Study Shows Minute Amounts of Water Drastically Slow Down Motion of Charges in Organic Electronic Devices

Organic semiconductors of poor quality can turn into high-quality semiconductors when properly manufactured.

In an article published in Nature Materials, scientists at Linköping University have shown that the motion of charges in organic electronic devices is markedly hindered by very small amounts of water.

The finding that organic materials, like polymers, have the ability to function as semiconductors led to a Nobel Prize in Chemistry in 2000. From then, investigation within organic electronics has certainly developed, particularly at Linköping University, which is the home of world-leading research in the field.

But organic semiconductors do not have the ability to conduct current as efficiently as, for instance, semiconductors of silicon or other inorganic materials. The researchers have found that one of the causes of this is the trap formation in the organic materials where the charge carriers get stuck. Many research teams worldwide have made great effort to understand where the traps are situated and how they can be eliminated.

Traps in Organic Semiconductors

There are traps in all organic semiconductors, but they are probably a greater problem in n-type materials, since these are generally poorer semiconductors than p-type materials.

Martijn Kemerink, Professor of Applied Physics, Division for Complex Materials and Devices, Linköping University..

P-type materials possess a positive charge and the charge carriers are in the form of holes, whereas n-type materials possess charge carriers in the form of electrons, offering the material a negative charge.

Martijn Kemerink and his team at Linköping University came to the conclusion that water is the perpetrator in the piece. In particular, it is believed that water is found in pores of nanometer size in the organic material and is absorbed from the environment.

In a p-type material the dipoles in the water align with their negative ends towards the holes, which are positively charged, and the energy of the complete system is lowered. You could say that the dipoles embed the charge carriers such that they cannot go anywhere anymore.

Martijn Kemerink, Professor of Applied Physics, Division for Complex Materials and Devices, Linköping University.

In the case of n-type materials, the water settles in the opposite way, but the effect is the same and the charge is captured.

The researchers performed experiments where the material is heated, dried, and made the water to disappear. It works satisfyingly for a period of time, but later, the material reabsorbs water from the surrounding atmosphere, and a greater portion of the advantage achieved by drying disappears.

Manufacture in a Dry Atmosphere

“The more water, the more traps. We have also shown that the drier the films can be manufactured, the better conductors they are. The theoretical work by Mathieu Linares quantitatively confirmed our ideas about what was going on, which was very satisfactory. Our article in Nature Materials shows not only how to get the water out, but also how to make sure that the water stays out, in order to produce an organic material with stable conductivity.”

To avoid the reuptake of water into the material once it has been dried, the researchers have also developed a method to remove the holes into which water molecules would have entered otherwise. This technique is based upon a fusion of heating the material in the presence of an appropriate organic solvent.

Materials that were previously believed to be extremely poor semiconductors can instead become good semiconductors, as long as they are manufactured in a dry atmosphere. We have shown that dry-prepared materials tend to remain dry, while materials that are made in the presence of water can be dried. The latter are, however, extremely sensitive to water. This is true of the materials we have tested, but there’s nothing to suggest that other organic semiconducting materials behave differently.

Martijn Kemerink, Professor of Applied Physics, Division for Complex Materials and Devices, Linköping University.