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.
At a bending radius of 1mm, leakage current occurred within the devices, and crack patterns, identical to the weave patterns of the fabric, appeared on the OLED cell. As a comparison, a flexible test OLED built on a PET substrate showed cracks starting at a 10mm bending radius, with total failure occurring beyond a 5mm bending radius.
The KAIST researchers expect such fabric-based OLEDs to integrate various e-textile industries, such as in the manufacturing of curtains and tablecloths and in automobiles as well as in the fashion and healthcare industries.