Just as the leaves on a tree capture sunlight and convert it to energy, so do solar cells – and now recyclable, sustainable and renewable photovoltaics can be made using natural substrates derived from trees. Organic solar cells have typically been fabricated on glass or plastic. Neither is easily recyclable, and petroleum-based substrates are not very eco-friendly. For instance, if cells fabricated on glass break during manufacturing or installation, the useless materials are dangerous to toss away. Paper substrates would be better for the environment, but they have shown limited performance in the past because of high surface roughness or porosity. The recyclable solar cell was made on nano-cellulose substrates made from trees. Also pictured are vials containing the different parts of the cell after being dissolved in water and the organic solvent. Images courtesy of Canek Fuentes-Hernandez, Georgia Tech. Researchers at Georgia Institute of Technology and Purdue University turned to cellulose nanocrystal (CNC) substrates made from wood, which are green, renewable and sustainable and have a low surface roughness of only about 2 nm. The organic solar cells have a 2.7 percent power conversion efficiency. “The development and performance of organic substrates in solar technology continues to improve, providing engineers with a good indication of future applications,” said Bernard Kippelen, a professor at Georgia Tech’s College of Engineering and director of the Center for Organic Photonics and Electronics (COPE). “But organic solar cells must be recyclable. Otherwise, we are simply solving one problem – [dependence] on fossil fuels – while creating another, a technology that produces energy from renewable sources but is not disposable at the end of its life cycle.” The CNC substrates are optically transparent, enabling light to pass through them before being absorbed by a very thin layer of organic semiconductor material. During the recycling process, the solar cells are simply immersed in water at room temperature. Within minutes, the CNC substrate dissolves, and the solar cell can be separated easily into its major components. A solar cell fabricated at Georgia Tech on nanocellulose substrates derived from trees. “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. Purdue School of Materials Engineering associate professor Jeffrey Youngblood collaborated with Kippelen on the research, which was published in Scientific Reports (doi: 10.1038/srep01536). A provisional patent on the technology has been filed with the US Patent Office.