Jung-Dae Kwon

Ultrasmooth, extremely deformable and shape recoverable Ag nanowire embedded transparent electrode.

Transparent electrodes have been widely used in electronic devices such as solar cells, displays, and touch screens. Highly flexible transparent electrodes are especially desired for the development of next generation flexible electronic devices. Although indium tin oxide (ITO) is the most commonly used material for the fabrication of transparent electrodes, its brittleness and growing cost limit its utility for flexible electronic devices. Therefore, the need for new transparent conductive materials with superior mechanical properties is clear and urgent. Ag nanowire (AgNW) has been attracting increasing attention because of its effective combination of electrical and optical properties. However, it still suffers from several drawbacks, including large surface roughness, instability against oxidation and moisture, and poor adhesion to substrates. These issues need to be addressed before wide spread use of metallic NW as transparent electrodes can be realized. In this study, we demonstrated the fabrication of a flexible transparent electrode with superior mechanical, electrical and optical properties by embedding a AgNW film into a transparent polymer matrix. This technique can produce electrodes with an ultrasmooth and extremely deformable transparent electrode that have sheet resistance and transmittance comparable to those of an ITO electrode.

Graphene-based gas sensor: metal decoration effect and application to a flexible device

Roles of metal nanoparticles (NPs) on graphene-based devices were investigated in terms of gas-sensing characteristics of NO2 and NH3, and flexible gas sensing was also realized for future applications. The synergistic combination of metal NPs and graphene modulates the electronic properties of graphene, leading to enhancement of selectivity and sensitivity in gas-sensing characteristics. Introduction of palladium (Pd) NPs on the graphene accumulates hole carriers of graphene, resulting in the gas sensor being sensitized by NH3 gas molecular adsorption. In contrast, aluminum (Al) NPs deplete hole carriers, which dramatically improves NO2 sensitivity. Furthermore, the sensitivity of flexible graphene-based gas sensors was also enhanced via the same approach, even after 104 bending cycles and was maintained after 3 months.

High Efficiency Inorganic/Organic Hybrid Tandem Solar Cells

Hybrid tandem solar cells comprising an inorganic bottom cell and an organic top cell have been designed and fabricated. The interlayer combination and thickness matching were optimized in order to increase the overall photovoltaic conversion efficiency. A maximum power conversion efficiency of 5.72% was achieved along with a V(oc) of 1.42 V, reaching as high as 92% of the sum of the subcell V(oc) values.

Surface Modification of a ZnO Electron-Collecting Layer Using Atomic Layer Deposition to Fabricate High-Performing Inverted Organic Photovoltaics

A ripple-structured ZnO film as the electron-collecting layer (ECL) of an inverted organic photovoltaic (OPV) was modified by atomic layer deposition (ALD) to add a ZnO thin layer. Depositing a thin ZnO layer by ALD on wet-chemically prepared ZnO significantly increased the short-circuit current (Jsc) of the OPV. The highest power conversion efficiency (PCE) of 7.96% with Jsc of 17.9 mA/cm2 was observed in the inverted OPV with a 2-nm-thick ALD-ZnO layer, which quenched electron-hole recombination at surface defects of ZnO ripples. Moreover, an ALD-ZnO layer thinner than 2 nm made the distribution of electrical conductivity on the ZnO surface more uniform, enhancing OPV performance. In contrast, a thicker ALD-ZnO layer (5 nm) made the two-dimensional distribution of electrical conductivity on the ZnO surface more heterogeneous, reducing the PCE. In addition, depositing an ALD-ZnO thin layer enhanced OPV stability and initial performance. We suggest that the ALD-ZnO layer thickness should be precisely controlled to fabricate high-performing OPVs.

Self-assembled monolayer as an interfacial modification material for highly efficient and air-stable inverted organic solar cells

Organic solar cells with inverted structures can greatly improve photovoltaic stability. This paper reports a method to lower the work function of indium tin oxide (ITO) in inverted organic solar cells by modification with ultrathin 3-aminopropyltriethoxysilane (APTES) monolayers. The device studies showed that the resulting photovoltaic efficiencies were significantly increased from 0.64% to 4.83% with the use of the APTES monolayer, which could be attributed to the dramatic enhancement in the open-circuit voltage and fill factor. The effective electron selectivity in the case of the APTES-modified ITO could be attributed to the reduction of the work function of ITO as a result of the electron-donating nature of the amine groups in the APTES monolayer. The power conversion efficiency of the unencapsulated inverted organic solar cells with APTES-modified ITO remained above 80% of their original values even after storage in air for thirty days. Our results provide a promising approach to improve the performance of highly efficient and air-stable inverted organic solar cells.

Fabrication of 3D ZnO hollow shell structures by prism holographic lithography and atomic layer deposition

Highly uniform 3D ZnO hollow shell structures were prepared by combining prism holographic lithography (PHL) and atomic layer deposition (ALD). As a dense ZnO film was obtained by using the ALD process, no volume shrinkage occurred during the subsequent calcination to remove the sacrificial polymer template. No volume shrinkage during heat treatment is crucial for achieving excellent optical properties and mechanical stability of inverse photonic crystals (PCs).

Control of Crystallinity in Nanocrystalline Silicon Prepared by High Working Pressure Plasma-Enhanced Chemical Vapor Deposition

The crystalline volume of nanocrystalline silicon (Si) films could be successfully controlled simply by changing the substrate scan speed at the high working pressure of 300u2009Torr. The Si crystalline volume fraction was increased from 30% to 57% by increasing the scan speed from 8 to 30u2009mm/s. When the Si film was prepared at a low scan speed (8u2009mm/s), Si crystals of size 5u2009nm grew homogeneously through the whole film. The higher scan speed was found to accelerate crystallization, and crystals of size up to 25u2009nm were deposited in the Si film deposited when the scan speed was 30u2009mm/s.

Low Temperature Two-Step Atomic Layer Deposition of Tantalum Nitride for Cu Diffusion Barrier

A cubic δ-TaN thin film with an electrical resistivity of 400 μΩ cm was successfully obtained by suppressing the formation of Ta 3 N 5 using two-step atomic layer deposition independent of NH 3 dosage. The deposition cycle involved two chemical reaction steps: The formation of elemental tantalum (Ta) by reducing tantalum pentafluoride (TaF 5 ) with hydrogen plasma and the subsequent nitridation of the preformed Ta with NH 3 at 200―350°C. The microstructure of the preformed Ta was β-Ta phase with an electrical resistivity of 220 μΩ cm, which was formed without regard to the deposition temperature. At a deposition temperature of less than 250°C, cubic δ-TaN with a Ta/N ratio of I was achieved independent of the NH 3 dosage. However, at a deposition temperature of greater than 300°C, the resistivity of Ta―N-based thin film increased abruptly as the NH 3 dosage exceeded 16.08 × 10 19 molecules/cm 3 due to the formation of Ta 3 N 5 .

Silver-palladium alloy deposited by DC magnetron sputtering method as lubricant for high temperature application

Abstract The silver-palladium(Ag-Pd) alloy coating as a solid lubricant was investigated for its application to the high temperature stud bolts used in nuclear power plants. A hex bolt sample was prepared in the following steps: 1) bolt surface treatment using alumina grit blasting for cleaning and increasing the surface area; 2) nickel(Ni) film coating as a glue layer on the surface of the bolt; and 3) Ag-Pd alloy coating on the Ni film. The films were deposited by using a direct current(DC) magnetron sputtering system. The thickness and composition of the Ag-Pd alloy film have effect on the friction coefficient, which was determined using axial force measurement. A 500 nm-thick Ag-Pd (80:20, molar ratio) alloy film has the lowest friction coefficient of 0.109. A cyclic test was conducted to evaluate the durability of bolts coated with either the Ag-Pd (80:20) alloy film or N-5000 oil. In a cycle, the bolts were inserted into a block using a torque wrench, which was followed by heating and disassembling. After only one cycle, it was not possible to remove the bolts coated with the N-5000 oil from the block. However, the bolts coated with the Ag-Pd (80:20) alloy could be easily removed up until 15 cycles.

3D multilayered plasmonic nanostructures with high areal density for SERS

Enhancing light–matter interactions is essential to improving nanophotonic and optoelectronic device performance. In the present work, we developed a new design for 3D plasmonic nanostructures with enhanced near-field interactions among the plasmonic nanomaterials. The 3D plasmonic nanostructures consisted of multilayered bottom Ag/polydimethylsiloxane (PDMS) nanostructures, an alumina middle layer, and top Ag nanoparticles (NPs). High areal density PDMS nanoprotrusions were self-organized by a simple maskless plasma etching process. The conformal deposition of alumina using atomic layer deposition and Ag deposition produced 3D plasmonic nanostructures. These structures induced multiple near-field interactions between the ultrahigh-areal-density (1400 μm−2) top Ag NPs and the underlying Ag nanostructures, and among the top Ag NPs themselves. The high density of hot spots across the 3D space yielded highly efficient and widely tunable plasmonic responses across the entire visible range. The SERS signal enhancement measured at the 3D plasmonic nanostructures was 3.9 times the signal measured at the 2D multilayered structures and 48.0 times the signal measured at a Ag NP layer deposited onto a Si substrate. Finally, the 3D plasmonic nanostructures exhibited excellent uniformity with a variation of 6.8%, based on a microscale Raman mapping analysis. The excellent Raman signal uniformity can be attributed to the ultrahigh areal density of the Ag NPs and the uniform thickness of the alumina spacing layer.