GIST FOCUS

Thinner, and cheaper: how far have the solar cells evolved?

You If you think about solar cells, you might imaginea blue color panels reflecting sunlight in a vast field.
Recent solar batteries however vary in their shapes as well as colors. They can operate not only outdoors but also indoors,
so they can be applied to a variety of fields such as clothing, packaging, wallpaper, and small electronic devices.
Recently, solar cells that can be inserted into a human body between 6 and 7μm thick have been developed, expanding their
range or applications. Currently, high efficiency and low cost are the keys to solar cell development, along with diversification of cells. We look at the current state of solar cell technology to solve problems.
Developing solar cells for insertion into humans
Expanding the applications for solar cells
Bio-Robotics Lab

Professor Jongho Lee
School of Mechanical Engineering
In May, a team led by School of Mechanical Engineering Professor Jongho Lee officially announced that they developed solar
cells for human implants. The new concept for solar cells was made with Korean technology and is expected to not only
expand the use of solar cells, which have mainly been been concentrated in the field of solar power, but will drastically improve the power shortage problem that has been a chronic problem in human-invasive medical devices. The research was supported by the Ministry of Science and ICT, the Core Space Technology Development Project, and the Research Institute for Solar and
Sustainable Energies. The results were published in the international journal 'Advanced Healthcare Materials.
What motivates you to research solar cells for human impants?
I was originally researching the solar cells by itself. When I accidentally turned on a flashlight over my skin, I saw part of the
light passing through. An idea then came to my mind that unlight passing through the body can be used for medical implants. Batters for edical electronic devices implanted inside the body, such as a cardiac pacemaker, only last 5 to 7 years.
The patient should have another surgery to replace the battery. If we can utilize the sunlight and produce electricity
autonomously, I thought that patients will suffer less financial and psychological burdens from not having more surgeries.
What parts have you focused on in your research?
I began my research with a question: "How much light can pass through the skin?" During my research however, there was a
paper published in Europe that found existing commercial silicon solar cells can be implanted into a body to produce energy.
The existing solar cells are thick and fragile, so there was always a risk of harm when inserted into a body. I was convinced that if we could solve this issue, we would succeed in developing a solar cell for human body implants.
To overcome these problems, a high-performance solar cell had to be separated from its hard substrate and incorporated into
film with a thickness of 6 to 7μm to provide skin-like flexibility. When inserted into a live mouse, the experiment showed that a
solar cell within 0.07㎠ (pure solar cell area) could produce a very high power of 647μW (micro-watts) with direct current. If the mouse receives sunlight for 2 hours a day, the power can operate a pacemaker 24 hours. But if it is combined with a small
rechargeable battery and a flexible pacemaker, we found that the pacemaker can be powered by a rechargeable battery even
in the absence of sunlight.
Which was most difficult about your research?
The research required a variety of background knowledge and skills, including machinery, electronics, materials, biology, and
medicine. For this reason, the research was conducted with various experts in Korea and abroad. In addition, during the development process, in order to analyze and verify the characteristics of human implanted solar cells, both positive (+) and negative (-) wires had to exposed through the outside of the mouse skin, and electrical characteristics analysis was difficult because some mice broke the electrical connections of the flexible wires.
Please tell us the research results and its expected benefits.
Solar cells have been developed for solar power generation. However, in this study, we designed and manufactured a flexible
solar cell in the form of a thin film for insertion into the human body, and it was meaningful to improve biocompatibility by
coating it with multiple transparent layers of film. In particular, analyzing and quantifying the electrical characteristics of the
solar cells under the skin through animal testing was the biggest achievement.
Lastly, please introduce your Bio-Robotics Lab.
Bio-robotics research aims to develop new types of robots,
nano-structures, and electronic devices with functions that
were previously impossible by basing them on biomimetic
engineering while developing useful technologies for real life.
The current research interests of the lab include bio-robotics
and controls, biomimicry nano/micro structures, and flexible/
neutral electronics. The ultimate goal is to integrate
technologies developed in each area into a biomimetric robot
or a biometric system to perform complex functions.
Issue Report

Solar cell catch two rabbits: Efficiency and practicality

Jung Hoon Kim, Donga Science reporter

You idiot, the problem is the cost!
The first generation of commercialized solar cells were created using the semiconductor. P-type semiconductor and the N-type semiconductor were attached, and the wire connected the other end. When the semiconductor was exposed to light, the
electrons move towards the anode and an electric current was made. It was a historical moment when the light was turned
into electric energy. The problem was efficiency. Only 6% of the solar energy could be converted into electricity. Research was
followed by more research to improve efficiency. Improving the semiconductor materials by making the silicon thinner
increased penetration of light and improved efficiency. Currently, the best solar cells are about 20% efficient. High-efficiency
solar cells are used in high-tech space industries such as satellites and robots for planetary exploration.
Semi-transparent organic solar cells installed on building windows
Solar cell panel manufactured by Hanhwa Q Cells
However, it was hampered again high production costs. Unit price were bound to increase as it needed to be made through
precise semiconductor processes. Although solar cells are an eco-friendly energy source, the process of making solar cells is not at all eco-friendly. In order to protect the environment, a paradoxical situation has occurred. Second generation solar cells therefore focused on lowering the production unit cost by manufacturing them with the affordable organic dyes rather than
expensive semiconductors. Solar cells can be created by covering a thin plastic plate with the organic dye, just like painting.
This is called as ‘the organic solar cells.’ Their unit cost is less than a half of the first generation, and they are flexible enough to
be bent. It can also be easily installed attached onto windows.
A leader in the next generation solar cells
Efficiency of the organic solar cells is relatively low. Solar cells are considered to be commercializable at around 7%, but
research has only recently begun to exceed these levels. Surprisingly, Korea has a team that leads the world's best efficiency
record for organic solar cells every time.
A team led by School of Materials Science and Engineering Professor Kwanghee Lee uses a strategy to increase efficiency of
solar cells by stacking organic materials into layers. It prevents light from being lost by depositing solar cells that absorb
different wavelengths of sunlight. However, it is difficult to commercialize layered organic solar cells because they have six or
more layers of complex structure. Professor Lee's team published a study on April 30 in "Advanced Functional Materials," which raised efficiency to 9.1 percent while simplifying the manufacturing process.
Layered organic solar cells developed by Professor Kwanghee Lee and his team at GIST
Manufacturing process has been greatly reduced compared to the existing organic solar cells while raising efficiency to 9%.
Earlier in January, the lab also improved the efficiency of large-area organic solar cell modules. Printed large-area organic solar cells show only 60 to 70 percent of the efficiency of small-area organic solar cells. This decrease in efficiency is the biggest
obstacle to commercialization. Professor Lee's team has developed a new process of printing stripes instead of complex
patterns, increasing the efficiency of organic solar cells by 7.5 percent, reaching a level where it can be commercialized.
Efficiency is further increased when the organic solar cell is combined with graphene. Graphene is nano-sized material with its carbon atoms arranged as a hexagonal net that resembles a beehive and is able to absorb light energy. If a solar cell is covered with graphene like a shell, its efficiency is increased to a maximum of 10.3%. Graphene coating on the organic material has the additional effect of preventing the organic solar cell from losing its endurance and efficiency over time. Dong Ik Son, a senior
researcher at Jeonbuk Institute of Advanced Composite Materials of the Korea Institute of Science and Technology, published
these findings in ‘Nano Energy’ last February.
Thinner and thinner
Efficiency is not the only issue to consider. Raising the applicability of solar-powered batteries by making them thinner is also
critical. Solar-powered batteries that can be inserted under the skin has been developed, and it has recently drawn people’s attention. Solar-powered batteries developed by Professor Jongho Lee and his team at GIST is thin, with its width of 6 to 7㎛
(micrometer, one millionth of a meter). Because it is manufactured with material that is as flexible as the skin, there is no
irritation once it is inserted underneath the skin. This is considered to be particularly useful for patients who needs to insert an
electronic medical device for medical purpose into their bodies.
Solar cells for insertion into the human body has been developed
by Professor Jongho Lee and his team at GIST (left) with a cardiac pacemaker (right)
Cardiac pacemakers that are installed near the heart treats arrhythmia or irregular hearbeats by providing regular electric
shocks to the heart. Such electronic devices should be replaced once the battery is low through surgery. If a solar cell inserted into the human body that can supply power, the pacemaker can be used semi-permanently without the need of having another surgery to replace the batteries. According to experimental results using a lab mouse, power for operating the cardiac
pacemaker for 24 hours can be generated if the mouse is exposed to sunlight for about 2 hours per day. This can solve the
power supply problem of the electronic devices that need to operate for a long time inside the human body, such as a real-time blood sugar analyzer or a disease diagnosis sensor.
Solar cells have previously been mocked as being environmentally harmful technology disguised as being eco-friendly,' but is
now regarded as a truly environmentally friendly technology thanks to recent developments. As solar cells can be made on
thin films, the use of solar cells is expanding rapidly. Next generation solar cells that are both energy efficient and practical are expected to drastically impact society in the future.