Jaewook Jeong

Spin-coated Ga-doped ZnO transparent conducting thin films for organic light-emitting diodes

Two corrections should be made to the previously published version of this article. On page 1, right-hand column, the radius of Zn should be read as 0.074 nm. This value is taken from Lide D R 1991 Handbook of Chemistry and Physics 71st edn (Boca Raton, FL: CRC Press). In the first paragraph of section 2, Diethanolammine and its acronym DEA should be corrected to Monoethanolamine and MEA.

Substrate thermal conductivity effect on heat dissipation and lifetime improvement of organic light-emitting diodes

We report substrate thermal conductivity effect on heat dissipation and lifetime improvement of organic light-emitting diodes (OLEDs). Heat dissipation behavior of top-emission OLEDs fabricated on silicon, glass, and planarized stainless steel substrates was measured by using an infrared camera. Peak temperature measured from the backside of each substrate was saturated to be 21.4, 64.5, and 40.5 °C, 180 s after the OLED was operated at luminance of 10 000 cd/m2 and 80% luminance lifetime was about 198, 31, and 96 h, respectively. Efficient heat dissipation through the highly thermally conductive substrates reduced temperature increase, resulting in much improved OLED lifetime.

Inkjet-printed stretchable silver electrode on wave structured elastomeric substrate

We report high performance and stable inkjet-printed stretchable silver electrodes on wave structured elastomeric substrates. Highly conductive silver electrodes were deposited directly on a ultraviolet ozone treated polydimethylsiloxane (PDMS) substrates having vertical wavy structures. Adhesion between printed silver lines and PDMS surface has been enhanced by intentionally roughened PDMS surface with wire-electro discharge machined aluminum mold. During slow (16.7 μm/s) stretching test, resistance of the printed silver electrode was increased only by three times at 30% tensile strain. Inkjet-printed silver electrodes also showed good mechanical stability during 1000-time fast (1 mm/s) cycling test with 10% tensile strain, showing maximum resistance change of less than three times.

All-solution-processed bottom-gate organic thin-film transistor with improved subthreshold behaviour using functionalized pentacene active layer

We report organic thin-film transistors (OTFTs) made by simple solution processes in an ambient air environment. Inkjet-printed silver electrodes were used for bottom-gate and bottom-contacted source/drain electrodes. A spin-coated cross-linked poly(4-vinylphenol) (PVP) and a spin-coated 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) were used as a gate dielectric layer and an active layer, respectively. A high-boiling-point solvent was used for TIPS-pentacene and the resulting film showed stem-like morphology. X-ray diffraction (XRD) measurement showed the spin-coated active layer was well crystallized, showing the (0 0 1) plane. The reasonable mobility, on/off ratio and threshold voltage of the fabricated device, which are comparable to those of the previously reported TIPS-pentacene OTFT with gold electrodes, show that the printed silver electrodes worked successfully as gate and source/drain electrodes. Furthermore, the device showed a subthreshold slope of 0.61 V/dec in the linear region (VDS = −5 V), which is the lowest value for spin-coated TIPS-pentacene TFT ever reported, and much lower than that of the thermally evaporated pentacene OTFTs. It is thought that the surface energy of the PVP dielectric layer is well matched with that of a well-ordered TIPS-pentacene (0 0 1) surface when a high-boiling-point solvent and a low-temperature drying process are used, thereby making good interface properties, and showing higher performances than those for pentacene TFT with the same structure.

Meyer–Neldel Rule and Extraction of Density of States in Amorphous Indium–Gallium–Zinc-Oxide Thin-Film Transistor by Considering Surface Band Bending

In this study, we analyzed the temperature-dependent characteristics of amorphous indium–gallium–zinc-oxide (a-IGZO) thin-film transistors (TFTs). We observed that a-IGZO TFTs obey the Meyer–Neldel rule (MN rule) at low gate-to-source voltage (VGS) and the inverse MN rule at high VGS, both of which can be explained by the statistical shift of Fermi level and electrostatic potential. Large Fermi level movement for small VGS change and the inverse MN rule, which are hardly observed for conventional amorphous TFTs, indicate that there is a very low density of state (DOS) in the sub-bandgap region for a-IGZO TFTs and the performance of TFTs is not affected by contact characteristics, respectively. By using the field-effect method and considering surface band bending, we extracted the DOS in the sub-bandgap region, the distribution of which is clearly distinguished by deep and tail states. The calculated parameters for tail and deep states were Nta = 3.5 ×1017 cm-3 eV-1, Eta = 0.18 eV, Nda = 1.6×1016 cm-3 eV-1, and σda = 0.21 eV.

Photonic crystal band edge laser array with a holographically generated square-lattice pattern

We fabricated a photonic band edge laser array based on a two-dimensional square-lattice photonic crystal (PC) slab waveguide using a laser holography (LH) method, instead of the commonly used electron-beam lithography (EBL). The nature of the LH process enabled high-throughput large-area fabrication of band edge lasers. Moreover, the laser performance was comparable to that of reported EBL counterparts. Careful examination of the spectral positions of the observed modes with respect to the calculated photonic band structure identified three main band edge modes as the origins of lasing: M2, X2, and M1. Owing to the gradual change in the air-hole size of the PC, the lasing modes shifted monotonically along the laser array, resulting in an M1 mode span of ∼30nm (centered at 1550nm) over a distance of 5mm.

MOSFET-Like Behavior of a-InGaZnO Thin-Film Transistors With Plasma-Exposed Source–Drain Bulk Region

In this paper, we analyzed electrical characteristics of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) with plasma-exposed source-drain (S/D) bulk region. The parasitic resistance and effective channel length characteristics exhibit similar behavior with that of crystalline silicon metal oxide-semiconductor field effect transistor (c-Si MOSFET) that has doped S/D bulk region. The transfer curves little changed with gate overlap variation, and the width-normalized parasitic resistance obtained from transmission line method was as low as 3 to 6 Omegamiddotcm. The effective channel length was shorter than the mask channel length and showed gate-to-source (VGS) voltage dependency that is frequently observed for lightly doped drain (LDD) MOSFET. Experimental and simulation results showed that the plasma exposure caused an LDD-like doping effect in the S/D bulk region by inducing oxygen vacancy in the a-IGZO layer.

Stable Stretchable Silver Electrode Directly Deposited on Wavy Elastomeric Substrate

We demonstrated a stable operation of stretchable silver (Ag) electrodes under fast cycling strain stress conditions. Relatively thick Ag electrodes (700 nm) were directly deposited on an elastomeric polydimethylsiloxane (PDMS) substrate with vertical wavy patterns that were formed by using an aluminum mold. The large surface roughness of PDMS substrate that was transferred from an intentionally roughened mold surface enhanced the adhesion between thick electrode and PDMS layer. Top PDMS layer was coated on the Ag electrode for encasement. During a slow stretching test (16.7 mum/s), the resistance of the Ag electrode on the wavy substrate was increased only by three times for 50% tensile strain. The change in Ag electrode resistance was monitored during 10 000 times of fast (1 mm/s) cycling of 10% tensile strain. The maximum resistance was increased by less than five times during the stress test and returned to its initial value after the stress was removed.

Contact Resistance of Inkjet-Printed Silver Source–Drain Electrodes in Bottom-Contact OTFTs

In this paper, we report contact resistance analysis between inkjet-printed silver source-drain (S/D) electrodes and organic semiconductor layer in bottom-contact organic thin-film transistors (OTFTs) using transmission line method (TLM). Inkjet-printed silver electrodes, spin-coated PVP and evaporated pentacene were used as gate and S/D electrodes, gate dielectric layer and semiconductor layer, respectively. On a common gate electrode, S/D electrodes with various channel length from 15 to 111 μm were printed for TLM analysis. The same bottom-contact OTFT with evaporated silver S/D electrodes was also fabricated for reference. We extracted contact resistances of 1.79 MΩ·cm and 0.55 MΩ·cm for inkjet-printed and evaporated silver electrodes, respectively. Higher contact resistance for inkjet-printed silver electrodes can be explained in terms of their relatively poor surface properties at electrode edge that can cause small pentacene molecule grain or slight oxidation of surface during the printed silver sintering process.

Self-Defined Short Channel Formation With Micromolded Separator and Inkjet-Printed Source/Drain Electrodes in OTFTs

We demonstrated fabrication of self-defined short channel using inkjet printing and micromolding in capillary (MIMIC) methods. Self-defined source/drain electrodes were successfully formed when silver ink drops were printed on a silicon dioxide/p+ heavily doped silicon substrate having a prepatterned narrow high-aspect-ratio hydrophobic teflon separator made by MIMIC. We fabricated well-defined channel region of pentacene organic thin film transistors (OTFTs) with 5 and 10 μm channel lengths, which showed good mobilities of 0.1 ~ 0.2 cm2/V · s. Characteristics of the fabricated OTFTs were also analyzed in accordance with channel length and surface wetting properties.