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

September 21, 2017 // By Julien Happich
At the annual International Smart Display and Touch Panel Exhibition, Touch Taiwan, in Taipei, Taiwan, Chunghwa Picture Tubes Ltd. (CPT) together with IMEC and Holst Centre are exhibiting a 1400x1400 pixels (1.96 megapixel) OLED microdisplay with orange and green sub-pixel pitches of 10μm (giving a 20μm pixel pitch) and providing a colour pixel resolution of 1250 ppi.

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 1900x600 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.