In the FOXIP project, two teams from the Swiss Federal Laboratories for Materials Science and Technology (Empa) attempted to print thin-film transistors with metal oxides on heat-sensitive materials such as paper and PET. The goal was ultimately not achieved, but those involved consider the project a success - thanks to a new printer ink and a transistor with a memory effect.
The bar was undoubtedly set high: In the research project "Functional Oxides Printed on Polymers and Paper" - FOXIP for short - the aim was to succeed in printing thin-film transistors on paper substrates or PET films. Electronic circuits with such elements play an important role in the growing Internet of Things, for example as sensors on documents, bottles and packaging - a global market worth billions beckons.
If it were possible to produce such transistors with inorganic metal oxides, this would open up new horizons. Compared to organic materials such as the semiconducting polymer polythiophenes, explains project leader Yaroslav Romanyuk from Empa's "Laboratory for Thin Films and Photovoltaics", the electrons in them are much more mobile. They could therefore significantly increase the performance of such elements and do not need to be protected against the influence of air and moisture with expensive encapsulation.
Heat as a challenge
The problem with inks with metal oxides: In order for them to become a stable transistor, they have to be sintered after printing - typically in an oven. Alternatively, light can be used for drying and sintering - for example with low-wave ultraviolet radiation or a xenon lamp: the printed layer is heated with very short flashes of light in order to protect the carrier material. Water, solvents and binders are then removed from the material.
An employee at the Empa Coating Competence Center (CCC) works on an inkjet printer used for printed electronic components
Nevertheless, such processes heat the carrier material to up to 200 degrees - far too hot for paper or PET, which begins to lose its strength at less than 80 degrees, while other plastics such as polyimides can withstand significantly higher temperatures.
From 2017 to 2021, experts from Empa, the "Soft Transducers Laboratory" at the École Polytechnique Féderale Lausanne (EPFL) and the "Polymer Nanotechnology Group" at the Paul Scherrer Institute worked on all steps of the process in a project of the "Strategic Focus Area - Advanced Manufacturing" (SFA-AM) research initiative, which was launched by the ETH Board: For example, on coatings to smooth the surface of paper, but also on ink formulations, irradiation ... - and achieved quite a few advances. However, the "ultimate wish", as Yaroslav Romanyuk says, to print functional thin-film transistors on paper was not fulfilled: the temperatures were too high, the material too rough. And the printed transistors on polymer films ultimately had too low an electrical output.
Many surprises
Flexible but robust: electronic circuits on a polyimide film from the Empa laboratory form synaptic transistorsDisappointed? No, says Jakob Heier from Empa's Functional Polymers department: "The project was by no means a failure." Not only because of new insights into technical details - but also because of unexpected side results: "It was a highly exciting project with many surprises." For example, there was an incident that was to have consequences - with the material graphene: conductive carbon in atom-thin layers, which is also well suited for printed transistors on flexible films.
One doctoral student in the team was not satisfied with the fact that graphene inks could no longer be printed at higher concentrations: The particles aggregate; they clump together - and a successful, thin film cannot be formed this way. Instead of using just one solvent, the employee tried a special emulsion of graphene and three solvents. But this coating also failed at the first attempt. However, when the ink was mixed evenly in the next attempt and then subjected to slight shear forces, the print succeeded.
Curious, the experts investigated the phenomenon and discovered that the shear forces fundamentally change the structure of the ink. The fine graphene leaflets in the liquid reform, so that van der Waals forces now become effective: relatively weak attractive forces between atoms or molecules. The result is a gel-like ink - without binders such as polymers, which otherwise ensure that the liquid retains its consistency and does not separate.
Process with market potential
A solution with practical benefits that also works at room temperature; the ink dries without heating. As it turned out, such van der Waals inks can be produced not only with graphene, but also with other two-dimensional substances for printing. The process has now been patented and, according to the experts, some companies are already showing interest in producing the coveted inks - all after a coincidence that the team pursued with healthy curiosity. This was not the only surprise in the FOXIP project, as Yaroslav Romanyuk explains. A field-effect transistor with an insulating layer of aluminum oxide, which was printed on a heat-resistant polyimide plastic in experiments, revealed a strange behavior. Instead of a constant signal, which would have been expected, rising waves appeared: The output signal became stronger because it "remembered" previous incoming signals. "This is actually undesirable when a transistor has such a memory," explains Romanyuk.
Van der Waals ink with the right consistency for extrusion printing
However, one student in the team had the idea of using the phenomenon in a different way: A transistor with such a memory effect works in a similar way to circuits in the human brain: synapses between nerve cells not only transmit signals, but also store them. Such a synaptic transistor could therefore be interesting for the vision of computers that mimic the human brain. But what could it do?
With Mozart's help
To explore its potential, the team built an electronic copy of the human auditory process together with the thin-film transistor - and fed it a popular Mozart melody: Rondo Alla Turca from Sonata No. 11 in A major. "It was supposed to be a lively piece," says Romanyuk with a grin. This experiment and further analyses showed that the synaptic function of the transistor is maintained from a few hertz up to almost 50,000 hertz - a significantly higher bandwidth than comparable printed transistors.
In contrast to the printed ink without binders, this basic research, which the team published in the online journal "Scientific Reports", does not yet have any concrete applications in sight. But on the road to new computer technologies, the insights may be a useful step that came as a surprise - as has often been the case in the history of science.
For Romanyuk and many other researchers, such coincidences are the salt in the soup, so to speak - especially in projects at the limits of what is feasible. "We deliberately set our goals very high," he says, "coincidences play a very big role! You set yourself a big challenge and then, suddenly and unexpectedly, these coincidences just happen."
Competence center for coatings
Closing the gap between laboratory research and industrial production for coatings: That is the goal of Empa's Coating Competence Center (CCC for short). Research is conducted there not only on printed electronics, but also on materials, processes and technologies for coatings: Methods with which thin layers are vapor-deposited onto substrates, or additive manufacturing, in which components are built up layer by layer. The CCC is structured as a "private-public partnership": The idea is that all partners along the value chain, from science to industry, work together to develop new technologies and find creative solutions.
