Beyond Silicon, indigo transistors grab interest for cost, performance, sustainability advantages

June 18, 2017 |

In Austria, a project funded by the Austrian Science Fund FWF, the physicist Serdar Sarıçiftçi had investigated possible uses in electronics of the semiconductor properties of indigo pigment.

For a number of years, however, an alternative to silicon has been available: certain hydrocarbons that also exhibit semiconductor properties are now the new standard in OLED displays of mobile phones and television sets. Moreover, these “organic” semiconductors, as these hydrocarbons are also called, can also be used for solar cells or transistors. Their big disadvantage is their lack of stability: atmospheric oxygen quickly destroys these elements, which is why they need to be packaged in an airtight cover. A research team led by the physicist Serdar Sarıçiftçi from the Johannes Kepler University Linz has now achieved a breakthrough in solving this problem. In a project funded by the Austrian Science Fund FWF, the team managed to produce semiconductors related to the indigo pigment which is not only stable when exposed to air, but also under water.

Processability was the problem in using indigo as a semiconductor: it is almost insoluble, which, incidentally, partly explains its durability. Many methods to produce organic semiconductor elements do, however, require the material to be first dissolved in some way and then deposited on a carrier medium. Sarıçiftçi and his group managed to render the pigment soluble by binding volatile side groups to the indigo molecule. When heated above 100°C these side groups split off again.

Does this mean the entire field of organic semiconductors can now shift to indigo compounds? Sarıçiftçi sounds a note of caution: “Owing to the hydrogen bonds, indigo has strong luminescence-quenching properties.”This weak bond between molecules, which plays an important role in ice, has a disruptive effect on optical applications.

The function of solar cells, for instance, is based on irradiating light interacting with the material, which releases electrons and initiates a current. In indigo molecules, however, such “excited” electronic states are quickly dissipated and converted into heat before they can be used. That means that both solar cells and light-emitting diodes will be difficult to realise with the indigo family of compounds. “We are trying to work around this problem, but there is no real solution to it”, explains. Sarıçiftçi. This is an aspect he is currently researching. Transistors are not affected by such problems.

Sarıçiftçi perceives great potential for indigo materials in medical uses. “We are devoting particular attention to the bio-compatibility of indigo transistors. We were able to show that they can operate even under water at different pH levels. “This means they can be used for implants in human tissue. “It opens the door for bio-applications”, observes Sarıçiftçi.

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