Sang Chul Lim

Hysteresis of pentacene thin-film transistors and inverters with cross-linked poly(4-vinylphenol) gate dielectrics

The authors report the effects of hydroxyl groups (OH bonds) on the electrical reliabilities of pentacene organic thin-film transistors (OTFTs) with poly-4-vinylphenol (PVP) gate dielectrics. PVP gate dielectric films mixed with different concentrations of methylated poly(melamine-co-formaldehyde) (MMF) were fabricated, and experiments on the hysteresis behavior of the OTFT device were conducted. Pentacene TFTs with the PVP (MMF 0wt.%) exhibited a large hysteresis, while in the PVP (MMF 125wt.%), nearly no hysteresis was observed. Large hysteresis observed in OTFT devices was confirmed to be strongly related to the hydroxyl groups existing inside of the polymeric dielectrics and could reduced by the decrease of OH group.

Hysteresis and threshold voltage shift of pentacene thin-film transistors and inverters with Al2O3 gate dielectric

The pentacene organic thin-film transistors (OTFTs) with plasma-enhanced atomic-layer-deposited 150nm thick Al2O3 as a gate dielectric and the inverters comprised of these OTFTs have been fabricated. The hysteresis in transfer characteristics of the OTFTs depends on the scan range of negative gate voltage, regardless of drain voltage. Negative gate-bias stress leads to the progressive negative shift of threshold voltage. The hysteresis of D inverter is similar to that of E inverter, but the former has a wider swing range and a higher gain in comparison with the latter.

Improved Contact Properties for Organic Thin-Film Transistors Using Self-Assembled Monolayer

Contact properties between the organic semiconductor and the metal source and drain contacts are one of the most important issues since the charge carrier injection through the contact has serious effects on the device performance. In this study, gold (Au) surfaces of the source and drain electrodes are modified using self-assembled monolayers (SAMs) using thiol derivatives. The charge carrier mobility was increased from 0.1 to 0.48 cm2/Vs after the modification of the source and drain electrodes. Mobilities and saturation currents seem to be governed more effectively by the contact properties than by the work function difference between the (modified) electrodes and organic semiconductor.

Low-voltage and high-gain pentacene inverters with plasma-enhanced atomic-layer-deposited gate dielectrics

The pentacene thin-film transistors with the plasma-enhanced atomic-layer-deposited 150nm thick Al2O3 or 120nm thick ZrO2 have been operated at gate voltages between −3 and 3V. The inverter with a ZrO2 gate dielectric shows a gain of 49 and a full swing from supply voltage (Vdd) to 0V, operating at input voltages (Vin) from 0to−1V and at Vdd of −1V. The hysteresis observed in the voltage transfer characteristic of the inverter depends on the scan range of Vin applied to the driver transistor, regardless of the Vdd applied to the load transistor.

Facile one-step synthesis of magnesium-doped ZnO nanoparticles: optical properties and their device applications

In this study, magnesium-doped (Mg-doped) zinc oxide (ZnO) nanoparticles were successfully synthesized by a sonochemical process under mild conditions. The x-ray diffraction pattern indicated that the Mg-doped ZnO nanoparticles maintain a wurtzite structure without impurities. We observed a blue-shift of the bandgap of the Mg-doped ZnO nanoparticles as the Mg-doping ratio increased. We also fabricated thin-film transistor (TFT) devices with the doped-ZnO nanoparticles. Devices using Mg-doped ZnO nanoparticles as a channel layer showed insensibility to white-light irradiation compared with undoped ZnO TFTs.

Electric characteristics of poly(3-hexylthiophene) organic field-effect transistors fabricated on O2 plasma-treated substrates

Soluble conjugated polymers such as poly(3-hexylthiophene) (P3HT) are very promising candidates for a cheap electronic device on various substrates. In this study we report the effects of O2 plasma treatment of the substrates on the electrical properties of P3HT organic thin-film transistors and metal/insulator/organic semiconductor capacitors. Based on the results of a capacitance–voltage measurement, the effective charge density in the interface between P3HT and SiO2 layer treated by O2 plasma for 30 s was approximately −18.3u2009nC/cm2. When the period of O2 plasma treatment was longer than 30 s, the field-effect mobility decreased since the amount of charge and the relaxation time constant of interface traps increased.

Stretchable Organic Thin-Film Transistors Fabricated on Wavy-Dimensional Elastomer Substrates Using Stiff-Island Structures

Stretchable organic thin-film transistors (OTFTs) were fabricated on the polydimethysiloxane (PDMS) elastomer substrates by employing the wavy-dimensional and polyimide stiff-island structures. A low-temperature solution process was also designed to obtain high strain profiles. The endurable maximum strains were estimated to be 2.28, 9.70, and 9.32% for the OTFTs formed on the flat, 1D-, and 2D-wavy PDMS elastomers, respectively. The field-effect mobilities were obtained to be 5 ~ 7 × 10-4 cm2 V-1 s-1 for all devices and they did not exhibit any degradation under the stretchable conditions before the fracture. The results suggest that the proposed methodologies were quite suitable for high-performance stretchable OTFTs.

Preparation and Characteristics of PMMA Microlens Array for a BLU Application by An Inkjet Printing Method

In this study, poly(methyl methacrylate)(PMMA) microlens array for a back light unit(BLU) application was fabricated by an inkjet printer, which loaded piezoelectric microchannel mounted 30 µm nozzles to deposit polymer droplets via drop-on-demand fashion. Before the inkjet printing, the surface of PMMA substrates was modified chemically by a phase separation technique. The surface topologies and aspect ratios of microlens were measured. A 5.2 × 7 cm2 BLU plate with microlens arrays printed by the inkjet method was fabricated for the application of a mobile phone.

Threshold voltage control of pentacene thin‐film transistor with dual‐gate structure

Abstract This paper presents a comprehensive study on threshold voltage (Vth) control of organic thin‐film transistors (OTFTs) with dual‐gate structure. The fabrication of dual‐gate pentacene OTFTs using plasma‐enhanced atomic layer deposited (PEALD) 150 nm thick Al2O3 as a bottom gate dielectric and 300 nm thick parylene or PEALD 200 nm thick Al2O3 as both a top gate dielectric and a passivation layer was investigated. The Vth of OTFT with 300 nm thick parylene as a top gate dielectric was changed from 4.7 V to 1.3 V and that with PEALD 200 nm thick Al2O3 as a top gate dielectric was changed from 1.95 V to ‐9.8 V when the voltage bias of top gate electrode was changed from ‐10 V to 10 V. The change of Vth of OTFT with dual‐gate structure was successfully investigated by an analysis of electrostatic potential.

Analysis of the charge transport in π-conjugated materials using generalized Langevin equation

The complex dielectric constants of several π-conjugated materials are measured, and generalized Langevin equation is used to analyze the dielectric behavior in the frequency domain. From the results of a fitting of the experimental data, all of the dielectric behaviors are well interpreted by the generalized Langevin equation which strongly suggests that the mechanism of the charge carrier transport in π-conjugated organic and polymeric materials is diffusion at dc and low frequencies and an oscillatory motion at (or near) phonon frequencies, and in between, a mixed mode of these two mechanisms.