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‘Double-doped’ polymer research provides breakthrough for organic electronics

Researchers from Chalmers University of Technology, Sweden, have made a discovery that could double the efficiency of organic electronics, which could benefit OLED-displays, plastic-based solar cells and bioelectronics and other technologies.

Most everyday electronics are based on inorganic semiconductors, such as silicon. “Doping”, which involves weaving impurities into the semiconductor to enhance its electrical conductivity, is crucial to their functioning and allows various components in solar cells and LED screens to work.

This process is also of importance in organic, carbon-based semiconductors. Doping in organic semiconductors operates through what is known as a redox reaction. This means that a dopant molecule receives an electron from the semiconductor, increasing the electrical conductivity of the semiconductor.

The more dopant molecules that the semiconductor can react with, the higher the conductivity – at least up to a certain limit, after which the conductivity decreases. Currently, the efficiency limit of doped organic semiconductors has been determined by the fact that the dopant molecules have only been able to exchange one electron each.

In an article recently published in in the scientific journal Nature Materials, Professor Christian Müller and his group from Chalmers University of Technology, together with colleagues from seven other universities, demonstrate that it is possible to move two electrons to every dopant molecule.

“Through this ‘double doping’ process, the semiconductor can therefore become twice as effective,” said David Kiefer, a PhD student working with the research group.

“The whole research field has been totally focused on studying materials, which only allow one redox reaction per molecule,” said Müller.

“We chose to look at a different type of polymer, with lower ionisation energy. We saw that this material allowed the transfer of two electrons to the dopant molecule. It is actually very simple.”

The discovery could allow further improvements to technologies which today are not competitive enough to make it to market. One problem is that polymers simply do not conduct current well enough, and so making the doping techniques more effective has long been a focus for achieving better polymer-based electronics. Now, this doubling of the conductivity of polymers, while using only the same amount of dopant material, over the same surface area as before, could represent the tipping point needed to allow several emerging technologies to be commercialised.

“With OLED displays, the development has come far enough that they are already on the market. But for other technologies to succeed and make it to market something extra is needed,” Müller said.

“With organic solar cells, for example, or electronic circuits built of organic material, we need the ability to dope certain components to the same extent as silicon-based electronics. Our approach is a step in the right direction.”

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