Showing papers by "Jaehoon Park published in 2015"

Pickering emulsion polymerized poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/polystyrene composite particles and their electric stimuli-response

Abstract: We report a facile synthesis of Pickering emulsion polymerized poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/polystyrene (PEDOT:PSS/PS) composite particles and their electro-responsive electrorheological (ER) characteristics when dispersed in silicone oil. The PEDOT:PSS particles act both as a solid surfactant for the Pickering emulsion polymerization and as an electro-responsive activator for PEDOT:PSS/PS composite particles. The morphology, chemical structure, and thermal properties of the synthesized PEDOT:PSS/PS composite particles were determined by scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis, respectively. Moreover, the formation of chain structures by the dispersed particles was confirmed directly by optical microscopy. The rheological response of PEDOT:PSS/PS-based ER fluid in the presence of an electric field, examined using a rotational rheometer, represented ER effects with typical Bingham fluid behavior.

Rapid curing of solution-processed zinc oxide films by pulse-light annealing for thin-film transistor applications

Abstract: In this study, a pulse-light annealing method is proposed for the rapid fabrication of solution-processed zinc oxide (ZnO) thinfilm transistors (TFTs). Transistors that were fabricated by the pulse-light annealing method, with the annealing being carried out at 90℃ for 15 s, exhibited a mobility of 0.05 cm2/Vs and an on/off current ratio of 106. Such electrical properties are quite close to those of devices that are thermally annealed at 165℃ for 40 min. X-ray photoelectron spectroscopy analysis of ZnO films showed that the activation energy required to form a Zn-O bond is entirely supplied within 15 s of pulse-light exposure. We conclude that the pulse-light annealing method is viable for rapidly curing solution-processable oxide semiconductors for TFT applications.

Stability study of solution-processed zinc tin oxide thin-film transistors

Abstract: In this study, the environmental dependence of the electrical stability of solution-processed n-channel zinc tin oxide (ZTO) thin-film transistors (TFTs) is reported. Under a prolonged negative gate bias stress, a negative shift in threshold voltage occurs in atmospheric air, whereas a negligible positive shift in threshold voltage occurs under vacuum. In the positive bias-stress experiments, a positive shift in threshold voltage was invariably observed both in atmospheric air and under vacuum. In this study, the negative gate-bias-stress-induced instability in atmospheric air is explained through an internal potential in the ZTO semiconductor, which can be generated owing to the interplay between H2O molecules and majority carrier electrons at the surface of the ZTO film. The positive bias-stress-induced instability is ascribed to electron-trapping phenomenon in and around the TFT channel region, which can be further augmented in the presence of air O2 molecules. These results suggest that the interaction between majority carriers and air molecules will have crucial implications for a reliable operation of solution-processed ZTO TFTs.

Pressure dependence of acoustic anomalies of polydimethylsiloxane studied by Brillouin spectroscopy

Abstract: The acoustic properties of polydimethylsiloxane elastomer was investigated as a function of pressure by using a multi-pass Fabry–Perot interferometer and a diamond anvil cell. Pressure dependence of the sound velocity, the Brillouin linewidth, and the refractive index was determined up to ~8.7 GPa. Acoustic properties exhibited a crossover behavior at approximately 1 GPa, which was attributed to the complete collapse of the free volume content in this polymer. The refractive index increased from 1.46 at ambient condition to ~1.63 at 8.67 GPa, which reflected the corresponding increase in density.

Light-induced Characteristic Variations in Organic Thin-film Transistors with a Poly(vinylphenol-co-methyl methacrylate)/Titanium-dioxide Nanocomposite Gate Dielectric

Abstract: We investigated the effect of light exposure on the electrical characteristics of organic thinfilm transistors (OTFTs) fabricated with a cross-linked poly(vinylphenol-co-methyl methacrylate) (PVP-co-PMMA)/titanium-dioxide (TiO2) nanocomposite gate dielectric. When illuminated, the off-state drain current increased significantly in the OTFTs containing the TiO2 nanocomposite gate dielectric, but changed negligibly in those containing the pristine PVP-co-PMMA gate dielectric. On the other hand, the dependence of the on-state drain current on the photon energy was similar in both cases. These results can be explained in terms of photogenerated charge carriers in the organic semiconductor and the TiO2 nanoparticles.

Trilayer metal electrode as an anode in red phosphorescent organic light-emitting diodes

Abstract: In this study, we fabricated red phosphorescent organic light-emitting diodes (PhOLEDs) on rigid glass and flexible plastic substrates by using a trilayer-structured metal anode (Ni/interlayer metal/Ni) and analyzed the optical and the electrical characteristics of these devices. The thickness of the electron transporting layer in red PhOLED with a trilayer metal anode was optimized by comparing its characteristics with those of the device with an indium-tin-oxide (ITO) anode. The performance of the red PhOLED was also analyzed according to interlayer metals (Al, Cu, and Ag) in the trilayer structure. Consequently, the trilayer metal electrode with the structure of Ni/Ag/Ni is suggested as an alternative to an ITO anode for PhOLEDs.

Temperature-dependent charge injection and transport in pentacene thin-film transistors

Abstract: The electrical characteristics of p-channel pentacene thin-film transistors (TFTs) were analyzed at different operating temperatures ranging from 253 to 353 K. An improvement in the drain current and field-effect mobility of the pentacene TFTs is observed with increasing temperature. From the Arrhenius plots of field-effect mobility extracted at various temperatures, a lower activation energy of 99.34 meV was obtained when the device is operating in the saturation region. Such observation is ascribed to the thermally activated hole transport through the pentacene grain boundaries. On the other hand, it was found that the Au/pentacene contact significantly affects the TFTs electrical characteristics in the linear region, which resulted in a higher activation energy. The activation energy based on the linear field-effect mobility, which increased from 344.61 to 444.70 meV with decreasing temperature, implies the charge-injection-limited electrical behavior of pentacene TFTs at low temperatures. The thermally induced electrical characteristic variations in pentacene TFTs can thus be studied through the temperature dependence of the charge injection and transport processes.

Effects of Au source/drain thickness on electrical characteristics of pentacene thin-film transistors

Abstract: We investigate the electrical characteristics of top-contact pentacene thin-film transistors (TFTs) fabricated with various thicknesses of the Au source and the drain (S/D) electrodes, i.e., 20, 30, 50, 70, and 105 nm. Pentacene TFTs exhibit enhancements in the drain current and the fieldeffect mobility with increasing thickness of Au S/D electrodes up to 50 nm, after which the TFT performance degrades with increasing Au thickness. A transmission line method is used to analyze the contact resistance between the Au electrode and the pentacene layer in the TFTs, and ultraviolet photoemission spectroscopy measurements are performed to determine the work function of the Au films. The lowest contact resistance, 73 kΩ·cm, is obtained for the 50-nm-thick Au case and is ascribed to the high work function (4.67 eV) of the film. Consequently, the effects of the Au S/D thickness on the performance of top-contact pentacene TFTs can be understood through the behavior of the charge injection at the Au electrode/pentacene interface.

Influence of the surface properties of polymeric insulators on the electrical stability of 6,13-bis(triisopropylsilylethynyl)-pentacene thin-film transistors

Abstract: We investigated the electrical stabilities of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) thin-film transistors (TFTs) fabricated with cross-linked polymeric insulators, i.e., poly(4-vinylphenol) (PVP) and poly(4-vinylphenol-co-methyl methacrylate) (PVP-co-PMMA). Compared to the cross-linked PVP insulator, the TIPS-pentacene TFTs containing a cross-linked PVP-co-PMMA insulator exhibit less hysteresis upon reversal of the gate-voltage sweep direction and a lower shift in the threshold voltage during consecutive operations, which is ascribed to the relatively hydrophobic surface of the cross-linked PVP-co-PMMA insulator. When these polymer solutions are mixed with yttrium-oxide nanoparticles, the rough surfaces of both nanocomposite insulators lead to larger shifts in the threshold voltage during consecutive operations, but its effect on the hysteretic behavior in the transfer characteristics of the TIPS-pentacene TFTs is negligible. Thus, the influence of the surface properties of the polymeric insulators on the electrical stability of TIPS-pentacene TFTs can be explained through hole-trapping and the delayed-depletion of the holes at the insulator/semiconductor interface.

Integrated optical refractometer based on bend waveguide with air trench structure

Abstract: This study proposes an optical sensor based on a refractometer integrating a bend waveguide and a trench structure. The optical sensor is a planar lightwave circuit device involving a bend waveguide with maximum optical loss. A trench structure was aligned with the partially exposed core layer’s sidewall of the bend waveguide, providing a quantitative measurement condition. The insertion losses of the proposed 1×2 single-mode optical splitter-type sensor were 4.38 and 8.67 dB for the reference waveguide and sensing waveguide, respectively, at a wavelength of 1550 nm. The optical loss of the sensing waveguide depends on the change in the refractive index of the material in contact with the trench, but the reference waveguide had stable optical propagating characteristics regardless of the variations of the refractive index.

Transfer printing of poly(vinylidene fluoride-trifluoroethylene) films for low-voltage ferroelectric field-effect transistors

Abstract: We report a viable method for localizing a ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] film by using the transfer-print process. Micro-scaled topographic P(VDF-TrFE) patterns were successfully produced by using the transfer-print method, and the transferred films exhibited inherent ferroelectric properties. The characteristic improvements in the ferroelectric field-effect transistors (FeFETs) with respect to the operation voltage and the field-effect mobility could be achieved by using the transfer-print method to fabricate a ferroelectric insulator. The results suggest that transferring a ferroelectric film would be useful for realizing high-performance nonvolatile memory devices composed of FeFETs.

Integrated optical refractometer based on bend waveguide with air trench structure

Abstract: This study proposed a novel optical sensor based on a refractometer integrating a bend waveguide and a trench structure. The optical sensor is a planar lightwave circuit (PLC) device involving a bend waveguide with maximum optical loss. A trench structure was aligned with the partially exposed core layer’s sidewall of the bend waveguide, providing a quantitative measurement condition. The insertion losses of the proposed 1 x 2 single-mode optical splitter-type sensor were 4.38 dB and 8.67 dB for the reference waveguide and sensing waveguide, respectively, at a wavelength of 1,550 nm. The optical loss of the sensing waveguide depends on the change in the refractive index of the material in contact with the trench, but the reference waveguide had stable optical propagating characteristic regardless of the variations of the refractive index.

Influence of Pentacene Film Thickness on Environmental Stability of Pentacene Thin-Film Transistors

Abstract: We investigated the influence of pentacene film thickness on the environmental stability of thin-film transistors (TFTs). In order to evaluate the environmental stability, the transfer and output characteristics in pentacene TFTs were measured in air and vacuum environments. The pentacene-thickness-dependent variation of transfer characteristics was shown to originate from the fixed charge at the pentacene/polymeric insulator interface induced by air molecules. In addition, an analysis of parasitic resistance suggests that air molecules degraded the contact property and increased channel conductivity. Particularly, a thick pentacene film was found to passivate the TFT channel region interrupting the penetration of air molecules.