Yejin Jo

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 10 000 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.

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.

Crystalline structure-tunable, surface oxidation-suppressed Ni nanoparticles: printable magnetic colloidal fluids for flexible electronics

In this study, we suggest the chemical methodology that allows for the facile controllability of phase transformation between face-centered cubic and hexagonal close-packed structures for Ni nanoparticles with a 0.4–2 nm thick shallow surface oxide layer, resulting in a maximum saturation magnetization of 33.2 emu g−1. As a first proof-of-concept of the potential for the formation of flexible, printed magnetic devices on cost-effective polyethylene terephthalate (PET) and paper substrates, it is demonstrated that the resulting Ni nanoparticles, prepared in the form of magnetic fluids, are transformed into bulk-like patterned Ni architectures via air-brush printing and instant photonic annealing in a timescale of 10−3 s, exhibiting highly flexible properties under the harsh conditions of 10 000 times repeated bending tests.

Study on thermal evolution of the CuSe phase in nanoparticle-based absorber layers for solution-processed chalcopyrite photovoltaic devices.

Nanoparticle-based, solution-processed chalcopyrite photovoltaic devices have drawn tremendous attraction for the realization of low-cost, large-area solar cell applications. In particular, it has been recently demonstrated that the CuSe phase plays a critical role in allowing the formation of device-quality, nanoparticle-based chalcopyrite absorber layers. For further in-depth study, with the aim of understanding the thermal behavior of the CuSe phase that triggers the vigorous densification reaction, a requisite for high-performance chalcopyrite absorber layers, both multiphase (CuSe-phase including) and single-phase (CuSe-phase free) CISe nanoparticles are investigated from the viewpoint of compositional variation and crystalline structural evolution. In addition, with CuSe-phase including CISe particulate layers, the basic restrictions in thermal treatment necessary for activating effectively the CuSe-phase induced densification reaction are suggested, in conjunction with consideration on the thermal decomposition of organic additives that are inevitably incorporated in nanoparticle-based absorber layers.

Polymeric mold soft-patterned metal oxide field-effect transistors: critical factors determining device performance

In recent decades, the use of high performance, soluble oxide semiconductors has been of paramount interest as a channel layer in device architectures of thin-film transistors. Their excellent device performance, which is even comparable to that of vacuum deposited counterparts, has been demonstrated; but, to date, the polymeric mold soft-patterning methods, applicable to roll-to-roll high throughput processes, have not been successfully implemented in creating the device-quality, soluble oxide transistors. In this study, we clarify the heretofore unrecognized origin of limited device performance in polymeric mold soft-patterned oxide transistors, with a model experiment based on the micro molding in capillary (MIMIC) method. In order to elucidate the chemical influence of precursor solutions, three kinds of representative precursor solutions, which undergo the characteristic synthetic pathways of a conventional sol–gel reaction, combustion chemistry reaction, and chemical additive mediated reaction, are employed. Through the comparative study in terms of device performance, in conjunction with the spectroscopic, microscopic, and rheological analyses, it is suggested that the gradual solvent evaporation in structurally confined polymeric molds triggers the additional sluggish chemical reaction unlike the case of evaporation free, semi-solid involved other patterning methodologies, resulting in the significant degradation of device performance. This newly suggested finding would pave the way to generate high performance oxide transistors in a high throughput way that has not been demonstrated so far due to the lack of in-depth study on chemical/physical structural evolution in polymeric molds.

Thermally-derived liquid phase involving multiphase Cu(In,Ga)Se2 nanoparticles for solution-processed inorganic photovoltaic devices

In the past decade, wet chemical strategies for solution-based Cu(In,Ga)Se2 (CI(G)Se) photovoltaic devices have gained a tremendous amount of attention in solar-cell research fields. In particular, nanoparticles allowing for liquid-phase densification have been recognized as viable candidates for advancements in photovoltaic devices. In this study, multiphase CIGSe nanoparticles are synthesized by the microwave-assisted solvothermal method, in which the chemically incorporated CuSe2 and Se phases form liquid phases for inducing vigorous reactions at elevated temperatures. The morphological/crystalline structural properties of multiphase nanoparticles are analyzed, in conjunction with the temperature dependent evolution in multiphase nanoparticle-incorporating functional layers. Furthermore, we examine physical parameters including the cell performance, shunt conductance, and series resistance for multiphase CIGSe nanoparticle-derived solar cells, from which the cell performance-limiting factors are discussed.

Unraveling the Issue of Ag Migration in Printable Source/Drain Electrodes Compatible with Versatile Solution-Processed Oxide Semiconductors for Printed Thin-Film Transistor Applications

In recent decades, solution-processable, printable oxide thin-film transistors have garnered a tremendous amount of attention given their potential for use in low-cost, large-area electronics. However, printable metallic source/drain electrodes undergo undesirable electrical/thermal migration at an interfacial stack of the oxide semiconductor and metal electrode. In this study, we report oleic acid-capped Ag nanoparticles that effectively suppress the significant Ag migration and facilitate high field-effect mobilities in oxide transistors. The origin of the role of surface-capped Ag nanoparticles is clarified with comparative studies based on X-ray photoelectron spectroscopy and X-ray absorption spectroscopy.

3D-printed origami electronics using percolative conductors

Recently, three-dimensional (3D) printing has garnered tremendous amounts of attention in various applications. In this study, we suggest a facile means of creating 3D-printed foldable electrodes on paper via the direct printing of composite pastes consisting of conductive fillers and a thermoplastic elastomer. The 3D-printability of the prepared composite pastes is investigated depending on the rheological properties. It is revealed that the composite paste with a high storage modulus would enable the formation of highly conductive features with a resistance of 0.4 Ω cm−1 on three-dimensional paper structures. The mechanical bending/folding stability levels of the printed electrodes are evaluated to judge the possibility of realizing 3D-printed origami electronics. The resistance is changed slightly with a normalized resistance value of 2.3, when the printed electrodes are folded with a folding angle of 150°. It is demonstrated that the 3D-printed composite electrodes are applicable to various origami electronics, including electrical circuits, strain sensors and electrochemical sensors.

High-performance solution-processable flexible and transparent conducting electrodes with embedded Cu mesh

Alternative transparent and conducting electrodes (TCEs) that can overcome the practical limitations of the existing TCEs have been explored. Although network structures of metal nanowires have been investigated for TCEs because of their excellent performance, characteristics such as high junction resistances, poor surface roughness, and randomly entangled NW networks still pose challenges. Here, we report cost-effective and solution-processable metallic mesh TCEs consisting of a Cu-mesh embedded in a flexible PDMS substrate. The unprecedented structures of the Cu-mesh TCEs offer considerable advantages over previous approaches, including high performance, surface smoothness, excellent flexibility, electromechanical stability, and thermal stability. Our Cu-mesh TCEs provide a transmittance of 96% at 550 nm and a sheet resistance of 0.1 Ω sq−1, as well as extremely high figures of merit, reaching up to 1.9 × 104, which are the highest reported values among recent studies. Finally, we demonstrate high-performance transparent heaters based on Cu-mesh TCEs and in situ color tuning of cholesteric liquid crystals (CLCs) using them, confirming the uniform spatial electrical conductivity as well as the reproducibility and reliability of the electrode.