Fabric-based OLEDs promise wearable displays

August 29, 2017 // By Julien Happich
Fabric-based OLEDs promise wearable displays
Moving away from the integration of LEDs into relatively stiff silicone or flexible plastic substrates, researchers at the Korea Advanced Institute of Science & Technology (KAIST) have managed to design fabric-based OLEDs that could be comfortable to wear as part of functional clothing.

Schematic illustration of the optimized fabric-based 
encapsulated OLEDs. 

Using a soft fabric made from heat-resistant polyethylene naphthalate (PEN) fibers as the substrate, they thermally laminated a thin planarization sheet onto the fabric through a roll-to-roll process to reduce surface roughness to only a couple of nanometres, so a functional 200nm-thick OLED could be layered on top.

The planarization sheet also integrated a gas barrier of 1.5 dyads to prevent the permeation of water vapour and oxygen. The sheets showed a moisture barrier property of nearly 10−3 g/m2/day.

With this approach, they could build an OLED stack consisting of two metal electrodes, carrier injection and transport layers, an emission layer with a host-guest system, and an out-coupling layer.

Their paper "Highly Flexible and Efficient Fabric-Based Organic Light-Emitting Devices for Clothing-Shaped Wearable Displays" published in Scientific Reports highlights nearly identical electrical and optical performance levels for the fabric-based OLED compared with a glass-based OLED.

The paper reports a maximum luminance and a current efficiency of 93,030 cd/m2 and 49.14 cd/A for non-encapsulated fabric-based OLEDs, to be compared with 169959 cd/m2 and 64.45 cd/A respectively for the glass-based OLEDs.

Adding MoO3/NPB/MoO3 encapsulation layers, they were able to yield a maximum current efficiency of 70.43 cd/A and a luminance of 35,844 cd/m2 for their fabric-based OLED.

The device on the fabric operated stably after harsh bending, even at a bending radius of 2mm for 3,000 cycles and a bending radius of 1cm after 30,000 cycles.

Top - Deviations of the electrical-optical performance capabilities according to the bending strain, (left) J-V-L characteristics, and (right) current efficiency.
Bottom – Comparing the fabric-based OLED emitting in bent condition, with a PET-based OLED under the same bending test.

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