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First Female Grand Prize Awardee of Samsung Humantech
Yeunhee Huh, PhD candidate (Professor Gyu-Hyeong Cho) from the School of Electrical Engineering received the grand prize of the 24th Humantech Paper Award. She is the first female recipient of this prize since its establishment in 1994. The Humantech Paper Award is hosted by Samsung Electronics and sponsored by the Ministry of Science and ICT with JoongAng Daily Newspaper. Her paper is titled, ‘A Hybrid Structure Dual-Path Step-Down Converter with 96.2％ Peak Efficiency using 250mΩ Large-DCR Inductor’. Electronic devices require numerous chips and have a power converter to supply energy adequately. She proposed a new structure to enhance energy efficiency by combining inductors and capacitors. Enhancing energy efficiency can reduce energy loss, which prolongs battery hours and solves overheating of devices; for instance, energy loss leads to the overheating issue affecting phone chargers. This technology can be applied to various electronic devices, such as cell phones, laptops, and drones. Huh said, “Power has to go up in order to meet customers’ needs; however the overheating problem emerges during this process. This problem affects surrounding circuits and causes other issues, such as malfunctions of electronic devices. This technology may vary according to the conditions, but it can enhance energy efficiency up to 4%.”During the ceremony, about eight hundred million KRW worth cash prizes was conferred to 119 papers. KAIST (44 papers) and Gyeonggi Science High School (6 papers) received special awards given to the schools.
Seoul Climate-Energy Conference Seeks Global Sustainability
(President Shin and Former UN Secretary General at the Seoul Climate Change-Energy Conference) Global leaders from both the private and public sectors discussed creative ways to seek inclusive green growth and sustainable development at the Seoul Climate-Energy Conference on November 24 in Seoul. The annual conference was co-hosted by KAIST and the Coalition for Our Common Future under the theme “Creating New Momentum for the Paris Agreement and a Sustainable Future.” More than 100 global leaders participated in the forum including the Director General Frank Rijsbermanof the Global Green Growth Institute and Executive Director Howard Bamsey of the Green Climate Fund. Former UN Secretary-General Ban Ki-Moon, who played a significant role in the signing of the Paris Agreement, was the keynote speaker. This year’s conference focused on Korea’s low carbon-energy transition and the Fourth Industrial Revolution to be aligned with green growth. At the conference, speakers and participants reviewed the progress of the decisions made by the UN Framework Convention on Climate Change (UNFCCC) COP23 in Bonn, Germany. The conference discussed topics of global collaboration for new climate regimes, green energy infrastructure, the Asia super grid, financing green energy, smart green cities, and new mobility. President Sung-Chul Shin emphasized global action and greater resilience toward climate change in his opening remarks. He said, “Today’s climate change can be attributed directly to the past three industrial revolutions. As industrialization continues, we must not make future generations pay the cost of this Fourth Industrial Revolution.” He explained that it is increasingly complicated to address climate change and energy issues because even though the use of energy consumption will continue to increase, energy policies are interwoven with global politics. He stressed three keywords to better address this global problem: innovation, collaboration, and speed. First he emphasized innovation as a priority for future success as it is hard to retain confidence without innovation. He noted KAIST has made sustainability initiatives in the fields of EEWS (energy, environment, water, sustainability) and green mobility. He also noted the importance of collaboration as industries are moving beyond a single discipline. KAIST is making collaborations in R&D and sustainability sectors, such as Saudi Aramco’s CO2 management center in KAIST. Finally, he explained that the speed of new transformation will be beyond our imagination, and governments should work efficiently to address issues in a fast manner. Meanwhile, Secretary-General Ban called for global unity in addressing climate change. He strongly emphasized that countries should make agreements not of willingness but of action, and that politicians should realize that this global agenda should be given top priority above domestic politics. He addressed how the world is experiencing the most powerful and destructive effects of climate change which makes active participation in the Paris Agreement increasingly important. He expressed his concern that the richest and most powerful countries are backing off, emphasizing the role of these countries as both global leaders and top producers of CO2. He also shared his hopes that the OECD will continue to work to fill the absence of the United States, and stressed the importance of acquiring 10 billion USD by 2020 to fund mitigation and adaptation technologies for developing countries’ CO2 emissions. Click for President Shin's opening remarks
Dr. Steven Chu Talks on Sustainable Energy Policy at KAIST
Nobel Laureate in physics and former US Energy Secretary Steven Chu called for concerted efforts to develop a more sustainable energy policy and portfolio at a lecture held at KAIST and a forum in Seoul on November 23. A policy with an energy mix including nuclear power and renewable energy could be ideal for retaining a stable energy supply given Korea’s very limited geographical conditions, Chu said during the Future Energy Forum in Seoul. He also held a lecture at KAIST’s Daejeon campus on “Climate Change, the Importance of Science and Policy in Achieving a Sustainable Future.” He said that unlike the United States, Korea and Japan have geographical limitations for generating enough renewable energy. "Wind speeds of more than 10 meters per second would allow wind power generation, but, South Korea's southernmost wind speed in Jeju is less than 8 meters per second, and the amount of sunshine is lower than in the Middle East. It is ideal to combine renewable energy with nuclear power plants," he said. Chu also stressed the role of science in achieving a sustainable future, citing many cases in foreign countries. For instance, Germany once decided to do away with nuclear power. However, their initial plan does not directly raise energy efficiency and the proportion of fossil fuels has led to an increase in the environmental issue of fine particular matter as well as carbon dioxide emission increases. He said that in the long term, renewable energy will emerge as major alternative resources, stressing the role of science in achieving a sustainable future. Without this alternative, we will eventually burn more fossil fuels and pollute the air. Chu also said that nuclear waste and safe plant operation will be a big concern, but it is technologically viable since Korea has already proven its prowess in nuclear power plant building and safety technology. Chu added, "Research in chemical energy storage through novel electrochemistry may lead to solutions, but for the next half century we will need additional energy-on-demand and carbon-free sources of energy from proven technologies." "While science, innovation and technology will no doubt lead to better solutions, sound government policies are needed to advance the transition to carbon-free energy needed to achieve a more sustainable world," he said. After serving as the US Secretary of Energy for four years from 2009 to 2013, Professor Chu returned to Stanford University, and currently holds a position of the William R. Kenan, Jr. Professor of Physics as well as Professor in the Department of Molecular and Cellular Physiology. Professor Chu is known for his research at Bell Labs and Stanford University regarding the cooling and trapping of atoms with laser light, for which he won the Nobel Prize in Physics in 1997.
Professor Lee's Research Selected as Top 100 National R&D Projects
A research project, led by Research Professor Ju Yong Lee from the KAIST Institute for IT Convergence, was selected as one of the Top 100 National Research and Development Projects 2017. This research project, titled LTE-A-based Single RF Small Base Station supporting Multiple Streams, developed 300Mbps low power, low complexity and broadband small base station technology that supports 4x4 MIMO (Multiple Input and Multiple Output) by proposing a new antenna structure and a new RF (Radio Frequency) structure based on LTE-A. Professors from the School of Electrical Engineering at KAIST, Dong Ho Cho, Songcheol Hong, and Yong Hoon Lee also collaborated on the project. The existing heterodyne method of communication systems generates the problems of increasing unit price and system complexity. In this project, however, Professor Lee directly modulated the baseband signal from the RF stage through an impedance loading-based RF chip. This method was designed to facilitate low power as well as low complexity while supporting broadband service. Based on this, his team developed source technology for RF that can be applied to fourth and even fifth generation networks. Furthermore, this base station is smallest among the small-cell stations so far, providing an eco-friendly installation environment. It contributes to the market for fifth generation mobile communications by reducing power consumption significantly yet providing high-capacity services. Professor Lee said, “This technology will contribute to creating a new market and additional jobs because business based on the fifth mobile generation can provide multi-functional services, including multiband. Requiring low power and providing high-capacity services anywhere at any time will enhance national competence and reduce costs for establishing a next generation mobile communication system. It is expected that this technology will help with disseminating mobile communication infrastructure through expanding information and communication system as well as the infrastructure of island areas.”
The Medici Effect: Highly Flexible, Wearable Displays Born in KAIST
(Ph.D. candidate Seungyeop Choi) How do you feel when technology you saw in a movie is made into reality? Collaboration between the electrical engineering and textile industries has made TVs or smartphone screens displaying on clothing a reality. A research team led by Professor Kyung Cheol Choi at the School of Electrical Engineering presented wearable displays for various applications including fashion, IT, and healthcare. Integrating OLED (organic light-emitting diode) into fabrics, the team developed the most highly flexible and reliable technology for wearable displays in the world. Recently, information displays have become increasingly important as they construct the external part of smart devices for the next generation. As world trends are focusing on the Internet of Things (IoTs) and wearable technology, the team drew a lot of attention by making great progress towards commercializing clothing-shaped ‘wearable displays’. The research for realizing displays on clothing gained considerable attention from academia as well as industry when research on luminescence formed in fabrics was introduced in 2011; however, there was no technology for commercializing it due to its surface roughness and flexibility. Because of this technical limitation, clothing-shaped wearable displays were thought to be unreachable technology. However, the KAIST team recently succeeded in developing the world’s most highly efficient, light-emitting clothes that can be commercialized. The research team used two different approaches, fabric-type and fiber-type, in order to realize clothing-shaped wearable displays. In 2015, the team successfully laminated a thin planarization sheet thermally onto fabric to form a surface that is compatible with the OLEDs approximately 200 hundred nanometers thick. Also, the team reported their research outcomes on enhancing the reliability of operating fiber-based OLEDs. In 2016, the team introduced a dip-coating method, capable of uniformly depositing layers, to develop polymer light-emitting diodes, which show high luminance even on thin fabric. Based on the research performance in 2015 and 2016, Ph.D. candidate Seungyeop Choi took the lead in the research team and succeeded in realizing fabric-based OLEDs, showing high luminance and efficiency while maintaining the flexibility of the fabric. The long-term reliability of this wearable device that has the world’s best electrical and optical characteristics was verified through their self-developed, organic and inorganic encapsulation technology. According to the team, their wearable device facilitates the operation of OLEDs even at a bending radius of 2mm. According to Choi, “Having wavy structures and empty spaces, fiber plays a significant role in lowering the mechanical stress on the OLEDs.” “Screen displayed on our daily clothing is no longer a future technology,” said Professor Choi. “Light-emitting clothes will have considerable influence on not only the e-textile industry but also the automobile and healthcare industries.” Moreover, the research team remarked, “It means a lot to realize clothing-shaped OLEDs that have the world’s best luminance and efficiency. It is the most flexible fabric-based light-emitting device among those reported. Moreover, noting that this research carried out an in-depth analysis of the mechanical characteristics of the clothing-spared, light-emitting device, the research performance will become a guideline for developing the fabric-based electronics industry.” This research was funded by the Ministry of Trade, Industry and Energy and collaborated with KOLON Glotech, INC. The research performance was published in Scientific Reports in July. (OLEDs operating in fabrics) (Current-voltage-luminance and efficiency of the highly flexible, fabric-based OLEDs;Image of OLEDs after repetitive bending tests;Verification of flexibility through mechanical simulation)
KATT Tops at Appropriate Technology Competition
The KAIST Appropriate Technology Team (KATT) consisting of KAIST international students received gold and bronze awards at ‘the 9th Creative Design Competition for the Other 90%’. This year’s competition was hosted by the Ministry of Science, ICT and Future Planning at Seoul National University’s Global Convention Plaza on May 26. Undergraduate and graduate students nationwide formed 65 teams to participate in the competition. The aim of the competition is to discover appropriate technology and sustainable design items to enhance quality of life for those with no or little access to science technology and its products around the world. This year’s competition categorized the designs into IT; water and energy; agriculture, hygiene, safety, and housing; and education. The teams were evaluated on their presentations and prototypes. KATT produced alarm warning bracelets for people in developing countries and smart hybrid dryers for agricultural products. The alarm warning bracelets were designed for those living in tsunami risk zones; they use wireless communication technology to receive and transmit warning signals and can be produced for less than $4. The smart hybrid dryers featured solar energy generation, aimed to help those with low income in subtropical, low-altitude regions with unstable climates, since there are currently no drying methods for agricultural products without direct exposure to sunlight. Therefore, the hybrid dryers allowed drying regardless of the weather, and thus increased the storage and distribution efficiency of agricultural products. Ashar Alam from India who participated in developing the alarm warning bracelet said, “Through the appropriate technology club, I recognized problems in India that also affect neighboring countries such as Indonesia and Bangladesh. I wanted to actively use the science and technology knowledge I have accumulated in KAIST for the less fortunate.” He continued, “It was meaningful to develop the product using the respective talents of students from various countries with the spirit of developing appropriate technology.” (Photo caption: Alarm warning bracelet team received the gold award)
Highly-Efficient Photoelectrochemical CO2 Reduction
Direct CO2 conversion has continuously attracted a great deal of attention as a technology to produce fuels and chemical building blocks from renewable energy resources. Specifically, substances such as carbon feedstocks and fuels can be produced by utilizing sunlight, water, and CO2 as semiconductors and a water interface through photoelectrochemical CO2 reduction. A KAIST research team demonstrated a novel photoelectrode structure for highly-selective and efficient photoelectrochemical CO2 reduction reactions. The research team led by Professor Jihun Oh of the Graduate School of EEWS (Energy, Environment, Water and Sustainability) presented a Si photoelectrode with a nanoporous Au thin film that is capable of reducing CO2 to CO with 90 percent selectivity in aqueous solution. The research team’s technology will provide a basic framework for designing the semiconductor photoelectrode structure necessary for photoelectrochemical conversion. In order to achieve steady conversion of CO2, it is necessary to use a high-performance catalyst to lower overpotential. Among the metal catalysts, Au is known to be an electrocatalyst that converts CO2 to CO. Conventionally, bare Au, as a catalyst, produces a lot of hydrogen gas due to its low CO selectivity. In addition, the high cost of Au remains a challenge in using the catalyst. Professor Oh’s research team addressed the issue by creating a nanoporous Au thin film formed by the electrochemical reduction of an anodized Au thin film. As a result, the team could demonstrate an efficient, selective photoelectrochemical reduction reaction of CO2 to CO using electrochemically-treated Au thin films on a Si photoelectrode. The electrochemical reduction on anodized Au thin films forms a nanoporous thin layer exhibiting many grain boundaries of nanoparticles on the Au surface. This dramatically improves the selectivity of the reduction reaction with a maximum CO faradaic efficiency of over 90% at low overpotential and durability. The research team also used an Au thin film of about 200 nanometers, 50,000 times thinner than previously reported nanostructured Au catalysts, resulting in a cost-effective catalyst. When depositing the catalyst on the semiconductor surface in the type of nanoparticles, the substrate of the thin film will be affected in the course of electrochemical reduction. Thus, the research team designed a new Si photoelectrode with mesh-type co-catalysts that are independently wired at the front and back of the photoelectrode without influencing the photoelectrode, and made it possible for electrochemical reduction. Due to the superior CO2 reduction reaction activity of the nanoporous Au mesh and high photovoltage from Si, the Si photoelectrode with the nanoporous Au thin film mesh shows conversion of CO2 to CO with 91% Faradaic efficiency at positive potential than CO equilibrium potential. Professor Oh explained, “This technology will serve as a platform for diverse semiconductors and catalysts. Researchers can further improve the solar-to-CO2 conversion efficiency using this technology. Dr. Jun Tae Song, the first author continued, “This new approach made it possible to develop a simple but very important type of electrode structure. It is the first time to achieve CO2 conversion at the potential lower than equilibrium potential. We believe that our research will contribute to efficient CO2 conversion.” This research was published in the inside front cover of Advanced Energy Materials on February 8, 2017. The research was funded and supported by the Korea Carbon Capture & Sequestration R&D Center. Professor Sung-Yoon Chung of the EEWS also participated in this research. (Figure: Schematic diagram of a Si photoelectrode that patterns with mesh-type nanoporous Au)
13 KAIST Faculty Named as Inaugural Members of Y-KAST
The Korean Academy of Science and Technology (KAST) launched the Young Korean Academy of Science and Technology (Y-KAST) and selected 73 scientists as its inaugural members on February 24. Among them, 13 KAIST faculty were recognized as the inaugural members of Y-KAST. Y-KAIST, made up of distinguished mid-career scientists under the age of 45, will take the leading role in international collaboration as well as innovative agenda-making in science and technology. The inaugural members include Professor Hyotcherl Ihee of the Department of Chemistry and Dr. Sung-Jin Oh of the Center for Mathematical Challenges at the Korea Institute for Advanced Study (KIAS), affiliated with KAIST. Professor Ihee is gaining wide acclaim in the fields of physics and chemistry, and in 2016, Dr. Oh was the youngest ever awardee of the Presidential Award of Young Scientist. The other Y-KAIST members are as follows: Professors Haeshin Lee of the Department of Chemistry; Mi Young Kim, Byung-Kwan Cho, and Ji-Joon Song of the Department of Biological Sciences; Song-Yong Kim of the Department of Mechanical Engineering; Sang-il Oum of the Department of Mathematical Sciences; Jung Kyoon Choi of the Department of Bio and Brain Engineering; Seokwoo Jeon, Sang Ouk Kim, and Il-Doo Kim of the Department of Materials Science and Engineering; Jang Wook Choi of the Graduate School of EEWS (Energy, Environment, Water and Sustainability); and Jeong Ho Lee of the Graduate School of Medical Science and Engineering. The leading countries of the Academy of Science, which include Germany, Sweden, Belgium, Canada, and Japan, have established the Young Academy of Science since 2010 in order to encourage the research activities of their young scientists and to establish a global platform for collaborative research projects through their active networking at home and abroad. President Myung-Chul Lee of KAST said, “We will spare no effort to connect these outstanding mid-career researchers for their future collaboration. Their networking will make significant impacts toward their own research activities as well as the global stature of Korea’s science and technology R&D. (Photo caption: Members of Y-KAST pose at the inaugural ceremony of Y-KAST on February 24.)
KAIST Ph.D. Candidate Wins the Next Generation of Engineers Award
Joo-Sung Kim, a doctoral student at the EEWS (Environment, Energy, Water and Sustainability) Graduate School won the inaugural Next Generation of Engineers Award in Leadership on December 14, 2016. The National Academy of Engineering of Korea hosts this award to support creative and ambitious students who have the potential to become leaders in engineering and who will serve as role models for future Korean engineers. Based on the recommendations of university professors in engineering and members of the academy, seven students are selected for the award in the categories of leadership and entrepreneurship. With his research focus on the development of high-performance, next-generation secondary cells for wearable devices such as smart watches, health bands, and smart eyewear, Joo-Sung created a startup, Lithium-ion Battery Energy Science and Technology (LiBEST), Inc. He plans to base his company at the Office of University and Industry Cooperation, KAIST, where he can receive assistance for launching the mass-production system for his technology. His adviser, Professor Jang-Wook Choi of the EEWS Graduate School, noted, “Joo-Sung has been a great student who has a strong sense of curiosity and perseverance. The award is the by-product of his hard work.” “I have always enjoyed my work and study as a researcher, but eventually would like to expand my career into business based on the results of my research. It would be wonderful if I could become a businessman like Elon Musk, Masayoshi Son, or Ma Yun and create a role model for aspiring engineers in Korea by combining science and technology with business demand to create social values that benefit many people,” Joo-Young said.
Mechanical Engineering Building on Campus Refurbished
KAIST’s Mechanical Engineering Department has finished the project to remodel its buildings and hosted an opening ceremony on December 12, 2016, which was attended by the university’s senior management and guests including President Steve Kang and Choong-Hwan Ahn, Architecture Policy Officer at the Ministry of Land, Infrastructure and Transport of Korea (MLIT). With an investment of approximately USD 10 million, the old buildings (each consisting of seven floors and one basement) were transformed into smart, green buildings. Among the upgrades were the establishment of LED lighting systems, the replacement of the exterior walls with insulated materials, and the installation of double-glazed windows, all resulting in the improvement of the buildings’ energy efficiency. Previously, offices and lecture halls in the buildings had individual cooling and heating systems, which consumed a great deal of energy, but they were replaced with a centralized smart energy control system that monitors the operation status as well as energy consumption in real time. With these new improvements, the Department was able to slash its energy consumption by 32%, for which it received Green Building Conversion Certification from MLIT. The ministry issues the certification to buildings that reduce their energy consumption by over 20% as a result of infrastructure upgrades. Beginning with the Mechanical Engineering buildings, KAIST will work on obtaining this certification for all of its buildings that are either under renovation or construction. President Kang said, “We are pleased to offer our students a comfortable environment for study and research and will continue improving outdated facilities and infrastructure to make the campus safer and nicer.” Picture 1: Ribbon-cutting ceremony for the refurbished Mechanical Engineering buildings on campus Picture 2: Mechanical engineering buildings
2016 KAIST EEWS Workshop
The Energy, Environment, Water and Sustainability (EEWS) Graduate School of KAIST hosted a workshop entitled “Progress and Perspectives of Energy Science and Technology” on October 20, 2016. The workshop took place at the Fusion Hall of the KAIST Institute on campus. About 400 experts in energy science and engineering participated in the event. Eight globally recognized scientists introduced the latest research trends in nanomaterials, energy theory, catalysts, and photocatalysts and led discussions on the current status and prospects of EEWS. Professors Yi Cui of Stanford University, an expert in nanomaterials, and William A. Goddard of California Institute of Technology presented their research experiments on materials design and recent results on the direction of theory under the topics of energy and environment. Dr. Miquel Salmeron, a former head of the Material Science Division of Lawrence Berkeley National Laboratory, and Professor Yuichi Ikuhara of Tokyo University introduced their analysis of catalysts and energy matters at an atomic scale. Professor Sukbok Chang of the Chemistry Department at KAIST, a deputy editor of ACS Catalysis and the head of the Center for Catalytic Hydrocarbon Functionalizations at the Institute of Basic Science, and Professor Yang-Kook Sun of Energy Engineering at Hanyang University, who is also a deputy editor of ACS Energy Letters, presented their latest research results on new catalytic reaction development and energy storage. The workshop consisted of three sections which addressed the design of energy and environment materials; analysis of energy and catalytic materials; and energy conversion and catalysts. The EEWS Graduate School was established in 2008 with the sponsorship of the Korean government’s World Class University (WCU) project to support science education in Korea. Professor J. Fraser Stoddart, the winner of the 2016 Nobel Prize in Chemistry, was previously worked at the KAIST EEWS Graduate School as a WCU visiting professor for two years, from 2011 to 2013. Professor Ali Coskun, who was a postdoctoral researcher in the laboratory of Professor Stoddart, now teaches and conducts research as a full-time professor at the graduate school. Dean Yousung Jung of the EEWS Graduate School said: “This workshop has provided us with a meaningful opportunity to engage in discussions on energy science and technology with world-class scholars from all around the world. It is also a good venue for our graduate school to share with them what we have been doing in research and education.”
KAIST Team Develops Semi-Transparent Solar Cells with Thermal Mirror Capability
A research team led by KAIST and Sungkyunkwan University professors has created semi-transparent perovskite solar cells that demonstrate high-power conversion efficiency and transmit visible light while blocking infrared light, making them great candidates for solar windows. Modern architects prefer to build exteriors designed with glass mainly from artistic or cost perspectives. Scientists, however, go one step further and see opportunities from its potential ability to harness solar energy. Researchers have thus explored ways to make solar cells transparent or semi-transparent as a substitute material for glass, but this has proven to be a challenging task because solar cells need to absorb sunlight to generate electricity, and when they are transparent, it reduces their energy efficiency. Typical solar cells today are made of crystalline silicon, but it is difficult to make them translucent. Semi-transparent solar cells under development use, for example, organic or dye-sensitized materials, but compared to crystalline silicon-based cells, their power-conversion efficiencies are relatively low. Perovskites are hybrid organic-inorganic halide-based photovoltaic materials, which are cheap to produce and easy to manufacture. They have recently received much attention as the efficiency of perovskite solar cells has rapidly increased to the level of silicon technologies in the past few years. Using perovskites, a Korean research team led by Professor Seunghyup Yoo of the Electrical Engineering School at KAIST and Professor Nam-Gyu Park of the Chemical Engineering School at Sungkyunkwan University developed a semi-transparent solar cell that is highly efficient and, additionally, functions very effectively as a thermal-mirror. The team has developed a top transparent electrode (TTE) that works well with perovskite solar cells. In most cases, a key to success in realizing semi-transparent solar cells is to find a TTE that is compatible with a given photoactive material system, which is also the case for perovskite solar cells. The proposed TTE is based on a multilayer stack consisting of a metal film sandwiched between a high refractive-index (high-index) layer and an interfacial buffer layer. This TTE, placed as a top-most layer, can be prepared without damaging ingredients used in perovskite solar cells. Unlike conventional transparent electrodes focusing only on transmitting visible light, the proposed TTE plays the dual role of passing through visible light while reflecting infrared rays. The semi-transparent solar cells made with the proposed TTEs exhibited average power conversion efficiency as high as 13.3% with 85.5% infrared rejection. The team believes that if the semi-transparent perovskite solar cells are scaled up for practical applications, they can be used in solar windows for buildings and automobiles, which not only generate electrical energy but also enable the smart heat management for indoor environments, thereby utilizing solar energy more efficiently and effectively. This result was published as a cover article in the July 20, 2016 issue of Advanced Energy Materials. The research paper is entitled “Empowering Semi-transparent Solar Cells with Thermal-mirror Functionality.” (DOI: 10.1002/aenm.201502466) The team designed the transparent electrode (TE) stack in three layers: A thin-film of silver (Ag) is placed in between the bottom interfacial layer of molybdenum trioxide (MoO3) and the top high-index dielectric layer of zinc sulfide (ZnS). Such a tri-layer approach has been known as a means to increase the overall visible-light transmittance of metallic thin films via index matching technique, which is essentially the same technique used for anti-reflection coating of glasses except that the present case involves a metallic layer. Traditionally, when a TE is based on a metal film, such as Ag, the film should be extremely thin, e.g., 7-12 nanometers (nm), to obtain transparency and, accordingly, to transmit visible light. However, the team took a different approach in this research. They made the Ag TE two or three times thicker (12-24 nm) than conventional metal films and, as a result, it reflected more infrared light. The high refractive index of the ZnS layer plays an essential role in keeping the visible light transmittance of the proposed TTE high even with the relatively thick Ag film when its thickness is carefully optimized for maximal destructive interference, leading to low reflectance (and thus high transmittance) within its visible light range. The team confirmed the semi-transparent perovskite solar cell’s thermal-mirror function through an experiment in which a halogen lamp illuminated an object for five minutes through three mediums: a window of bare glass, automotive tinting film, and the proposed semi-transparent perovskite solar cell. An infrared (IR) camera took thermal images of the object as well as that of each window’s surface. The object’s temperature, when exposed through the glass window, rose to 36.8 Celsius degrees whereas both the tinting film and the cell allowed the object to remain below 27 Celsius degrees. The tinting film absorbs light to block solar energy, so the film’s surface became hot as it was continuously exposed to the lamp light, but the proposed semi-transparent solar cell stayed cool since it rejects solar heat energy by reflection, rather than by absorption. The total solar energy rejection (TSER) of the proposed cell was as high as 89.6%. Professor Yoo of KAIST said, “The major contributions of this work are to find transparent electrode technology suitable for translucent perovskite cells and to provide a design approach to fully harness the potential it can further deliver as a heat mirror in addition to its main role as an electrode. The present work can be further fine-tuned to include colored solar cells and to incorporate flexible or rollable form factors, as they will allow for greater design freedom and thus offer more opportunities for them to be integrated into real-world objects and structures such as cars, buildings, and houses.” The lead authors are Hoyeon Kim and Jaewon Ha, both Ph.D. candidates in the School of Electrical Engineering at KAIST, and Hui-Seon Kim, a student in the School of Chemical Engineering at Sungkyunkwan University. This research was supported mainly by the Climate Change Research Hub Program of KAIST. Picture 1: Semi-transparent Perovskite Solar Cell This picture shows a prototype of a semi-transparent perovskite solar cell with thermal-mirror functionality. Picture 2: A Heat Rejection Performance Comparison Experiment This picture presents thermal images taken by an infrared camera for comparing the heat rejection performance of bare glass, automotive tinting film, and a semi-transparent perovskite solar cell after being illuminated by a halogen lamp for five minutes.
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