Organic materials present an alternative to inorganic materials such as silicon because as they are light, flexible and relatively inexpensive to make, even offering the possibility for printable manufacture.
For organic photovoltaics to become a realistic replacement for fossil fuels, they must improve their efficiency when converting the fraction of incident solar energy to electrical energy and key to achieving this is choosing the right combination of materials.
PhD student Yuanbao Lin and Thomas Anthopoulos have now achieved this by developing what Lin describes as "an efficient molecular dopant to improve the performance and stability of organic solar cells." The team’s findings are published in ACS Energy Letters.
Most photovoltaic devices have an n-type region and a p-region, so-called because each region has a net negative and positive electric charge. These charges can be achieved by adding impurities to the semiconductor. An impurity that creates an n-type material is a donor, while an acceptor impurity makes a p-type material.
Lin, Anthopoulos and their team are said to have used diquat (C12H12Br2N2) as a molecular donor dopant to enhance the conversion efficiency of high-performance organic solar cells.
The dopant was added to two organic material systems that have previously shown favourable photovoltaic performance. In one case, the power conversion efficiency was improved from 16.7 per cent to 17.4 per cent, while they were able to attain a maximum efficiency of 18.3 per cent in the other. According to KAUST, 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.
Like many organic n-type dopants, diquat is reactive in an ambient atmosphere and this 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, which is stable in air.
This ability 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," Lin said in a statement. "We will try our best to reach this."