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Photolithography-patterned OLEDs could break 10kppi barrier, says imec

Photolithography-patterned OLEDs could break 10kppi barrier, says imec

Technology News |
By eeNews Europe



Circumventing the resolution bottleneck of fine metal masking (FMM) or inkjet printing (IJP) technologies, the international team of researchers used a “FMM-free” technique leveraging photolithography on an i-line (365nm UV) chemically amplified photoresist system in a process flow dedicated to OLED stacks.

The unit on display exhibits uniform electroluminescence and each colour can be lit separately since all pixels are addressed individually. Preliminary lifetime investigation shows the colours can operate up to a few hundred hours at more than 50% of the original brightness, after patterning.

Electroluminescence image of a patterned 1250ppi
two-color passive display (top) and the optical
microscope electroluminescence detail (bottom)
of the pixels.

In an invited paper titled “Photolithography as Enabler of AMOLED Displays Beyond 1000 ppi” presented last May at SID 2017, IMEC and Holst Center in cooperation with Fujifilm had revealed their path to very high resolution OLED displays. In their paper, the researchers detailed how they had explored the feasibility of realizing resolution above 1000 ppi in functional OLED arrays, starting with a 1900×600 pixels passive display with 10μm red and blue subpixel pitches, resulting in a 2500ppi monochrome resolution and 1250 ppi in bi-colour arrays.

This had been done by patterning small molecule OLED stacks (phosphorescent red and fluorescent blue) on top of a metal bottom contact wrapped by an inorganic pixel definition layer (PDL), then capping the pixels with a semi-transparent top contact and glass encapsulation.


Optical microscope picture of the photoresist after
development (top left) and photoluminescence image
after photoresist strip (bottom right). Pixel size is 2μm
and pixel spacing is 1μm (equivalent to a 8466ppi
resolution monochrome).

As part of their study, the researchers also realized OLED arrays with a 3μm pixel pitch (2μm pixel / 1μm spacing), yielding a monochrome resolution of 8466ppi and showing that their technique could possibly cross the 10,000ppi barrier once optimized. Photolithography, the researchers explain, allows for very high aperture ratios by minimizing the unused area between emitting elements. Reversely, it could be used to create semi-transparent displays by selectively removing the OLED stack, hence limiting the area of the non-transparent OLED stack to the necessary minimum.

On their roadmap, the authors envisaged the integration of organic photodetector subpixels to enable the display with adaptive luminance calibration or gesture recognition.

“Our goal is to identify and explore technology bottlenecks in the extremely competitive and fast-paced display industry. With research activities spanning from material and device science, through design, fabrication and integration all the way to tech transfer, we make use of our semiconductor and thin-film background,” stated Paul Heremans, program director of thin-film electronics at Holst Centre/imec.

”The collaboration with CPT enables us to improve the manufacturability readiness level of the OLED photolithography patterning method. This will enable production of ultra-high resolution OLED displays for next-generation user interfaces” Heremans added.

imec – www.imec-int.com.
Holst Centre – www.holstcentre.com

Chunghwa Picture Tubes Ltd – www.cptt.com.tw

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