A team of researchers led by Seunghyup Yoo from the Korea Advanced Institute of Science and Technology (KAIST) and Tae-Woo Lee from Pohang University of Science and Technology (POSTECH) have developed highly flexible organic light-emitting diodes (graphene as a transparent electrode. The results, published in Nature Communications, show how graphene-based OLEDs offer notably improved properties compared to traditional OLEDs.
How do OLEDs Work?
OLEDs consist of multiple layers of organic compounds which are stacked between two electrodes, cathode and anode, and housed on glass, plastic or foil substrates. The application of a voltage between the two electrodes results in the movement of electrons from the cathode and holes (positive charges) from the anode towards each other. The electrons and holes meet and recombine, resulting in the emission of photon energy and the emittance of light from a transparent electrode.
The use of graphene as a transparent electrode has been regarded as the most practical entry point for graphene in consumer products, but up until now the material has not yielded a greater efficiency than indium tin oxide (OLED displays. Although ITO offers low sheet resistance, high transparency and a dedicated manufacturing process, the material is very stiff and highly susceptible to cracking when bent.
Instead of using ITO as a transparent electrode, the team proposed a new method:
- Create a transparent anode within a composite structure
- Create a titanium dioxide layer with a high refractive index
- Create a hole-injection layer of conductive polymers with a low refractive index
- Insert the graphene layer between the other two, creating a ‘graphene sandwich’
The process increases the reflectivity of the electrode structure, resulting in a higher efficiency, greater color gamut and far higher flexibility than ITO. The team’s tests found that the graphene-based OLEDs achieved a maximum external quantum efficiency of close to 41% and 160 lumens per watt luminous efficiency, the highest efficiency yet from a graphene-based device. The team tested the robustness of the graphene-based OLEDs by bending the devices more than 1000 times without any sign of cracking or damage.
The inclusion of graphene in an OLED display enables the display to be highly flexible and able to withstand numerous bends, improving the practicality of OLEDs and increasing the range of applications. Examples given by the team include highly flexible and monitoring health.
The team’s findings represent a crucial step towards the mass adoption of OLEDs in both the consumer and business worlds. While OLEDs have seen increased use over the past few years, the team’s graphene-based OLEDs offer a notable improvement in efficiency, luminous efficiency and an impressive rate of high flexibility. As a result, we should expect devices featuring graphene-based OLED displays to become very common over the next few years.
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