient solar cells using natural substrates derived from plants such as trees are now under development from Georgia Institute of Technology and Purdue University researchers. By fabricating them on cellulose nanocrystal (CNC) substrates, the solar cells can end up quickly recycled in water at the end of their lifecycle.

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Organic solar cells so far reach a power conversion efficiency of 2.7 percent, an unprecedented figure for cells on substrates derived from renewable raw materials, the researchers said. The CNC substrates on which they fabricate the solar cells are optically transparent, enabling light to pass through them before it ends up absorbed by a very thin layer of an organic semiconductor. During the recycling process, the solar cells simply immerse in water at room temperature. Within only minutes, the CNC substrate dissolves and the solar cell separates easily into its major components.

“The development and performance of organic substrates in solar technology continues to improve, providing engineers with a good indication of future applications,” said study leader Georgia Tech College of Engineering Professor Bernard Kippelen, who is also the director of Georgia Tech’s Center for Organic Photonics and Electronics (COPE). “But organic solar cells must be recyclable. Otherwise we are simply solving one problem, less dependence on fossil fuels, while creating another, a technology that produces energy from renewable sources but is not disposable at the end of its lifecycle.”

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To date, organic solar cells usually end up made on glass or plastic. Neither is easily recyclable, and petroleum-based substrates are not very eco-friendly. For instance, if cells fabricated on glass were to break during manufacturing or installation, the useless materials would be difficult to dispose of. Paper substrates are better for the environment, but have limited performance because of high surface roughness or porosity. However, cellulose nanomaterials made from wood are green, renewable and sustainable. The substrates have a low surface roughness of only about two nanometers.

“Our next steps will be to work toward improving the power conversion efficiency over 10 percent, levels similar to solar cells fabricated on glass or petroleum-based substrates,” Kippelen said. The group plans to achieve this by optimizing the optical properties of the solar cell’s electrode.

A provisional patent on the technology is on file with the U.S. Patent Office.

There’s also another positive impact of using natural products to create cellulose nanomaterials. The nation’s forest product industry projects that tens of millions of tons of them could end up produced once large-scale production begins, potentially in the next five years.

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