A group of researchers at the Laboratory of Organic Electronics, Linköping University, Sweden, have developed a thermoelectric organic transistor that demonstrates a strong dependence on temperature. The function is based on the use of poly(3-hexylthiophene-2,5-diyl) (P3HT) as the active semiconducting layer and poly(vinylphosphonic acid- co-acrylic acid) (P(VPA-AA)) as the polyanionic electrolyte insulator, according to the team.

The high sensitivity to heat of this electrolyte, more than 100 times greater than traditional thermoelectric materials, allows small areas of electrolyte, which acts as sensor, to be connected to the transistor circuit to create a "smart pixel," the researchers said.

The researchers forecast ionic thermoelectric sensors will go beyond the limitations of traditional thermopiles and pyroelectric detectors and that their work paves the way for new infrared-gated electronic circuits with potential applications in IR cameras, medical electronics and electronic-skins.

The heat-driven transistor builds on research that led to an organic supercapacitor that was reported a year ago. In the capacitor heat, potentially from direct sunlight, is converted to electricity that can then be stored in the capacitor until it is needed.

The liquid electrolyte consists of ions and conducting polymer molecules. The positively charged ions are small and move rapidly, while the negatively charged polymer molecules are large and heavy. When one side is heated, the small ions move rapidly towards the cold side and a voltage difference arises. This large Soret-induced open voltage gates the transistor. Hence, this ionic thermoelectric-gated transistor converts a change in temperature to a change in the drain current.

"When we had shown that the capacitor worked, we started to look for other applications of the new electrolyte," said researcher Xavier Crispin, in a statement.

The work was reported in a paper titled, "Ionic thermoelectric gating organic transistors," authored by Dan Zhao, Simone Fabiano, Magnus Berggren and Xavier Crispin, Linköping University, Campus Norrköping, Nature Communications 2017.

This article first appeared on EE Times Europe.