Hongseok Youn

Continuous and scalable fabrication of flexible metamaterial films via roll-to-roll nanoimprint process for broadband plasmonic infrared filters

We demonstrate the continuous fabrication of large-area flexible metamaterial films via roll-to-roll (R2R) nanoimprint lithography (NIL) technique that can be conducted in an ambient environment at high speed. The plasmonic metal-insulator-metal structure is successfully fabricated by R2R NIL to continuously pattern the sub-wavelength scale metal disk array on flexible substrates. The patterned metal disks having varying diameters and sub-micron spacing with few defects lead to the desired broadband IR filtering performance at the designed dual-band, which correlates well with simulation analysis. Our method realizes a simple and high-throughput fabrication of plasmonic metamaterials for scalable and flexible optoelectronic and photonic applications.

Photo-roll lithography (PRL) for continuous and scalable patterning with application in flexible electronics.

A novel nanofabrication methodology for continuous, scalable, and geometry-tunable lithography is developed, named photo-roll lithography (PRL), by integrating photolithography with rollable processing. As a flexible mask attached to a quartz cylinder containing a UV source rolls over a photoresistcoated substrate, PRL realizes continuous photolithographic fabrication of various micro/nanoscale patterns with geometry that is tunable by controlling mask-substrate motions.

Organic Photovoltaic Cells: From Performance Improvement to Manufacturing Processes

Organic photovoltaics (OPVs) have been pursued as a next generation power source due to their light weight, thin, flexible, and simple fabrication advantages. Improvements in OPV efficiency have attracted great attention in the past decade. Because the functional layers in OPVs can be dissolved in common solvents, they can be manufactured by eco-friendly and scalable printing or coating technologies. In this review article, the focus is on recent efforts to control nanomorphologies of photoactive layer and discussion of various solution-processed charge transport and extraction materials, to maximize the performance of OPV cells. Next, recent works on printing and coating technologies for OPVs to realize solution processing are reviewed. The review concludes with a discussion of recent advances in the development of non-traditional lamination and transfer method towards highly efficient and fully solution-processed OPV.

Fabrication and characterization of WO3/Ag/WO3 multilayer transparent anode with solution-processed WO3 for polymer light-emitting diodes

The dielectric/metal/dielectric multilayer is suitable for a transparent electrode because of its high-optical and high-electrical properties; however, it is fabricated by an expensive and inefficient multistep vacuum process. We present a WO3/Ag/WO3 (WAW) multilayer transparent anode with solution-processed WO3 for polymer light-emitting diodes (PLEDs). This WAW multilayer not only has high transmittance and low resistance but also can be easily and rapidly fabricated. We devised a novel method to deposit a thin WO3 layer by a solution process in an air environment. A tungstic acid solution was prepared from an aqueous solution of Na2WO4 and then converted to WO3 nanoparticles (NPs) by a thermal treatment. Thin WO3 NP layers form WAW multilayer with a thermal-evaporated Ag layer, and they improve the transmittance of the WAW multilayer because of its high transmittance and refractive index. Moreover, the surface of the WO3 layer is homogeneous and flat with low roughness because of the WO3 NP generation from the tungstic acid solution without aggregation. We performed optical simulation and experiments, and the optimized WAW multilayer had a high transmittance of 85% with a sheet resistance of 4 Ω/sq. Finally, PLEDs based on the WAW multilayer anode achieved a maximum luminance of 35,550 cd/m2 at 8 V, and this result implies that the solution-processed WAW multilayer is appropriate for use as a transparent anode in PLEDs.

Solution processed polymer light-emitting diodes utilizing a ZnO/organic ionic interlayer with Al cathode

This letter reports polymer light-emitting diodes that employ a soluble zinc oxide (ZnO) nanoparticle (NP) and organic ionic interlayer as an electron-injection layer exhibits remarkable enhancement of device performance despite aluminum cathode. The ionic solution infiltrated into ZnO NP layer, which contains poly(ethylene oxide) and tetra-n-butylammonium tetrafluoborate, significantly lowers the large electron-injection barrier by forming a permanent interfacial dipole. The polymer, phenyl substituted poly(para-phenylene vinylene) known as “Super Yellow,” yellow light-emitting diodes employing the ZnO NP and ionic interlayer show a maximum efficiency of 6.3 cd/A at a 1209 cd/m2 and 5.4 V. The maximum brightness of the device reached 24 000 cd/m2 at 9 V.

Roll-to-Roll Cohesive, Coated, Flexible, High-Efficiency Polymer Light-Emitting Diodes Utilizing ITO-Free Polymer Anodes

This paper reports solution-processed, high-efficiency polymer light-emitting diodes fabricated by a new type of roll-to-roll coating method under ambient air conditions. A noble roll-to-roll cohesive coating system utilizes only natural gravity and the surface tension of the solution to flow out from the capillary to the surface of the substrate. Because this mechanism uses a minimally cohesive solution, the roll-to-roll cohesive coating can effectively realize an ultra-thin film thickness for the electron injection layer. In addition, the roll-to-roll cohesive coating enables the fabrication of a thicker polymer anode film more than 250 nm at one time by modification of the surface energy and without wasting the solution. It is observed that the standard sheet resistance deviation of the polymer anode is only 2.32 Ω/□ over 50 000 bending cycles. The standard sheet resistance deviation of the polymer anode in the different bending angles (0 to 180°) is 0.313 Ω/□, but the case of the ITO-PET is 104.93 Ω/□. The average surface roughness of the polymer anode measured by atomic force microscopy is only 1.06 nm. Because the surface of the polymer anode has a better quality, the leakage current of the polymer light-emitting diodes (PLEDs) using the polymer anode is much lower than that using the ITO-PET substrate. The luminous power efficiency of the two devices is 4.13 lm/W for the polymer anode and 3.21 lm/W for the ITO-PET. Consequently, the PLEDs made by using the polymer anode exhibited 28% enhanced performance because the polymer anode represents not only a higher transparency than the ITO-PET in the wavelength of 560 nm but also greatly reduced roughness. The optimized the maximum current efficiency and power efficiency of the device show around 6.1 cd/A and 5.1 lm/W, respectively, which is comparable to the case of using the ITO-glass.

Multi-film roll transferring (MRT) process using highly conductive and solution-processed silver solution for fully solution-processed polymer solar cells

To produce practical large area polymer solar cells (PSCs), it is highly desirable that the Ag (silver) top electrodes be made by a printing process rather than by vacuum evaporation. However, directly printing electrodes using highly conductive metal inks, such as organometallic and nanoparticle inks, has risks which can cause the infiltration and contamination of the underlying polymer layers during the printing and annealing processes. Moreover, the metal inks usually require high sintering temperatures to achieve high-conductivity electrodes. To overcome these limitations, we introduce a multi-layer roll transferring (MRT) approach, in which a high performance solution processed Ag electrode is prepared separately from the rest of the organic layers, and the device is completed by a final transferring process. By optimizing the processing conditions of the reductive organometallic Ag solution, the resulting metal electrode has an excellent resistivity (3.4 μΩ cm−1) and a morphology comparable to that of a thermally evaporated silver film. The performances of the devices fabricated by the MRT process were comparable to those of metal evaporated devices. Furthermore we achieved fully solution processed devices fabricated by integrating the roll-to-roll coating of the polymer cathode, polymer semiconductor and charge extraction layer and the MRT process.

Semitransparent and Flexible Mechanically Reconfigurable Electrically Small Antennas Based on Tortuous Metallic Micromesh

A novel optically transparent, flexible, and mechanically reconfigurable zeroth-order resonant (ZOR) antenna using stretchable micromesh structure is presented in this paper. The size reduction of the antenna is achieved by using the ZOR property, and the uniform metallic patches of the antenna are replaced with the tortuous micromesh. The tortuous micromesh structures provide a high degree of freedom for stretching when encapsulated in elastomeric polymers with added feature of semitransparency. Accordingly, the structure can undergo mechanical deformation such as stretching (up to 40%), folding, or twisting without breakage. The resonant frequency of the antennas is linearly reconfigurable from 2.94 to 2.46 GHz upon stretching.

A photonic sintering derived Ag flake/nanoparticle-based highly sensitive stretchable strain sensor for human motion monitoring

Recently, the demand for stretchable strain sensors used for detecting human motion is rapidly increasing. This paper proposes high-performance strain sensors based on Ag flake/Ag nanocrystal (NC) hybrid materials incorporated into a polydimethylsiloxane (PDMS) elastomer. The addition of Ag NCs into an Ag flake network enhances the electrical conductivity and sensitivity of the strain sensors. The intense localized heating of Ag flakes/NCs is induced by intense pulsed light (IPL) irradiation, to achieve efficient sintering of the Ag NCs within a second, without damaging the PDMS matrix. This leads to significant improvement in the sensor sensitivity. Our strain sensors are highly stretchable (maximum strain = 80%) and sensitive (gauge factor = 7.1) with high mechanical stability over 10 000 stretching cycles under 50% strain. For practical demonstration, the fabrication of a smart glove for detecting the motions of fingers and a sports band for measuring the applied arm strength is also presented. This study provides an effective method for fabricating elastomer-based high-performance stretchable electronics.

Defect-Free, Highly Uniform Washable Transparent Electrodes Induced by Selective Light Irradiation

A simple route to fabricate defect-free Ag-nanoparticle-carbon-nanotube composite-based high-resolution mesh flexible transparent conducting electrodes (FTCEs) is explored. In the selective photonic sintering-based patterning process, a highly soft rubber or thin plastic substrate is utilized to achieve close and uniform contact between the composite layer and photomask, with which uniform light irradiation can be obtained with diminished light diffraction. This well-controlled process results in developing a fine and uniform mesh pattern (≈12 μm). The mesh patternability is confirmed to be dependent on heat distribution in the selectively light-irradiated film and the pattern design for FTCE could be adopted for more precise patterns with desired performance. Moreover, using a very thin substrate could allow the mesh to be positioned closer to the strain-free neutral mechanical plane. Due to strong interfacial adhesion between the mesh pattern and substrate, the mesh FTCE could tolerate severe mechanical deformation without performance degradation. It is demonstrated that a transparent heater with fine mesh patterns on thin substrate can maintain stability after 100 repeated washing test cycles in which a variety of stress situations occurring in combination. The presented highly durable FTCE and simple fabrication processes may be widely adoptable for various flexible, large-area, and wearable optoelectronic devices.