Most of our everyday electronics are based on inorganic semiconductors, such as silicon. Crucial to their functionality is a process called doping, which involves weaving impurities into the semiconductor to enhance its electrical conductivity. It’s this that allows various components in solar cells and LED screens to work.
For organic—that is, carbon-based—semiconductors, this doping process is similarly important. Since the discovery of electrically conducting plastics and polymers, a field in which a Nobel Prize was awarded in 2000, research and development of organic electronics has accelerated quickly. Organic LED (OLED) displays are one example of such that’s already on the market; for instance, check out the latest generation of smartphones. Other applications haven’t yet been fully realized, due in part to the fact that organic semiconductors have so far not been efficient enough.
Examples of organic electronics: flexible solar cells (left, supplied by Epishine AB), electronic paper (center) and piezoelectric textiles (right). (Courtesy of Johan Bodell/Chalmers University of Technology)
Doping in organic semiconductors operates through what’s 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.
But now, in an article in the scientific journal Nature Materials, Christian Müller, Professor of Polymer Science at Chalmers University of Technology, and his group, together with colleagues from seven other universities, demonstrate that it’s possible to move two electrons to every dopant molecule.
"Through this ‘double-doping’ process, the semiconductor can therefore become twice as effective," says David Kiefer, PhD student in the group and first author of the article.
According to Christian Müller, this innovation isn’t built on some great technical achievement. Instead, it’s simply a case of seeing what others have not seen.
When can we expect to see the new “double-doped” solar cells? Says Professor Müller, “Giving you a specific number would not be correct; some types of solar cells may work a little better, and some a lot. I would rather like to say that more efficient doping enables many semiconductor technologies.”
"The whole research field has been totally focused on studying materials, which only allow one redox reaction per molecule. We chose to look at a different type of polymer, with lower ionization energy. We saw that this material allowed the transfer of two electrons to the dopant molecule. It is actually very simple," says Müller.
The discovery could allow for further improvements to technologies that aren’t yet competitive enough to make it to market. One problem is that polymers simply don’t conduct current well enough, therefore, 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 enable the commercialization of several emerging technologies.
“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. 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,” says Müller.
The discovery offers fundamental knowledge and could help thousands of researchers to achieve advances in flexible electronics, bioelectronics, and thermoelectricity. Müller’s research group is researching several different applied areas, with polymer technology at the center. Among other things, his group is looking into the development of electrically conducting textiles and organic solar cells.
For further details, read the article in Nature Materials: "Double Doping of Conjugated Polymers with Monomer Molecular Dopants"
The research was funded by the Swedish Research Council, the Knut and Alice Wallenberg Foundation, and the European Research Council (ERC), and was carried out in collaboration with colleagues from Linköping University (Sweden), King Abdullah University of Science and Technology (Saudi Arabia), Imperial College London (UK), the Georgia Institute of Technology and the University of California, Davis (USA), and the Chemnitz University of Technology (Germany).