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New cost-effective system solutions to drive growth of flexible electronic devices

New cost-effective system solutions to drive growth of flexible electronic devices

29-Sep-14

 Basic electronic structure is composed of a substrate, backplane electronics, a front plane, and encapsulation. To make the structure flexible, all the components must bend up to some degree without losing their function. Two basic approaches have been adopted to make flexible electronics, that is; transfer and bonding of completed circuits to a flexible substrate and fabrication of the circuits directly on the flexible substrate.

Fraunhofer FEP has worked on cost-effective system solutions for flexible electronics- a roll-to-roll process line which enables the application of organic materials for OLED (organic light-emitting diodes), OPD (organic photodiodes) or OPV (organic photovoltaic) on flexible substrates in one complete technology. The process includes the structuring, automatic inspection of the initial substrates, the vapor deposition of the organics and, finally, the encapsulation of the coated films or glasses. Organic electronics certainly require flexible electrical contacts. Therefore, Fraunhofer FEP implemented an additional printing process of metal contacts for the reliable contacting of, for example, large-area flexible OLED on metal, polymer and thin glass substrates. In cooperation with printing paste manufacturers and other suppliers, e.g. machine manufacturers, adhesive manufacturers, encapsulation film suppliers, the scientists are now able to develop optimized products for required process steps under production conditions. Dr. Jacqueline Bruckner, Project Manager Surface Analysis for the roll-to-roll organic technology, says: "Our customers have different requirements to design and mechanical stability of devices. With our know-how and our process equipment we provide an unique development platform for all these demands." For example, there are various solutions for the device contacting. Currently, several contacting solutions with flat ribbon cable, like ACF (Anisotropic Conductive Film)-Bonding, ACA (Anisotropically Conductive Adhesive)-Bonding or ACP (Anisotropic Conductive Paste)-Bonding are evaluated.

The first flexible display device based on graphene has been unveiled by scientists in the UK, who say it is the first step on the road towards next generation gadgets that can be folded, rolled or crumpled up without cracking the screen. The device is the result of a collaboration between Plastic Logic, a company that specialises in flexible displays, and researchers led by Andrea Ferrari at the University of Cambridge. Although others have successfully used graphene to make screen components before, this is the first example of a flexible screen that uses graphene-based electronics. What we have done here is to include graphene in the actual backplane pixel technology,says Ferrari. This shows that in principle the properties of graphene - conductivity, flexibility and so on - can be exploited within a real-world display.' The prototype is an electrophoretic display containing the kind of electronic ink found in e-readers that works by reflecting - rather than emitting light. Plastic Logic have been working on making these displays flexible for some time by replacing the glass with bendy plastic, and using non-brittle components in the electronic layer. Graphene is an ideal material for this, as it is more flexible and more conductive than the metals currently used. The team managed to make the graphene electrode in a way that is compatible with electronics manufacturing, using solution processing rather than chemical vapour deposition, which often requires temperatures exceeding 1000°Celcius "All the major companies are trying to make bendable and flexible gadgets, says Ferrari. "We think that graphene will be a powerful addition to that, and if we manage to make the process easy, scalable and cheap enough, then it should be considered very strongly by industry. As current displays go, the team's prototype is basic, capable of showing images in black and white at a resolution of 150 pixels per inch - akin to that of a basic e-reader. But Ferrari's team are working on applying the same technology to make a graphene-based LCD and OLED displays like those used in smartphones and tablets, capable of showing full colour images and playing video. Their goal is to have these ready within the next 12 months.

Researchers from the University of Texas at Austin and Northwestern University have demonstrated a new method to improve the reliability and performance of transistors and circuits based on carbon nanotubes (CNT), a semiconductor material that has long been considered by scientists as one of the most promising successors to silicon for smaller, faster and cheaper electronic devices. The result appears in a paper published in the journal Applied Physics Letters from AIP Publishing.
To overcome the drawbacks of single-walled carbon nanotube field-effect transistors and improve their performance, the researchers deposited PVDF-TrFE on the top of self-fabricated single-walled carbon nanotube transistors by inkjet printing, a low-cost, solution based deposition process with good spatial resolution. The fluoropolymer coated film was then annealed or heated in air at 140° Celcius for three minutes. Later, researchers observed the differences of device characteristics. We found substantial performance improvements with the fluoropolymer coated single-walled carbon nanotube both in device level and circuit level, Dodabalapur noted. On the device level, significant decreases occur in key parameters such as off-current magnitude, degree of hysteresis, variation in threshold voltage and bias stress degradation, which means a type of more energy-efficient, stable and uniform transistors with longer life time. On the circuit level, since a transistor is the most basic component in digital circuits, the improved uniformity in device characteristics, plus the beneficial effects from individual transistors eventually result in improved performance of a five-stage complementary ring oscillator circuit, one of the simplest digital circuits "The oscillation frequency and amplitude [of the single-walled carbon nanotube ring oscillator circuit] has increased by 42% and 250% respectively" said Dodabalapur. The parameters indicate a faster and better performing circuit with possibly reduced power consumption. The improvements have been attributed to the polar nature of PVDF-TrFE.

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