Byeong-Soo Bae

High performance solution-processed amorphous zinc tin oxide thin film transistor

Thin film transistors (TFTs) with amorphous zinc tin oxide (ZTO) channel layer were fabricated by a simple and low-cost solution process. The ZTO thin films are highly transparent (>90% transmittance) in the visible region. The ZTO TFTs fabricated at 400 and 500 °C are operated in enhancement mode. The TFT annealed at 500 °C shows a mobility of 14.11 cm2 V−1 s−1, a threshold voltage of 1.71 V, a subthreshold slope of 0.4 V dec−1 and an on–off current ratio greater than 108. In addition, we investigated the gate bias stability of the TFT. Positive gate bias results in a positive shift of the threshold voltage due to the charge trapping in the channel/dielectric interface.

Solution-Processed Indium-Zinc Oxide Transparent Thin-Film Transistors

Transparent thin-film transistors (TTFTs) with an indium-zinc oxide (IZO) active layer by the solution-processed deposition method were fabricated and their TFT characterization was examined. Solution-processed IZO thin films were amorphous and highly transparent with >90% transmittance in the visible region with an optical bandgap of 3.1 eV. Spin-coated IZO TTFTs were operated in depletion mode and showed a field-effect mobility as high as 7.3 cm 2 /V s, a threshold voltage of 2.5 V, an on/off current ratio greater than 10 7 , and a subthreshold slope of 1.47 V/decade.

Raman spectroscopy of copper phosphate glasses

Abstract Raman spectra of binary copper phosphate glass samples, xCuO · (1 − x)P2O5, having x = 0.4, 0.5, and 0.55 and various [Cu2+]/[Cutotal] ratios have been measured. The Raman spectra of the samples changed with changing [Cu2+]/[Cutotal] ratio as well as CuO content. The smaller ionic size and the greater charge of the Cu2+ ion than those of the Cu1+ ion create larger in-chain POP bond angles and smaller out-of-chain PO2 angle as well as greater CuO field strength. Differing local structures of Cu1+ and Cu2+ ions in phosphate glass results in a variation of Raman spectra of the glasses depending on the [Cu2+]/[Cutotal] ratio. Also, a decrease in phosphate chain length with increasing CuO content affected the Raman spectra of the glasses. Although the crystallization of the samples is not detected in the X-ray diffraction, the overlapping bands in the Raman spectra of the 0.55 CuO samples are attributed to crystalline phases.

Stretchable and Transparent Electrodes using Hybrid Structures of Graphene–Metal Nanotrough Networks with High Performances and Ultimate Uniformity

Transparent electrodes that can maintain their electrical and optical properties stably against large mechanical deformations are essential in numerous applications of flexible and wearable electronics. In this paper, we report a comprehensive analysis of the electrical, optical, and mechanical properties of hybrid nanostructures based on graphene and metal nanotrough networks as stretchable and transparent electrodes. Compared to the single material of graphene or the nanotrough, the formation of this hybrid can improve the uniformity of sheet resistance significantly, that is, a very low sheet resistance (1 Ω/sq) with a standard deviation of less than ±0.1 Ω/sq, high transparency (91% in the visible light regime), and superb stretchability (80% in tensile strain). The successful demonstration of skin-attachable, flexible, and transparent arrays of oxide semiconductor transistors fabricated using hybrid electrodes suggests substantial promise for the next generation of electronic devices.

Flexible Transparent Conducting Hybrid Film Using a Surface-Embedded Copper Nanowire Network: A Highly Oxidation-Resistant Copper Nanowire Electrode for Flexible Optoelectronics

We report a flexible high-performance conducting film using an embedded copper nanowire transparent conducting electrode; this material can be used as a transparent electrode platform for typical flexible optoelectronic devices. The monolithic composite structure of our transparent conducting film enables simultaneously an outstanding oxidation stability of the copper nanowire network (14 d at 80 °C), an exceptionally smooth surface topography (R(rms) < 2 nm), and an excellent opto-electrical performances (Rsh = 25 Ω sq(-1) and T = 82%). A flexible organic light emitting diode device is fabricated on the transparent conducting film to demonstrate its potential as a flexible copper nanowire electrode platform.

Solution-Processed Flexible Fluorine-doped Indium Zinc Oxide Thin-Film Transistors Fabricated on Plastic Film at Low Temperature

Transparent flexible fluorine-doped indium zinc oxide (IZO:F) thin-film transistors (TFTs) were demonstrated using the spin-coating method of the metal fluoride precursor aqueous solution with annealing at 200°C for 2 hrs on polyethylene naphthalate films. The proposed thermal evolution mechanism of metal fluoride aqueous precursor solution examined by thermogravimetric analysis and Raman spectroscopy can easily explain oxide formation. The chemical composition analysed by XPS confirms that the fluorine was doped in the thin films annealed below 250°C. In the IZO:F thin films, a doped fluorine atom substitutes for an oxygen atom generating a free electron or occupies an oxygen vacancy site eliminating an electron trap site. These dual roles of the doped fluorine can enhance the mobility and improve the gate bias stability of the TFTs. Therefore, the transparent flexible IZO:F TFT shows a high mobility of up to 4.1 cm2/V·s and stable characteristics under the various gate bias and temperature stresses.

Effect of increased surface area of stainless steel substrates on the efficiency of dye-sensitized solar cells

In order to increase the electrical contact area between TiO2 particles and stainless steel (StSt) substrates of the dye-sensitized solar cells (DSSCs), StSt foil was roughened electrochemically using sulfuric acid with some additives. Compared with the DSSC with nontreated StSt substrate, DSSC with this roughened StSt substrate showed a 33% increase in light-to-electricity conversion efficiency with negligible effect on open circuit voltage (Voc) and fill factor. Electrochemical impedance spectroscopy clearly confirmed that the increased performance was due to a decreased electrical resistance at the TiO2/StSt interface.

Measurement of the thermo-optic coefficients in sol-gel derived inorganic–organic hybrid material films

Thermo-optic coefficients (dn/dT) of inorganic–organic hybrid material films prepared by sol-gel process of organoalkylsilanes are measured using the prism coupler equipped with autocontrolled hot stage. In order to validate the reliability of this method, dn/dT of polymethylmethacrylate film is measured. dn/dT of inorganic–organic hybrid material films are negative and as high as the order of 10−4, which are comparable to those of optical polymers. Their dn/dT increase with increasing organic content in the film and mainly depend on their thermal expansion.

Chitin Nanofiber Transparent Paper for Flexible Green Electronics

A transparent paper made of chitin nanofibers (ChNF) is introduced and its utilization as a substrate for flexible organic light-emitting diodes is demonstrated. Given its promising macroscopic properties, biofriendly characteristics, and availability of the raw material, the utilization of the ChNF transparent paper as a structural platform for flexible green electronics is envisaged.

Er3+ luminescence and cooperative upconversion in ErxY2−xSiO5 nanocrystal aggregates fabricated using Si nanowires

Er3+ luminescence and cooperative upconversion in ErxY2−xSiO5 nanocrystal aggregates fabricated using Si nanowires is investigated. X-ray diffraction and photoluminescence spectroscopy indicate that the composition of the final nanocrystals can be varied continuously from pure Y2SiO5 to pure Er2SiO5 while keeping the crystal structure the same. Analysis of concentration and pump-power dependence of the Er3+ photoluminescence intensity and decay time shows that while cooperative upconversion occurs at high Er concentrations, the cooperative upconversion coefficient is only (2.2±1.1)×10−18cm3∕s at a Er concentration of 1.2×1021cm−3. This is nearly ten times lower at more than ten times higher Er concentration than that reported from Er-doped silica and demonstrates the viability of using such silicates for compact, high-gain Si-based optical material for Si photonics.