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

Sept. 1, 2011
The push to create wearable electronic devices is gaining steam.

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. 

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

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

Sponsored Recommendations

Request a free Micro 3D Printed sample part

April 11, 2024
The best way to understand the part quality we can achieve is by seeing it first-hand. Request a free 3D printed high-precision sample part.

How to Tune Servo Systems: The Basics

April 10, 2024
Learn how to tune a servo system using frequency-based tools to meet system specifications by watching our webinar!

Motion Scan and Data Collection Methods for Electro-Optic System Testing

April 10, 2024
Learn how different scanning patterns and approaches can be used in measuring an electro-optic sensor performance, by reading our whitepaper here!

How Precision Motion Systems are Shaping the Future of Semiconductor Manufacturing

March 28, 2024
This article highlights the pivotal role precision motion systems play in supporting the latest semiconductor manufacturing trends.

Voice your opinion!

To join the conversation, and become an exclusive member of Laser Focus World, create an account today!