Stretchy Displays Sport Better Resolution

Ultrathin OLEDs still shine when stretched

3 min read

A glowing square of material shines red on the top of a persons hand as it is pulled up by another hand.

A stretchable OLED that conforms to the skin can be used as a phototherapeutic patch.

Sangin Hahn/KAIST/Nature Communications

Stretchy displays could be used in new kinds of health-monitoring devices and wearable computers, or make it possible to turn curved or spherical objects into touchscreens and lights. But pulling on these displays typically reveals gaps between islands of pixels.

Now a team of researchers has found a way around this resolution problem in a stretchy organic light-emitting diode (OLED) display. Their design includes hidden ultrathin OLEDs that sit in the folds of a display, popping out when the device is stretched. Seunghyup Yoo, an engineer who develops organic electronics at the Korea Advanced Institute of Science and Technology (KAIST), says these hidden OLEDs make for a display filled with a higher proportion of pixels. When stretched to a strain of 30 percent along two axes, 87 percent of the device still emits light, maintaining most of its resolution. This work is described in the journal Nature Communications.

There are several approaches to making stretchy electronics. Researchers can chemically engineer novel electronic and optical materials that are inherently stretchy, for instance. Another way is to pattern a stretchy surface with rigid “islands” of electronic devices made from traditional materials and connect them with serpentine wires.

Four rows of images that individually spell out "Display" show the initial and stretched states, then proposed stretched state to show how clearly the word can then be read by comparison.An OLED display with extra light-emitting area hidden in its folds loses less resolution when stretched.Sangin Hahn/KAIST/Nature Communications

Reached over email, Yoo said that the key to making this design work is the use of ultrathin OLEDs. Working with islands of ultrathin OLEDs, which are inherently flexible, allows for a middle path—the devices are robust enough to survive getting folded into hidden areas of the device. Donggyun Lee, a Ph.D. student in Yoo’s lab, came up with the idea. At the time, he was using ultrathin OLEDs to make phototherapy devices. It took the team about three years to make the stretchy display idea work, says Yoo.

To make the devices, Yoo’s team starts by molding the display’s stretchy substrate using an elastomer. The substrate is patterned with a series of square islands that stick up from the surface. Separately, the team creates an array of OLEDs on a glass sheet. The OLEDs are made using conventional materials, but they are carefully engineered and encased so that they will be mechanically robust. Then the researchers bring the two together. The elastomer substrate is prestretched along 4 axes, and the OLED array is carefully placed on top of it. Some of the light-emitting area falls on the islands and some on the parts in between. When the stretch is relaxed, this “hidden active area” gets folded in between the islands.

When the resulting display is stretched, its active area decreases by about 10 percent; previous devices that used this island design lose about 60 percent of their resolution.

Two diagrams compare a conventional stretchable OLED with the proposed version.Previous stretchable displays that place OLEDs on islands become patchy and lose resolution when stretched [left]. A new design that hides active display area in folds between islands maintains 87 percent active OLED coverage when stretched [right].Nature Communications

“It’s very interesting work, based on a clever idea and executed at a high level of technical sophistication,” says John Rogers, a materials scientist at Northwestern University who works on stretchable electronics.

So far, Yoo says, the pixel density of these displays is not very high, measuring about 11 pixels per square inch. By contrast, most smartphone screens offer 300 to 450 ppi. He says the team was limited by their lab equipment, but since they used conventional OLED materials, the devices should be amenable to mass manufacturing—and that using higher quality equipment should also lead to better resolution. He says the resolution could be improved to 100 to 200 ppi by miniaturizing the islands and making each one an individual pixel.

Yoo notes that the first applications of flexible OLEDs were not in foldable phones but in curved screens. “We can learn a lesson from the past,” he says. Similarly, stretchy screens might find their earliest applications in covering rigid objects, but they should make it possible to extend to 3D form factors like spheres. Yoo also hopes stretchy OLEDs will enable new kinds of wearable electronics. His group is currently working on integrating the stretchable displays into health-monitoring devices and biometric sensors.
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