Nanolaminate boost for flexible screens and printed electronics
The nanostructured gate dielectric is composed of a fluoropolymer layer followed by a nanolaminate made from two metal oxide materials. This serves as gate dielectric and simultaneously protects the organic semiconductor – which had previously been vulnerable to damage from the environment – and enables the transistors to operate with signficantly higher stability than inorganic versions, including 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 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, the Joseph M. Pettit professor in Georgia Tech’s School of Electrical and Computer Engineering (ECE) 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.”
A key part of the development is that the dielectric layer uses 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,” said Canek Fuentes-Hernandez, senior research scientist. “We developed complex nanolaminate barriers which could be produced at temperatures below 110 degrees Celsius and that when used as gate dielectric, enabled transistors to sustain being immersed in water near its boiling point.”
The architecture uses alternating layers of aluminum oxide and hafnium oxide, five of each repeated 30 times on top of the fluoropolymer, to make the dielectric. The oxide layers are produced with atomic layer deposition (ALD) and the resulting nanolaminate is 50nm thick and essentially 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 degrees Celsius. 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 but newer processes such as spatial ALD that uses multiple heads with nozzles delivering the precursors could speed up 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,” said Kippelen.
Internet of things (IoT) devices could also benefit from fabrication enabled 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. It can also be applied to flexible OLED displays.
“We will get better image quality, larger size and better resolution,” said Kippelen. “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.”
The demonstration used a model organic semiconductor with a relatively low carrier mobility of 1.6 cm2/Vs isn’t the fastest available. As a next step, the researchers are looking at organic semiconductors that provide higher charge mobility and testing the nanolaminate under different bending conditions, across longer time periods, and in other device platforms such as photodetectors.
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