ATLANTA — By using natural substrates derived from plants, researchers from the Georgia Institute of Technology and Purdue University developed recyclable solar cells that are partially made from trees. Plus, by fabricating them on cellulose nanocrystal (CNC) substrates, the solar cells can be recycled at the end of their lifecycle.
Published in the journal Scientific Reports in January, the research reports that the organic solar cells reach a power conversion efficiency of 2.7 percent, a higher rate compared to other studies. The CNC substrates on which the solar cells are fabricated are optically transparent, allowing light to pass through them before being absorbed by a very thin layer of an organic semiconductor. During the recycling process, the solar cells are immersed in water at room temperature. Within 30 minutes, the CNC substrate dissolves and the solar cell can be separated easily into its major components. It is potentially reusable, leaving nothing but residue once the water evaporates.
Georgia Tech College of Engineering Professor Bernard Kippelen led the study, which is just the beginning to finding a truly recyclable, sustainable and renewable solar cell technology. “The development and performance of organic substrates in solar technology continues to improve, providing engineers with a good indication of future applications,” he said in a statement. “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.”
Previous developments of solar cells have been fabricated on glass or plastic, which are not very eco-friendly because the material would be difficult to toss out if broken. Paper substrates have also been used, but do not perform as well because of high surface roughness. According to Ars Technica, “research into paper substrates in 2010 resulted in a power conversion efficiency of just 0.13 percent.”
Cellulose nanomaterials made from wood, however, have a low surface roughness of about two nanometers proved to be green, renewable and sustainable. The CNC films used as the basis of the solar cells were created at the US Forest Service’s Forest Products Laboratory.
According to Kippelen, the research team plans to work toward improving the power conversion efficiency to more than 10 percent, which would be similar to the levels fabricated on glass or petroleum-based substrates. He plans to achieve this by optimizing the optical properties of the solar cell’s electrode. “We will also coat these cells with an eco-friendly thin environmental barrier coating to protect the cells from water and oxygen when operating in the field,” he said in a statement.
A provisional patent on the technology has been filed with the U.S. Patent Office, and the forest product industry predicts that tens of millions of tons of them could be produced once large-scale production begins, possibly in the next five years.