Yunseok Jang

Effect of the phase states of self-assembled monolayers on pentacene growth and thin-film transistor characteristics.

To investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers (SAMs) on the growth mode of pentacene films and the performance of organic thin-film transistors (OTFTs), we deposited pentacene molecules on SAMs of octadecyltrichlorosilane (ODTS) with different alkyl-chain orientations at various substrate temperatures (30, 60, and 90 degrees C). We found that the SAM phase state played an important role in both cases. Pentacene films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a better interconnectivity between the pentacene domains, which directly serves to enhance the field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in crystallinity and field-effect mobility between pentacene films grown on ordered and disordered substrates increased with increasing substrate temperature. These results can be possibly explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer and (2) the temperature-dependent alkyl chain mobility of the ODTS monolayers.

Low-voltage and high-field-effect mobility organic transistors with a polymer insulator

A key issue in research into organic thin-film transistors (OTFTs) is low-voltage operation. In this study, we fabricated a pentacene thin-film transistor with an ultrathin layer of polyvinyl alcohol (9nm) as a gate insulator, and obtained a device with excellent electrical characteristics at low operating voltages (below 2V). This device was found to have a field-effect mobility of 1.1cm2∕Vs, a threshold voltage of −0.98V, an exceptionally low subthreshold slope of 180mV/decade, and an on/off current ratio of 106. This favorable combination of properties means that such OTFTs can be operated successfully at voltages below 2V.

Solution-processable pentacene microcrystal arrays for high performance organic field-effect transistors

The authors report the fabrication of one-dimensional crystal arrays of triisopropylsilylethynyl pentacene (TIPS PEN) via simple drop casting on a tilted substrate. By pinning a solution droplet on the tilted substrate, an array of ribbon-shaped crystals aligned in the tilted direction was formed on the substrate. X-ray diffraction analysis revealed that these crystals were oriented in the crystal growth direction. A thin film transistor based on such an array of TIPS PEN crystals was found to have a high field-effect mobility of 0.3cm2∕Vs, which results from the directed organization of the π-conjugated molecules.

Change of molecular ordering in soluble acenes via solvent annealing and its effect on field-effect mobility

The increase of the molecular ordering and field-effect mobility in triethylsilylethynyl anthradithiophene (TES ADT) thin film transistors by solvent annealing was investigated. X-ray diffraction data revealed that TES ADT molecules crystallize with silyl groups on the substrate surface. After solvent annealing, spherulites were formed and the field-effect mobilities dramatically increased up to 0.43cm2∕Vs (over 100-fold) as result of maximized overlap of π electron clouds along the in-plane direction and the formation of a continuous crystal.

Effects of the permanent dipoles of self-assembled monolayer-treated insulator surfaces on the field-effect mobility of a pentacene thin-film transistor

In order to investigate the effects of permanent dipoles on insulator surfaces on the electrical properties of organic thin-film transistors, the authors fabricated insulators with various self-assembled monolayers and similar surface energies. Surprisingly, they found that the field-effect mobility of pentacene thin-film transistors increases by a factor of approximately 20 for insulators with an electron-withdrawing group. This remarkable increase in the field-effect mobility is due to the increase in the hole density of the insulator surface that arises from the increased band bending of the insulator/semiconductor interface.

Effects of metal penetration into organic semiconductors on the electrical properties of organic thin film transistors

To investigate the effects of metal penetration into organic semiconductors on the electrical properties of organic thin film transistors, gold was deposited onto pentacene films at various deposition rates. The sharp interface between the gold electrode and the pentacene film that results from a fast deposition rate was found to produce lower contact resistance and an increase in the field-effect mobility.

Fabrication of a bionic superhydrophobic metal surface by sulfur-induced morphological development

We describe the fabrication of lotus leaf-like superhydrophobic metal surfaces by using the simple electrochemical reaction of Cu or Cu–Sn alloy plated on steel sheets with sulfur gas, and subsequent perfluorosilane treatment. The microstructure of these surfaces was obtained through the nonelectric chemical plating of the copper onto the steel sheets, and the nanotexturing of the surfaces was achieved via an electrochemical reaction of copper in a sulfur-containing environment at 150 °C, resulting in the formation of a copper sulfide nanostructure on the microstructure. The chemical composition of this metal surface was confirmed using X-ray photoelectron spectroscopy. The water contact angles of the bionic metal surfaces were found to be over 160°, and this surface exhibits a low contact angle hysteresis. To our knowledge, this is the first time this approach has been used with a simple chemical reaction to fabricate an artificial superhydrophobic metal surface.

Influence of the dielectric constant of a polyvinyl phenol insulator on the field-effect mobility of a pentacene-based thin-film transistor

The mobility of pentacene thin-film transistors (TFTs) is correlated with the dielectric properties of their insulators. We varied the dielectric properties of the poly(4-vinylphenol) insulators of such TFTs by changing the molar ratio of the prepolymer/cross-linking agent while keeping the surface potential of the insulator surface constant. It was found that the field-effect mobility of the pentacene TFTs increases with increases in the dielectric constant of the insulators. A small increase in the dielectric constant of the insulator (a 20% increase, 3.6–4.3) was found to result in a dramatic increase in the field-effect mobility of pentacene TFTs by a factor of 3 (0.26to0.81cm2∕Vs).

Low-voltage polymer thin-film transistors with a self-assembled monolayer as the gate dielectric

By a simple process, we manufactured polymer thin-film transistors (PTFTs) using a 2.6 nm thick self-assembled monolayer (SAM) of alkyl chains as the gate dielectric to reduce the operating voltage of the device. These manufactured PTFTs operate with supply voltages of less than 2 V. A densely packed SAM of docosyltrichlorosilanes (DCTS) was a very efficient insulating barrier due to the very limited penetration of polymer transistor molecules into the SAM insulator. The present results show that a DCTS monolayer is suitable for use as a gate dielectric. These results enhance the prospects of using polymer TFTs with a SAM gate dielectric in low-power applications such as identification tags.

Low-voltage organic transistors with titanium oxide/polystyrene bilayer dielectrics

Pentacene-based low-voltage organic transistors were realized with titanium oxide/polystyrene (TiO2/PS) bilayer dielectrics. Significantly, the TiO2 bottom layer was fabricated by a layer-by-layer deposition procedure and had an ultrathin thickness ( 107).