Donghoon Kwak

Evaporation-Induced Self-Organization of Inkjet-Printed Organic Semiconductors on Surface-Modified Dielectrics for High-Performance Organic Transistors

We demonstrate the influence of the surface wettability of a dielectric substrate on the crystalline microstructure and film morphology of an inkjet-printed organic semiconductor, namely 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS_PEN), using various self-assembled monolayers (SAMs). Self-aligned crystals with highly ordered crystalline structure are developed by printing on hydrophilic surfaces with high surface energy. It is found that the pinning of the contact line induces an outward convective flow as the evaporation proceeds, which results in the nucleation of crystals and the self-assembly of TIPS_PEN molecules from the periphery to the central region of the droplet. However, for hydrophobic surfaces with low surface energy, small agglomerates with random orientation of molecules are formed, which is induced by depinning of the contact line. The field-effect transistors fabricated with self-organized crystals printed on hydrophilic surfaces exhibit a high field-effect mobility of 0.15 cm(2) V(-1 )s(-1). These results indicate that the control of both the evaporation behavior and the contact line dynamics in a drying droplet plays an important role in the printing of organic semiconductor films with uniform morphology and desired molecular orientation for the direct-write fabrication of high-performance organic transistors.

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.

Solubility-driven polythiophene nanowires and their electrical characteristics

We demonstrated that interconnected nanofibrillar networks of poly(3-hexylthiophene) (P3HT) thin films with improved crystallinity can be easily fabricated by aging the precursor solution with marginal solvent. Structural analysis revealed that these benefits arise from the improvements in the crystallinity of P3HT in the precursor solution. At dilute concentrations, P3HT molecules grew into near-spherical particles during the aging time. As the aging time increased further, P3HT molecules exhibited one-dimensional growth into rod-like structures. At higher P3HT concentrations and longer P3HT solution aging times, dense nanowires were observed to form gradually, thereby improving the electronic properties of field-effect transistors (FETs) based on these films. This improvement was due to the change in P3HT organization in the precursor solution from a random-coil conformation to an ordered aggregate as a result of aging in a marginal solvent, methylene chloride. At high temperatures, the P3HT molecules were completely solvated and adopted a random-coil conformation, as is observed in good solvents. Whereas upon aging the solution at room temperature, methylene chloride poorly solvated the P3HT molecules such that ordered aggregates of P3HT grew in solution, which improved the molecular ordering of P3HT thin films produced from these solutions. The field-effect mobility of the thin films was, therefore, enhanced without the need for post-treatments.

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 (1.2 cm2 V−1 s−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.

pH-Induced Antireflection Coatings Derived From Hydrogen-Bonding-Directed Multilayer Films

Hydrogen-bonding-directed layer-by-layer assembled films, based on polystyrene-block-poly(acrylic acid) (PS-b-PAA) block copolymer micelles and poly(4-vinylpyridine) (P4VP), were successfully fabricated in methanol. Varying the PAA content in the PS-b-PAA micelles afforded control over the film growth properties, especially the multilayer film thickness. Interestingly, antireflection films with refractive indices that could be tuned between 1.58 and 1.28 were obtained by treatment with an aqueous HCl solution (pH 2.27), and the transmittance obtained was as high as 98.4%. In acid solution, the pyridine group was protonated, destroying the hydrogen bonding between P4VP and PAA. A concomitant pH-induced polymer reorganization in the multilayers resulted in a porous honeycomb-like texture on the substrate.

Transparent carbon nanotube patterns templated by inkjet-printed graphene oxide nanosheets

Partially reduced graphene oxide (GO) nanosheets served as interfacial adhesive layers for the fabrication of single-walled carbon nanotube (SWCNT) patterns on hydrophilic substrates.

Effect of HAuCl4 Doping on the Contact Properties of Polymer Thin-Film Transistors

We show that the electrical properties of polymer thin-film transistors can be enhanced by doping poly(3-hexylthiophene) (P3HT) with HAuCl 4 . Specifically, the addition of HAuCl 4 causes an increase in the two-dimensional molecular ordering of P3HT and a remarkable reduction in the contact resistance at the electrode/semiconductor interface with no pre- or post-treatment process. This phenomenon is understood in terms of broadening of the transport manifold in the organic semiconductor, induced by HAuCl 4 , which results in a reduction in the hole-injection barrier and an enhancement of the interfacial stability at the contact between the printed electrode and the semiconductor layers.