Kee-Hyun Shin

A Novel Method to Guarantee the Specified Thickness and Surface Roughness of the Roll-to-Roll Printed Patterns Using the Tension of a Moving Substrate

Roll-to-roll (R2R) continuous printing is an attractive technology for mass-producing flexible printed electronics. Many studies have been conducted in this field. The application of the R2R printing process, however, requires information pertaining to system parameters such as substrate flexibility, ink formulation, and printing method as well as the curing method for conductive ink. We show that the quality of a printed pattern (thickness and surface roughness) could be affected by tension variation of the flexible bare substrate in spite of the optimal settings of the ink, substrate, and printing method. In addition, an ink-transfer mechanism for the R2R printed electronics is analyzed to reveal the relationships between the dynamic surface roughness and tension of a moving web. Since the dynamics of the physical problem are complex, simple meta models using a design of experiment are developed. The experimental results are found to be in agreement with the meta models. It is found that the two important factors for achieving the desired thickness and surface roughness of the R2R printed patterns are optimal tension and control accuracy of the operating tension.

An Investigation of the Ink-Transfer Mechanism During the Printing Phase of High-Resolution Roll-to-Roll Gravure Printing

Reducing the linewidth of electrodes is of high importance for increasing not only the efficiency of photovoltaic devices but also the performance of organic thin-film transistors. In particular, controlling the line pattern in printing processes has been difficult. In this paper, we report an analytical approach for obtaining high-resolution control over the ink-transfer mechanism in roll-to-roll (R2R) gravure printing. A dimensionless adhesion-force difference was defined for a simple ink-transfer model, and it was used to predict and evaluate the ink-transfer mechanism with respect to several parameters, such as the surface tension of the ink, surface energy of the substrate, and surface energy and aspect ratio (AR) of the cell. It was found that the low-surface-tension inks, high-surface-energy substrates, and low-surface-energy and high-AR cells are preferable to increasing the ink-transfer ratio during the printing phase. Finally, a matching-logic flowchart was developed for controlling the ink-transfer mechanism and fidelity of R2R gravure printing. The printed patterns obtained had an average width as small as 15.3 μm (standard deviation = 0.9 μm).

Flexible ultraviolet photodetectors with ZnO nanowire networks fabricated by large area controlled roll-to-roll processing

Ultraviolet (UV) photodetectors containing flexible films were fabricated by a roll-to-roll processing method. Gravure printing and slot-die coating techniques were utilized for manufacturing Ag sensing electrodes and Zn acetate seeds, respectively, while well-organized Zn oxide nanowire networks with a high density were grown hydrothermally on flexible substrates. The maximum photocurrent of 62.1 μA, a response time of 9.1 s, and a recovery time of 56 s were obtained at a bias voltage of 10 V under UV irradiation with a density of 127 μW cm−2. The resulting correlation between the seeding rate, the nanowire diameter, and the photocurrent indicates that high seeding rates lead to the formation of dense nanowire structures and excellent response times.

Modeling and Matching Design of a Tension Controller Using Pendulum Dancer in Roll-to-Roll Systems

Dancer systems are typical equipment for attenuation of tension disturbances. The two kinds of dancer, active dancer and passive dancer, are distinguished by an external actuator. In the active dancer, the position of the dancer roll is measured and the roll is forced by the external actuator to regulate tension disturbances. However, the passive dancer, composed of a spring, damper and roll has no external actuator. Tension disturbance generates movements of the roll of the passive dancer and the displacements regulate the tension variation. However, the hybrid dancer, which is a mixture of passive and active dancer, is applied for the roll-to-roll systems. It regulates tension disturbances indirectly in the manner of keeping a constant position for the roll by changing the velocity of the driven roller adjacent to the dancer roll. It has different characteristics from those of the passive or active dancer. In this paper, the modeling of the pendulum dancer is derived. The dynamics of the hybrid dancer, which feedbacks the position of the dancer roll, and the PI control of a driven roll are analyzed. The matching logic for tuning gains is developed and experimentally verified.

Effect of process parameters on fidelity of printed line width in high resolution roll-to-roll gravure printing

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.

Fast sintering of silver nanoparticle and flake layers by infrared module assistance in large area roll-to-roll gravure printing system.

We present fast sintering for silver (Ag) nanoparticle (NP) and flake layers printed using roll-to-roll (R2R) gravure printing. An infrared (IR) sintering module was applied to an R2R system to shorten the sintering duration of an R2R gravure-printed Ag layer. IR sintering of the conductive layer was improved by optimising the process condition. After printing of the Ag NP and Ag flake layers, additional IR sintering was performed in the R2R system. The lowest sheet resistance obtained in the Ag NP layer was 0.294 Ω/□, the distance between the substrate and lamp was 50-mm long, the IR lamp power was 500 W, and the sintering time was 5.4 s. The fastest sintering of 0.34 Ω/□ was achieved with 50-mm distance, 1,000-W IR lamp power, and 1.08-s sintering time. In the Ag flake layer, the lowest sheet resistance obtained was 0.288 Ω/□ with a 20-mm distance, 1,000-W IR lamp power, and 10.8-s sintering time. Meanwhile, the fastest sintering was obtained with a 3.83 Ω/□ sheet resistance, 20-mm distance, 1000-W IR lamp, and 1.08-s sintering time. Thus, the IR sintering module can easily be employed in an R2R system to obtain excellent layer sheet resistance.

Novel Modeling of Correlation between Two-Dimensional Registers in Large-Area Multilayered Roll-to-Roll Printed Electronics

For the adaption of a roll-to-roll printing method to printed electronics, it is mandatory to increase the register control resolution. Therefore, i ti s necessary to derive a mathematical model of the register and to develop a controller to reduce register error. A correlation between the machinedirection (MD) and cross-direction (CD) registers was derived by considering both the lateral motion of a moving substrate and the transverse position of a printing roll. By the analysis of the correlation between MD and CD registers, a novel mathematical model of an oblique-machinedirection (OMD) register was derived. The OMD register is more critical in large-area roll-to-roll printing, as the OMD register error is proportional to the width of the substrate. The proposed model can be used to design an effective two-dimensional (2D) register controller that reduces the correlation between MD and CD registers. The proposed mathematical model was validated by numerical simulations and experimental verifications under various operation conditions using a multilayered direct gravure printing machine. The results show that the proposed OMD register model reasonably exposes the MD–CD register correlation. # 2011 The Japan Society of Applied Physics

Two-Dimensional Register Modeling and Control in Multi-Layer Roll-To-Roll E-Printing Systems

Abstract For the adaption of roll-to-roll printing method to the printed electronics, it is mandatory to increase the resolution of register control. A two-dimensional register was derived considering both the lateral motion and strain of a moving web, and the transverse position of a printing roll. Also, a correlation between machine directional register and cross directional register was analyzed and the correlation was modeled as a oblique machine directional register. A feedforward control method was proposed to cancel out the disturbance of 2-D register from upstream span. The mathematical modeling and proposed control method were validated by numerical simulations and experimental verification in various operating conditions using a multi-layer direct gravure printing machine. These results show that the proposed feedforward control scheme greatly improved the control performance of register control in overcoming the upstream disturbances

Optimization of calendering process using Taguchi method to improve the performance of printed capacitor

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.

Roll-to-roll infrared and hot-air sintering of gravure-printed Ag layer based on in situ tension measuring and analysis

This study presents a method developed to achieve the roll-to-roll sintering of printed Ag patterns based on exposure to hot air, near-infrared, and mid-infrared sources. The sintering energy was quantified and evaluated based on theoretical and experimental calculations. Moreover, the effect of the sintering energy on the web tension was simultaneously considered.