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Graphene-Based Transparent Electrodes for Highly Efficient Flexible OLEDs
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.
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).
2016.06.07 View 12528 -
Professor Keon-Jae Lee Lectures at IEDM and ISSCC Forums
Professor Keon-Jae Lee of KAIST’s Materials Science and Engineering Department delivered a speech at the 2015 Institute of Electrical and Electronics Engineers (IEEE) International Electron Devices Meeting (IEDM) held on December 7-9, 2015 in Washington, D.C.
He will also present a speech at the 2016 International Solid-State Circuits Conference scheduled on January 31-February 4, 2016 in San Francisco, California.
Both professional gatherings are considered the world’s most renowned forums in electronic devices and semiconductor technology. It is rare for a Korean researcher to be invited to speak at these global conferences.
Professor Lee was recognized for his research on flexible NAND chips. The Korea Times, an English language daily newspaper in Korea, reported on his participation in the forums and his recent work. An excerpt of the article follows below:
“KAIST Professor to Lecture at Renowned Tech Forums”
By Lee Min-hyung, The Korea Times, November 26, 2015
Recently he has focused on delivering technologies for producing flexible materials that can be applied to everyday life. The flexible NAND flash memory chips are expected to be widely used for developing flexible handsets. His latest research also includes flexible light-emitting diodes (LED) for implantable biomedical applications.
Lee is currently running a special laboratory focused on developing new flexible nano-materials. The research group is working to develop what it calls “self-powered flexible electronic systems” using nanomaterials and electronic technology.
Lee’s achievement with flexible NAND chips was published in the October edition of Nano Letters, the renowned U.S.-based scientific journal.
He said that flexible memory chips will be used to develop wearable computers that can be installed anywhere.
2015.11.26 View 9428 -
Open KAIST 2015
KAIST’s research environment and its most recent achievements were open to the public.
KAIST hosted “Open KAIST 2015” over two days from November 5-6, 2015 in which its 17 departments and three research centers were open to the public. The event is one of the largest events that KAIST holds, which permits such public viewings of its facilities. It is the eighth time it has taken place.
During this event, the departments and centers offered 64 programs including laboratory tours, research achievement exhibitions, department introductions, and special lectures.
The “Motion Capture System”of Professor Jun-Yong Noh’s lab (Graduate School of Culture Technology) drew particular attention.
The “Motion Capture System” expresses human and animal motion in three-dimensional (3D) space using infrared cameras and optic markers, which can then be applied to various industries such as movies, games, and animation. During the program, researchers themselves demonstrated the recording of the movement and its conversion into 3D characters.
Professor Yong-Hoon Cho’s laboratory introduced the scientific mechanism behind the Light Emitting Diode (LED) as well as its manufacturing process under the topic:“A to Z of LED Production.” The reserachers explained that how green LED is much more efficient compared to previous light sources and presented applications that how it is widely used in everyday life in smart phones, electronic displays, and other mobile gadgets.
Professor Jun-tani of the Department of Electronic and Electrical Engineering introduced “Humanoid Robot Nao’s Imitation of Human Motions.” Nao is an autonomous, programmable humanoid robot developed by a French robotics company based in Paris. Nao has an artificial neural circuit, which is the functional equivalent of a human brain, and can thus mimic the subject’s motions through learning.
In addition, Professor Hyo-Choong Bang (Department of Aerospace Engineering) in his lecture on “Unmanned Vehicle Research and Nano Satellites” and Professor Hyun Myung (Department of Civil and Environmental Engineering) on his lecture on “Future Civilization Robot System: the Jellyfish Elimination Robotic Swarm and the Wall-Climbing Drone” provided information on the progress of their respective research.
KAIST also displayed its most recent research achievements. A lecture on “Information Technology Convergence” offered a showroom for “Dr. M,” which is a mobile healthcare platform. Dr. M is a mobile healthcare system that collects and analyzes biosignals via a smart sensor attached to the human body that shows around 20 advanced technologies.
The Satellite Technology Research Center introduced the public to its “Get to Know Satellites” program on Korea’s first satellite “Our Star 1” in addition to showing the satellite assembly room and the satellite communication center.
Special lectures were also held for visitors. Professor Min-Hyuk Kim and Hye-Yeon Oh of the School of Computing talked about “Computer Graphics and Advanced Video Technology” and “Man and the Computer,” respectively, from the perspective of non-experts.
Another interesting feature was the “Wearable Computer Competition” in which college students held fashion shows with computers attached to their clothes.
Professor Jung Kwon Lee, the Dean of the College of Engineering, who led this event, said that “the Open KAIST, which is being held for the eighth time this year, is an excellent opportunity for the general public to experience KAIST’s research environment.” He hoped this could motivate young adults to widen their spectrum of scientific knowledge and raise affection for science.
2015.11.13 View 9368 -
Professors Sukbok Chang and Jang-Wook Choi Receive the 2015 Knowledge Award from the Korean Government
The Ministry of Science, ICT and Future Planning (MISP) of the Republic of Korea announced the 2015 Knowledge Awards on October 20, 2015. Two KAIST professors received the award.
Established in 2009, the awards are presented to Korean scientists whose publications have contributed to the international science community. Specifically, the MISP used the two biggest science databases, Science Citation Index Expanded (SCIE) and Scopus, to identify ten highly cited papers ranked in the top 1% by total citations in the past ten years.
Professor Sukbok Chang of Chemistry (left in the picture below) is a global authority in the field of catalytic hydrocarbon functionalization. His paper entitled “Palladium-catalyzed C-H Functionalization of Pyridine N-Oxides: Highly Selective Alkenylation and Direct Arylation with Unactivated Arenes,” which was published in the Journal of the American Chemical Society in 2008, was once selected by Thomson Reuters as one of the “Most Influential Research Papers of the Month.” In 2011, the American Chemical Society included his paper in the list of the top 20 research papers that were most frequently cited in the last three years.
Professor Jang-Wook Choi of the Graduate School of EEWS (Energy, Environment, Water, and Sustainability) has been known for his leading research in rechargeable battery, supercapacitor, and materials chemistry. In particular, his work on secondary fuel cells attracted significant attention from academia and industry in Korea. Professor Choi developed a super-thin flexible lithium-ion battery this year, thinner than a credit card, which lasts longer than the existing batteries and with greater performance. He also developed new electrode materials for next-generation sodium-ion and magnesium secondary fuel cells.
Professor Sukbok Chang (left) and Professor Jang-Wook Choi (right)
2015.10.23 View 9957 -
KAIST holds the 2015 KAIST-MIT-Technion International Symposium on Nano Science
The 2015 KAIST-MIT-Technion International Symposium on Nano Science was held on August 11, 2015 at the KAIST campus. The event took place under three subtopics: Materials for Production and Storage of Renewable Energy, Functional Materials, and Multiferroic Materials.
The joint symposium invited more than 300 experts in material science and engineering including ten speakers and panelists. From MIT (Massachusetts Institute of Technology), Professors Harry L. Tuller and Geoffrey S. Beach in the Department of Material Engineering, as well as Professor Gregory Rutledge in the Department of Chemical Engineering joined the symposium. Professor Avner Rothschild in the Department of Material Engineering and Professor Yair Ein-Eli in the Department of Chemical Engineering at Technion Institute of Technology in Israel also participated.
From KAIST, Professors Il-Doo Kim, Byong-Guk Park, and Yeon-Sik Jung in the Department of Materials Science and Engineering, Professor Chan-Ho Yang in the Department of Physics, and Professor Doh-Chang Lee in the Department of Chemical and Biomolecular Engineering spoke at the event. The list of topics included “Next Generation Lithium-Air Battery,” “Nano Materials for High Performance Energy Storage System,” and “Iron Oxide Nanoparticles Photoelectrode for the Conversion and Storage of Solar Energy.”
In its efforts to promote cooperation among the three universities, KAIST plans to send six students to MIT and one student to Technion for joint research.
Professor Il-Doo Kim, who organized the symposium said, “We believe this kind of international gathering will serve as an opportunity for scholars from leading universities to share their expertise in material science and help them better understand on the recent trends in nanoscience and its related technology.”
2015.08.12 View 7321 -
The College of Information Science & Technology names its Alumnus of the Year 2014
The College of Information Science & Technology (CIST), KAIST, selected Tae-Kyung Yoo, the Chief Executive Officer of Lumens, Inc., a Korean company producing semiconductors and light emitting diodes (LEDs), as its Alumnus of the Year 2014.
The award ceremony took place on September 19, 2014 at the KAIST Institute with the participation of the university’s senior management and students.
Mr. Yoo was recognized for his pioneering work to develop the LED industry in Korea as the next-generation growth engine for the nation’s economy.
After the ceremony, he gave a talk entitled “The Past and Future of the LED Industry: Its Important Role in the Change of Korean Industry.”
The CIST created the Alumnus of the Year 2014 award, for the first time this year, to appreciate its alumni’s contribution to the advancement of the industrial and academic sectors of Korean information science and technology, and it will continue presenting the award from this year onwards.
2014.09.22 View 6970 -
Opening of "Education Donation Center" for Knowledge Sharing
KAIST is a leader in knowledge-sharing services for the educationally underprivileged. KAIST held the opening ceremony of the Education Donation Center at its Munji Campus on July 4, 2014 and was appointed as a “2014 Chungcheong-Gangwon province model local education donation center” sponsored by the Education Ministry of Korea and Korea Foundation for the Advancement of Science and Creativity.
The Education Donation Center (EDC) will integrate programs run by the Chungcheong-Gangwon province education donation organizations and introduce them to local chapter organizations. The EDC will recommend new donation organizations to the organizations in need. To ensure efficient donation, the EDC will consult programs of donation organizations. The EDC will also suggest regionally suitable education donation programs. KAIST will provide three staff members, six graduate students, and one to manage the career counselor to the center.
Joo-Sung Lee of the Business & Technology Management Department is the professor in charge of the EDC. He explained, “The center is the medium between the newly participating education donation organizations and the demand organizations. I will invest every effort to provide high-quality educational benefits to educationally underprivileged people."
KAIST has also been running an education service group called ‘Midam Scholarship’, an online education donation group called ‘Chalk’, and science camp programs for youth to establish the sharing and collaboration culture of KAIST.
2014.07.10 View 8100 -
Professor Won Do Heo on LED Light Technology for Controlling Proteins in Living Cells
With the newly developed LED technology, Professor Won Do Heo at the College of Life Science and Bioengineering, KAIST, was able to suppress cell migration and division when cells are exposed to LED light. This suggests a breakthrough to apply in future cancer cell research.
Professor Heo talked about the impact of his research in the following excerpt from a news article:
“We are already conducting research on the spread of cancer, as well as brain science in animal models with the Light-Activated Reversible Inhibition by Assembled Trap. I believe this technology will be a breakthrough in investigating cancer treatments and the function of neurons in a complex neural network, which existing technologies have not been able to do.”
From EE Times Europe, June 19, 2014
“LED Light Technology Controls Proteins in Living Cells”
http://www.ledlighting-eetimes.com/en/led-light-technology-controls-proteins-in-living-cells.html?cmp_id=7&news_id=222909336
2014.06.22 View 7557 -
Professor Kyu-Young Whang receives the PAKDD Distinguished Contributions Award
Professor Kyu-Young Whag
Dr. Kyu-Young Whang, Distinguished Professor from the Department of Computer Science, KAIST, has received the 2014 Distinguished Contributions Award from the Pacific-Asia Conference on Knowledge Discovery and Data Mining (PAKDD). PAKDD is the leading academic international conference on data mining held in Asia/Pacific. This year’s international conference was held from 13th to 15th May at Tainan, Taiwan.
As a life member of the PAKDD Steering Committee, Professor Whang worked for the development of the data mining field in the Asia-Pacific region, and his contribution to the international database and data-mining field has been widely recognized.
The PAKDD Distinguished Contributions Award has been awarded to a total of six people until now, including Professor Whang, and he is the first Korean to receive this award.
Professor Whang has also a history of receiving the Outstanding Contributions Award in 2011 from the Database Systems for Advanced Applications (DASFAA), the prestigious database academic conference in the Asia-Pacific region. The database and data mining field in the region was barren 20 years ago, but through the efforts and contributions of many researchers, including Professor Whang, it has now leapt to the level of being the equal of North American and European researchers.
In fact, three academic organizations in the current international database field are led by professors in the Asia-Pacific region. The IEEE ICDE (Institute of Electrical and Electronics Engineers Technical Committee on Data Engineering) is led by Professor Whang; the VLDB (Very Large Data Base) Endowment by Professor Beng Chin Ooi from National University of Singapore (NUS); and the ACM SIGMOD (Association for Computing Machinery Special Interest Group on Management of Data) by Professor Don Kossmann from ETH Zurich.
2014.05.26 View 8064 -
An Education Donation Club at KAIST Received the Education Minister's Award in 2013
Chalk, one of the student clubs at KAIST, shares knowledge by providing free online classes to teenage students in Korea via Internet.
Chalk, a KAIST student club which donates their educational knowledge, received an award from the Education Minister of the Republic of Korea at the 2nd Donation for Education Award held on December 16th, 2013 at the Plaza Hotel in Seoul.
The Donation for Eduation Award aims to find and reward corporations, organizations, and individuals, which have actively contributed to growing the dreams and talents of students, as well as revitalizing the culture of donation for education. It has been awarded by the Ministry of Education since 2012 to promote the awareness and participation of students for education.
Chalk provides free online video lectures on www.playchalk.com and runs a mentoring-based education program. The club has been recognized for offering online courses, math and science classes in particular, to teenage students who are from socially and economically less privileged backgrounds.
Chalk was founded by five KAIST students in 2011. Their ultimate goal is to create a society where students can fully enjoy the benefits of education, regardless of their economic conditions. About 60 undergraduate students currently participate in the club, with more than 5,000 students attending over 160 lectures without any cost.
2014.01.07 View 8941 -
Mechanism in regulation of cancer-related key enzyme, ATM, for DNA damage and repair revealed
Professor Kwang-Wook Choi
A research team led by Professor Kwang-Wook Choi and Dr. Seong-Tae Hong from the Department of Biological Sciences at KAIST has successfully investigated the operational mechanism of the protein Ataxia Telangiectasia Mutated (ATM), an essential protein to the function of a crucial key enzyme that repairs the damaged DNA which stores biometric information. The results were published on December 19th Nature Communications online edition.
All organisms, including humans, constantly strive to protect the information within their DNA from damages posed by a number of factors, such as carbonized materials in our daily food intake, radioactive materials such as radon emitting from the cement of buildings or ultraviolet of the sunlight, which could be a trigger for cancer.
In order to keep the DNA information safe, the organisms are always carrying out complex and sophisticated DNA repair work, which involves the crucial DNA damage repair protein ATM. Consequently, a faulty ATM leads to higher risks of cancer.
Until now, academia predicted that the Translationally Controlled Tumor Protein (TCTP) will play an important role in regulating the function of ATM. However, since most of main research regarding TCTP has only been conducted in cultured cells, it was unable to identify exactly what mechanisms TCTP employs to control ATM.
The KAIST research team identified that TCTP can combine with ATM or increase the enzymatic activity of ATM. In addition, Drosophilia, one of the most widely used model organisms for molecular genetics, has been used to identify that TCTP and ATM play a very important role in repairing the DNA damaged by radiation. This information has allowed the researchers to establish TCTP’s essential function in maintaining the DNA information in cell cultures and even in higher organisms, and to provide specific and important clues to the regulation of ATM by TCTP.
Professor Kwang-Wook Choi said, “Our research is a good example that basic research using Drosophilia can make important contributions to understanding the process of diseases, such as cancer, and to developing adequate treatment.”
The research has been funded by the Ministry of Science, ICT and Future Planning, Republic of Korea, and the National Research Foundation of Korea.
Figure 1. When the amount of TCTP protein is reduced, cells of the Drosophila's eye are abnormally deformed by radiation. Scale bars = 200mm
Figure 2. When the amount of TCTP protein is reduced, the chromosomes of Drosophilia are easily broken by radiation. Scale bars = 10 mm.
Figure 3. When gene expressions of TCTP and ATM are reduced, large defects occur in the normal development of the eye. (Left: normal Drosophilia's eye, right: development-deficient eye)
Figure 4. ATM marks the position of the broken DNA, with TCTP helping to facilitate this reaction. DNA (blue line) within the cell nucleus is coiled around the histone protein (green cylinder). When DNA is broken, ATM protein attaches a phosphate group (P). Multiple DNA repair protein recognizes the phosphate as a signal that requires repair and gathers at the site.
2014.01.07 View 11849 -
KAIST Student Awarded Prize from Energy Saving Contest
Jun-Min Kwon, an undergraduate student in the Department of Chemistry at KAIST, was awarded a prize from the Ministry of Trade, Industry and Energy, Republic of Korea, at the 35th Energy Saving Contest which was held on November 20.
The student club he has been leading was also selected as one of the best groups by the Save Energy Save Earth (SESE), a volunteer organization supported by the Korea Energy Management Corporation and the Ministry of Knowledge Economy, Republic of Korea.
Kwon began promoting energy conservation through a blog and participated in related meetings and workshops as a high school student to improve the understanding on the importance of energy saving and recycling.He also received awards from the Second National Assembly Forum on Climate Change, the Korean National Science Fair, as well as the Samsung Human Tech Paper Award.
2013.12.24 View 10882