Traditionally made from inorganic materials such as silicon, solar cells have been proving increasingly good at converting solar energy into electricity. However, solar cells made from organic materials have become more and more attractive to manufacturers, due to being a lighter, more flexible, and inexpensive alternative.
For organic solar cells to become a viable replacement for their inorganic cousins (and, indeed, fossil fuels), scientists need to find a way to improve their efficiency when converting solar energy into electrical energy. Now, researchers from KAUST (King Abdullah University of Science and Technology) in Saudi Arabia say that they have successfully demonstrated that diquat, which is widely used in herbicides and is one of the many quaternary ammonium compounds, can improve such conversion efficiency.
Doping Organic Solar Cells
In the study, the KAUST researchers reported that they have managed to increase the conversion efficiency of organic solar cells by up to 1.6%. This is through the use of diquat, which is a quaternary ammonium compound that is widely used in herbicides.
In this research, the compound was used as a donor molecular dopant in two organic photovoltaic devices, leading to a reported efficiency rate of 16.7%. And in the two organic solar cells, moreover, the efficiency rose to 17.4% and 18.3% in the first and second device respectively.
As explained in the researchers’ study: “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”.
According to KAUST researchers, by adding diquat (a known herbicide) as a dopant, the conversion efficiency of high-performance organic solar cells can be enhanced. Pictured: a graphic depiction of the inside and outside of KAUST’s organic solar cell
Image credit: Yuanbao Lin, KAUST
Stabilising Neutral Diquat
As is typical of organic n-type dopants, diquat is reactive in an ambient atmosphere. This means that it’s unstable in such environmental conditions, which is perhaps the chief reason that the molecular dopant is yet to see widespread adoption. However, the KAUST team reports that it has been able to develop a process that has enabled the development of a stable neutral diquat. This was by electrochemically reducing charged diquat, which is in fact stable in ambient atmospheric conditions.
This development makes diquat a promising choice for the next generation of organic solar cells. “The predicted maximum efficiency of the organic solar cell is around 20 per cent," explains KAUST PhD student Yuanbao Lin. “We will try our best to reach this."
This work is the latest in a series of organic solar cell-related research that has come out of KAUST in recent years. In December 2019, KAUST researchers reported the use of an inexpensive material made from tungsten disulfide that helped to improve the performance of organic solar cells.
At the time, Thomas Anthopoulos, one of the researchers working on the project, said that the team’s immediate goal was to push the efficiency of organic solar cells well beyond 17%—an achievement which represents a big step towards reaching the theoretical maximum efficiency of such solar cells: 20%.