Organic electronics printed on CDs, DVDs

Ashley N. Paddock, ashley.paddock@photonics.com

A new patterning technique uses commercially available technology to print conducting polymers using an infrared laser onto standard CDs and DVDs, a feat that could propel current proof-of-concept organic devices – organic LEDs, thin-film transistors and microactuators – into large-scale manufacturing.

Developed by engineers at Hewlett-Packard, LightScribe uses the infrared laser inside a CD/DVD drive to record data onto a CD or DVD as well as to print labels containing text and images onto the surface of the disks. To make these labels, the laser pulses up and down to chemically activate a specialized dye coating on the disk’s surface.

Instead of printing on this specialized coating, researchers at the University of California and the University of Wollongong in Australia covered the disk with a film of conducting polyaniline nanofibers, which then can be directly printed upon. The scientists found an unusual photothermic effect that occurs when light from the infrared laser is absorbed by the nanostructured polyaniline. The study was published online July 5 in Nano Letters (doi: 10.1021/nl2011593).

Lithography

“Electronic flexible organics can eventually replace, if not be complementary to, metal base electronics, but in order to help these new types of materials achieve this goal, we still require a simple and inexpensive lithography technique,” said Veronica Strong, a UCLA doctoral candidate. “We ideally wanted to develop a simple lithography technique that is not only comparable to other more elaborate methods, but that also does not require expensive masks, additional processing, expensive materials or even cleanroom operations.”

The generated heat “laser welds,” or cross-links, the molecular chains and supramolecular fibers together to change the surface morphology of a small area of the polymer. Upon completion of the laser treatment, the welded area of the polymer changes from a tangled nanofibrous mat to a smooth, continuous film, a result of the chemical cross-linking. Because the nanofibers are poor heat conductors, the heat generated from the laser does not spread beyond the laser lines, resulting in a well-defined separation between the welded and nonwelded areas.

The welding method provides both high resolution and a high degree of control over the conductivity and optical properties of the welded conducting polymer, which is not possible with previous approaches to patterning. The researchers found that welded areas demonstrate a significant decrease in film conductivity, which they have attributed to the loss of dopants and pi-conjugation during the cross-linking process. In addition, they demonstrated how printing in gray scale could tune the polymer’s conductivity, with lighter gray-scale colors possessing higher conductivity. With the ability to print gray-scale colors from white to black, they could tune the film from semiconductor to insulator, representing a change in conductivity of about seven orders of magnitude.

This new technique could lead to applications through doping, the scientists say. They found that the welded areas no longer responded to reversible doping or de-doping with acids or bases, while the nonwelded areas continued to be affected by doping. This property could be useful for patterning rechargeable electrodes for batteries or supercapacitors, they noted.

The researchers plan to further develop the idea by fabricating electronically active devices. With the simple welding technique, they predict that it will serve as an important step toward patterning polymer-based organic electronics on a large scale.

“Currently, we are looking into other photothermically active and low-luminescence-efficiency materials that can benefit from this simple type of lithography,” Strong said. “We are also considering using the welding properties generated from laser-welding polyaniline to use in dielectrics and membranes.”