In a sunny office, Ji-Seon Kim holds up a sheet of stripy plastic. In the middle of dark blue and transparent bands, a small red glow catches the eye, clearly visible even against the bright daylight. There are no sockets or chargers, but that little light is no magic trick.
“It’s a printed solar cell from my industrial collaborator,” Kim explains. “This blue material is the organic semiconductor printed in the plastic. It absorbs indoor light and generates electricity to power the LED.”
Kim is a professor in the Department of Physics at Imperial College London, and was director of the university’s EPSRC Plastic Electronics Centre for Doctoral Training, which closed in 2023. She researches carbon-based semiconductors, sometimes called organic, molecular or plastic semiconductors. In 2023 the Institute of Physics (IOP) awarded her the Nevill Mott Medal and Prize in recognition of her “outstanding contributions to the materials physics” of this area.
Yet she came to the field almost by accident. After completing her master’s degree in theoretical physics in Seoul in 1994, Kim was about to embark on a theory-focused PhD studying nonlinear optics at Imperial, when her master’s supervisor told her about some exciting work happening at the University of Cambridge.
A team there had just created the first organic light-emitting diodes (OLEDs) based on conjugated polymers, successfully stimulating carbon-based molecules to glow under an applied voltage. Intrigued by the nascent field, Kim contacted Richard Friend, who led the research and, following an interview, he offered her a PhD position. Friend himself won the IOP’s Isaac Newton Medal and Prize in 2024.
I was really lucky to be in the right place at the right time, just after this new discovery
Ji-Seon Kim
“I spent almost six months learning how to use certain equipment in the lab,” Kim recalls of the tricky transition from theory to experimental work. “For example, there’s a big glove box you have to put your hands in to make the devices inside it, and I wasn’t sure whether I was even able to open the chamber.”
But as she found her feet, she became increasingly passionate about the work. “I was really lucky to be in the right place at the right time, just after this new discovery.”
Seeing the light
You could hardly find a clearer example of fundamental research moving into consumer applications in recent years than OLEDs – now a familiar term in the world of TVs and smartphones. But when Kim joined the field, the first OLEDs were inefficient and degraded quickly due to high electric fields, heat and oxygen exposure. So, during her PhD, Kim focused on making the devices more efficient and last for longer.
She also helped to develop a better understanding of the physics underlying the phenomenon. At the time, researchers disagreed about the fundamental limit of device efficiency determined by excited state (singlet vs triplet) formation under charge injection. Drawing on her theoretical background, Kim developed innovative simulation work on display device outcoupling, which provided a new way of determining the orientation of emitting molecules and the device efficiency, which is now commonly used in the OLED community.
Kim completed her PhD in 2000 and continued studying organic semiconductors, moving to Imperial in 2007. Besides display screens, she is interested in numerous other potential applications of the materials, including sustainable energy. After all, just as the molecules can emit light in response to injected charges, so too can they absorb photons and generate electricity.
Organic semiconductors have several advantages over traditional silicon-based photovoltaic materials. As well as being lightweight, carbon molecules can be tuned to absorb different wavelengths. Whereas silicon solar cells only work with sunlight, and must be installed as heavy panels on roofs or in fields, organic semiconductors offer more options. They could be inconspicuously integrated into buildings, capturing indoor office light that is normally wasted and using it to power appliances. They could even be made into a transparent film and incorporated into windows to convert sunlight into electricity.
Plastic fabrication methods offer a further benefit. Unlike silicon, carbon-based semiconductors can be dissolved in common organic solvents to create a kind of ink, opening the door to low-cost, flexible printing techniques.
And it doesn’t stop there. “A future direction I am particularly interested in is using organic semiconductors for neuromorphic applications,” says Kim. “You can make synaptic transistors – which mimic biological neurons – using molecular semiconductors.”
With all the promise of these materials, the field has flourished. Kim’s group is currently tackling the challenge of the high binding energy between the electron–hole pair in organic semiconductors, which resists separation into free charges, increasing the intrinsic energy cost of using them. Kim and her team are exploring new small molecules, which create an energy level offset by simply changing their packing and orientations, providing an extra driving force to separate the charges.
Building bridges
Alongside her work at Imperial, Kim was also a visiting professor at KAIST in Korea, and is actively involved in strengthening UK–Korea research ties. In 2016 she co-established the GIST-ICL Research and Development Centre for Plastic Electronics, a collaboration between the Gwangju Institute for Science and Technology and Imperial.
“International interactions are critical not only for scientific development but also for future technology,” Kim says. “The UK is really strong in fundamental science, but we don’t have many manufacturing sites compared to Asian countries like Korea. For a fundamental discovery to be applied in a commercial device, there’s a transition from the lab to the manufacturing scale. For that we need a partner, and those partners are overseas.”
Kim is also seeking to build bridges across disciplines. She will soon be moving to the University of Oxford to work on physical chemistry as part of a research initiative focused on sustainable materials and chemistry. She will draw on her expertise in spectroscopic techniques to study and engineer molecules for sustainable applications.
“These days physics is multidisciplinary,” she notes. “For future technology and science, you have to be able to integrate different disciplines. I hope I can contribute as a physicist to bridge different disciplines in molecular semiconductors.”
But one constant is how Kim mentors undergraduate students. Her advice is to engage them with innovations from the lab, which is why she likes to get out the plastic sheet powering the LED. The emphasis on tangible experience is inspired by the excitement and motivation she remembers feeling when she saw organic semiconductors glowing at the start of her PhD.
“Even though the efficiency was so poor that we had to turn the overhead light off and use a really high voltage to see the faint light, that exposure to the real physics was really important,” she says. “That was for me a Eureka moment.”
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