Winning JSID Outstanding Student Paper Describes Solution-Processed Metal Oxide on Flexible Foil

by Jan Genoe and Jenny Donelan

The current flat-panel active-matrix organic light-emitting-diode (AMOLED) display market is dominated by low-temperature polysilicon (LTPS) backplane technology on rigid glass plates.  LTPS as a backplane for larger display sizes, such as TVs, has some drawbacks.  The excimer laser used for LTPS does not scale to larger sizes.  LTPS is fairly costly to implement and also requires relatively high processing temperatures (>300°C).  These temperatures limit the integration of LTPS directly onto desirable, lighter weight substrates such as flexible foil or plastic, which degrade in high heat.

In order to create a backplane technology that could accommodate fabrication on flexible foil at a low post-annealing temperature, a multidisciplinary, cross-organizational student research team from Europe demonstrated the use of high-performance solution-based n-type metal-oxide thin-film-transistors (TFTs).  These were fabricated directly on polyimide foil at a post-annealing temperature of only 250°C.  This work, as detailed in the paper, “Solution-processed and low-temperature metal oxide n-channel thin-film transistors and low-voltage complementary circuitry on large-area flexible polyimide foil,” appeared in the October 2012 issue of the Journal of the SID, earning its authors the award for the JSID Outstanding Student Paper of 2012.

The team consisted of Maarten Rockelé, Duy-Yu Pham, Jürgen Steiger, Silviu Botnaras, Dennis Weber, Jan Vanfleteren, Tom Sterken, Dieter Cuypers, Soeren Steudel, Kris Myny, Sarah Schols, Bas van der Putten, Jan Genoe, and Paul Heremans.  Members hailed from institutions including the research center IMEC in Belgium, specialty chemical manufacturer Evonik DeGussa GmbH in Germany, Katholieke Universiteit Leuven and Universiteit Gent in Belgium, and the independent research entity the Holst Center in The Netherlands.

Developing and Executing the Idea

This research group had already been working for more than 10 years on the topic of thin-film electronics on foil, explains Genoe.  “We first started exploring RFID tags using pentacene as an organic p-type semiconductor,” he says.  “In parallel, we started exploring n-type semiconductors on flexible foil, both organic and oxide n-type semiconductors.  When n- and p-type semiconductors became available, it was clear that an attempt could be made to merge both in order to yield a CMOS (complementary metal-oxide semiconductor) technology with superior performance.”  He adds that the approach described in the paper, together with Evonik’s work as part of the EU-financed R&D project ORICLA (formed to develop RFID tags based on hybrid organic-oxide complementary thin-film technology), is among several parallel tracks the team has been investigating.

The authors were also intrigued by recent research on metal-oxide semiconductors processed from solution (as opposed to methods such as vacuum sputtering).  Advanced solution-processing techniques like printing promised to enable simple, inexpensive, high throughput.  However, the annealing temperature required to convert soluble precursors into semiconductor oxide films was still in the 300–500°C range and hence difficult to process on foil.

The researchers succeeded in fabricating spin-coated indium-based oxide n-type TFTs at annealing temperatures as low as 250°C.  These n-type TFTs were processed directly on top of polyimide foil, resulting in high-performance (saturation mobilities exceeding 2 cm²/V-sec) and low-voltage flexible uni-polar digital circuitry.  To make the flexible line-drive circuitry more robust, a hybrid complementary inorganic–organic technology was developed.  The hybrid technology was further optimized and for the first time transferred to a flexible substrate, the polyimide foil.

The authors claim that both in terms of stability and speed, this hybrid complementary technology is directly applicable for complex digital circuitry, such as the line-drive circuitry of future rollable AMOLED displays, for which it can be easily embedded at the borders of the display.  In this way, the conventional and rigid driving circuitry can be replaced, which should dramatically improve the flexibility of the display.

Challenges

“Lowering this annealing temperature to 250°C, while maintaining high performance and reliable transistors directly on foil, was by far the most challenging part of this work,” says Genoe.  “Moreover, we integrated these solution-based metal-oxide transistors together with our baseline organic transistors into a scalable hybrid complementary technology.  The main challenge here was the development of an integration path where both the oxide and organic TFTs are processed next to each other and with a high density, without sacrificing the individual transistor performances.”

Award-Winning Results and a Look Ahead

The paper easily itself earned the annual outstanding student paper award.  “I think the committee was impressed with a high level of technical sophistication and the novelty of the approach taken by the authors.  The group also provided a clear explanation of their work and provided a comprehensive set of results,” says JSID editor John Kymissis.

“The results we obtained with the solution-based metal-oxide transistors on polyimide foil are state of the art,” says Genoe¸ adding, “We are ready to make robust flexible line-drive circuitry for a future generation of flexible AMOLED displays.  In the framework of the ORICLA project, we successfully realized the most complex thin-film RFID systems on foil so far.  We think that thin-film electronics on foil, particularly for thin-film RFID tags, is a very exciting topic from the point of R&D as well as from the business prospect point of view.”  In recent years, he says, the speed of development in this particular field has lagged behind the promises of the R&D community.  But now, the consistent progress the group has demonstrated in realizing complex and fast circuits on foil makes them optimistic that this technology can be pushed to commercialization.  •