How to Double the Efficiency of OLEDs

Double doping could improve the light-harvesting efficiency of flexible organic solar cells (Epishine AB, left), the switching speed of electronic paper (center) and the power density of piezoelectric textiles. (Source: J. Bodell, Chalmers U)

Researchers from Chalmers Uni­versity of Tech­nology, Sweden, have discovered a simple new tweak that could double the effi­ciency of organic elec­tronics. OLED-displays, plastic-based solar cells and bioelec­tronics are just some of the tech­nologies that could benefit from their new disco­very, which deals with double-doped polymers.

The majority of our everyday elec­tronics are based on inorganic semi­conductors, such as silicon. Crucial to their function is doping, which involves weaving impuri­ties into the semi­conductor to enhance its electrical conduc­tivity. It is this that allows various components in solar cells and LED screens to work. For organic semi­conductors, this doping process is similarly of extreme importance. OLED-displays are one example which are already on the market, for example in the latest genera­tion of smart­phones. Other appli­cations have not yet been fully realised, due in part to the fact that organic semi­conductors have so far not been effi­cient enough.

Doping in organic semi­conductors operates through 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, Christian Müller and his group, 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,” says David Kiefer, PhD student in the group. Accor­ding to Christian Müller, this inno­vation is not built on some great technical achieve­ment. Instead, it is simply a case of seeing what others have not seen.

“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 ioni­sation 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 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. 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 themselves are researching several different applied areas, with polymer tech­nology at the centre. Among other things, his group is looking into the develop­ment of elec­trically con­ducting textiles and organic solar cells. (Source: Chalmers TU)

Reference: D. Kiefer et al.: Double doping of conjugated polymers with monomer molecular dopants, Nat. Mat., online 14 January 2019; DOI: 10.1038/s41563-018-0263-6

Link: Dept. of Chemistry and Chemical Engineering, Chalmers University of Technology, Göteborg, Sweden

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