Woo Soon Jang

Low-temperature, high-performance solution-processed thin-film transistors with peroxo-zirconium oxide dielectric.

We demonstrated solution-processed thin film transistors on a peroxo-zirconium oxide (ZrO(2)) dielectric with a maximum temperature of 350 °C. The formation of ZrO(2) films was investigated by TG-DTA, FT-IR, and XPS analyses at various temperatures. We synthesized a zirconium oxide solution by adding hydrogen peroxide (H(2)O(2)). The H(2)O(2) forms peroxo groups in the ZrO(2) film producing a dense-amorphous phase and a smooth surface film. Because of these characteristics, the ZrO(2) film successfully blocked leakage current even in annealing at 300 °C. Finally, to demonstrate that the ZrO(2) film is dielectric, we fabricated thin-film transistors (TFTs) with a solution-processed channel layer of indium zinc oxide (IZO) on ZrO(2) films at 350 °C. These TFTs had a mobility of 7.21 cm(2)/(V s), a threshold voltage (V(th)) of 3.22 V, and a V(th) shift of 1.6 V under positive gate bias stress.

Boron-Doped Peroxo-Zirconium Oxide Dielectric for High-Performance, Low-Temperature, Solution-Processed Indium Oxide Thin-Film Transistor

We developed a solution-processed indium oxide (In2O3) thin-film transistor (TFT) with a boron-doped peroxo-zirconium (ZrO2:B) dielectric on silicon as well as polyimide substrate at 200 °C, using water as the solvent for the In2O3 precursor. The formation of In2O3 and ZrO2:B films were intensively studied by thermogravimetric differential thermal analysis (TG-DTA), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FT IR), high-resolution X-ray diffraction (HR-XRD), and X-ray photoelectron spectroscopy (XPS). Boron was selected as a dopant to make a denser ZrO2 film. The ZrO2:B film effectively blocked the leakage current at 200 °C with high breakdown strength. To evaluate the ZrO2:B film as a gate dielectric, we fabricated In2O3 TFTs on the ZrO2:B dielectrics with silicon substrates and annealed the resulting samples at 200 and 250 °C. The resulting mobilities were 1.25 and 39.3 cm(2)/(V s), respectively. Finally, we realized a flexible In2O3 TFT with the ZrO2:B dielectric on a polyimide substrate at 200 °C, and it successfully operated a switching device with a mobility of 4.01 cm(2)/(V s). Our results suggest that aqueous solution-processed In2O3 TFTs on ZrO2:B dielectrics could potentially be used for low-cost, low-temperature, and high-performance flexible devices.

Driving vertical phase separation in a bulk-heterojunction by inserting a poly(3-hexylthiophene) layer for highly efficient organic solar cells

A desirable vertical phase separation of a bulk-heterojunction was achieved by inserting a P3HT layer between the blend layer and the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer. According to the high (PEDOT:PSS) and low (P3HT) surface energies of substrate, it might be possible to modulate the vertical phase separation in the bulk-heterojunction. The result of vertical phase separation was determined using time-of-flight secondary-ion mass spectroscopy analysis. A controlled thickness of 50 nm for the inserted P3HT layer prevented undesirable light absorption and the power conversion efficiency of this condition was increased by 44% compared to that of a reference device.

Efficient Hydrogen Evolution by Mechanically Strained MoS2 Nanosheets

We demonstrated correlations between mechanically bent tensile-strain-induced two-dimensional MoS2 nanosheets (NSs) and their electrochemical activities toward the hydrogen evolution reaction (HER). The tensile-strain-induced MoS2 NSs showed significantly steeper polarization curves and lower Tafel slopes than the strain-free ones, which is consistent with the simple d-band model. Furthermore, the mechanical strain increased the electrochemical activities of all the NSs toward the HER except those loaded with high MoS2 mass. Mechanically bending MoS2 NSs to induce tensile strain enables the production of powerful, efficient electrocatalysis systems for evolving hydrogen.

Effects of solution temperature on solution-processed high-performance metal oxide thin-film transistors

Herein, we report a novel and easy strategy for fabricating solution-processed metal oxide thin-film transistors by controlling the dielectric constant of H2O through manipulation of the metal precursor solution temperature. As a result, indium zinc oxide (IZO) thin-film transistors (TFTs) fabricated from IZO solution at 4 °C can be operated after annealing at low temperatures (∼250 °C). In contrast, IZO TFTs fabricated from IZO solutions at 25 and 60 °C must be annealed at 275 and 300 °C, respectively. We also found that IZO TFTs fabricated from the IZO precursor solution at 4 °C had the highest mobility of 12.65 cm2/(V s), whereas the IZO TFTs fabricated from IZO precursor solutions at 25 and 60 °C had field-effect mobility of 5.39 and 4.51 cm2/(V s), respectively, after annealing at 350 °C. When the IZO precursor solution is at 4 °C, metal cations such as indium (In3+) and zinc ions (Zn2+) can be fully surrounded by H2O molecules, because of the higher dielectric constant of H2O at lower temperatures. These chemical complexes in the IZO precursor solution at 4 °C are advantageous for thermal hydrolysis and condensation reactions yielding a metal oxide lattice, because of their high potential energies. The IZO TFTs fabricated from the IZO precursor solution at 4 °C had the highest mobility because of the formation of many metal-oxygen-metal (M-O-M) bonds under these conditions. In these bonds, the ns-orbitals of the metal cations overlap each other and form electron conduction pathways. Thus, the formation of a high proportion of M-O-M bonds in the IZO thin films is advantageous for electron conduction, because oxide lattices allow electrons to travel easily through the IZO.

Highly efficient inverted polymer solar cells with reduced graphene-oxide-zinc-oxide nanocomposites buffer layer

In this study, we reported a 36% improvement in the performance of inverted solar cells as a result of increased short-circuit current (JSC) obtained using a composition of zinc oxide (ZnO) and reduced graphene oxide (RGO) as an n-type buffer layer. RGO-ZnO nanocomposites show higher electron conductivity than intrinsic ZnO; moreover, they show reduced contact resistance at the interface between the active layer and n-type buffer layer. These factors prevent carrier loss resulting from defects and recombinations in the device, thereby significantly increasing the JSC value for the device. Thus, an efficiency of 4.15% was achieved for inverted solar cells with a controlled RGO-ZnO nanocomposites layer.

Playing with dimensions: rational design for heteroepitaxial p-n junctions.

A design for a heteroepitaxial junction by the way of one-dimensional wurzite on a two-dimensional spinel structure in a low-temperature solution process was introduced, and its capability was confirmed by successful fabrication of a diode consisting of p-type cobalt oxide (Co(3)O(4)) nanoplate/n-type zinc oxide (ZnO) nanorods, showing reasonable electrical performance. During thermal decomposition, the 30° rotated lattice orientation of Co(3)O(4) nanoplates from the orientation of β-Co(OH)(2) nanoplates was directly observed using high-resolution transmission electron microscopy. The epitaxial relations and the surface stress-induced ZnO nanowire growth on Co(3)O(4) were well supported using the first-principles calculations. Over the large area, (0001) preferred oriented ZnO nanorods epitaxially grown on the (111) plane of Co(3)O(4) nanoplates were experimentally obtained. Using this epitaxial p-n junction, a diode was fabricated. The ideality factor, turn-on voltage, and rectifying ratio of the diode were measured to be 2.38, 2.5 V and 10(4), respectively.

Kinetically controlled way to create highly uniform mono-dispersed ZnO sub-microrods for electronics

Highly uniform mono-dispersed ZnO sub-microrods were synthesized under the condition of a limited injection of Zn divalent ions into an aqueous solution of hexamethylenetetramine at 85 °C. The mechanisms governing the initial seed formation and growth of the ZnO sub-microrods were revealed as an oriented aggregation and classical growth, respectively, from an analytic observation with a high-resolution transmission electron microscope. To demonstrate the capability and possibility of forming uniform microrods for electronic applications, a monolayer of thermally annealed rods was produced on a flexible substrate.

Ultrathin self-powered artificial skin

Herein, we introduce an ultra-thin self-powered artificial skin (SPAS) based on a piezoelectric nanogenerator, which harvests stored elastic deformation energy produced by the bending and stretching actions of the skin. This finding is an important step toward building a self-powered “smart skin”.

Homogeneous liquid crystal alignment on inorganic–organic hybrid silica thin films derived by the sol–gel method

We developed an inorganic–organic hybrid thin film via the sol–gel method for a new liquid crystal alignment layer and investigated the influence of an organic species on the alignment characteristics of the liquid crystals (LCs). A thin film of methyl-doped amorphous silicon oxide (a-SiOx:CH3) was fabricated from the hydrolysis and condensation reaction of the initial precursors of methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS) at a proper ratio. A low-energy ion beam (IB) treatment gives rise to the homogeneous alignment of LC in an IB condition on a-SiOx:CH3 thin film; however, it is difficult to control the LC alignment on a-SiOx thin film derived only from TEOS as a precursor. The LC alignment depending on the chemical structure of the silica thin film was investigated and analyzed in terms of the sensitivity of the axis-selective destruction of the chemical bonding on the surface of the thin silica film.