Hyeon-Gyun Im

Optical and electrical properties of Ga2O3-doped ZnO films prepared by r.f. sputtering

Abstract Optical and electrical properties of Ga2O3doped ZnO films prepared by r.f. sputtering have been investigated as functions of preparation conditions and dopant concentration in an attempt to develop transparent films with low electrical resistivity and with good stability at higher temperatures. The electrical resistivity of the sputtered films depends strongly on the r.f. power density, the argon gas pressure, the Ga2O3 concentration and the thickness of the films when the thickness is less than about 2500 A. The optical transparency depends on the thickness and dopant concentration and is almost independent of the sputtering conditions. Ga2O3-doped ZnO films become degenerate semiconductors when the carrier concentration exceeds about 1019 cm−3 and the optical band gap increases with increasing electron concentration owing to the increase in the Fermi level in the conduction band. Ga2O3-doped ZnO films 3000 A thick with an optical transmission higher than 85% and with electrical resistivity lower than 10−3 ohmcm can be produced by sputtering a ZnO target containing 5 wt.% Ga2O3 with an r.f. power density of 0.84 W cm−2 and an argon gas pressure of 5 m Torr.

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.

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.

Effects of annealing conditions on the properties of tantalum oxide films on silicon substrates

The formation of a SiO2 layer at the Ta2O5/Si interface is observed by annealing in dry O2 or N2 and the thickness of this layer increases with an increase in annealing temperature. Leakage current of thin (less than 40 nm thick) Ta2O5 films decreases as the annealing temperature increases when annealed in dry O2 or N2. The dielectric constant vs annealing temperature curve shows a maximum peak at 750 or 800° C resulting from the crystallization of Ta2O5. The effect is larger in thicker Ta2O5 films. But the dielectric constant decreases when annealed at higher temperature due to the formation and growth of a SiO2 layer at the interface. The flat band voltage and gate voltage instability as a function of annealing temperature can be explained in terms of the growth of interfacial SiO2. The electrical properties of Ta2O5 as a function of annealing conditions do not depend on the fabrication method of Ta2O5 but strongly depend on the thickness of Ta2O5 layer.

High-performance hybrid plastic films: a robust electrode platform for thin-film optoelectronics

We report a novel flexible hybrid plastic film that can be used as a robust electrode platform for typical thin-film optoelectronic devices. Silver nanowires (AgNWs) were embedded on the surface of a glass-fabric reinforced transparent composite (GFRHybrimer) film to form a flexible transparent conducting substrate with excellent opto-electrical properties, superior thermal stability, and impressive mechanical flexibility. A highly efficient and flexible inverted organic solar cell with a power conversion efficiency (PCE) of 5.9% under 100 mW cm−2 AM 1.5G illumination was fabricated on the AgNW–GFRHybrimer film. The AgNW–GFRHybrimer film exhibits potential as an excellent transparent electrode for low cost flexible optoelectronic devices.

Moisture Barrier Composites Made of Non-Oxidized Graphene Flakes

Graphene flakes (GFs) with minimized defects and oxidation ratios are incorporated into polyethylene (PE) to enhance the moisture barrier. GFs produced involving solvothermal intercalation show extremely low oxidation rates (3.17%), and are noncovalently functionalized in situ, inducing strong hydrophobicity. The fabricated composite possesses the best moisture barrier performance reported for a polymer-graphene composite.

Reliable thin-film encapsulation of flexible OLEDs and enhancing their bending characteristics through mechanical analysis

Thin film encapsulation of flexible organic light-emitting diodes (FOLEDs) with a moisture barrier, incorporating a silica nanoparticle-embedded sol–gel organic–inorganic hybrid nanocomposite (S–H nanocomposite) and Al2O3 were demonstrated, and their reliability and mechanical characteristics were assessed. The bending stress of the multi-layer structure for both the case of moisture barriers and encapsulated FOLEDs was investigated based on nonlinear finite-element analysis (FEA). To minimize the bending stress at the desired region, the neutral axis (NA) position could be strategically adjusted by the introduction of a buffer layer of UV-curable cycloaliphatic epoxy hybrid materials (hybrimer), synthesized via a sol–gel reaction. The optimized multi-layer structure, proposed as a result of FEA was validated by related experiments. Regarding the bending characteristics of the moisture barrier structure, the water vapor transmission rate (WVTR) of the hybrimer-coated moisture barrier was much lower than that of a non-coated sample, as a result of calcium corrosion tests after bending. The structure of encapsulated FOLEDs, which are coated by the hybrimer achieved an almost identical performance to that of non-bending samples in spite of 30 days exposure to 30 °C and 90% R.H. after a bending test with a radius of 1 cm. During this period, the occurrence of dark spots caused by moisture penetration was effectively suppressed. Collectively, these results suggest that the bending characteristics of hybrimer-coated multi-layer structures are remarkably improved with the theoretical prediction of the NA position.

Flexible Hard Coating: Glass‐Like Wear Resistant, Yet Plastic‐Like Compliant, Transparent Protective Coating for Foldable Displays

A flexible hard coating for foldable displays is realized by the highly cross-linked siloxane hybrid using structure-property relationships in organic-inorganic hybridization. Glass-like wear resistance, plastic-like flexibility, and highly elastic resilience are demonstrated together with outstanding optical transparency. It provides a framework for the application of siloxane hybrids in protective hard coatings with high scratch resistance and flexibility for foldable displays.

Metal-containing thin-film encapsulation with flexibility and heat transfer

The thin-film encapsulation (TFE) technology is a salient technique for the realization of flexible organic light-emitting diodes. To reliably fabricate bendable and lightweight displays, ultra-thin and flexible encapsulation is required. Reported herein is a moisture-resistant, flexible, and thermally conductive TFE technology created by inserting a metal thin film with an inorganic–organic multibarrier structure to resolve the reliability and heat dissipation issues. Silica-nanoparticle-embedded sol-gel organic/inorganic hybrid nanocomposite (S-H) and Al2O3 were used as organic and inorganic materials, respectively. A silver (Ag) thin film used as a metal was deposited through thermal evaporation, and it had slight barrier properties, outstanding ductility, and high thermal conductivity. The proposed structure, which consists of three materials, resulted in a low water vapor transmission rate of 10−5 g/m2/day for a 240-nm-thick thin film, and showed improvement of the resistance to bending stress compared with the previous structure formed without an Ag thin film in terms of flexibility. A comparative analysis of the heat transfer properties of encapsulation structures was also performed through the investigation of the thermal conductivity of the materials, and thermal imaging measurement. The heat dissipation performance was confirmed to have been improved by the insertion of Ag thin films into the inorganic/organic multibarrier.

Flexible Transparent Conductive Films with High Performance and Reliability Using Hybrid Structures of Continuous Metal Nanofiber Networks for Flexible Optoelectronics

We report an Ag nanofiber-embedded glass-fabric reinforced hybrimer (AgNF-GFRHybrimer) composite film as a reliable and high-performance flexible transparent conducting film. The continuous AgNF network provides superior optoelectronic properties of the composite film by minimizing transmission loss and junction resistance. In addition, the excellent thermal/chemical stability and mechanical durability of the GFRHybrimer matrix provides enhanced mechanical durability and reliability of the final AgNF-GFRHybrimer composite film. To demonstrate the availability of our AgNF-GFRHybrimer composite as a transparent conducting film, we fabricated a flexible organic light-emitting diode (OLED) device on the AgNF-GFRHybrimer film; the OLED showed stable operation during a flexing.