Jeong Ho Cho

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.

High-mobility low-temperature ZnO transistors with low-voltage operation

Low voltage high mobility n-type thin film transistors (TFTs) based on sol-gel processed zinc oxide (ZnO) were fabricated using a high capacitance ion gel gate dielectric. The ion gel gated solution-processed ZnO TFTs were found to exhibit excellent electrical properties. TFT carrier mobilities were 13u2002cm2/Vu2009s, ON/OFF current ratios were 105, regardless of the sintering temperature used for the preparation of the ZnO thin films. Ion gel gated ZnO TFTs are successfully demonstrated on plastic substrates for the large area flexible electronics.

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.

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).

High field-effect mobility pentacene thin-film transistors with nanoparticle polymer composite/polymer bilayer insulators

High field-effect mobility organic thin-film transistors with nanoparticle polymer composite/polymer bilayer insulators were fabricated. A device composed of a polyvinylphenol (PVP) insulator filled with barium strontium titanate (BST) nanoparticles and coated with a thin polystyrene film was found to exhibit a field-effect mobility (1.2u2002cm2u2009V−1u2009s−1 for a pentacene device) and a field-induced current that are superior to those of devices with only a PVP insulator or a BST-filled PVP insulator. These improvements in the performance are due to an increase in the capacitance and a reduction in the interface roughness of the insulator.

Energy-Level Alignment at Interfaces Between Gold and Poly(3-hexylthiophene) Films with Two Different Molecular Structures

The electronic structures of the interfaces between Au and poly(3-hexylthiophene) (P3HT) films with two different molecular orientations and orderings were investigated using synchrotron radiation photoemission spectroscopy. We found that, depending on whether thermal treatment was used, the P3HT thin film adopts two different molecular orientations, parallel and perpendicular to the silicon substrate, which result in different values of the vacuum level shift and hole-injection barrier. Thus, the molecular orientation and ordering of the P3HT material strongly affect the energy level alignment at the P3HT/Au interface.