Researchers make plastic electronics breakthrough

The team combined top gate organic field effect transistors with a bilayer gate insulator, which allowed the transistor to perform with stability. They also found it could be mass produced in a regular atmosphere and could be created using lower temperatures, making it compatible with the plastic devices it will power. The findings have been published in the online journal Advanced Materials. "Rather than using a single dielectric material, as many have done in the past, we developed a bilayer gate dielectric," said Bernard Kippelen, director of the Centre for Organic Photonics and Electronics and a professor at Georgia Tech. "This was made up of a fluorinated polymer known as CYTOP and a high-k metal oxide layer created by atomic layer deposition." Kippelen noted that whilst CYTOP is known to form few defects at the interface of the organic semiconductor, it also has a very low dielectric constant, which requires an increase in drive voltage. On the other hand, the high-k metal oxide uses low voltage, but doesn't have good stability because of a high number of defects on the interface. "This meant there were two different degradation mechanisms happening at the same time," he explains. "But the effects are such that they compensate for one another. So if you use one it leads to a decrease of the current, if you use the other it leads to a shift of the threshold voltage and over time to an increase of the current. But if you combine them, their effects cancel each other out." The team performed a battery of tests to see how stable the bilayer was. They cycled the transistors 20,000 times, tested it under a continuous bias stress and even placed it in a plasma chamber for five minutes, but there were no signs of degradation. "When we started to do the test experiments, the results were stunning," commented Kippelen. "We were expecting good stability, but not to the point of having no degradation at all." The transistor conducts current and runs at a voltage comparable to amorphous silicon, the current industry standard used on glass substrates. It can also be manufactured at temperatures below 150^°C in line with the capabilities of plastic substrates. Possible applications include smart bandages, rfid tags, plastic solar cells and light emitters for smart cards.