Indigo: From ancient dye to modern high-performance organic electronics circuits

We propose here to use the ancient dye Indigo as a high performance organic semiconductor in organic electronic circuits based on organic field effect transistors (OFETs). In the ancient history of Indigo dye and its derivative Tyrian Purple played a major role due to their natural origin and stability. Recent results from our group showed that Indigo and Tyrian Purple perform as stable p- and n-type organic semiconductors. Due to hydrogen bonding, Indigo and Tyrian Purple form highly-ordered crystalline thin films which stack very regularly enabling efficient charge transport along the stacking direction. Such films have been used to fabricate high-performance organic field-effect transistors which even show ambipolar charge transport, i.e. electrons and holes are transported in positive and negative gate voltages, respectively. With performance on-par with the best available organic semiconductors, Indigoids demonstrate the potential of sustainable electronics based on biodegradable and biocompatible materials. It is the purpose of this research to bring basic electronic circuits with megahertz switching frequencies and operate in air based on Indigo OFETs.

Indigo is one of the oldest dye stuff. The history of Indigo goes back to antic cultures of Old Egypt and far east. This substance was named after the plant, indigofera tinctoria, which grows in eastern India. In Europa was this molecule extracted from Waid. This dye was preferred because of its exceptional stability and biocompatibility. The product of indigo after heating to high temperatures is called aniline (Annil in portugese language means blue color of Indigo. Aniline was the main chemical product, which lead to the development of modern chemical industry in Germany 150 years ago. Many of the major chemical companies started their names containing the word Aniline (like BASF and AGFA etc). We have been searching for bio-compatible and bio-organic Semiconductors since 2006. After many studies on other systems we discovered in 2012 that Indigo and its derivatives are promising organic semiconductors. The body of scientific work coming from our Institute since 2012 is not limited to but mainly based on this project of FWF. We have fabricated and characterized many different electronic devices like diodes, solar cells, detectors, light emitting diodes, transistors and inverters, using these indigo derivatives. These electronic devices using indigo-derivatives are competitive in performance to the state of art organic electronic devices and by far more superior in terms of environmental stability. We can safely claim that these indigo derivatives may play the role of game changer in the whole area of organic electronics. We have also shown that functionalization of the hydrogen bonds lead to soluble indigo derivaties. Furthermore, we have investigated the performance of such organic transistors under water in different pH values, resulting in remarkable stability of such devices even in harsh under water conditions. This discovery is especially important for the introduction of such devices for integration in living organisms and human body. This created a focus point in our research which can be widely described as "bio-integrated electronic". Since 2014 we have established an international winterschool on this topic, which takes place every March in Tyrol (see www.bioel.at). Already repeated for the third time, this international winterschool established to be the most important meeting in the field of "bio-organic electronics". Last but not least, with patent applications we tried to protect some IPR of this research and companies show ample interest in applying these results in industrial scale.