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Optoelectronics/Displays??

New process saves OLED cost, improves radiance

Posted: 14 May 2013 ?? ?Print Version ?Bookmark and Share

Keywords:OLED? colour filter? LCD screen?

Researchers from Fraunhofer Research Institution for Organics, Materials and Electronic Devices COMEDD, together with Von Ardenne Anlagentechnik GmbH, have announced a process that producing mini-OLED screens without colour filters and a method that increases the luminosity of the microdisplays. Until now, the colour filter suppressed the self-radiance of the OLED, so that only about 20 per cent of the emitted light could be used. Two negative effects from the filter sheet being used are responsible for this: First, it suppresses two of the three colour ranges of an OLED subpixel; second, as an additional layer applied over the OLED, it automatically dims the generated light.

"In 2008, the first manufacturers introduced displays that were less than a millimeter thick," stated Rigo Herold from Fraunhofer. "OLEDs emit light themselves, and unlike the ordinary LCD screens of today, they work without background lighting. For this reason, it will soon be possible to manufacture very thin and simultaneously very flexible, bendable displays."

Yet the technology is still in its infancy stages. Beside the minimal lifespan, up to now the extremely high acquisition costs are impeding a widespread breakthrough. "Producing organic light diodes is still very expensive. This is why you still cannot purchase large-scale OLED TV screens. Right now, the technology is being used primarily for very small screen sizes of just a few square centimeters. Examples include the ViewFinder on digital cameras or, even smaller, on cell phone beamers and data glasses."

Microdisplays are barely larger than the human eye.

Microdisplays are barely larger than the human eye. A new, cost-effective process now lets them shine markedly brighter. Source: Fraunhofer COMEDD.

In order to circumvent the use of the colour filter, the red, green and blue subpixelswhich are integral to the depiction of a colour imagemust be loaded onto the OLED directly. That was previously impossible. "The subpixels in the tiny display are typically about 8?m2. However, conventional technology only allowed for the processing of units greater than 50?m2," noted Herold, illustrating the challenge to be mastered. In order to resolve this set of problems, scientists employed a special technology made by VON ARDENNE, their partner company. This technology facilitates the targeted vaporisation of organic layers locally, under heat. In this manner, surfaces can be processed that are smaller than 10?m2. "In order to use the technology for OLED microdisplays, we redesigned the entire manufacturing process. It is therefore possible to load the red, green and blue colour pixels directly. The use of the colour filter is no longer necessary and it is possible to use 100 per cent of the light emitted," added Herold.

Still, the OLED not only shine brighter, the new production process is also less expensive. Colour filters are very expensive to produce. Depending on the application, they have to be custom-designed, consist of suitable materials and be mounted properly. If the filter shifts, for instance, that could have a negative impact on the image quality. "Ultimately, the consumer benefits as well: We all know that our mobile devices, like smartphones and digital cameras, consume a lot of energy each day. The less is used for the colour presentation of the displays, the longer our batteries will last for telephone calls, surfing or taking pictures," Herold stated.





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