Sun Sook Lee

Stable Aqueous Based Cu Nanoparticle Ink for Printing Well-Defined Highly Conductive Features on a Plastic Substrate

With the aim of inkjet printing highly conductive and well-defined Cu features on plastic substrates, aqueous based Cu ink is prepared for the first time using water-soluble Cu nanoparticles with a very thin surface oxide layer. Owing to the specific properties, high surface tension and low boiling point, of water, the aqueous based Cu ink endows a variety of advantages over conventional Cu inks based on organic solvents in printing narrow conductive patterns without irregular morphologies. It is demonstrated how the design of aqueous based ink affects the basic properties of printed conductive features such as surface morphology, microstructure, conductivity, and line width. The long-term stability of aqueous based Cu ink against oxidation is analyzed through an X-ray photoelectron spectroscopy (XPS) based investigation on the evolution of the surface oxide layer in the aqueous based ink.

Air-stable, surface-oxide free Cu nanoparticles for highly conductive Cu ink and their application to printed graphene transistors

Air-stable, surface-oxide free Cu nanoparticles are, for the first time, synthesized by surrounding completely the Cu surface with oleic acid incorporated as a capping molecule. XPS analysis, in conjunction with TEM analysis, revealed that the oleic acid is chemisorbed to the Cu surface via a chemical interaction wherein a monodentate bond is included, without leaving behind free (non-interacting) oleic acid, thereby providing complete surface protection against oxidation. By eliminating the surface oxide layer that critically degrades the electrical properties, the surface-oxide free Cu nanoparticle ink facilitates the realization of a solution-processed Cu electrode layer with resistivity as low as 4 μΩ cm, comparable to the resistivity of noble metal-based, solution-processed counterparts. In addition, high resolution Cu electrode patterns with 5 μm line-width are directly printed using an electrohydrodynamic inkjet technique, and graphene transistors with the printed Cu electrodes demonstrate potential applications in printed electronics.

Extremely flexible, printable Ag conductive features on PET and paper substrates via continuous millisecond photonic sintering in a large area

The development of highly conductive, flexible metallic constituents in patterned geometries has been of paramount interest in various optoelectronic applications. Among a variety of materials, silver nanoparticles have been considered as candidates that meet the physical/chemical requirements for practical applications; but, the issues for applicability to roll-to-roll processes on inexpensive substrates have not been yet resolved. In this study, we demonstrate that the highly flexible, rollable, printable Ag structures, with an electrical resistivity of 8.0 μΩ cm, are easily formed on a timescale of 10−3 s on polyethylene terephthalate and paper substrates, by supplying highly intensive photon energies on olate-Ag nanoparticle assemblies. The precise control of the amount of carbon residues, by virtue of sophisticatedly adjustable input of photon energies, allows the formation of well-adhesive metallic films on plastic substrates, without incorporating any additional procedures, enabling extreme flexibility during 10u2006000 cycles with a bending radius of 1.5 mm. The continuous approach with a moving stage also suggests the potential toward a practical sintering process for instantly generating highly flexible, conductive metallic architectures in a large area.

Metal salt-derived In–Ga–Zn–O semiconductors incorporating formamide as a novel co-solvent for producing solution-processed, electrohydrodynamic-jet printed, high performance oxide transistors

We report the previously unrecognized co-solvent, formamide (FA), which can comprehensively improve both the device performance and bias stability of metal salt-derived, solution-processed In–Ga–Zn–O (IGZO) TFTs. By incorporating FA in IGZO precursor solutions, the chemical structures are tailored adequately for reducing the content of hydroxide and encouraging the oxygen vacancy formation, which has not been fulfilled in conventional chemical/physical approaches. Owing to such distinct chemical structural evolution, the field-effect mobility is enhanced dramatically by a factor of 4.3 (from 2.4 to 10.4 cm2 V−1 s−1), and the threshold voltage shift during a positive-bias stress test is suppressed effectively by a factor of 2.3 (from 9.3 to 4.1 V) for unpassivated devices. The addition of formamide to IGZO precursor solutions also facilitates electrohydrodynamic-jet (e-jet) printability, with which the directly printed device with a channel width of ∼30 μm is demonstrated successfully. In addition, a high performance, solution-processed IGZO transistor with a mobility of 50 cm2 V−1 s−1 is suggested through coupling a FA-added IGZO oxide semiconductor with a solution-processed zirconium aluminum oxide ((Zr,Al)2Ox) gate dielectric.

Chemically improved high performance printed indium gallium zinc oxide thin-film transistors

With the aim of facilitating the high performance printed In-Ga-Zn-O (IGZO) thin-film transistors (TFTs), we present the heretofore unrecognized chemical methodology for tailoring the chemical structures of printable IGZO semiconductors through incorporation of ethylene glycol in sol–gel derived precursor solutions. With the optimal composition of ethylene glycol, the device performance of TFT employing the printed IGZO semiconducting layer annealed at 400 °C is significantly improved with the field-effect saturation mobility of 4.9 cm2 V−1s−1. In addition, by lowering the contact resistance between the source/drain electrode and printed IGZO semiconducting layer, the device performance is further improved with the field-effect saturation mobility of 7.6 cm2 V−1s−1.

Printed Cu source/drain electrode capped by CuO hole injection layer for organic thin film transistors

Using conductive functional ink comprising Cu/CuO core–shell nanoparticles, we present the printed Cu source/drain electrode surrounded by a CuO hole injection layer for organic thin-film transistors. The inherent CuO hole injection layer facilitates the transistor with electrical performance comparable to that of a transistor based on a vacuum deposited Au electrode, without additional physical or chemical treatments for obtaining an energetically compatible interface between the electrode and p-type organic semiconductor.

Enhanced performance of solution-processed organic thin-film transistors with a low-temperature-annealed alumina interlayer between the polyimide gate insulator and the semiconductor.

We studied a low-temperature-annealed sol-gel-derived alumina interlayer between the organic semiconductor and the organic gate insulator for high-performance organic thin-film transistors. The alumina interlayer was deposited on the polyimide gate insulator by a simple spin-coating and 200 °C-annealing process. The leakage current density decreased by the interlayer deposition: at 1 MV/cm, the leakage current densities of the polyimide and the alumina/polyimide gate insulators were 7.64 × 10(-7) and 3.01 × 10(-9) A/cm(2), respectively. For the first time, enhancement of the organic thin-film transistor performance by introduction of an inorganic interlayer between the organic semiconductor and the organic gate insulator was demonstrated: by introducing the interlayer, the field-effect mobility of the solution-processed organic thin-film transistor increased from 0.35 ± 0.15 to 1.35 ± 0.28 cm(2)/V·s. Our results suggest that inorganic interlayer deposition could be a simple and efficient surface treatment of organic gate insulators for enhancing the performance of solution-processed organic thin-film transistors.

Independent chemical/physical role of combustive exothermic heat in solution-processed metal oxide semiconductors for thin-film transistors

The development of high performance, solution-processed metal-oxide semiconductors has been of paramount interest in various fields of electronic applications. Among the variety of methodologies for synthesizing solution-processed precursor solutions, the combustion chemistry reaction, which involves an internal exothermic heat reaction, has drawn a tremendous amount of attraction as one of the most viable chemical approaches. In this paper, we report the synthesis of new zinc–tin oxide (ZTO) precursor solutions that can be used to independently adjust the amount of combustive exothermic heat. Through comparative analyses based on X-ray photoelectron spectroscopy, spectroscopic ellipsometry, and X-ray absorption spectroscopy, the independent influence of combustive heat is elucidated in indium-free, solution-processed oxide semiconductors, in conjunction with an interpretation of observed variations in the device performance.

3D-stacked carbon composites employing networked electrical intra-pathways for direct-printable, extremely stretchable conductors.

The newly designed materials for stretchable conductors meeting the demands for both electrical and mechanical stability upon morphological elongation have recently been of paramount interest in the applications of stretchable, wearable electronics. To date, carbon nanotube-elastomeric polymer mixtures have been mainly developed; however, the method of preparing such CNT-polymer mixtures as stretchable conductors has been limited to an ionic liquid-mediated approach. In this study, we suggest a simple wet-chemical method for producing newly designed, three-dimensionally stacked carbon composite materials that facilitate the stable morphological elongation up to a strain of 300% with normalized conductivity variation of only 0.34 under a strain of 300%. Through a comparative study with other control samples, it is demonstrated that the intraconnected electrical pathways in hierarchically structured composite materials enable the generation of highly stretchable conductors. Their direct patternability is also evaluated by printing on demand using a programmable disperser without the use of prepatterned masks.

Graphene electrodes transfer-printed with a surface energy-mediated wet PDMS stamp: impact of Au doped-graphene for high performance soluble oxide thin-film transistors

Unlike conventional dry stamp-based printing methodologies, the suggested wet PDMS stamping technique allows for the generation of well-patterned graphene source/drain electrode structures. It is clarified that the electrical characteristics of soluble In–Ga–Zn–O (IGZO) TFTs can be improved effectively by adjusting the work function of transfer-printed graphene electrodes with a simple, facile Au doping technique. By implementing the transfer-printed, Au-doped graphene layers as a source/drain electrode and the newly developed chemical structure-tailored IGZO semiconductors as a soluble channel layer, the high performance graphene/IGZO TFTs are demonstrated with a field-effect mobility of 3.2 cm2 V−1 s−1, which is far superior to the previously reported TFTs employing the printable electrode and a soluble oxide semiconductor.