Herbicide helps boost efficiency of organic solar cells

Researchers from the King Abdullah University of Science and Technology (KAUST) have shown how a known herbicide can be used to improve solar energy conversion in organic devices, which should help the development of higher efficiency solar cells. Their breakthrough has been published in the journal ACS Energy Letters.

While solar cells have traditionally been made from inorganic materials such as silicon, organic materials are starting to break through as an alternative because they are light, flexible and relatively inexpensive to make, even offering the possibility for printable manufacture. But for organic photovoltaics to become a realistic replacement for fossil fuels, they must improve their efficiency when converting incident solar energy into electrical energy. Key to achieving this is choosing the right combination of materials.

Most photovoltaic devices have two important elements: an n-type region and a p-region, so called because each region has a net negative and positive electric charge, respectively. These charges can be achieved by adding impurities to the semiconductor. An impurity that creates an n-type material is known as a donor, while an acceptor impurity makes a p-type material.

Led by Yuanbao Lin and Thomas Anthopoulos, KAUST researchers used the herbicide diquat (C₁₂H₁₂Br₂N₂) as a molecular donor dopant to enhance the conversion efficiency of high-performance organic solar cells. Like many organic n-type dopants, diquat is reactive in an ambient atmosphere; its lack of stability has prevented its use as a molecular dopant so far. However, the KAUST team were able to develop a process that stably created neutral diquat by electrochemically reducing charged diquat.

The dopant was added to two organic material systems that have previously shown excellent photovoltaic performance. The researchers were able to increase one system’s power conversion efficiency from 16.7% to 17.4%, while they were able to attain a maximum efficiency of 18.3% in the other. These improvements were possible because the molecular diquat dopant increased both the materials’ optical absorption and the lifetime of the electrical charges when light was absorbed.

“The predicted maximum efficiency of the organic solar cell is around 20%,” said Lin. “We will try our best to reach this.”

Image caption: Adding diquat, a known herbicide, as a dopant enhances the conversion efficiency of high-performance organic solar cells. Image ©2021 Yuanbao Lin.

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