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Wednesday, 26 Jul 2017

KAIST and POSTECH researchers develop graphene-based transparent electrodes for highly efficient flexible OLEDs

Under this approach, graphene-based OLEDs exhibit 40.8% of ultrahigh external quantum efficiency (EQE) and 160.3 lm/W of power efficiency

3 Jun 2016 | Editor

A Korean research team led by Professor Seunghyup Yoo from the School of Electrical Engineering, KAIST and Professor Tae-Woo Lee from the Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH) has developed highly flexible OLEDs with excellent efficiency by using 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 (cathode and anode). When voltage is applied across the electrodes, electrons from the cathode and holes (positive charges) from the anode draw toward 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. However, ITO can potentially be expensive, and moreover, is brittle, being susceptible to bending-induced formation of cracks.

KAIST/POTECH - OLED with the composite structure of TiO2/graphene/conducting polymer electrode in operation KAIST/POTECH - The new architecture to develop highly flexible OLEDs with excellent efficiency by using graphene as a transparent electrode

Figure: KAIST/POTECH - The new architecture to develop highly flexible OLEDs with excellent efficiency by using graphene as a transparent electrode

According to the researchers graphene is emerging as an alternative to ITO, as it has outstanding electrical, physical, and chemical properties, coupled with atomic thinness leading to a high degree of flexibility and transparency makes it an ideal candidate for transparent electrodes. However, the efficiency of graphene-based OLEDs reported to date has been, at best, about the same level of ITO-based OLEDs.

As a solution, the Korean research team, which further includes Professors Sung-Yool Choi (Electrical Engineering) and Taek-Soo Kim (Mechanical Engineering) of KAIST and their students, proposed a new device architecture that can maximize the efficiency of graphene-based OLEDs. They fabricated a transparent anode in a composite structure in which a TiO2 layer with a high refractive index (high-n) and a hole-injection layer (HIL) of conducting polymers with a low refractive index (low-n) sandwich graphene electrodes.

KAIST/POTECH - OLED with the composite structure of TiO2/graphene/conducting polymer electrode in operation

Figure: KAIST/POTECH - OLED with the composite structure of TiO2/graphene/conducting polymer electrode in operation

This is an optical design that induces a synergistic collaboration between the high-n and low-n layers to increase the effective reflectance of transparent electrodes. As a result, the enhancement of the optical cavity resonance is maximised. The optical cavity resonance is related to the improvement of efficiency and colour gamut in OLEDs. At the same time, the loss from surface plasmon polariton (SPP), a major cause for weak photon emissions in OLEDs, is also reduced due to the presence of the low-n conducting polymers.

Under this approach, graphene-based OLEDs exhibit 40.8% of ultrahigh external quantum efficiency (EQE) and 160.3 lm/W of power efficiency, which is unprecedented in those using graphene as a transparent electrode. Furthermore, these devices remain intact and operate well even after 1,000 bending cycles at a radius of curvature as small as 2.3 mm.

The research team said that 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 that TiO2 has a crack-deflection toughening mechanism that tends to prevent bending-induced cracks from being formed easily.

Professor Yoo said, "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." He added, "Our work was the achievement of collaborative research, transcending the boundaries of different fields, through which we have often found meaningful breakthroughs."
Professor Lee added, "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."

Synergetic electrode architecture for efficient graphene-based flexible organic light-emitting diodes

Jaeho Lee | Tae-Hee Han | Min-Ho Park | Dae Yool Jung | Jeongmin Seo | Hong-Kyu Seo | Hyunsu Cho | Eunhye Kim | Jin Chung | Sung-Yool Cho | Taek-Soo Kim | Tae-Woo Lee | Seunghyup Yoo

Nature Communications | Article number: 11791 | doi:10.1038/ncomms11791

Received 15 March 2016 | Accepted 27 April 2016 | Published 02 June 2016

Abstract

Graphene-based organic light-emitting diodes (OLEDs) have recently emerged as a key element essential in next-generation displays and lighting, mainly due to their promise for highly flexible light sources. However, their efficiency has been, at best, similar to that of conventional, indium tin oxide-based counterparts. We here propose an ideal electrode structure based on a synergetic interplay of high-index TiO2 layers and low-index hole-injection layers sandwiching graphene electrodes, which results in an ideal situation where enhancement by cavity resonance is maximized yet loss to surface plasmon polariton is mitigated. The proposed approach leads to OLEDs exhibiting ultrahigh external quantum efficiency of 40.8 and 62.1% (64.7 and 103% with a half-ball lens) for single- and multi-junction devices, respectively. The OLEDs made on plastics with those electrodes are repeatedly bendable at a radius of 2.3 mm, partly due to the TiO2 layers withstanding flexural strain up to 4% via crack-deflection toughening.

www.kaist.edu    www.postech.ac.kr   

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