Hwa Sung Lee

Directly Drawn Organic Transistors by Capillary Pen: A New Facile Patterning Method using Capillary Action for Soluble Organic Materials

A capillary pen drawing technique, developed as a new patterning methodology for the large-area patterning and fabrication of organic electronics, provides several advantages over conventional approaches: the method is simple and versatile, there are no restrictions on the patterning shapes that could be produced, and the method can be tailored to a variety of substrates.

Electrohydrodynamic printing of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) electrodes with ratio-optimized surfactant

An electrohydrodynamic (EHD) printing process was optimized for the printing of a (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) conductive polymer by manipulating the surface tension of a PEDOT:PSS solution. A stable cone-jet mode was confirmed by adjusting the process parameters of the EHD printing process, such as applied voltage, flow rate, and working distance. The addition of a nonionic surfactant, Triton X-100, enabled both printing of PEDOT:PSS conductive lines with widths ranging from 335 μm to 90 μm in a low-power operation (0.5 kV), as well as a 100-fold increase in conductivity of the PEDOT:PSS film compared with that of the pristine one. To utilize printed PEDOT:PSS lines for high functional applications, a multi-deposition technique was carried out, which results in a decrease in line resistance from 1.3 × 104 Ω mm−1 to 0.2 × 103 Ω mm−1.

Flexible piezocapacitive sensors based on wrinkled microstructures: toward low-cost fabrication of pressure sensors over large areas

Flexible pressure sensors are a key component of electronic skin (e-skin) for use in future applications ranging from human healthcare monitoring to robotic skins and environmental risk detection. Here, we demonstrated the development of a highly sensitive, simple, and low-cost capacitive pressure sensor, which acted as a flexible capacitive dielectric, based on a microstructured elastomeric template that could be fabricated over a large area. To achieve this goal, the dielectric template was prepared simply by stretching and releasing a flexible Ecoflex film to produce wrinkled surface microstructures with a feature size on the order of tens of micrometers. The effects of the wrinkled surface microstructure on the sensing performance were systematically investigated by comparing the nonwrinkled film, one-side wrinkled film, and double-side wrinkled film. The response and release times of the double-side wrinkled pressure sensor were improved by 42% and 25% in comparison with the values obtained from the unwrinkled case, respectively. These results showed that the introduction of wrinkled surface microstructures to the elastomeric template efficiently enhanced the pressure sensor performance. We also demonstrated that our sensor could be used to detect a variety of changes in the surroundings, such as variations in the angle of a stimulus, object loading/unloading, or an exhaled breath.

Marginal solvents preferentially improve the molecular order of thin polythiophene films

The crystalline order within π-conjugated polymer films prepared using solution processing methods determines the electrical properties of the film. A channels morphology is particularly important to device performance. The molecular order and morphology within a channel region near the underlying active layer have not yet been examined systematically. Here, we characterize the crystal order homogeneity as a function of the solvent penetration depth after applying simple solvent post-treatment. The morphological, optical, and electrical properties of poly(3-hexylthiophene) (P3HT) films could be profoundly improved by casting the films in methylene chloride solutions. The impact of the solvent application was most pronounced in the thin P3HT films, especially in the center of the film. During solvent casting, the central region of the film was exposed to methylene chloride for a longer period of time than the edge region of the film, thereby producing a thinner and more ordered film structure in the central region. Concomitant with the improved order, the charge carrier transport in the resulting field-effect transistors increased.

Tuning the Work Function of Printed Polymer Electrodes by Introducing a Fluorinated Polymer To Enhance the Operational Stability in Bottom-Contact Organic Field-Effect Transistors

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a promising electrode material for organic electronic devices due to its high conductivity, good mechanical flexibility, and feasibility of easy patterning with various printing methods. The work function of PEDOT:PSS needs to be increased for efficient hole injection, and the addition of a fluorine-containing material has been reported to increase the work function of PEDOT:PSS. However, it remains a challenge to print PEDOT:PSS electrodes while simultaneously tuning their work functions. Here, we report work function tunable PEDOT:PSS/Nafion source/drain electrodes formed by electrohydrodynamic printing technique with PEDOT:PSS/Nafion mixture solutions for highly stable bottom-contact organic field-effect transistors (OFETs). The surface properties and work function of the printed electrode can be controlled by varying the Nafion ratio, due to the vertical phase separation of the PEDOT:PSS/Nafion. The PEDOT:PSS/Nafion electrodes exhibit a low hole injection barrier, which leads to efficient charge carrier injection from the electrode to the semiconductor. As a result, pentacene-based OFETs with PEDOT:PSS/Nafion electrodes show increased charge carrier mobilities of 0.39 cm2/(V·s) compared to those of devices with neat PEDOT:PSS electrodes (0.021 cm2/(V·s)). Moreover, the gate-bias stress stability of the OFETs is remarkably improved by employing PEDOT:PSS/Nafion electrodes, as demonstrated by a reduction of the threshold voltage shift from -1.84 V to -0.28 V.

Thermally Crosslinked Biocompatible Hydrophilic Polyvinylpyrrolidone Coatings on Polypropylene with Enhanced Mechanical and Adhesion Properties

We developed a stable hydrophilic biocompatible hydrogel-forming coating for polypropylene (PP)-based disposal medical applications. Although PP has a variety of advantages, including good stability and inertness in medical applications, tissue damage and insertion resistance are observed upon insertion of PP-based devices into the human body due to the high hydrophobicity of the PP surface. These issues limit the utility of PP in medical applications. To address these problems, we sought to develop a stable hydrophilic and biocompatible hydrogel-forming layer using polyvinyl pyrrolidone (PVP) combined with a crosslinked polyethyleneglycolacrylate (PEGDA) matrix. Systematic studies of the blended hydrogel-forming PVP:PEGDA were conducted using a variety of blending ratios between the two polymers. The hydrophilicity and water-affinity of the hydrogel-forming layer improved significantly as the PEGDA-to-PVP blending ratio increased. Importantly, the tensile strain at the break point increased by a factor of more than 7, and the strength of adhesion to the PP surface for the 1:1 PVP:PEGDA (PVP(1):PEGDA(1)) blend ratio was 54 times that of the PVP film, determined using tensile strain–stress and peel tests. The water stability of the PVP(1):PEGDA(1) improved significantly. This approach is potentially useful as a biocompatible hydrophilic polymer coating in a variety of low-priced consumable PP commercial medical applications.

Chemically Robust Superhydrophobic Poly(vinylidene fluoride) Films with Grafting Crosslinkable Fluorinated Silane

A superhydrophobic surface with excellent chemical stability was fabricated using the spraying method, one of the most efficient technologies for producing large-area coatings at low cost. Poly(vinylidene fluoride) (PVDF) was used as a hydrophobic polymer material, and heptadecafluoro-1,1,2,2,-tetra-hydrodecyl)trichlorosilane (FTS), which reacts with moisture during curing, was used to improve the water repellency and durability. Spray coating of PVDF alone yielded PVDF nanostructures described by the Cassie-Baxter model. The water contact angle of a water droplet on this surface, however, was 128°, indicating that the surface was not superhydrophobic. On the other hand, spray-coating a mixed PVDF-FTS solution provided a complex and homogeneous nanostructured surface with excellent water repellency and a contact angle of up to 159°. Immersion of the PVDF-only film for 20 min in N,N-dimethylformamide (DMF), a good solvent for PVDF, led to complete dissolution of the film. By contrast, the PVDF-FTS film maintained its superhydrophobicity with a water contact angle of 151° after 20 min of immersion in DMF, and still exhibited a high contact angle of 142° after 1 h. The PVDF-FTS film developed in the present work should enable the production of large-area superhydrophobic coatings at low cost using a simple spray process. Moreover, the PVDF-FTS film displayed excellent stability against solvents, thus increasing its suitability for robust superhydrophobic applications.

Crisscross-Designed Piezoresistive Strain Sensors with Cracked Microtectonics for Direction-Selective Tensile Perception

Flexible strain sensors are a key component of electronic skin (e-skin), a technology that is currently receiving considerable research attention with a view to future applications ranging from human healthcare monitoring to robotic skins and environmental risk detection. Here, we developed a highly sensitive, simple, and low-cost piezoresistive strain sensor, which acted as a flexible reactive resistor with a cracked microtectonic architecture that could be fabricated over a large area. In particular, our strain sensor recognizes the direction of tensile stimulation through its rational crisscross electrode design, allowing it to overcome some of the shortcomings of traditional flexible strain sensors. Under a given stress, the strain sensor developed here showed a variation in the relative resistance (ΔR/R0) of up to 24-fold depending on the direction of the applied stress. For example, application of a 1% strain changed ΔR/R0 by 0.11 when the strain was applied parallel to the direction of current flow, but by only 0.012 when the strain was applied perpendicular to that direction. Similarly, a 5% strain changed ΔR/R0 by 0.85 and 0.062, and a 20% strain changed ΔR/R0 by 2.37 and 0.098, depending on whether the strain was applied parallel or perpendicular to the current flow, respectively. In addition, ΔR/R0 varied approximately linearly as a function of the strain over the operational range. The results thus show that the proposed sensor is sensitive to the direction in which an external stress is applied. Finally, we demonstrated that our sensor could be used to detect the bending of a human finger.

Enhancement of Solvent-Resistance by Forming Interpenetrating Network for High-Performance Polymer Field-Effect Transistors

To enhance the solvent-resistance of polymer semiconductor film in organic field-effect transistors, bis(trichlorosily)hexane (BTH) as a cross-linkable agent was mixed with polymer semiconductors, poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2) and poly(3-hexylthiophene) (P3HT). The solvent-resistance was dramatically enhanced in both the F8T2/BTH and P3HT/BTH cases, even for the 1% addition of BTH. However, clear differences in the field-effect mobilities with increasing BTH-blend ratio were observed between the F8T2 and the P3HT cases. For the F8T2-FETs, the field-effect mobility was maintained by level of 90% at the 1% BTH-blend ratio, and decreased gradually above 1% blend ratio. In contrast, the field-effect mobilities of P3HT-FETs were dramatically decreased by blending the BTH, although the solvent-resistance was increased. This obvious difference is a result of the difference in crystalline properties between the amorphous F8T2 and the crystalline P3HT. This approach to improve the solvent-resistance of polymer films provides a facile method for the enhancement of the environmental stability in response to humidity and oxygen.