A Miracle Material Difficult to Process
“We were actually looking for biodegradable semiconductor materials,” explains Sarıçiftçi. “In the process, we stumbled upon this biblical material known as indigo. Indigo and its derivatives exhibit true semiconductor properties.” It was no surprise that indigo showed great stability: “Indigo was used, for example, in the tombs of the pharaohs, where it is still visible after thousands of years, and the blue in denim is well known for its durability,” notes Sarıçiftçi.
Processability was the problem in using indigo as a semiconductor: it is almost insoluble, which, incidentally, partly explains its durability. However, many methods for producing organic semiconductors require the material to be dissolved in some way first and then deposited onto a carrier medium. Sarıçiftçi and his group managed to make the pigment soluble by attaching volatile side groups to the indigo molecule. When heated above 100°C, these side groups separate again.
That has removed the main obstacle to using indigo as a semiconductor, says Sarıçiftçi: "We see this stability of indigo as a game-changer. We encourage everyone working on organic transistors to focus on this class of materials from now on."
Unanswered Questions About Solar Cells and Light-Emitting Diodes
: Does this mean the entire field of organic semiconductors can now shift to indigo compounds? Sarıçiftçi sounds cautious: "Due to hydrogen bonding, indigo has strong luminescence-quenching properties." This weak bond between molecules, which plays a significant role in ice, has a disruptive effect on optical applications.
The function of solar cells, for example, relies on light irradiation interacting with the material, which releases electrons and initiates a current. In indigo molecules, however, such "excited" electronic states dissipate rapidly and are converted into heat before they can be used. This means that both solar cells and light-emitting diodes will be difficult to process using the indigo compound family. "We are trying to avoid this problem, but there is no real solution yet," Sarıçiftçi explains. This is an aspect that is currently being researched. Transistors are not affected by these problems.
a researcher in the field of implant electronics,
sees great potential for indigo materials in medical applications. "We are paying special attention to the biocompatibility of indigo transistors; we were able to demonstrate that they can function even underwater at different pH levels." This means they can be used for implants in human tissue. "It opens the door to bio-applications," Sarıçiftçi observes. Recently, his group published several articles on this topic in prestigious journals and obtained a patent. In 2014, they began organizing an annual conference on bioelectronics. (www.bioel.at).
The low cost of the base material could also be a decisive advantage. "This will be a compelling argument for future mass applications," Sarıçiftçi notes.
