Sungjun Park

Bio: Sungjun Park is an academic researcher from Ajou University. The author has contributed to research in topics: Oxide & Materials science. The author has an hindex of 24, co-authored 67 publications receiving 3282 citations. Previous affiliations of Sungjun Park include Samsung & Global Alliance in Management Education.

Flexible metal-oxide devices made by room-temperature photochemical activation of sol–gel films

Abstract: A method for annealing metal-oxide semiconductor films with deep-ultraviolet light yields thin-film transistors with performance comparable to that of thermally annealed devices, and widens the range of substrates on which such devices can be fabricated. Solution-processable metal-oxide semiconductors are attractive materials for low-cost, flexible electronics, but the need to anneal the deposited materials at relatively high temperatures limits the range of substrates on which such devices can be fabricated. Now Yong-Hoon Kim and colleagues demonstrate that irradiating the solution-cast films with deep ultraviolet light can obviate the need for an annealing step. In this system, photochemical activation serves essentially the same purpose as annealing, and the resulting semiconducting materials have device performance levels comparable to those produced using the high-temperature techniques. Amorphous metal-oxide semiconductors have emerged as potential replacements for organic and silicon materials in thin-film electronics. The high carrier mobility in the amorphous state, and excellent large-area uniformity, have extended their applications to active-matrix electronics, including displays, sensor arrays and X-ray detectors1,2,3,4,5,6,7. Moreover, their solution processability and optical transparency have opened new horizons for low-cost printable and transparent electronics on plastic substrates8,9,10,11,12,13. But metal-oxide formation by the sol–gel route requires an annealing step at relatively high temperature2,14,15,16,17,18,19, which has prevented the incorporation of these materials with the polymer substrates used in high-performance flexible electronics. Here we report a general method for forming high-performance and operationally stable metal-oxide semiconductors at room temperature, by deep-ultraviolet photochemical activation of sol–gel films. Deep-ultraviolet irradiation induces efficient condensation and densification of oxide semiconducting films by photochemical activation at low temperature. This photochemical activation is applicable to numerous metal-oxide semiconductors, and the performance (in terms of transistor mobility and operational stability) of thin-film transistors fabricated by this route compares favourably with that of thin-film transistors based on thermally annealed materials. The field-effect mobilities of the photo-activated metal-oxide semiconductors are as high as 14 and 7 cm2 V−1 s−1 (with an Al2O3 gate insulator) on glass and polymer substrates, respectively; and seven-stage ring oscillators fabricated on polymer substrates operate with an oscillation frequency of more than 340 kHz, corresponding to a propagation delay of less than 210 nanoseconds per stage.

Self-powered ultra-flexible electronics via nano-grating-patterned organic photovoltaics.

Abstract: Next-generation biomedical devices1-9 will need to be self-powered and conformable to human skin or other tissue. Such devices would enable the accurate and continuous detection of physiological signals without the need for an external power supply or bulky connecting wires. Self-powering functionality could be provided by flexible photovoltaics that can adhere to moveable and complex three-dimensional biological tissues1-4 and skin5-9. Ultra-flexible organic power sources10-13 that can be wrapped around an object have proven mechanical and thermal stability in long-term operation13, making them potentially useful in human-compatible electronics. However, the integration of these power sources with functional electric devices including sensors has not yet been demonstrated because of their unstable output power under mechanical deformation and angular change. Also, it will be necessary to minimize high-temperature and energy-intensive processes10,12 when fabricating an integrated power source and sensor, because such processes can damage the active material of the functional device and deform the few-micrometre-thick polymeric substrates. Here we realize self-powered ultra-flexible electronic devices that can measure biometric signals with very high signal-to-noise ratios when applied to skin or other tissue. We integrated organic electrochemical transistors used as sensors with organic photovoltaic power sources on a one-micrometre-thick ultra-flexible substrate. A high-throughput room-temperature moulding process was used to form nano-grating morphologies (with a periodicity of 760 nanometres) on the charge transporting layers. This substantially increased the efficiency of the organophotovoltaics, giving a high power-conversion efficiency that reached 10.5 per cent and resulted in a high power-per-weight value of 11.46 watts per gram. The organic electrochemical transistors exhibited a transconductance of 0.8 millisiemens and fast responsivity above one kilohertz under physiological conditions, which resulted in a maximum signal-to-noise ratio of 40.02 decibels for cardiac signal detection. Our findings offer a general platform for next-generation self-powered electronics.

Stretchable and waterproof elastomer-coated organic photovoltaics for washable electronic textile applications

Abstract: Textile-compatible photovoltaics play a crucial role as a continuous source of energy in wearable devices. In contrast to other types of energy harvester, they can harvest sufficient electricity (on t he order of milliwatts) for wearable devices by utilizing the cloth itself as the platform for photovoltaics. Three features are important for textile-compatible photovoltaics, namely environmental stability, sufficient energy efficiency and mechanical robustness. However, achieving these simultaneously remains difficult because of the low gas barrier properties of ultrathin superstrates and substrates. Here, we report on ultraflexible organic photovoltaics coated on both sides with elastomer that simultaneously realize stretchability and stability in water whilst maintaining a high efficiency of 7.9%. The efficiency of double-side-coated devices decreases only by 5.4% after immersion in water for 120 min. Furthermore, the efficiency of the devices remains at 80% of the initial value even after 52% mechanical compression for 20 cycles with 100 min of water exposure. Organic solar cells can be thin, bendable and strechable. Now, Jinno et al. develop flexible organic photovoltaic devices that can also be washed in water and detergent, opening future integration routes into everyday objects such as fabric.

Influence of PEDOT:PSS crystallinity and composition on electrochemical transistor performance and long-term stability.

Abstract: Owing to the mixed electron/hole and ion transport in the aqueous environment, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)-based organic electrochemical transistor has been regarded as one of the most promising device platforms for bioelectronics. Nonetheless, there exist very few in-depth studies on how intrinsic channel material properties affect their performance and long-term stability in aqueous environments. Herein, we investigated the correlation among film microstructural crystallinity/composition, device performance, and aqueous stability in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) films. The highly organized anisotropic ordering in crystallized conducting polymer films led to remarkable device characteristics such as large transconductance (∼20 mS), extraordinary volumetric capacitance (113 F·cm−3), and unprecedentedly high [μC*] value (∼490 F·cm−1V−1s−1). Simultaneously, minimized poly(styrenesulfonate) residues in the crystallized film substantially afforded marginal film swelling and robust operational stability even after >20-day water immersion, >2000-time repeated on-off switching, or high-temperature/pressure sterilization. We expect that the present study will contribute to the development of long-term stable implantable bioelectronics for neural recording/stimulation.

Ultrasoft electronics to monitor dynamically pulsing cardiomyocytes

Abstract: In biointegrated electronics, the facile control of mechanical properties such as softness and stretchability in electronic devices is necessary to minimize the perturbation of motions inherent in biological systems1–5. For in vitro studies, multielectrode-embedded dishes6–8 and other rigid devices9–12 have been widely used. Soft or flexible electronics on plastic or elastomeric substrates13–15 offer promising new advantages such as decreasing physical stress16–18 and/or applying mechanical stimuli19,20. Recently, owing to the introduction of macroporous plastic substrates with nanofibre scaffolds21,22, three-dimensional electrophysiological mapping of cardiomyocytes has been demonstrated. However, quantitatively monitoring cells that exhibit significant dynamical motions via electric probes over a long period without affecting their natural motion remains a challenge. Here, we present ultrasoft electronics with nanomeshes that monitor the field potential of human induced pluripotent stem cell-derived cardiomyocytes on a hydrogel, while enabling them to move dynamically without interference. Owing to the extraordinary softness of the nanomeshes, nanomesh-attached cardiomyocytes exhibit contraction and relaxation motions comparable to that of cardiomyocytes without attached nanomeshes. Our multilayered nanomesh devices maintain reliable operations in a liquid environment, enabling the recording of field potentials of the cardiomyocytes over a period of 96 h without significant degradation of the nanomesh devices or damage of the cardiomyocytes. Ultrasoft nanomesh electronics enable monitoring of the field potential of cardiomyocytes without interference with their natural motion.

Scalable fabrication of high-power graphene micro-supercapacitors for flexible and on-chip energy storage

Abstract: Microscale supercapacitors are promising alternative energy-storage devices; however, their use has been limited by the need for complicated fabrication techniques. This work reports the scalable fabrication of graphene supercapacitors with planar geometry that achieve power densities of up to 200 W cm−3.

Metal oxides for optoelectronic applications

Abstract: Optical transparency, tunable conducting properties and easy processability make metal oxides key materials for advanced optoelectronic devices. This Review discusses recent advances in the synthesis of these materials and their use in applications. Metal oxides (MOs) are the most abundant materials in the Earth's crust and are ingredients in traditional ceramics. MO semiconductors are strikingly different from conventional inorganic semiconductors such as silicon and III–V compounds with respect to materials design concepts, electronic structure, charge transport mechanisms, defect states, thin-film processing and optoelectronic properties, thereby enabling both conventional and completely new functions. Recently, remarkable advances in MO semiconductors for electronics have been achieved, including the discovery and characterization of new transparent conducting oxides, realization of p-type along with traditional n-type MO semiconductors for transistors, p–n junctions and complementary circuits, formulations for printing MO electronics and, most importantly, commercialization of amorphous oxide semiconductors for flat panel displays. This Review surveys the uniqueness and universality of MOs versus other unconventional electronic materials in terms of materials chemistry and physics, electronic characteristics, thin-film fabrication strategies and selected applications in thin-film transistors, solar cells, diodes and memories.

25th Anniversary Article: Rational Design and Applications of Hydrogels in Regenerative Medicine

Abstract: Hydrogels are hydrophilic polymer-based materials with high water content and physical characteristics that resemble the native extracellular matrix. Because of their remarkable properties, hydrogel systems are used for a wide range of biomedical applications, such as three-dimensional (3D) matrices for tissue engineering, drug-delivery vehicles, composite biomaterials, and as injectable fillers in minimally invasive surgeries. In addition, the rational design of hydrogels with controlled physical and biological properties can be used to modulate cellular functionality and tissue morphogenesis. Here, the development of advanced hydrogels with tunable physiochemical properties is highlighted, with particular emphasis on elastomeric, light-sensitive, composite, and shape-memory hydrogels. Emerging technologies developed over the past decade to control hydrogel architecture are also discussed and a number of potential applications and challenges in the utilization of hydrogels in regenerative medicine are reviewed. It is anticipated that the continued development of sophisticated hydrogels will result in clinical applications that will improve patient care and quality of life.

Molecular-Scale Electronics: From Concept to Function

Abstract: Creating functional electrical circuits using individual or ensemble molecules, often termed as “molecular-scale electronics”, not only meets the increasing technical demands of the miniaturization of traditional Si-based electronic devices, but also provides an ideal window of exploring the intrinsic properties of materials at the molecular level. This Review covers the major advances with the most general applicability and emphasizes new insights into the development of efficient platform methodologies for building reliable molecular electronic devices with desired functionalities through the combination of programmed bottom-up self-assembly and sophisticated top-down device fabrication. First, we summarize a number of different approaches of forming molecular-scale junctions and discuss various experimental techniques for examining these nanoscale circuits in details. We then give a full introduction of characterization techniques and theoretical simulations for molecular electronics. Third, we highlig...

Building devices from colloidal quantum dots.

Abstract: BACKGROUND The Information Age was founded on the semiconductor revolution, marked by the growth of high-purity semiconductor single crystals. The resultant design and fabrication of electronic devices exploits our ability to control the concentration, motion, and dynamics of charge carriers in the bulk semiconductor solid state. Our desire to introduce electronics everywhere is fueled by opportunities to create intelligent and enabling devices for the information, communication, consumer product, health, and energy sectors. This demand for ubiquitous electronics is spurring the design of materials that exhibit engineered physical properties and that can enable new fabrication methods for low-cost, large-area, and flexible devices. Semiconductors, which are at the heart of electronics and optoelectronics, come with high demands on chemical purity and structural perfection. Alternatives to silicon technology are expected to combine the electronic and optical properties of inorganic semiconductors (high charge carrier mobility, precise n- and p-type doping, and the ability to engineer the band gap energy) with the benefits of additive device manufacturing: low cost, large area, and the use of solution-based fabrication techniques. Along these lines, colloidal semiconductor quantum dots (QDs), which are nanoscale crystals of analogous bulk semiconductor crystals, offer a powerful platform for device engineers. Colloidal QDs may be tailored in size, shape, and composition and their surfaces functionalized with molecular ligands of diverse chemistry. At the nanoscale (typically 2 to 20 nm), quantum and dielectric confinement effects give rise to the prized size-, shape-, and composition-tunable electronic and optical properties of QDs. Surface ligands enable the stabilization of QDs in the form of colloids, allowing their bottom-up assembly into QD solids. The physical properties of QD solids can be designed by selecting the characteristics of the individual QD building blocks and by controlling the electronic communication between the QDs in the solid state. These QD solids can be engineered with application-specific electronic and optoelectronic properties for the large-area, solution-based assembly of devices. ADVANCES The large surface-to-volume ratio of QDs places a substantial importance on the composition and structure of the surface in defining the physical properties that govern the concentration, motion, and dynamics of excitations and charge carriers in QD solids. Recent studies have shown pathways to passivate uncoordinated atoms at the QD surface that act to trap and scatter charge carriers. Surface atoms, ligands, and ions can serve as dopants to control the electron affinity of QD solids. Surface ligands and surrounding matrices control the barriers to electronic, excitonic, and thermal transport between QDs and between QDs and matrices. New ligand chemistries and matrix materials have been reported that provide freedom to control the dynamics of excitons and charge carriers and to design device interfaces. These advances in engineering the chemical and physical properties of the QD surface have been translated into recent achievements of high-mobility transistors and circuits, high-quantum-yield photodetectors and light-emitting devices, and high-efficiency photovoltaic devices. OUTLOOK The dominant role and dynamic nature of the QD surface, and the strong motive to build novel QD devices, will drive the exploration of new surface chemistries and matrix materials, processes for their assembly and integration with other materials in devices, and measurements and simulations with which to map the relationship between surface chemistry and materials and device properties. Challenges remain to achieve full control over the carrier type, concentration, and mobility in the QD channel and the barriers and traps at device interfaces that limit the gain and speed of QD electronics. Surface chemistries that allow for both long carrier lifetime and high carrier mobility and the freedom to engineer the bandgap and band alignment of QDs and other device layers are needed to exploit physics particular to QDs and to advance device architectures that contribute to improving the performance of QD optoelectronics. The importance of thermal transport in QD solids and their devices is an essential emerging topic that promises to become of greater importance as we develop QD devices.