WEARABLE PHOTONICS: Polymer light-emitting device stretches up to 45%

The push to create wearable electronic devices is gaining steam. At the forefront of this concept, which includes consumer electronics as well as medical monitoring devices, is the idea that electronics can be made to stretch. The end results could range from small stick-on patches that display heart rate to shirts that serve as mobile phones or computers. Crucial to all these, however, is the display—it must be stretchable, too.

Although rigid light emitters like LEDs can be made into flexible displays, the result is too bulky to wear like clothing. If possible, the best approach is to make a display’s light source itself stretchable. Researchers at the University of California Los Angeles (UCLA) have done just that by fabricating an intrinsically stretchable polymer organic LED (OLED) device that uses carbon nanotubes as the conductor.¹ While the prototype is nonpixelated, it is truly elastic: It can be reversibly stretched by up to 50% strain without damage (see figure).

Made on glass, then peeled off

Polymer light-emitting electrochemical cells (PLECs) were spin-cast on glass, laminated, and then peeled off; the layer was then roll-laminated with additional layers to form the finished prototype. The luminescent layer, which includes a blue emissive fluorene copolymer, an ionic conductor, and a salt, was sandwiched between two single-walled carbon-nanotube (SWNT)-polymer electrodes. When PLECs operate, they form p-i-n junctions that allow the light-emitting polymer to make electrical contact with both the SWNT anodes and cathodes.

In addition to being stretchable, the finished device can be transformed and locked into many shapes by heating it to elevated temperatures above the glass-transition temperature of the polymer (70ºC). The device can be pulled into the desired shape and then cooled to room temperature; the newly shaped material maintains its stretchability. This shape-memory quality is important for future manufacturability of practical devices.

The PLEC emits light with a sky-blue color. Its emission stability was tested over a seven-hour period under constant-current operation: The intensity reached a peak of 170 cd/m² at 30 min, then gradually decreased to 96 cd/m² at 7 h. The operating voltage followed the intensity, which suggests an approximately constant power efficiency (lumens per watt). The researchers attribute this change in brightness to an excess of ionic conductor added to the PLEC, with the solution being the uses of a light-emitting polymer conductive enough to eliminate the need for an added ionic conductor.

Interestingly, although the emission intensity drops as the device is stretched, the luminous efficiency increases. In addition, the light becomes polarized during stretching: At a 45% strain, the intensity of light polarized along the stretch direction was up to three times that of the perpendicular direction. 

REFERENCE
1 Z. Yu et al., Adv. Mat., doi:10.1002/adma.201101986 (2011).