PolymerVision showcased their truly rollable 6" display technology capable of showing animated images (10Hz) at the IDTechEx Printed Electronics USA event earlier this month - the world's largest event on the topic. This is a significant advance and promises to open the door for a plethora of different printed components and materials. The potentially printed applications include ITO replacement, printed TFTs, printed emissive or reflective displays etc.
Rollable displays dislodge the notion that a big screen necessarily corresponds to a big device. This is because an unbreakable large screen can now be rolled out from a tightly-rolled scroll. These displays therefore comfortably plug the market gap between higher volume mobile screen and lower volume laptop or desktop screens.
Dr Edzer Huitema (CTO) explained the technology behind the display in his talk. The rollable displays are made using a bond-/de-bond technology. Here the display is initially processed when attached atop a rigid carrier substrate. The trick here lies in a proprietary de-bonding process which releases the very thin (0.1mm) and light (7 grams) display from the substrate without damage.
The displays are currently driven using a proven and mature organic TFT technology. Here mobility values in the range of 0.2-0.3 cm2/Vs are sufficient (the dominant backplane technology in the LCD world is hydrogenated amorphous silicon which has a mobility of 0.5-1 cm2/Vs). Moreover, these TFTs are optimised to have a turn-on voltage of around 0 V, resulting in low-voltage and low-power operation. Critically also, this devices are encapsulated from the ambient in order to improve their stability during operation.
The initial products have a reflective electrophoretic display. This is the case because electrophoretic display are bi-stable, meaning that power consumption can be kept at a minimum in applications requiring mainly static images or low refresh rates. But more critically, electrophoretic displays are more tolerant of variations in the TFT backplane process. This means that long lifetimes can be achieved even when the TFT backplane is not highly stable or very high performance. Faster refresh rate may require a move towards an electrowetting display technology.
Moving towards a coloured emissive OLED display however represents significant challenges. In particular, the backplane technology must improve to meet the stringent performance and stability requirements that OLED displays impose. This is the case because OLED are current driven, whereas liquid crystal and electrophoretic ones are voltage drive. This means that high mobility values are required to supply the current, and high stability is required to keep the changes in current to a minimum.
The answer to realising OLED display lies in improving oxide semiconductor TFTs, which are the main candidate for the next-generation TFT technology. The oxide semiconductor TFTs offer high mobility (>10 cm2/Vs) and high stability in the dark. There are however challenges remaining in terms of encapsulation, dielectric choice, photostability, etc.
Potential for Printed Electronics
One of the most immediate potential for printed electronics is in ITO replacement for touch screen. ITO is brittle and therefore not a good candidate for flexible electronics. This opens up a platform for a variety of different conductive inks materials including silver and gold, and different shapes including nanoparticles, nanowires and various flakes.
In the long term the backplane OLEDs and TFTs can become printed. The challenges here are significant however, mainly because these devices are highly sensitive to their interfacial properties, which is hard to control.
Printed electronics are currently making progress, particularly with printed nanosilicon semiconductors. But we anticipate that printed TFTs will first target low-performance applications, which neither require high TFT counts nor high-performance TFTs.
For more information, Printed Electronics Europe 2012 in Berlin on April 3-4 will feature candid discussions and presentations comparing ITO replacement alternatives. Printed Electronics Europe 2012.