Hyun-Woo Koo

Stretchable Hydrogel Electronics and Devices.

Stretchable hydrogel electronics and devices are designed by integrating stretchable conductors, functional chips, drug-delivery channels, and reservoirs into stretchable, robust, and biocompatible hydrogel matrices. Novel applications include a smart wound dressing capable of sensing the temperatures of various locations on the skin, delivering different drugs to these locations, and subsequently maintaining sustained release of drugs.

Mechanical Flexibility of ZnSnO/Ag/ZnSnO Films Grown by Roll-to-Roll Sputtering for Flexible Organic Photovoltaics

The mechanical flexibility of ZnSnO (ZTO)/Ag/ZTO multilayer films prepared on flexible poly(ethylene terephthalate) (PET) substrates by roll-to-roll (R2R) sputtering was investigated for use in cost-efficient flexible organic solar cells (FOSCs). The change of resistance (ΔR) of the ZTO/Ag/ZTO films was measured by means of lab-made outer/inner bending, twist bending, and stretching test systems. The failure bending radii of the ZTO/Ag/ZTO film in the outer and inner bending tests were 3 and 4.5 mm, respectively. In addition, the twisting test showed that the resistance of the ZTO/Ag/ZTO multilayer began to increase at an angle of 38°. Furthermore, the stretching test showed that the strain failure of the ZTO/Ag/ZTO multilayer film was 3%. The superior flexibility of the ZTO/Ag/ZTO films is attributed to the existence of a ductile Ag layer and the mechanical stability of the amorphous ZTO film. Similar performances of the FOSCs with flexible ZTO/Ag/ZTO anodes to reference OSCs with ITO anodes indicate that the R2R sputter-grown ZTO/Ag/ZTO multilayer is promising as an indium-free flexible anode for cost-efficient FOSCs.

47.1: Invited Paper: Highly Robust Flexible AMOLED Display on Plastic Substrate with New Structure

A new flexible TFT backplane structure with improved mechanical reliability has been fabricated on a plastic substrate using amorphous indium-gallium-zinc-oxide (a-IGZO) thin film transistors. The panel withstood 10,000 bending cycles at a bending radius of 5 mm without any noticeable TFT degradation. After the bending test, change of Vth, mobility, sub-threshold slope, and gate leakage current were only −0.10V, −0.11cm2/V-s, 0.01V/decade, and −1.03×10-−12A, respectively. No line defects, dark spots, or bright spots appeared after 10K bending cycles at a bend radius of 10 mm.

Antireflective ZnSnO/Ag bilayer-based transparent source and drain electrodes for transparent thin film transistors

We report on antireflective ZnSnO (ZTO)/Ag bilayer and ZTO/Ag/ZTO trilayer source/drain (S/D) electrodes for highly transparent ZTO channel-based thin film transistors (TFTs). Although both bilayer and trilayer films have a similar sheet resistance (3–5 Ω/sq), the ZTO/Ag bilayer is a more effective transparent S/D electrode for ZTO channel layer than the ZTO/Ag/ZTO trilayer S/D electrode, due to the direct contact of the Ag layer on the ZTO channel layer and a desirable oxide-metal-oxide multilayer structure for antireflection effects. ZTO channel-based all-transparent TFTs with ZTO/Ag bilayer S/D electrodes exhibited a saturation mobility of 4.54 cm2/Vs and a switching value (1.31 = V/decade), comparable to those of a ZTO channel-based TFT with metallic Ag S/D electrodes. This indicates that the antireflective ZTO/Ag bilayer is a promising transparent S/D electrode for use in all-transparent TFTs as a substitute for conventional opaque metal S/D electrodes.

Flexible ZnSnO3/Ag/ZnSnO3 multilayer electrodes grown by roll-to-roll sputtering on flexible polyethersulfone substrates

The authors report on the characteristics of flexible ZnSnO3 (ZTO)/Ag/ZTO multilayer electrodes sputtered on a flexible polyethersulfone substrate using roll-to-roll sputtering at room temperature. The sheet resistance and optical transmittance of the flexible ZTO/Ag/ZTO multilayer were mainly affected by the thickness and morphology of the Ag layer inserted between the ZTO layers. Dependence of photoluminescence peak intensity on the Ag thickness indicates that the high transmittance of the flexible ZTO/Ag/ZTO electrode can be explained by surface plasmon resonance as well as antireflection effects. At the optimized Ag thickness (10 nm), the flexible ZTO/Ag/ZTO exhibited a resistivity of 4.15 × 10−5 Ω-cm, an optical transmittance of 86.03%, and a figure of merit value of 42.83 × 10−3 Ω−1 comparable to those of conventional crystalline ITO electrodes. In addition, the optimized ZTO/Ag/ZTO sample showed constant resistance change (ΔR/R) during repeated bending cycles, indicating the robustness of the ZTO/A...

Simple brush painted Ag nanowire network on graphene sheets for flexible organic solar cells

The authors report on highly flexible and transparent Ag nanowire (NW) coated graphene hybrid electrodes for flexible organic solar cells (FOSCs). Brush painted Ag NW percolating network on the transparent graphene sheet led to an invisible Ag NW/graphene hybrid electrode with a sheet resistance of 15.25 Ω/sq. and a high optical transmittance of 77.4% as well as superior flexibility. In particular, similar bending radius of Ag NW/graphene hybrid electrode to graphene bilayer electrode during outer bending test demonstrated the superior mechanical flexibility of the Ag NW/graphene hybrid electrodes. FOSCs fabricated on Ag NW/graphene hybrid electrode showed higher power conversion efficiency (2.681%) than that (1.681%) of FOSC with graphene bilayer electrode due to lower sheet resistance and improved wettability for hole extracting layer. This indicates that brush painting of conductive Ag NW is a critical solution to solve the problem of high resistance and hydrophobic graphene electrode for use in FOSC.

Transmission electron microscopy study of degradation in transparent indium tin oxide/Ag/indium tin oxide multilayer films

The degradation mechanism and structural evolution of transparent ITO/Ag/ITO (IAI) multilayer films caused by rapid thermal annealing (RTA) were investigated by high resolution transmission electron microscopy (HRTEM) and synchrotron X-ray scattering analysis. The IAI multilayer with low sheet resistance of 9.51 Ω/square and high transmittance of 88.24% was significantly degraded after 600 °C RTA. Discontinuity, agglomeration of the embedded Ag layer at the interface region of the IAI multilayer, and oxygen diffusion through crystalline ITO grain boundaries into Ag layers led to electrical and optical degradation of the IAI multilayer. Using HRTEM analysis, the microstructures and interfaces of as-deposited and 600 °C annealed IAI multilayer films were compared to explain their electrical and optical degradation mechanisms.

Brush-paintable and highly stretchable Ag nanowire and PEDOT:PSS hybrid electrodes

Highly transparent and stretchable Ag nanowire (NW)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hybrid electrodes were prepared on stretchable polyurethane substrates by using simple and cost-effective brush painting technique. The optimized Ag NW/PEDOT:PSS hybrid electrode showed a sheet resistance of 19.7 Ohm/square and a high optical transmittance of 88.64% comparable to conventional ITO electrode. It was found that shear stress of the paintbrush led to an effective lateral alignment of the Ag NWs into the PEDOT:PSS matrix during brush painting process. In addition, we investigated mechanical properties of the brush painted Ag NW/PEDOT:PSS hybrid electrode using inner/outer bending test, stretching tests, twisting test and rolling test in detail. The optimized brush painted Ag NW/PEDOT:PSS electrode showed a higher strain (~30%) than brush painted Ag NW or sputtered ITO electrode. Furthermore, we demonstrated the outstanding stretchability of brush painted Ag NW/PEDOT:PSS hybrid electrode in two applications: stretchable interconnectors and stretchable electrodes for stretchable and wearable thin film heaters. These results provide clear evidence for its potential and widespread applications in next-generation, stretchable displays, solar cells, and electronic devices.

36.2: Invited Paper: Materials and Components for Flexible AMOLED Display

Materials and Components for Flexible AMOLED Display were reviewed in this paper. Plastic substrates need high thermal resistance and low coefficient of thermal expansion, which would be compatible with mass production line. Flexible modules such as chip on flexible substrate, slim polarizer, and touch screen panel are also important to realize flexible AMOLED display.