Janghoon Park

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.

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.

Practical Design Guidelines for the Development of High-Precision Roll-to-Roll Slot-Die Coating Equipment and the Process

This paper proposes practical design guidelines for roll-to-roll (R2R) slot-die coating equipment with a precision mechanism enabling high-accuracy coating performance. The equipment is developed for specially fabricating organic photovoltaic devices. An R2R transportation module (capable of unwinding, in-feeding, out-feeding, and rewinding), a slot-die coating module, a far-infrared drying module, and a cleanliness control module are incorporated into the equipment. When used to apply an organic solution, the developed equipment can provide an operating tension of 2.3%-11.3%, a coating velocity of 2-7 m/min, a coating thickness of 28-95 nm, a uniformity of 8%-23%, and a roughness of 2.86-7.71 nm. A review of this equipment on the basis of theoretical and experimental approaches is included in the detailed equipment specifications and design guidelines for R2R slot-die coating machines.

Solution processed flexible organic thin film back-gated transistors based on polyimide dielectric films

An organic thin film back-gated transistor (OBGT) was fabricated and characterized. The gate electrode was printed on the back side of substrate, and the dielectric layer was omitted by substituting the dielectric layer with the polyimide (PI) film substrate. Roll-to-roll (R2R) gravure printing, doctor blading, and drop casting methods were used to fabricate the OBGT. The printed OBGT device shows better performance compared with an OTFT device based on dielectric layer of BaTiO3. Additionally, a calendering process enhanced the performance by a factor of 3 to 7 (mobility: 0.016 cm2/V·s, on/off ratio: 9.17×103). A bending test was conducted to confirm the flexibility and durability of the OBGT device. The results show the fabricated device endures 20000-cyclic motions. The realized OBGT device was successfully fabricated and working, which is meaningful for production engineering from the viewpoint of process development.

Register mark measurement errors in high-precision roll-to-roll continuous systems: The effect of register mark geometry on measurement error

It is important to achieve high-precision register control in roll-to-roll continuous printing systems. Thus far, many studies on the dynamics of registers and tension and on register control techniques have identified register control as a problem of controlling and minimizing the disturbance of strain of the substrate. However, register control using printed register marks is necessary, and printing defects in creating these marks cause measurement errors. This study demonstrates by experimental verification that the measurement error is generated by the widening and agglomeration of the register mark. Furthermore, the error is shown to differ with the size and shape of the mark under identical printing conditions. The results illustrate the importance of improving the printing quality of the register mark, selecting the desired geometry for register marks with regard to printability, and utilizing an edge-detection algorithm in the control program for high-precision register control.

A study on model development of the slot-die coating process through the statistical analysis

In roll-to-roll slot-die coating process, the characteristics such as coated layer thickness or roughness which determines the performance of the devices such as OPV, OLED are important. And it is determined by the interaction of coating variables which are coating gap, operating velocity, flow rate. In this study, sensitivity analysis for each variable was conducted by using D.O.E methodology. As a result, some dominant factors affecting output were revealed and a mathematical model was derived as a meta model.