Press Release Archive

  • home_icon HOME
  • News
  • Press Release Archive

A Korean research team developed an ideal electrode structure composed of graphene and layers of titanium dioxide and conducting polymers, resulting in highly flexible and efficient OLEDs.

 

The arrival of a thin and lightweight computer that even rolls up like a piece of paper will not be in the far distant future. Flexible organic light-emitting diodes (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.

 

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. The research results were published online on June 2, 2016 in Nature Communications.

 

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.

 

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. With outstanding electrical, physical, and chemical properties, its atomic thinness leading to a high degree of flexibility and transparency makes it an ideal candidate for TEs. Nonetheless, 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. 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. As a result, the enhancement of the optical cavity resonance is maximized. The optical cavity resonance is related to the improvement of efficiency and color 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 TE. 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. 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 said, “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.”

 

The research paper is entitled “Synergistic Electrode Architecture for Efficient Graphene-based Flexible Organic Light-emitting Diodes” (DOI. 10.1038/NCOMMS11791). The lead authors are Jae-Ho Lee, a Ph.D. candidate at KAIST; Tae-Hee Han, a Ph.D. researcher at POSTECH; and Min-Ho Park, a Ph.D. candidate at POSTECH.

 

This study was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF) through the Center for Advanced Flexible Display (CAFDC) funded by the Ministry of Science, ICT and Future Planning (MSIP); by the Center for Advanced Soft-Electronics funded by the MSIP as a Global Frontier Project; by the Graphene Research Center Program of KAIST; and by grants from the IT R&D Program of the Ministry of Trade, Industry and Energy of Korea (MOTIE).

 

Figure 1: Application of Graphene-based OLEDs

This picture shows an OLED with the composite structure of TiO2/graphene/conducting polymer electrode in operation. The OLED exhibits 40.8% of ultrahigh external quantum efficiency (EQE) and 160.3 lm/W of power efficiency. The device prepared on a plastic substrate shown in the right remains intact and operates well even after 1,000 bending cycles at a radius of curvature as small as 2.3 mm.

F_20160607185448_639.jpg


Figure 2: Schematic Device Structure of Graphene-based OLEDs

This picture shows the new architecture to develop highly flexible OLEDs with excellent efficiency by using graphene as a transparent electrode (TE).

F_20160607185510_727.jpg



List of Articles
No. Category Subject Author Date Views
15 KINC 전기적 자기적 성질 동시에 갖는 신물질 물성 규명 [2012-01-17] file KAIST_INSTITUTE 2016.06.10 7309
14 KIHST 이제는 세포 내부도 훤히 들여다본다 [2012-01-27] file KAIST_INSTITUTE 2016.06.10 5003
13 KIHST KIOST 김승우 연구소장 올해 국가과학자 선정 [2012-09-20] KAIST_INSTITUTE 2016.06.10 5115
12 KINC 최성율 교수 도장 찍듯이 자유롭게 그래핀 옮기는 기술 개발 [2015-08-05] file KAIST_INSTITUTE 2016.06.10 8586
11 KIITC 박인규 교수, 공기오염 측정 센서 원천기술 개발 [2015-08-05] file KAIST_INSTITUTE 2016.06.10 5927
10 KINC 조병진, 임성갑, 유승협 교수 휘어지는 10나노미터 고분자 절연막 개발 [2015-08-05] file KAIST_INSTITUTE 2016.06.10 8254
9 KINC KAIST Develope a Credit-Card-Thick Flexible Lithium Ion Battery [2015-08-06] file KAIST_INSTITUTE 2016.06.10 61225
8 KIHST Anti-Cancer Therapy Delivering Drug to an Tumor Developed by KAIST Researchers [2015-08-06] file KAIST_INSTITUTE 2016.06.10 24652
7 KIHST Fast and Accurate 3-D Imaging Technique to Track Optically-Trapped Particles [2015-08-06] file KAIST_INSTITUTE 2016.06.10 22699
6 KIR BBC News Channel's Technology Program "Click"Aired KAIST's Jellyfish Robot [2015-08-06] file KAIST_INSTITUTE 2016.06.10 8550
5 Others 융합연구기획과제 성과 - 대학 캠퍼스 내 실내외 통합 내비게이션 개발 [2015-09-17] file KAIST_INSTITUTE 2016.06.10 9606
4 Others A Firefighter Drone That Flies and Crawls Up Walls [2016-01-20] file KAIST_INSTITUTE 2016.06.18 66826
3 KINC KAIST Researchers Develop a Technology to Enable Unzipping of the Graphene Plane [2016-03-22] file KAIST_INSTITUTE 2016.06.18 15315
2 KIHST Next-generation Holographic Microscope for 3D Live Cell Imaging [2016-03-29] file KAIST_INSTITUTE 2016.06.18 50080
» KINC Graphene-based Transparent Electrodes for Highly Efficient Flexible OLEDs [2016-06-07] file KAIST_INSTITUTE 2016.06.18 48259
Board Pagination Prev 1 2 Next
/ 2

KAIST 291 Daehak-ro, Yuseong-gu, Daejeon (34141)
T : +82-42-350-2381~2384
F : +82-42-350-2080
Copyright (C) 2015. KAIST Institute