Organic Light Emitting Displays (OLEDs) are being introduced very rapidly in rigid form into television, camcorders, mobile phones and other applications and they are dearly needed in flexible form for even larger markets. The potential for flexible OLEDs has received such huge publicity that consumer packaged goods companies and many others are sold on the potential for low cost, flexible, light emitting displays and seek to carry out many trials and rollouts. However, providing OLEDs suitable for most of the envisaged applications continues to prove troublesome because of product life, availability and cost.
Inorganics fill the market gap
Inorganic light emitting displays are therefore filling the market gap. For example, DNP in Japan, formerly known as Dai Nippon Printing, has a program to leapfrog today's ac electroluminescent displays on T-shirts, billboards, packaging and so on.
DNP developed a material that emits light at a low voltage and can be printed onto paper or cloth in any desired pattern, such as letters or images. In recent years, luminescent materials have been used increasingly to boost the advertising effectiveness of posters, point-of-purchase materials and other products. DNP's new light emitting material is made from a special metal alloy and ionic liquid. It can be printed with a regular screen of offset printing press, thereby making it possible to print luminescent posters or other point of purchase materials more quickly and cheaply than is possible with organic EL or other materials that require large-scale, specialised equipment such as a clean room or vacuum equipment. DNP plans to improve the material's luminescence and launch it commercially by 2014.
Meanwhile, materials suppliers, device printers and product integrators have changed their view that ac electroluminescence (ACEL), usually based on printed zinc sulfide doped with manganese, is yesterday's technology. The transparent electrode is RF sputtered indium tin oxide as we await suitable printed transparent electrodes that are cheaper and more tightly rollable. Opaque contacts are carbon, aluminium or silver and outer barrier layers - relatively undemanding compared to those essential for OLEDs or organic photovoltaics - can involve metal oxides or Honeywell ClarusTM treated film. Far from being yesterday's product, ACEL is benefiting from a wave of innovation and the forty or so printers making these today will soon be fifty, excluding those that simply print non-electronic colours on feedstock. Many companies now offer reels of white light feedstock that any printer can overlay with colours to achieve desired effects. Several companies can switch as many as 50 independent images and enhance colours with repeater phosphors and other approaches.
Rapid advances in ac electroluminescence
Here are just a few of the advances in ac electroluminescence. They also apply to so-called dc electroluminescence which refers to battery operated ac electroluminescence as seen in the fast-selling animated illuminated T-shirts sold by about ten companies in China. Ac electroluminescent displays, both driven by ac as with billboards and posters in China (a $15 million market inside China for example) and dc driven (a larger market in China) are now a fast growing market. Flexible OLEDs are not, but they still look like being the end game, perhaps in ten years time. Rigid OLEDs have kicked out much of the ACEL backlighting in mobile phones and this is a sign of things to come. However, in the flexible world it is a longer wait.
Images: Various materials shine after light-emitting material is printed onto them. Above image: Printed cloth; Top image: Printed Glass sheet. Source: DNP.
For more attend Printed Electronics Europe 2010.