Table of contents

In this paper, graphene/ZnO composite film is fabricated by fairly rapid and easy approach. Zinc oxide (ZnO) (<50 nm particle size) has been immediately deposited on uncured pre-deposited randomly oriented graphene flakes (less than four layers) thin film, using electrohydrodynamic atomization technique (EHDA). After curing, graphene/ZnO composite film morphology and structure have been characterized. For electrical behavior analysis, graphene/ZnO composite thin film is employed as cathode in a diode device indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/poly(dioctylfluorene–benzothiadiazole) (F8BT)/(graphene/ZnO). Obtained J–V curve shows diode behavior i.e., at voltage of 0.3 V, the current density in organic structure is at low value of 1.4 × 10⁻⁵ A/cm² and by further increasing the applied voltage to 2 V, the device current density is increased by 50 times and has reached up to 6.3 × 10⁻⁴ A/cm².

In this study, we have successfully fabricated In–Ga–Zn–O (IGZO) thin-film transistors (TFTs) with various Ga₂O₃ deposition powers prepared using a two radio-frequency (RF) (ceramics targets: In₂O₃ and Ga₂O₃) and one direct-current (DC) (metallic target: Zn) magnetron cosputtering system at room temperature. The carrier concentration for the IGZO films decreases to less than 3 × 10¹⁶ cm⁻³ when the Ga₂O₃ deposition power is 175 W and Hall mobility decreases from 12.8 cm² V⁻¹ s⁻¹ and saturates at 4.6 cm² V⁻¹ s⁻¹ with increasing Ga₂O₃ deposition power. The increase in the resistivity of the cosputtered films correlates with the decrease in the crystallinity of the InGaZn₇O₁₀ phase and the phase transformation from InGaZn₇O₁₀ to InGaZn₂O₅ with increasing Ga₂O₃ deposition power. With an optimum Ga₂O₃ deposition power of 150 W, cosputtered IGZO TFTs with a higher, saturated drain current of 4.5 µA, good saturation mobility, μ_sat of 4.92 cm² V⁻¹ s⁻¹, I_on/I_off of 10⁹, a low subthreshold swing (SS) of 0.27 V/decade, and R_SD of 30 kΩ have been successfully fabricated.

Methods to evaluate powder injection molding feedstock without performing the complete processing were developed to reduce the development-to-commercialization cycle time for feedstock. The mixing homogeneity of the powder and binder, rheological properties, mechanical property and shape retainability during debinding were characterized to evaluate the feedstock. The validity of using these parameters was investigated through comparison with various feedstocks. Based on these results, the evaluation methods developed in this work have potential to aid in the selection of proper feedstock without undergoing the subsequent processing.

In this study, we examined the characteristics of Ni/Au (20 nm/80 nm) ohmic contacts to non-polar a-plane p-GaN as a function of annealing temperature. The current–voltage (I–V) curves were showed an upward curve with annealing when Ni/Au metals were used as ohmic metals to nonpolar p-GaN, which was similar to those of the other crystalline planes. The contact resistivity decreased from 2.36 to 6.95 × 10⁻³ Ω cm². Secondary ion mass spectroscopy showed that the Ga atoms out-diffused from the GaN substrate after annealing at 400 °C, which led to the generation of Ga vacancies. The formation of Ga and N vacancies was found to be a competing process during annealing.

Here we reported a new strategy based on the use of bidentate polymer ligands to exchange the long alkyl chain surfactants adsorbed on Ag nanocrystals to facilely achieve stable dispersions of Ag nanocrystals in alcohol. Amazedly, bidentate ligand-protected Ag nanocrystals showed a complete precipitation in water. We found that the tightly packing self-assembled bidentate polymer ligands were adsorbed on Ag nanocrystals, therefore leading to unusual dispersion ability. Another free functional group provided the potential reactivity for further modification.

Growth behaviors of intermetallic compounds (IMCs) and Kirkendall voids in Cu/Sn/Cu microbump were systematically investigated by an in-situ scanning electron microscope observation. Cu–Sn IMC total thickness increased linearly with the square root of the annealing time for 600 h at 150 °C, which could be separated as first and second IMC growth steps. Our results showed that the growth behavior of the first void matched the growth behavior of second Cu₆Sn₅, and that the growth behavior of the second void matched that of the second Cu₃Sn. It could be confirmed that double-layer Kirkendall voids growth kinetics were closely related to the Cu–Sn IMC growth mechanism in the Cu/Sn/Cu microbump, which could seriously deteriorate the mechanical and electrical reliabilities of the fine-pitch microbump systems.

Metal-assisted chemical etching (MACE) using a nanosphere lithography (NSL) technique is regarded as a general fabrication method for silicon nanowire (SINW) arrays. However, morphology control of SiNWs using this method has not been reported. In this study, silicon nanowire (SINW) and silicon nanocone (SINC) arrays were fabricated by MACE using a NSL. Depending on the concentration of etchants in the etching solution, the morphology of the wires and etching rate were systemically changed. At high concentrations, the wires were etched cylindrically and at low concentrations, tapered (cone-like) wires were obtained. To quantify the degree of tapering, the volume ratio of the etched part was calculated from their morphology. The degree of tapering increased as the concentrations of etchants decreased. We suggest that the mechanism of the formation of nanocone is related to the degree of hole diffusion under the metal layer, which was supported by field emission scanning electron microscope (FE-SEM) images.

Dye-sensitized solar cell (DSC) is highly focused because of low-cost. Generally, DSC was composed of photoelectrode, dye, electrolyte, and counter electrode (CE). Sputter-coated Pt on the fluorine doped tin oxide (FTO) electrode was mainly used as CE, however it was expensive and low speed fabrication. In this study, we tried to apply the spray mode of the electrostatic inkjet printing for fabrication of CE. Fundamental characteristics were investigated with the spray-coated Pt CE, the sputter-coated Pt CE, and FTO CE. Conversion efficiency was very low in case of FTO CE. Efficiency of spray-coated Pt CE was 3.6%. The efficiency was a little lower than that of commonly used sputter-coated Pt CE because Pt was not completely coated on the FTO glass by the inkjet printing. These results indicated that spray-coated Pt on FTO glass had possibility to be used as CE of DSC.

Computational fluid dynamics simulations incorporating supersonic turbulent gas flow models and a droplet breakup model are performed to study supersonic gas atomization for producing micron-sized metal powder particles. Generally such atomization occurs in two stages: a primary breakup and a secondary breakup. Since the final droplet size is primarily determined by the secondary breakup, parent droplets of certain sizes (1 to 5 mm) typically resulting from the primary breakup are released at the corner of the nozzle and undergo the secondary breakup. A comparison of flow patterns with and without the introduction of a liquid melt clearly indicates that the mass loading effect is quite significant as a result of the gas–droplet interactions. The flow pattern change reasonably explains why the final droplets have a bimodal mass size distribution. The transient size changes of the droplets are well described by the behavior of the Weber number. The present results based on the 1 mm parent droplets best fit previous experimental results. Moreover, the effects of inlet gas pressure and temperature are investigated in an attempt to further reduce droplet size.

Here we study the interaction of a nanosecond laser pulse with a nanoparticle to explore the mechanism of energetic ion formation and in particular the particle size dependence. Multiphoton ionization and the subsequent electron impact ionization accompanied by inverse Bremsstrahlung process are determined appropriate for generation of multiple charged ions. The Coulomb expansion of a positive ion cloud is then calculated with molecular dynamics simulations, resulting in temporal evolution of ions, a radial distribution of kinetic energy of ions, and size-dependency of the ion kinetic energy. A mass spectrum peak of ion simulated by the present model is found comparable to the experimental data. Alternatively, a direct measurement of kinetic energy is also explained by the model.

Two copper–zinc (Cu–Zn) alloy wires with different copper contents (90Cu/10Zn and 65Cu/35Zn) were used as sacrificed electrodes in submerged arc discharge process at ambient pressure. Three types of dielectric liquids including ethylene glycol, ethanol and deionized water were investigated. The microstructure, composition and size of the particles obtained were examined by high-resolution transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM/EDS) and X-ray diffraction (XRD) analysis. The particles synthesized from both wires in all dielectric liquids were spherical with size in the range of 5–50 nm. Ethylene glycol yielded the smallest particles, in comparison deionized water yielded the largest particles. These particles contained Cu, Cu–Zn, and zinc oxide without copper oxide. l-Ascorbic acid was then used as a reducing agent to eliminate zinc oxide. Thus, the 5–50 nm spherical Cu/Cu–Zn nanoparticles with similar composition to the wires used, can be synthesized. However, ethanol cannot be used as dielectric liquid in case of 65Cu/35Zn wire, because high zinc content in ethanol with l-ascorbic acid formed zinc oxalate. Conductive ink was prepared from the particles synthesized by using 90Cu/10Zn in ethylene glycol, and screen printed on poly(ethylene terephthalate) (PET) substrate. The patterns printed could be sintered in air at 150 °C for 60 min. The patterns fabricated were characterized by scanning electron microscope with energy dispersive X-ray spectroscopy (SEM/EDS). The resistivity of the conductive pattern was measured by two-point probe at room temperature. The lowest volume resistivity of the pattern obtained was 125 µΩ·cm.

We fabricated a thin-film transistor (TFT) using amorphous indium gallium zinc oxide (a-IGZO), which was formed through annealing of an IGZO precursor film with a single-mode cavity microwave at 2.45 GHz. The transisitor fabricated with the a-IGZO film prepared by microwave annealing for 15 min showed higher device performance, i.e., a field effect mobility of 5.75 × 10⁻² cm²·V⁻¹·s⁻¹, an on/off ratio of 10⁶, and a threshold voltage of 20 V, than that prepared by annealing with a conventional oven for 120 min. The Raman spectra confirm that the device improvement originates from the decrease in the number of –OH groups and removal of organic species for 15 min by microwave annealing. These results suggest that the microwave annealing method has an advantage as the annealing process of solution-processed oxide semiconductors to reduce the process time. It can be applied to the fabrication of TFTs.

In this paper, we present an inverted organic solar cell fabricated on a cylindrical glass rod. Because of the nonconventional geometry of the substrate, a variety of deposition processes were employed, including atomic layer deposition, hydrothermal growth, dip coating, and thermal evaporation. ZnO nanorod array used as an electron-selective layer improved the device performance compared with devices without ZnO nanorods owing to its role as a vertically guided electron pathway. The thickness and morphology of the active layer [poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM)] created by the dip coating process were optimized by varying the substrate lifting speed. In addition, thermal annealing of the active layer induced an appropriate level of microphase separation of donors and acceptors, improving the charge separation and resulting in a maximum power conversion efficiency (PCE) of 1.38%. Finally, we conclude that solar cells on a curved surface improve the normalized PCE dependence on the incident angle of sunlight compared with planar solar cells.

The fabrication and characterization of a temperature sensor on flexible poly(ethylene terephthalate) (PET) substrate is discussed. The device was printed by drop-on-demand (DOD) electrohydrodynamic (EHD) Patterning method using silver nanoparticles ink on a roll-to-roll (R2R) system on a mass scale. EHD patterning was performed at atmospheric pressure and room temperature in a single step. The ink viscosity was 300 cps and it contained 55 wt % of silver nanoparticles. The parameters for printing head speed, and DOD were optimized to create connected lines. The printed temperature sensor has the resistivity of 25.35 µΩ·cm and the temperature coefficient of resistance (TCR) is 0.0007687 °C⁻¹. The micro sensor can be applied to many applications to measure temperature accurately of flat or curved surfaces.

For flexible displays in manufacture, the full-field residual stress inspection for films deposited on flexible substrates is an important issue. According to the literature, the material phase retardation is related to its residual stress. This study proposes a novel method of the fast full-field phase-retardation distribution inspection with a flat large-area twisted nematic liquid crystal modulator based on a white light source by using a common-path interferometry. To fast inspect two-dimension (2D) phase-retardation distribution, a tensile testing machine is utilized to provide flexible substrates with tensional forces to enhance the residual stresses for measuring phase retardations in the tensional force direction. Accordingly, a 2D phase-retardation distribution concerning a 125-dpi line-pattern poly(ethylene terephthalate)/indium tin oxide substrate is inspected and analyzed by using a full-field flat liquid-crystal modulating common-path interferometry. 2D phase-retardation distribution inspection results depicted on flexible displays help a designer or maker of flexible displays design useful and comfortable products.

A thin electric connector has been required for use in mobile electronics. Although an anisotropic conducting adhesive technology, e.g., the use of an anisotropic conductive film (ACF), has been developed to meet the demand, the technology is not suitable for flexible electronic devices based on plastic materials, since the plastic films used will change their mechanical properties by pressure and heat. To address these limitations, we have developed a novel film-type connector composed of a base material such as polyimide, an adhesive layer that deforms elastically against pressure, and electrodes arranged on the adhesive layer. In this study, the contact resistance of a film-type connector is investigated and compared with that of an ACF joint through four tests related to reliability. Results of the tests show that the film-type connector is more useful than the ACF joint in areas such as future flexible fine-pitch interconnections.

We incorporated a dual-gate structure into organic crystal field-effect transistors (OCFETs). An organic semiconductor crystal was laminated on the comb-shaped interdigitated Cr/Au source and drain that were preliminarily deposited on SiO₂ (used as a bottom-gate insulator) covering Si (a bottom-gate contact). On top of the crystal, we successively formed a parylene top-gate insulator and an Au top-gate contact. High hole mobilities were observed by applying voltages to both gate contacts and by applying a certain voltage to the top-gate contact with the bottom-gate contact floated. The drain currents of dual-gate devices are larger than those of single-gate devices with only the bottom-gate contact. Dual-gate OCFETs produce light emissions with the application of alternating-current (AC) voltages to both gate contacts. The dual-gate OCFETs require gate voltages lower than those of single-gate devices. These results indicate that dual-gate structures are useful for realizing high hole mobilities and current-injected light emissions by facilitating carrier injections into the OCFETs.

The research is focused on the development of a customized inkjet printed WLAN antenna for the application, e.g., in smartphones. Therefore a new three dimensional antenna for communication devices was designed. This antenna is facing requirements such as high functionality (high communication quality) by being manufactured in a cost efficient digital printing process (inkjet printing). In order to manufacture such an antenna, the processes of contactless inkjet printing, thermal sintering and substrate folding need to be applied-always in direct relation to the antenna performance (antenna parameter). The three dimensional shape of the antenna allows an optimized antenna performance and a decreased antenna size. For the development of printed three dimensional WLAN antennas, we are focusing on advanced antenna design engineering, antenna parameter simulation (to speed up the development process), inkjet printing on flexible polymer substrates and antenna parameter measurements for validation. Our research results proof the suitability of printing as a beneficial manufacturing method for highly integrated smartphone antennas. We could also show the dependency of inkjet printing parameters (such as drop space) and the antenna performance afterwards.

Effects of both wet chemical treatments and line pattern densities on the interfacial bonding characteristics of Cu–Cu pattern direct bonds were systematically investigated. The silicon oxide (SiO₂) on a parallel Cu lines patterned wafer could be removed effectively by using a solution of buffered oxide etch and sulfuric acid (BOE/H₂SO₄) to improve the bonding quality of Cu–Cu pattern direct bonds. The Cu surface after BOE/H₂SO₄ wet pretreatment revealed the complete removal of both the residue particles and the surface oxide layer. After BOE/H₂SO₄ wet pretreatment, the interfacial adhesion energies of 0.06, 0.09, and 0.23 pattern densities were 7.9, 4.8, and 4.1 J/m², respectively, where the dielectric erosion due to chemical–mechanical polishing (CMP) process with increasing pattern density significantly affected the Cu–Cu pattern direct bonding quality. Therefore, CMP planarization performance is critical for reliable Cu pattern direct bonding.

We have demonstrated NAND and NOR logic circuit operations of stacked-structure complementary thin-film transistors (TFTs) using 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) and soluble ZnO as active layers. Bottom-gate-type TIPS-pentacene TFTs, as p-channel transistors, were formed on n-channel ZnO TFTs with common gate electrodes. Solution-processed silicone-resin layers were used as gate dielectric and electrical interconnection layers between lower and upper TFTs. The stacked-structure integrated circuits have several advantages such as ease of active layer formation, compact device area per stage, and the short length of the interconnection compared with the planar configuration in a conventional logic circuit.

The paper aims to investigate the influence of bottom electrode configurations in all-inkjet-printed organic thin film transistors (OTFTs) on crystalline structures of semiconductor layers as well as the electrical performance of OTFTs. Two different types of all-inkjet-printed OTFTs were fabricated: one type with a new bottom-top contact configuration and the other type with a conventional bottom-bottom contact configuration. The crystal growth of semiconductor layer and the electrical characteristics of OTFTs were evaluated and compared between two different bottom electrode configurations. The resulting crystalline structures of semiconductor layers are mostly affected by the evaporation process of semiconductor layer. Randomly-oriented crystalline structures are generated in the semiconductor layer with a conventional bottom-bottom contact configuration due to irregular evaporation. However, a new bottom-top contact configuration leads to well-oriented crystalline structures because of controlled contact line movement. The average mobility of OTFTs with well-oriented crystalline structures is about 11 times as high as that with randomly-oriented crystalline structures.

We report on solution-processed n-channel ZnO thin-film transistors. We fabricated by a low-temperature process to improve their performance using inkjet printing under various conditions. The resulting films were inkjet-printed with a resolution 200 dpi using droplets of 50 µm diameter and 35 pl volume. The characteristics of the inkjet-printed TFTs were improved significantly at an annealing temperature of 150 °C. The field-effect mobility, V_th, and on/off current ratio were 3.03 cm² V⁻¹ s⁻¹, −3.3 V, and 10⁶, respectively. These results indicate that annealing at 150 °C is sufficient to obtain a mobility (μ_sat) as large as 3.03 cm² V⁻¹ s⁻¹.

EMI shielding metal-mesh pattern was prepared on the plastic film by electrohydrodynamic (EHD) jet printing with Ag ink. The printed Ag line width in the mesh was 9.72 µm which was small enough not to be distinctly detected by human eyes. The line-to-line spacing (pitch) in the mesh pattern was modulated in order to investigate the electrical and optical properties of the EHD jet-printed metal-mesh layers. The pitch of 300 µm in the mesh led to the sheet resistance of 7 Ω/sq, the transmittance of 88.2%, and the haze less than 1%. Even though the decrease in the pitch resulted in the improvement on the electrical property of the metal-mesh layer, it was found that the decrease in the pitch simultaneously degraded the optical property such as transparency and haze. Electromagnetic interference shielding effectiveness (EMI SE) of the EHD jet-printed metal-mesh was investigated over the X-band frequency range (8–12 GHz). It turned out that the EHD jet-printed metal-mesh showed a high EMI SE of 20 dB which indicated that the mesh layer can be used in the various application of electronics for the EMI shielding purpose.

The formation of relaxed charge-transfer (CT) excitons and its effect on photocarrier generation in a series of donor–acceptor (DA)-type polymers were examined with a focus on the two unique absorption bands that originate from low-energy charge-transfer (CT) and high-energy main-chain (MC) excitations, respectively. The photoluminescence in response to the CT excitation demonstrated Stokes shift, which is a clear evidence of the formation of bound CT excitons associated with a strong lattice relaxation. Results indicate that photocarriers are generated indirectly by CT excitation through the formation of bound electron–hole pairs, the binding energy and lifetime of which correlate with the donor–acceptor CT degree in the DA-type polymers.

We demonstrated organic field-effect transistors (OFETs) using nylon 11, poly(γ-methyl-l-glutamate) (PMLG), and poly(ε-benzyloxycarbonyl-l-lysine) [Plys(z)] as gate dielectrics. By a Fourier-transform IR (FT-IR) measurement, the secondary structure of nylon 11 was determined to be a β-sheet, and those of PMLG and Plys(z) have an α-helix. The orientation of the α-helix of PMLG and Plys(z) and its crystallinity were determined by FT-IR and X-ray diffraction (XRD) measurements, respectively. The OFET using nylon 11 showed no hysteresis in the transfer characteristic (on/off ratio is 1.2). In contrast, OFETs using PMLG and PLys(z) showed hysteresis and it operated as ferroelectric memories (on/off ratios are 2.2 × 10⁴ and 53, respectively). This difference is attributed to the difference in the secondary structure and the crystal system. The memory retention property in OFETs using PMLG and PLys(z) suggested that high crystallinity of the film and highly ordered dipoles are not necessary for the memory retention.

One of the difficulties in printed electronics is the robustness and reliability of the thin film transistor gate dielectrics. Thin layers are needed for low voltage operation. Proportionally thicker layers can be used if a higher permittivity (high-k) dielectric is chosen. Here we present the experimental results of partly inkjet and gravure printed thin film transistors with a commercial blend of barium titanate and poly(methyl methacrylate) (PMMA). Poly(triarylamine) (PTAA) or single walled carbon nanotube (SWCNT) inks were deposited as the semiconductor layer. Gravure printing of the dielectric material worked very well. On the other hand the SWCNT ink printing resulted in very thin layers which did not work well in transistors. Large hysteresis and high off current was present in most cases. The reference PTAA transistor measurements show that also a polymer semiconductor can work without hysteresis on top of the high-k dielectric.

We report on the device characteristics of a sheetlike flexible pressure sensor array consisting of poly(amino acid)-based piezoelectric elements fabricated on a plastic-film substrate by a printing technique. Poly(amino acid)-based piezoelectric elements were fabricated using poly(amino acid) as the piezoelectric material, silver paste ink as the electrodes, and polyimide sheet as the substrate. When dropping the load of 200 g from a height of 8.5 cm to the piezoelectric element, +1.7 and −1.4 V were observed owing to the displacement of the dipole moment in the poly(amino acid) film. We confirmed that pressures from 0 to 10,000 N were successfully measured from the change in the capacitance of the piezoelectric element, which underwent mechanical strain caused by the pressure application. The flexible pressure sensor element exhibited no noticeable degradation after repeated pressure tests at 10,000 N. Moreover, the spliced large-area pressure sensor sheet successfully detected human-stepping pressure and monitored positional information.

High frequency poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)] copolymer ultrasonic transducers were prepared by screen printing as an all-printed device with conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) used the electrode material. A large number of prototypes with different electrode areas and electrode thickness were obtained by printing single or dual layers of the conductive polymer. The printed transducers were characterized using impedance and acoustic backscattering measurements. Effects imposed by the PEDOT:PSS conductivity were also evaluated both through experiments and finite element method (FEM) modeling, pointing out important limitations of conductive polymers when applied in high frequency ultrasonic transducers.

The precision printing of biomaterials is essential for fabricating bio devices, and two-dimensional (2D) and three-dimensional (3D) cell structures. To fabricate 3D cell structure artificially, biomaterials should be installed between cells to support the gravity force of cells. In general, the viscosity of ink from biomaterials is relatively high. An electrostatic inkjet is used for the bioprinting of cells and biomaterials because it has good merits, i.e., high printing resolution and good capability to eject highly viscous ink. In this paper, gelatin, an important biomaterial is printed using an electrostatic inkjet. The width of the finest printed line is 6 µm. The precisely printed line can be used as a scaffold of living cells.

A micro-contact printing (µCP) method has been applied to the fabrication of microstructures and electronics. We have successfully fabricated a copper wiring structure on flexible substrates using a combination of µCP, electroless plating, and electroplating. A nucleating agent ink pattern was printed by µCP onto polyethylene terephthalate, and polyethylene naphthalate. Nickel and copper thin films were electrolessly plated onto the pattern, and used as a conductive layer for subsequent copper electroplating. Copper wiring structures with a half-pitch of a 100 µm and a 250 µm line-and-space pattern, were successfully produced on flexible films at temperatures below 100 °C.

The resonant oscillation of a sessile droplet on the electrohydrodynamic (EHD) jetting nozzle was studied using a series of sinusoidal and square pulse voltage signal. At small amplitude oscillations, the lowest mode resonant frequency depended on a contact angle and the explicit relationship was developed assuming one-dimensional standing capillary wave. When the meniscus was oscillated with a higher voltage amplitude for jetting, its height became maximum at the phase delay of about π/2 from the maximum applied voltage signal, which can be predicted in a harmonic oscillator model. In addition, there existed an effective resonant frequency where the meniscus oscillation was amplified quickly to initiate a jetting earlier, and the jetting continued every cycle afterwards. Therefore, the resonant oscillation would be effectively used for high speed printing of a simple regular pattern where the jetting should occur at a constant frequency.

Recently, printing pattern in high resolution (line width <20 µm) has been given a great interest in the field of printed electronics for application of organic photovoltaic and fast integrated circuits. Improving the grid electrode could reduce the optical loss and the resistance loss thereby increasing conversion efficiency of solar cells. For higher performance transistors, it is necessary to scale channel length below 10 µm to achieve MHz operations. To enable to build up pattern with high fidelity demands a study on the effect of process parameters in gravure printing. In this paper, a mathematical model is proposed to analyze the mechanism of the ink spreading on the substrate based on squeezing flow theory between parallel plates. It was proven that process parameters such as nip force, printing speed and viscosity of ink are significant factors contributing to the resulting printed line width. Finally, the experimental investigation on the effect of such parameters demonstrated that a high printing speed, low nip force and high viscosity of ink could decrease the ink spreading thereby gaining high fidelity. This work could be utilized as a guideline to set up the operating conditions to maintain the fidelity of printed line width in high resolution roll-to-roll gravure printing.

In this study, experiments on liquid transfer were performed to observe the change of the surface contact angle with respect to the process speed. The liquid transfer ratio from the experiments was compared with that from numerical simulation for low speed ranges. While the surface contact angle on the lower plate was almost constant in the experiment regardless of operating speed, the surface contact angle on the upper plate was found to be significantly changed during the process at the low speed ranges. This resulted in a reduction of the transfer ratio. By applying the time-dependent values of contact angle to the numerical simulation model, the transfer ratio showed better agreement with the experimental results. The dynamic effect of surface contact angle should be considered as an additional variable, especially for the analysis of liquid transfer in a low speed printing process.

One of the major factors that determine the performance of printed parallel-plate capacitors is the thickness of the dielectric layer; the thickness is affected significantly by the surface roughness of the conductive layer underneath the dielectric layer. In this study, we employed a calendering process to reduce the surface roughness of the first conductive layer. After a set of experiments involving a calendering process, the Taguchi method was applied to determine the optimal setting of the process parameters in terms of speed, nip pressure, and temperature. In the Taguchi method, a three-level orthogonal array was used to decide the signal-to-noise ratio. In addition, analysis of variance and F-test values were used to determine the most significant process parameters that affect the output, i.e., capacitance. Validation tests were carried out using the optimal levels of the process parameters and the results were presented. These results can be used as a practical guideline for determining optimal parameters in a calendering process for printed electronics applications.

An innovative electrical current sintering technique is applied to joule-heat conductive ink by increasing current gradually through a stepwise-form. This stepwise electrical sintering technique is devised to overcome thermal damage of printed conductive ink line during electrical sintering due to its high initial resistance. To monitor a stepwise electrical sintering method, in-situ specific resistance was measured. Surface morphology of the sample was observed by field-emission scanning electron microscope. By increasing the current gradually, the conductive line can endure higher current because the specific resistance has dropped gradually during the process. Finally, enhanced final-step current produces lower specific resistance of the conductive line than that obtained from a constant current-supplying electrical sintering method without damaging printed conductive line.

Previously fabricated electronic devices were used for vacuum manufacturing processes such as conventional semiconductor manufacturing. However, they are difficult to apply to continuous processes such as roll-to-roll printing, which results in very high device manufacturing and processing costs. Therefore, many developers have been interested in applying continuous processes to contact printing or noncontact printing technologies and they proposed various continuous printing techniques instead of conventional batch coating. In this paper, a hybrid spray coating technique is proposed for use in various organic electronic device fabrication processes on the basis of noncontact printing technique. This hybrid spray coating technique is a combination of electrostatic spraying and air pressure spraying. Using this technique we obtained smaller particles and shorter deposition time than by the conventional spray coating using a single nozzle.

This paper reports a very precise lateral control system for organic photovoltaic (OPV) fabrication that uses a roll-to-roll process. A roll-to-roll fabrication method was employed to increase the efficiency of an OPV module by reducing the aperture loss of a one-dimensional coating. Instead of edge of a web, a line pattern printed using inkjet printing technology was used as a reference for lateral control. Lateral displacement of the pattern was measured with a camera and determined with a pattern recognition algorithm. A proportional integral derivative (PID) controller was designed and tested through computer simulation. The lateral control system was also validated through experiments on a roll-to-roll simulator with 200 m of web transfer and web transfer speed of 15 mpm. Experimental results show that lateral displacement of a web can be controlled within ±50 µm, which can be considered as meaningful progress toward roll-to-roll fabrication of OPV.

Double-shot inkjet printing (DS-IJP) is a unique technique for manufacturing molecular semiconductor films by an alternate microdroplet deposition of semiconductor solution and its antisolvent. Here, we investigated patterned thin-film formation of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) by the DS-IJP technique. We found that uniform and pinhole-free thin films can be grown using TIPS-PEN; the films are composed of large crystal domains and with thickness uniformity over the whole area of the films. To obtain uniform films, it is necessary to reverse the deposition sequence between semiconductor solution and the antisolvent for TIPS-PEN, as compared with the deposition sequence in the case of using 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), most possibly owing to the strong preference of TIPS-PEN to form plate like crystals. On the basis of the results, we discuss that the crystalline preference should be a key factor for optimizing the printing processes using molecular materials by the DS-IJP.

One of the most important issues concerning the handling of the web in a roll-to-roll printing system is tension control of the web. In particular, the tension controls for the winders, including both the un-winder and re-winder, are required for the stable supply of the web and collecting of the printed electronic devices in the form of a roll without causing defects in the devices, respectively. The web tension control of the winder span requires time-variable control gains because the dynamics of the web vary with the winder roller radius. If fixed gains are used to control the web tension of the winder span regardless of changes in the radius of the winder roller, the tension could be unstable. Therefore, previous studies have tried to control the web tension of the winder span while considering the time-variable characteristics of the web dynamics by changing proportional-integral-derivative (PID) gains according to the roll radius or by applying adaptive control methods. However, most of these studies were limited to simulations. In this study, different types of fuzzy-PID controllers were applied to control winder tensions considering the dynamic characteristics of winders in a roll-to-roll system. A modified fuzzy-PID method is suggested that adjusts PID gains to correspond to changes in the dynamics of the web of the re-winder. PID gains based on these fuzzy rules vary within a range of positive values according to changes in the un-winder roller radius as opposed to the conventional fuzzy-PID controller, in which the fuzzy rules vary from negative to positive. On the other hand, the conventional fuzzy-PID controller is applied to the re-winder control to improve the response considering dynamics characteristics of re-winder that differs from that of re-winder. The proposed method was applied to the tension controls of the un-winder and re-winder of an actual roll-to-roll system and was verified through experiments. Both the simulation and experimental results showed that the proposed method can successfully control the web tension of winder systems.

The purpose of this paper is that evaluate of developed micro-gravure coater system and manufacture of thin film to be applied printed electronics, such as organic light emitting diodes (OLED), organics photovoltaic (OPV), organic thin film transistor (OTFT), etc. The micro-gravure coater has various modules such as web tension controller, micro-gravure coating unit, doctor blade, ink dispenser and hybrid dry units for NIR, hot air. Especially, this coating system is very suitable for mass or continuous process due to the applied for roll-to-roll (R2R) web moving method. So, this micro-gravure coating system have the advantage of higher processing time and lower manufacturing cost compare with conventional vacuum process. Also, this system is suitable for coated large area and controlled coating thickness compare with non-contact coating process, such as electrostatic spray deposition (ESD), aerosol-jet, etc. on the other hand, this system have disadvantage of substrate damage due to the contact with the substrate and micro-gravure patterned roll. However, from the view of productivity and cost, there are a lot more benefits than disadvantages. For this reason, we designed and manufactured the micro-gravure coating system based with R2R and we performed coating test with the various micro-gravure patterned roll and then through the various coating test, we ascertain whether or not developed the micro-gravure coating system for printed electronics.