Seoul | Ultra thin and light weight computers that roll up like a piece of paper may be closer to reality, thanks to highly flexible organic light emitting diodes (OLEDs) developed by Korean scientists. The OLEDs have excellent efficiency and make use of graphene as a transparent electrode, researchers said.
OLEDs, built upon a plastic substrate, have received greater attention lately for their use in nextgeneration displays that can be bent or rolled while still operating.
Researchers led by Seunghyup Yoo from Korea Advanced Institute of Science (KAIST) and TaeWoo Lee from Pohang University of Science and Technology (POSTECH) in South Korea used graphene as a transparent electrode (TE) which is placed in between titanium dioxide (TiO2) and conducting polymer layers.
OLEDs are stacked in several ultra thin layers on glass, foil, or plastic substrates, in which multilayers of organic compounds are sandwiched between two electrodes. When voltage is applied across the electrodes, electrons from the cathode and holes (positive charges) from the anode draw towards each other and meet in the emissive layer.
OLEDs emit light as an electron recombines with a positive hole, releasing energy in the form of a photon. One of the electrodes in OLEDs is usually transparent, and depending on which electrode is transparent, OLEDs can either emit from the top or bottom.
In conventional bottomemission OLEDs, an anode is transparent in order for the emitted photons to exit the device through its substrate. Indiumtinoxide (ITO) is commonly used as a transparent anode because of its high transparency, low sheet resistance, and wellestablished manufacturing process.
However, ITO is expensive and brittle, being susceptible to bending induced formation of cracks. Graphene, a two dimensional thin layer of carbon atoms tightly bonded together in a hexagonal honeycomb lattice, has recently emerged as an alternative to ITO. However, efficiency of graphene based OLEDs reported to date has been about the same level of ITO based OLEDs.
Researchers proposed a new device architecture that can maximise the efficiency of graphene based OLEDs. They fabricated a transparent anode in which a TiO2 layer with a high refractive index (highn) and a hole injection layer of conducting polymers with a low refractive index (lown) sandwich graphene electrodes.
This is an optical design that induces a synergistic collaboration between the highn and lown layers to increase the effective reflectance of TEs. Under this approach, graphene based OLED devices remain intact and operate well even after 1,000 bending cycles at a radius of curvature as small as 2.3 mm.
We expect that our technology will pave the way to develop an OLED light source for highly flexible and wearable displays, or flexible sensors that can be attached to the human body for health monitoring, for instance, Lee said. The research was published in the journal Nature Communications.
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