Polymers take charge to produce conductive ink

A research group led by Simone Fabiano at Linköping University in Sweden has created an organic material with superb conductivity that doesn't need to be doped. They have achieved this by mixing two non-conducting polymers with different properties.

In order to increase the conductivity of polymers, and thus produce more efficient organic solar cells, light-emitting diodes and other bioelectronic applications, researchers have until now doped them with various substances. Typically, this is done by either removing or donating an electron to a semiconducting polymer by adding a dopant molecule, a strategy that increases the number of charge carriers and thereby the conductivity of the material.

"We normally dope our organic polymers to improve their conductivity and the device performance," says Fabiano, who is head of the Organic Nanoelectronics group within the Laboratory of Organic Electronics at Linköping University. "The process is stable for a while, but the material degenerates and the substances we use as doping agents can eventually leach out. This is something that we want to avoid at any cost in, for example, bioelectronic applications, where the organic electronic components can give huge benefits in wearable electronics and as implants in the body."

By combining two different polymers, Fabiano and his research group, including scientists from five countries, have now succeeded in producing an ink that does not require any doping to conduct electricity. Because the energy levels of the two polymers perfectly match, charge carriers are spontaneously transferred from one polymer to the other. The researchers report their work in a paper in Nature Materials.

"The phenomenon of spontaneous charge transfer has been demonstrated before, but only for single crystals on a laboratory scale," says Fabiano. "No one has shown anything that could be used at an industrial scale. Polymers consist of large and stable molecules that are easy to deposit from solution, and that's why they are well suited for large-scale use as ink in printed electronics."

Polymers are simple and relatively cheap materials, and are commercially available. No foreign substances leach out from the new polymer mixture, which also remains stable for a long time and can withstand high temperatures. These properties are important for energy harvesting/storage devices, as well as wearable electronics.

"Since they are free of doping agents, they are stable over time and can be used in demanding applications," Fabiano explains. "The discovery of this phenomenon opens completely new possibilities for improving the performance of light-emitting diodes and solar cells. This is also the case for other thermoelectric applications, and not least for research within wearable and close-body electronics."

"We have involved scientists at Linköping University and Chalmers University of Technology, and experts in the US, Germany, Japan and China," he adds. "It has been a really great experience to lead this work, which is a large and important step in the field."

"Fundamentally, doping in conducting polymers, generating high electrical conductivity, has so far only been achieved by combining a non-conducting dopant with a conducting polymer," says Magnus Berggren, director of the Laboratory of Organic Electronics. "Now, for the first time, the combination of two polymers renders a composite system that is highly stable and highly conducting. This discovery defines a major new chapter in the field of conducting polymers, and will spark many novel applications and interest world-wide."

This story is adapted from material from Linköping University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.