Minkyu Hwang

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.

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.

Solvent effect of inkjet printed source/drain electrodes on electrical properties of polymer thin-film transistors

We show that the electrical properties of polymer thin-film transistors (PTFTs) can be enhanced by controlling the solvent properties of poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT/PSS) solution used as the inkjet-printed source and drain electrodes. Specifically, addition of dimethyl sulfoxide (DMSO) into the PEDOT/PSS solution increased the conductivity of the inkjet-printed PEDOT electrodes and remarkably reduced the contact resistance of the electrodes. The lower contact resistance of the DMSO-treated PEDOT electrode compared to the corresponding electrode without DMSO treatment may be due to enhanced interfacial stability at the contact between the printed PEDOT electrodes and the semiconductor layers.

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.

Enhancement of hole injection in organic TFTs by ozone treatment of indium tin oxide electrodes

Indium tin oxide electrodes of polymer thin-film transistors (TFTs) were treated with ozone, with the aim of enhancing their hole injection properties. Synchrotron radiation photoelectron spectroscopy results showed that the ozone treatment of the electrodes resulted in an increase in their work function by about 0.4 eV. This increase was found to lower the hole injection barrier and produce an increase in the field-effect mobility.

Effect of Phase State of Self‐Assembled Monolayers on Pentacene Growth and Thin Film Transistors Characteristics

With the aim of investigating the effect of alkyl chain orientation of self‐assembled monolayers (SAMs) on the pentacene growth mode and the performances of organic thin‐film transistors (OTFT), pentacene films were deposited on octadecyltrichlorosilane (ODTS) with different alkyl chain orientation. The alky chain orientation of ODTS was controlled by adjusting the reaction temperature. Pentacene films on ordered ODTS were found to have higher crystallinity and better device performances than those on the disordered ODTS, and their differences increased with the substrate temperature. These results can be explained by (1) lattice matching effect between the pentacene crystals and ODTS, and (2) temperature‐dependent alkyl chains mobility of ODTS.