Georgia Tech’s Canek Fuentes-Hernandez (left) and Bernard Kippelen (right) examine an organic thin-film transistor created with the new nanostructured gate dielectric. Photo: Rob Felt, Georgia Tech.
Georgia Tech’s Canek Fuentes-Hernandez (left) and Bernard Kippelen (right) examine an organic thin-film transistor created with the new nanostructured gate dielectric. Photo: Rob Felt, Georgia Tech.

A nanostructured gate dielectric – comprising a fluoropolymer layer beneath a nanolaminate made from two metal oxide materials – may have addressed the most significant obstacle to expanding the use of organic semiconductors in thin-film transistors. This nanostructure can both serve as a gate dielectric and protect the organic semiconductor, which had previously been vulnerable to damage from the ambient environment, allowing the resultant thin-film transistor to operate with unprecedented stability.

This stability is comparable to that of thin-film transistors made with inorganic materials, allowing these new transistors to operate in ambient conditions – even underwater. Organic thin-film transistors can be made inexpensively at low temperature on a variety of flexible substrates using techniques such as inkjet printing, potentially opening up new applications that take advantage of simple, additive fabrication processes.

"We have now proven a geometry that yields lifetime performance that for the first time establish that organic circuits can be as stable as devices produced with conventional inorganic technologies," said Bernard Kippelen, a professor in the School of Electrical and Computer Engineering (ECE) at Georgia Institute of Technology and director of Georgia Tech's Center for Organic Photonics and Electronics (COPE). "This could be the tipping point for organic thin-film transistors, addressing long-standing concerns about the stability of organic-based printable devices."

The research, which is the culmination of 15 years of development within COPE and was supported by sponsors including the US Office of Naval Research, the US Air Force Office of Scientific Research and the National Nuclear Security Administration, is reported in a paper in Science Advances.

Transistors comprise three electrodes. The source and drain electrodes pass current to create the ‘on’ state, but only when a voltage is applied to the gate electrode, which is separated from the organic semiconductor material by a thin dielectric layer. A unique aspect of the architecture developed at Georgia Tech is that this dielectric layer is made up of two components: a fluoropolymer and a metal-oxide layer.

"When we first developed this architecture, this metal oxide layer was aluminum oxide, which is susceptible to damage from humidity," explained Canek Fuentes-Hernandez, a senior research scientist and co-author of the paper at Georgia Tech. "Working in collaboration with Georgia Tech professor Samuel Graham, we developed complex nanolaminate barriers which could be produced at temperatures below 110°C and that when used as gate dielectric enabled transistors to sustain being immersed in water near its boiling point."

The new Georgia Tech architecture uses alternating layers of aluminum oxide and hafnium oxide – five layers of one, then five layers of the other, repeated 30 times atop the fluoropolymer – to make the dielectric. The oxide layers are produced with atomic layer deposition (ALD). The nanolaminate, which ends up being about 50nm thick, is virtually immune to the effects of humidity.

"While we knew this architecture yielded good barrier properties, we were blown away by how stably transistors operated with the new architecture," said Fuentes-Hernandez. "The performance of these transistors remained virtually unchanged even when we operated them for hundreds of hours and at elevated temperatures of 75°C. This was by far the most stable organic-based transistor we had ever fabricated."

For the laboratory demonstration, the researchers used a glass substrate, but many other flexible materials – including polymers and even paper – could also be used.

In the lab, the researchers used standard ALD growth techniques to produce the nanolaminate. But newer processes referred to as spatial ALD – utilizing multiple heads with nozzles delivering the precursors – could accelerate production and allow the devices to be scaled up in size. "ALD has now reached a level of maturity at which it has become a scalable industrial process, and we think this will allow a new phase in the development of organic thin-film transistors," Kippelen said.

One obvious application is to produce the transistors that control pixels in the organic light-emitting displays (OLEDs) used in such devices as the iPhone X and Samsung phones. These pixels are currently controlled by transistors fabricated with conventional inorganic semiconductors, but with the additional stability provided by the new nanolaminate, they could perhaps be made with printable organic thin-film transistors instead.

Internet of things (IoT) devices could also benefit from the fabrication made possible by the new technology, allowing production with inkjet printers and other low-cost printing and coating processes. The nanolaminate technique could also allow development of inexpensive paper-based devices, such as smart tickets, that would use antennas, displays and memory fabricated on paper through low-cost processes.

But the most dramatic applications could be in very large flexible displays that could be rolled up when not in use.

"We will get better image quality, larger size and better resolution," Kippelen said. "As these screens become larger, the rigid form factor of conventional displays will be a limitation. Low processing temperature carbon-based technology will allow the screen to be rolled up, making it easy to carry around and less susceptible to damage.

For their demonstration, Kippelen's team – which also included Xiaojia Jia, Cheng-Yin Wang and Youngrak Park at Georgia Tech– used a model organic semiconductor. This material has well-known properties, but, with carrier mobility values of 1.6cm2/Vs, isn't the fastest available. As a next step, the researchers would like to test their process on newer organic semiconductors with higher charge mobilities. They also plan to continue testing the nanolaminate under different bending conditions, across longer time periods and in other device platforms such as photodetectors.

Though carbon-based electronics are expanding their device capabilities, traditional materials like silicon have nothing to fear.

"When it comes to high speeds, crystalline materials like silicon or gallium nitride will certainly have a bright and very long future," said Kippelen. "But for many future printed applications, a combination of the latest organic semiconductor with higher charge mobility and the nanostructured gate dielectric will provide a very powerful device technology."

This story is adapted from material from Georgia Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.