Organic transistors: The color blue for green electronics

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Searching for materials is an “organic” electronics specialist’s favorite thing. And it can lead to some surprising results. After blue jeans, the centuries old indigo dye may be experiencing a second renaissance as the bio-semiconductor.

Pharaohs used it to decorate the dead for their graves, and the most frequently sold trousers in the world carry its name. We are talking about the color blue, or to be exact, indigo blue. It is the last recognizable shade of blue before things start turning purple is now supposed to give organic transistors greater endurance.

Certain organic compounds (hydrocarbons) with semiconductor properties have been used in OLED displays in smartphones and televisions for years. In principle, they are also suitable for solar cells and transistors. Provided there is no oxygen in the air. That is because it destroys them very quickly. Which is why they have to be packaged in elaborate air-tight housings.

That is not the case with semiconductors made of indigo dye. Not only do they defy the air, they also stand up to liquids such as water. Which is particularly interesting for applications in the medical sector.

Organic transistors that don’t tend to disintegrate

Unfortunately, that kind of resilience also has a down side: As a rule, the methods used to manufacture organic semiconductor elements require that the material being used somehow be dissolved so that it can be deposited on a substrate. Now a group of researchers at Johannes Kepler University in Linz working in a project financed by the FWF Austrian Science Fund have managed to bond volatile side groups to the indigo molecules, which makes them soluble. The groups split off again when heated to 100 degrees.

That would finally eliminate the greatest obstacle for using indigo as a semiconductor. And many of those working on organic transistors are likely to concentrate on this “blue” class of materials.

Still, solar cells and light diodes are difficult to realize using this method. That is because indigo has strong luminescence-extinguishing properties due to hydrogen bonds (H bonds). This weak form of bond between molecules, which plays an important role in ice, for example, interferes with optical applications.
And in solar cells, current does not flow until the incoming light releases electrons in the material. But in indigo molecules, those kind of “excited” electronic states are quickly dissipated and converted into heat before they can be used.

There is, however, a great deal of potential for indigo transistors in the medical sector because of their biocompatibility. They can also be operated under water with various pH values. Two important prerequisites, say, for implants in human tissue. The only thing left to mention is the low price of the base material, and it wouldn’t be the first time that price is the most important aspect when introducing a new technology.

Knowledge Base

Royal Society of Chemistry: Organic Field Effect Transistors




Organic transistors (Image: Fraunhofer IPMS)

Organic electronics can be produced using cost-effective roll-to-roll techniques. (Photo: Fraunhofer EMFT).