This photo shows the abilities of the novel graphene-based OLED. Photo: KAIST.
This photo shows the abilities of the novel graphene-based OLED. Photo: KAIST.

A Korean research team has developed highly flexible organic light-emitting diodes (OLEDs) with excellent efficiency by using graphene as a transparent electrode (TE) placed between layers of titanium dioxide (TiO2) and a conducting polymer. 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), the team published its results in Nature Communications.

OLEDs are produced by stacking multiple layers of organic compounds between two electrodes (cathode and anode) on glass, foil or plastic substrates. Applying a voltage between the electrodes causes electrons from the cathode and holes (positive charges) from the anode to move toward each other. When these electrons and holes meet in the emissive layer, they recombine to release energy in the form of a photon. One of the electrodes is usually transparent, allowing the OLEDs to emit light either from their top or bottom.

In conventional bottom-emission OLEDs, indium-tin-oxide (ITO) is commonly used as a transparent anode because of its high transparency, low sheet resistance and well-established manufacturing process. However, ITO is expensive and brittle, being susceptible to cracking when bent.

Graphene, a two-dimensional, atom-thick layer of carbon atoms tightly bonded together in a hexagonal honeycomb lattice, has recently emerged as an alternative to ITO. With outstanding electrical, physical and chemical properties, its atomic thinness leads to a high degree of flexibility and transparency, making it an ideal candidate for transparent electrodes. Nevertheless, the efficiency of the graphene-based OLEDs developed to date has been, at best, about the same as ITO-based OLEDs.

The Korean research team, which further included Sung-Yool Choi and Taek-Soo Kim of KAIST and their students, has now proposed a new device architecture for maximizing the efficiency of graphene-based OLEDs. They designed a composite structure in which a TiO2 layer with a high refractive index (high-n) and a hole-injection layer with a low refractive index (low-n), made from a conducting polymer, are stacked on top of a transparent graphene anode.

This design induces a synergistic collaboration between the high-n and low-n layers that increases the effective reflectance of the graphene electrode and maximizes the resonance of the optical cavity, thereby improving the efficiency of OLED. At the same time, the loss from surface plasmon polariton (SPP), a major cause of weak photon emission in OLEDs, is also reduced due to the presence of the low-n conducting polymers.

Using this approach, the team developed graphene-based OLEDs that exhibit an ultrahigh external quantum efficiency (EQE) of 40.8% and 160.3 lm/W of power efficiency, which is unprecedented in OLEDS that use graphene as a transparent electrode. Furthermore, these devices remained intact and fully operational even after 1000 bending cycles at a radius of curvature as small as 2.3 mm. This is a remarkable result for OLEDs containing oxide layers such as TiO2, because oxides are typically brittle and prone to bending-induced fractures even at a relatively low strain. The research team discovered, however, that TiO2 has a crack-deflection toughening mechanism that helps to prevent the formation of bending-induced cracks.

"What's unique and 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," explained Yoo. "Our work was the achievement of collaborative research, transcending the boundaries of different fields, through which we have often found meaningful breakthroughs."

"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 added.

This story is adapted from material from KAIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.