Bernard Kippelen and his research team at the Center for Organic Photonics and Electronics have demonstrated a new transistor for use on flexible plastic electronics, known as a top-gate organic field-effect transistor with a bilayer gate insulator. The transistor’s properties give it incredible stability while exhibiting good performance. Photo credit: Canek Fuentes-Herandez/Georgia Tech |
In the quest to develop flexible plastic electronics, one of the stumbling
blocks has been creating transistors with enough stability for them to function
in a variety of environments while still maintaining the current needed to
power the devices. Online in the journal Advanced Materials,
researchers from the Georgia Institute of Technology describe a new method of
combining top-gate organic field-effect transistors with a bilayer gate
insulator. This allows the transistor to perform with incredible stability
while exhibiting good current performance. In addition, the transistor can be
mass produced in a regular atmosphere and can be created using lower
temperatures, making it compatible with the plastic devices it will power.
The research team used an existing semiconductor and changed the gate
dielectric because transistor performance depends not only on the semiconductor
itself, but also on the interface between the semiconductor and the gate
dielectric.
“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 Center for Organic Photonics and Electronics and professor in Georgia
Tech’s School of Electrical and Computer Engineering.
The bilayer dielectric is made of a fluorinated polymer known as CYTOP and a
high-k metal-oxide layer created by atomic layer deposition. Used
alone, each substance has its benefits and its drawbacks.
CYTOP is known to form few defects at the interface of the organic semiconductor,
but it also has a very low dielectric constant, which requires an increase in
drive voltage. The high-k metal-oxide uses low voltage, but doesn’t
have good stability because of a high number of defects on the interface.
So, Kippelen and his team wondered what would happen if they combined the
two substances in a bilayer. Would the drawbacks cancel each other out?
“When we started to do the test experiments, the results were stunning. We
were expecting good stability, but not to the point of having no degradation in
mobility for more than a year,” said Kippelen.
The team performed a battery of tests to see just how stable the bilayer
was. They cycled the transistors 20,000 times. There was no degradation. They
tested it under a continuous biostress where they ran the highest possible
current through it. There was no degradation. They even stuck it in a plasma
chamber for five minutes. There was still no degradation.
The only time they saw any degradation was when they dropped it into acetone
for an hour. There was some degradation, but the transistor was still
operational.
No one was more surprised than Kippelen.
“I had always questioned the concept of having air-stable field-effect
transistors, because I thought you would always have to combine the transistors
with some barrier coating to protect them from oxygen and moisture. We’ve
proven ourselves wrong through this work,” said Kippelen.
“By having the bilayer gate insulator we have two different degradation
mechanisms that happen at the same time, but the effects are such that they
compenstate for one another,” explains Kippelen. “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
thereshold voltage and over time to an increase of the current. But if you
combine them, their effects cancel out.”
“This is an elegant way of solving the problem. So, rather than trying to
remove an effect, we took two processes that compliment one another and as a
result you have a result that’s rock stable.”
The transistor conducts current and runs at a voltage comparable to
amorphous silicon, the current industry standard used on glass substrates, but
can be manufactured at temperatures below 150°C, in line with
the capabilities of plastic substrates. It can also be created in a regular
atmosphere, making it easier to fabricate than other transistors.
Applications for these transistors include smart bandages, RFID tags,
plastic solar cells, light emitters for smart cards—virtually any application
where stable power and a flexible surface are needed.
In this paper the tests were performed on glass substrates. Next, the team
plans on demonstrating the transistors on flexible plastic substrates. Then
they will test the ability to manufacture the bilayer transistors with ink jet
printing technologies.
