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| Unroll your display |
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Imagine a TV that is not just thin like a plasma screen, but thin like a birthday card. That lives in a narrow box near the ceiling and has a string you pull to unroll it.
Something from the future?
No, it has already happened. Martin Cobb of Surrey-based Trident Displays saw it. "I was shown a demo. It pulled down just like a projector screen," he says.
Trident is a display distributor and product design house. Cobb was on a visit to Taiwan-based Ritek as part of his brief to watch organic LED (OLED) display developments. "It is the biggest factory I have been in this side of Korea," says Cobb. "I've never seen investment of the like that has been put into OLEDs."
Investors in Ritek's OLED business, which include Intel, obviously see a future in OLEDs, as does the US government. It is providing military funding to USDC, the US display consortium, which shares out cash amongst companies with likely OLED technologies.
At the moment, flexible OLED display demonstrators like Ritek's are just that - demonstrators. Before glass OLED displays get big enough for TV use, let alone flexible enough for roll-up TV use, there are some significant hurdles to jump.
The first is display lifetime. OLED materials from all manufacturers have a life which is dependent on both how hard the display is driven, and what environment the material is operating in. A life of 10,000 hours for a display is considered commercially viable.
This may not seem much - under two years continuous use - but comparing it to the 250,000 mile life expectancy of a quality car (8,300 hours at 30mph), puts this into perspective.
Absolute life expectance is not actually the biggest issue with OLED as, unlike LCDs which use colour filters over identical pixels, OLEDs are vulnerable to differential aging.
"The big problem for colour is red, green and blue emitters degrade at different rates," says Cobb. "Two years ago, one firm was getting through four displays a day on their stand at a show." They had to swap displays as colour-shift was obvious within hours of switch-on even though the life of its weakest OLED material was rated at 2,000 hours, explains Cobb.
In glass displays, lifetime is becoming less of an issue.
Cambridge-based display technology firm CDT is developing polymer-based OLEDs which it calls PLEDs. Blue PLEDs have the shortest life on the CDT pallet.
"Blue life has increased eight or ten fold in the last 18 months," CDT marketing manager Terry Nicklin tells Electronics Weekly. "At the May SID conference this year we showed 35,000 hours lifetime [from 100cd/m² to half brightness for blue, last month we demonstrated 70,000 hours for blue."
These figures compare with 210,000 hours for red and 200,000 for green, he says.
Ritek, says Trident's Cobb, is claiming 50,000 hours for its monochrome production OLED displays once they are removed from their protective packaging.
The need for such packaging hints at another issue. OLED's chemicals and some of the electrode metals necessary to make an OLED display are ruined by exposure to water or oxygen.
Glass is virtually impervious to these, largely removing them from the life equation in ridged OLED displays except at the edge-seals. Plastics however are porous to both and need to be coated with impermeable barrier layers to block water and oxygen penetration.
Without effective barrier layers, flexible displays with existing and foreseen OLED materials are commercially dead ducks.
"Industry is looking for better plastics and barrier systems," says Janice Mahon, v-p of technology commercialisation at US OLED firm Universal Display Corporation (UDC).
To work, a barrier layer has to block while remaining flexible and transparent. UDC is working with Californian OLED barrier specialist Vitex Systems on the problem.
Vitex's solution is to use normally brittle metal and semiconductor oxides or nitrides in very thin layers interspersed with polymer layers.
The polymer layers compensate for feature height differences, allowing the barrier layers to be flat, and limit the bend radius of the barrier layers, preventing them cracking.
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| UDC's flexible OLED |
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As the barrier is too thin to be fully impervious, multiple layers are used forming a structure that is very slightly porous, although any penetrating molecule takes a very long time to get through. "GE and Dow Corning are also developing product technology in this area. All three are funded by USDC. There is some wonderful work going on," says UDC's Mahon.
"It is not a matter of if, it is a matter of when" oxygen and water ingress problems are solved, she says.
Unlike LCDs which only need a tiny charge current and are essentially voltage-driven, OLEDs need a current drive just like their conventional LED cousins.
Resistance of line and row-driving conductors is already an issue in large LCDs and with current drive it becomes a problem to which there is no ready solution, particularly if metre-size displays are to be made.
ITO (indium tin oxide), industry's favourite transparent electrode material, is too brittle for flexible OLEDs unless they are only bent slightly. Conductive polymer PEDOT is flexible, but too resistive.
One possible option is thin metallic lines deposited in between pixels, perhaps in a matrix of PEDOT to bridge any cracks.
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| Plastic Logic's silver nano-tracks |
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For conductors where transparency is not required, Cambridge-based Plastic Logic prints tracks with silver nano-particles, then laser anneals them to cut resistance. "One of the really hard things to do with a flexible substrate is to align lines," warns Plastic Logic's Cranch Lamble.
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| HP's prototype LCD on plastic substrate |
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Hewlett Packard has been developing LCDs on plastic substrates at its Bristol Labs and has skipped over the alignment issue by depositing metal-laced conductors in the same pass as it constructs other pixel features. How it does it, HP is not saying.
By adding two or more transistors to each pixel, an active matrix OLED display is possible. Not only does this reduce row and column drive requirements to something like LCD levels, it also allows OLED current to be delivered by power planes, one on top and one below the OLED layer.
With two-dimensional conduction, power planes will go some way to address voltage drop issues, but again suitable materials are not obvious - although a metal foil backing halves the problem.
An active matrix also reduces high peak currents needed by passive addressing, potentially increasing pixel life.
Unfortunately, conventional fast active matrix thin-film transistors (TFTs) are polysilicon which is not compatible with plastic.
"Low-temperature polysilicon processes need several hundred degrees centigrade," says UDC's Mahon. "We really need ultra-low temperature polysilicon which uses a very rapid laser anneal to convert amorphous silicon to poly. This is under development."
She also points out that metal foil substrates are much more resistant to temperature and are more stable mechanically. "Metals are at a very exciting early stage," she says.
So when will we see large commercial flexible colour OLEDs? "Three to five years", for small ones and "five to ten" before reel-to-reel processing really cuts the cost out of these, says Mahon. "Five to ten years for large ones."
"We learn to crawl, then walk, then run the marathon," she adds. "Large displays are running the marathon."
www.cdtltd.co.uk
www.ritek.com.tw
www.tridentdisplays.co.uk
www.universaldisplay.com
www.usdc.org
www.vitexsys.com