Kipyo Hong

Multiwall Carbon Nanotube and Poly(3,4-ethylenedioxythiophene): Polystyrene Sulfonate (PEDOT:PSS) Composite Films for Transistor and Inverter Devices

Highly conductive multiwalled carbon nanotube (MWNT)/Poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) films were prepared by spin coating a mixture solution. The solution was prepared by dispersing MWNT in the PEDOT:PSS solution in water using ultrasonication without any oxidation process. The effect of the MWNT loading in the solution on the film properties such as surface roughness, work function, surface energy, optical transparency, and conductivity was studied. The conductivity of MWNT/PEDOT:PSS composite film was increased with higher MWNT loading and the high conductivity of MWNT/PEDOT:PSS films enabled them to be used as a source/drain electrode in organic thin film transistor (OTFT). The pentacene TFT with MWNT/PEDOT:PSS S/D electrode showed much higher performance with mobility about 0.2 cm²/(V s) and on/off ratio about 5 × 10⁵ compared to that with PEDOT:PSS S/D electrode (∼0.05 cm²/(V s), 1 × 10⁵). The complementary inverters exhibited excellent characteristics, including high gain value of about 30.

Bending-stress-driven phase transitions in pentacene thin films for flexible organic field-effect transistors

The effects of bending strain on the structure and electrical characteristics of pentacene films in flexible devices were investigated. It was found that the volume fraction of bulk phase in the pentacene film increases from 10.7% to 27.7% under 1.1% of tensile strain but decreases to 3.5% under 1.0% of compressive strain. These bending-stress-driven phase transitions between the bulk phase and the thin-film phase in the pentacene film resulted in the changes in field-effect mobility, and were driven by the differences between the in-plane dimensions of the crystal unit cells of the two phases to reduce the external bending stress.

Effect of the hydrophobicity and thickness of polymer gate dielectrics on the hysteresis behavior of pentacene-based field-effect transistors

We demonstrate the origin and mechanism of the hysteresis behavior that is frequently observed during the operation of organic field-effect transistors (OFETs) based on polymer gate dielectrics. Although polar functionalities, such as hydroxyl groups, present in the polymer gate dielectrics are known to induce hysteresis, there have only been a few detailed investigations examining how the presence of such end functionalities both at the polymer surface—forming an interface with the semiconductor layer—and in the bulk influences the hysteresis. In this study, we control the hydrophobicity of the polymer by varying the number of hydroxyl groups, and use an ultrathin polymer/SiO2 bilayer and a thick single polymer as the gate dielectric structure so that the hysteresis behavior is divided into contributions from hydroxyl groups present at the polymer surface and in the bulk, respectively. Electrical characterizations of the OFETs, performed both in vacuum (≈10−3 Torr) and in ambient air (relative humidity o...

Origin of high mobility within an amorphous polymeric semiconductor: Space-charge-limited current and trap distribution

To elucidate the origin of the high field-effect mobility (≈0.02cm2∕Vs) of amorphous poly[(1,2-bis-(2′-thienyl)vinyl-5′,5″-diyl)-alt-(9,9-dioctyldecylfluorene-2,7-diyl], we investigated the current density–voltage (J-V) and mobility–voltage (μ-V) relationships as a function of temperature. By using the power law model and the Gaussian hopping model, we determined a characteristic trap energy of 67meV, an energetic disorder parameter of 64meV, and a total trap density of 2.5×1016cm−3, comparable to that of poly(3-hexylthiophene). We conclude that the relatively low trap density, which originates from the grain-boundary-free amorphous nature of the semiconductor, enables this high field-effect mobility.

Solution-processed flexible ZnO transparent thin-film transistors with a polymer gate dielectric fabricated by microwave heating

We report the development of solution-processed zinc oxide (ZnO) transparent thin-film transistors (TFTs) with a poly(2-hydroxyethyl methacrylate) (PHEMA) gate dielectric on a plastic substrate. The ZnO nanorod film active layer, prepared by microwave heating, showed a highly uniform and densely packed array of large crystal size (58 nm) in the [002] direction of ZnO nanorods on the plasma-treated PHEMA. The flexible ZnO TFTs with the plasma-treated PHEMA gate dielectric exhibited an electron mobility of 1.1 cm(2) V(-1) s(-1), which was higher by a factor of approximately 8.5 than that of ZnO TFTs based on the bare PHEMA gate dielectric.

Hysteresis behaviour of low-voltage organic field-effect transistors employing high dielectric constant polymer gate dielectrics

Here, we report on the fabrication of low-voltage-operating pentacene-based organic field-effect transistors (OFETs) that utilize crosslinked cyanoethylated poly(vinyl alcohol) (CR-V) gate dielectrics. The crosslinked CR-V-based OFET could be operated successfully at low voltages (below 4 V), but abnormal behaviour during device operation, such as uncertainty in the field-effect mobility (μ) and hysteresis, was induced by the slow polarization of moieties embedded in the gate dielectric (e.g. polar functionalities, ionic impurities, water and solvent molecules). In an effort to improve the stability of OFET operation, we measured the dependence of μ and hysteresis on dielectric thickness, CR-V crosslinking conditions and sweep rate of the gate bias. The influence of the CR-V surface properties on μ, hysteresis, and the structural and morphological features of the pentacene layer grown on the gate dielectric was characterized and compared with the properties of pentacene grown on a polystyrene surface.

High-Performance Triisopropylsilylethynyl Pentacene Transistors via Spin Coating with a Crystallization-Assisting Layer

The effects of spin speed and an amorphous fluoropolymer (CYTOP)-patterned substrate on the crystalline structures and device performance of triisopropylsilylethynyl pentacene (TIPS-PEN) organic field-effect transistors (OFETs) were investigated. The crystallinity of the TIPS-PEN film was enhanced by decreasing the spin speed, because slow evaporation of the solvent provided a sufficient time for the formation of thermodynamically stable crystalline structures. In addition, the adoption of a CYTOP-patterned substrate induced the three-dimensional (3D) growth of the TIPS-PEN crystals, because the patterned substrate confined the TIPS-PEN molecules and allowed sufficient time for the self-organization of TIPS-PEN. TIPS-PEN OFETs fabricated at a spin speed of 300 rpm with a CYTOP-patterned substrate showed a field-effect mobility of 0.131 cm(2) V(-1) s(-1), which is a remarkable improvement over previous spin-coated TIPS-PEN OFETs.

Photopatternable Poly(4-styrene sulfonic acid)-Wrapped MWNT Thin-Film Source/Drain Electrodes for Use in Organic Field-Effect Transistors

We describe the cross-linking of poly(4-styrene-sulfonic acid) (PSS) by exposure to ultraviolet (UV) light (λ = 255 nm) under a vacuum. Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) showed that the photo-crosslinking of PSS resulted from coupling between radicals that were generated in the polymer chains by UV excitation. The photo-cross-linkable characteristics of PSS were employed to fabricate solution-processable, photopatternable, and conductive PSS-wrapped multiwalled carbon nanotube (MWNT) composite thin films by wrapping MWNTs with PSS in water. During photo-cross-linking, the work function of the PSS-wrapped MWNTs decreased from 4.83 to 4.53 eV following cleavage of a significant number of sulfonic acid groups. Despite the decreased work function of the PSS-wrapped MWNTs, the photopatterned PSS-wrapped MWNTs produced good source/drain electrodes for OFETs, yielding a mobility (0.134 ± 0.056 cm²/(V s)) for the TIPS-PEN field-effect transistors fabricated using PSS-wrapped MWNTs as source/drain electrodes that was higher than the mobility of gold-based transistors (0.011 ± 0.004 cm²/(V s)).

Photo-Curable Polymer Blend Dielectrics for Advancing Organic Field-Effect Transistor Applications

A solution method of photo-curable and -patternable polymer gate dielectrics was introduced by using blend solutions of poly(4-dimethylsilyl styrene) (PDMSS) and poly(melamine-co-formaldehyde) acrylate (PMFA). The fabrication was optimized to produce a smooth hydrophobic gate dielectric with good insulating and solvent-resistant properties. On the optimized PDMSS/PMFA blend gate dielectric, pentacene could grow into highly ordered structure, showing high electric performances for the resulting OFETs, as well as PTCDI-C13 and TES-ADT.

Improved n-type bottom-contact organic transistors by introducing a poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) coating on the source/drain electrodes

We improved the device performance of N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) n-type field-effect transistors, increasing electron-mobility from 0.003 to 0.101 cm2/V s, by applying a coating of poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) to gold source/drain (S/D) electrodes, thereby reducing contact resistance in the devices. Crystallinity and electronic structure studies suggested that the improved device performance resulted from higher crystallinity of PTCDI-C13 on the PEDOT:PSS-coated S/D electrodes at the interface between the electrode and the channel.