Flexible displays: What's the holdup?

A few manufacturing issues must still be overcome before devices with screens that flex, bend or roll up hit the mainstream.

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Display manufacturers have been showing exciting prototypes of flexible electronic screens at technology trade shows for several years. So why are we all still using rigid, flat-panel LCDs? What happened to the displays that conform to curved surfaces, that can flex and fold like a newspaper or that you simply roll up when you're done using them?

It's not the display media that are keeping flexible displays off the market. Instead, the delay is caused largely by manufacturing issues, including how to affix thin film transistor arrays to a flexible substrate and how to produce the displays at large volumes cost effectively.

UDC prototyperollable OLED display
Prototype of a rollable OLED display from UDC. Source: UDC.

Then there's the issue of the rest of the electronics in a device. A flexible display embedded in a smartphone can't flex unless the rest of the electronics -- and the housing around it -- can move as well. But a flexible display could be designed to roll out of a redesigned phone, as Universal Display Corp. has demonstrated with its tube-shaped organic light-emitting diode (OLED) phone prototype.

This story accompanies our series on red-hot display technologies to watch in 2011.

Building a better backplane

High-performance display media used in personal computing and communications products, whether liquid crystal, OLED or e-paper technologies, require an active-matrix thin film transistor array to drive them. The traditional way to do this is by fabricating a layer of thin film transistors (TFTs) onto a glass panel, which forms the foundation, or substrate, on which the rest of the display is constructed (see "The display sandwich").

While a flexible backplane won't work very well with LCDs, which tend to distort the image when the screen bends, it can work for OLEDs and e-paper displays. "The hard part is, how do you make that array of thin film transistors onto something you can bend?" says Nicholas Colaneri, director of the Flexible Display Center (FDC) at Arizona State University.

Today there are two approaches to doing this. One, supported by Samsung, LG Display and a few other display manufacturers, takes the existing the silicon-on-glass etching processes used in LCD factories and applies it to a flexible substrate. But traditional inorganic semiconductors based on silicon or other non-carbon elements must be deposited at high temperatures onto the substrate, a process that works fine on glass but melts ordinary plastic.

ITRI flexible OLED
This flexible OLED display was created using ITRI's FlexUPD technology, which uses a plastic substrate that can withstand high temperatures. Source: ITRI.

Instead, these manufacturers are working with heat-resistant materials such as a paper-thin foil made from stainless steel. And Taiwanese research outfit Industrial Technology Research Institute (ITRI) has developed a plastic that can withstand the high temperatures of the deposition process without melting. The plastic layer is affixed to a glass backer during the fabrication process and then peeled off once the TFT array has been applied to it.

The advantage of this approach, ITRI says, is that it can be adapted to existing LCD manufacturing lines that fabricate the TFT on glass with relatively minor changes when compared to building an entirely new fabrication facility.

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