Industrial + Specialty Printing

This article provides an in-depth look at what it takes to print organic light-emitting diodes successfully.

By Markus Tuomikoski, Riikka Suhonen

The successful launch of OLED flat-panel displays into the high-end market is a reality, and companies involved in solid-state lighting are now releasing their first OLED-based products. Both of these applications benefit from the unique properties of the technology as a thin, flat, area source of diffuse light. Concurrently, both of these applications also envision using the most appealing feature of OLEDs: their flexibility.

Organic light-emitting diodes (OLEDs) traditionally are divided into two classes: small-molecule and polymer-based OLEDs, according to the active materials used in the OLED stack. Commercially available products are based mostly on small-molecule materials because of their enhanced performance, stability, and luminous efficiency. Vacuum-based deposition techniques are used in current manufacturing processes, with glass as a substrate, to ensure adequate product life. Even though a rigid glass substrate functions as an excellent barrier and protects the organic materials from deg-radation, it also inhibits the flexibility of the organic materials.

Vacuum-based techniques currently used in the deposition of small-molecule-based OLEDs require high investments and, therefore, high throughput to decrease the share of manufacturing cost in the final product. Thus, vacuum evaporation can be seen as a transitional deposition technique that will be partly or totally replaced by more cost-efficient, solution-based deposition techniques such as printing.

The potential for lowered fabrication costs that result from the solubility of polymer-based OLED materials in common solvents has been the motivation for their use. Suppliers of small-molecule materials have recently indicated interest in the development of soluble small-molecule materials, and many research projects have been launched focusing on the solubility of and ability to process small-molecule-based materials via solution-based deposition techniques.

Fabrication processes
The main advantages of printing as a deposition method include the high speed of fabrication, low material wastage, well established deposition techniques, possibility for direct patterning of the printed films, and the low processing temperatures that enable the use of flexible substrates. The most commonly used printing-deposition techniques in OLED processing to date include gravure, flexo, screen, and inkjet printing, as well as slot die coating. This article emphasizes the use of gravure printing for producing OLED devices.