Ultrathin and lightweight 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 next-generation displays that can be bent or rolled while still operating. Also Read - Facebook for Android will soon get dark mode and coronavirus tracking featureAlso Read - Scientists develop soft contact lens that can zoom with a blink
Researchers led by Seunghyup Yoo from Korea Advanced Institute of Science (KAIST) and Tae-Woo 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 multi-layers of organic compounds are sandwiched between two electrodes. When Also Read - Increasing smartphone usage may be resulting in growing horns on our skull; research suggests
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 bottom-emission OLEDs, an anode is transparent in order for the emitted photons to exit the device through its substrate. Indium-tin-oxide (ITO) is commonly used as a transparent anode because of its high transparency, low sheet resistance, and well-established manufacturing process.
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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 (high-n) and a hole-injection layer of conducting polymers with a low refractive index (low-n) sandwich graphene electrodes. This is an optical design that induces a synergistic collaboration between the high-n and low-n 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.