In conventional bottom emission OLEDs, an anode is transparent so emitted photons can exit the device through its substrate. While indium tin oxide (ITO) is commonly used as a transparent anode, it can be expensive and is susceptible to cracking induced by bending.
Graphene, however, can endure a high degree of flexibility and transparency makes it suitable as a transparent electrode. But graphene based OLEDs have only shown the same level of effiency as ITO based OLEDs.
As a solution, the team fabricated a composite 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.
Using this approach, graphene based OLEDs were said to exhibit 40.8% of an external quantum efficiency of 40.8% and a power efficiency of 160.3lm/W, results said to be ‘unprecedented’. The electrodes remained intact and operating well after more than 1000 bending cycles at a radius of curvature as small as 2.3mm. Prof Yoo said: “What’s advanced about this technology, compared with previous graphene based OLEDs, is the synergistic collaboration of high- and low-index layers that enables optical management of both resonance effect and SPP loss, leading to significant enhancement in efficiency, all with little compromise in flexibility.”
Prof Lee added: “We expect 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.”