메탈메쉬용 Nano Ag/Cu@Ag 혼합 페이스트의 물성 및 전극 형성 방법 연구

Abstract

Transparent conducting electrode film is highly desirable for application to touch screen panels, flexible OLED and wearable displays, sensors and actuators1-5). Metal nanoparticles have been used for the conducting electrodes by using various fabrication process. One-step direct nanoimprinting of gold nanoparticles was reported by using hexanethiol self-assembled monolayer(SAM) protected gold particles and polydimethylsilexane(PDMS) mold6). The gold nanoparticles were also imprinted by PDMS mold on the polyimide film and organic field effect transistor device was fabricated7). The direct imprinting process using PDMS mold leave metal nanoparticles on top of the film substrate so that it has limited application to the transparent touch screen panel(TSP) fabrication. Silver conductive ink was patterned on the PDMS strechable substrate by stencil printing to make organic thin film transistor(OTFT) devices8). The OTFT device had excellent strechability up to 150% however the pattern width was 50㎛ due to the stencil printing process thus limiting the application to the TSP. The direct imprinting and screen printing processes have merit of simple process operated at ambient condition. However they result in embossed pattern on the film substrate so that may not withstand such hard mechanical stresses as bending and streching. In this work we synthesized both the Ag and cost effective Cu@Ag nanoparticles for application to the large size touch screen panels. Indium tin oxide(ITO) has been widely used as the transparent conducting electrode(TEC) to make small size TSPs used in the smart phones. ITO conductor however could not be used in the large size TSPs over 15 inches due to the trade-off between high transparency and low electrical resistance required for the operation of touch screen panels. Since the thickness of ITO thin film has to be increased to lower the electrical resistance. Of the transparent conducting materials such as carbon nanotube(CNT), graphene, conducting polymer(PEDOT : PSS) and metal nanostructures, the silver nanoparticle and silver coated cupper(Cu@Ag) nanoparticles have high potential for application to large size TSP including flexible and stretchable versions. This is due to the suitability of the silver nanoparticle paste to the metal mesh type transparent conducting electrode. In the metal mesh method the transparent electrodes are patterned by using the trench filling process. In this method narrow engraved trench(~2.5 ㎛) pattern is formed by coating UV curing resin on the substrate film and then pressing with transparent mold followed by UV exposure and demolding. The gap between the trenches is over 100㎛ so that the visible light transmittance of the metal mesh film is over 88% while the resistivity of the Ag nanoparticle paste is less than 10Ω.cm by the percolation mechanism. Metal mesh TCEs also have merit of easy hard coating layer formation on top of trench pattern layer filled with silver particle than the imprinting or screen printing process with embossed silver electrode patterns. Transparent conducting electrode film is highly desirable for application to touch screen panels, flexible and wearable displays, sensors and actuators. A sputtered film of indium tin oxide(ITO) shows high transmittance(90%) at low sheet resistance(50Ω/sq). However, ITO films lack mechanical flexibility especially under bending stress and have limitation in application to large area TSP (over 15 inches) due to the trade-off in high transmittance and low sheet resistance properties. One promising solution is to use metal mesh type transparent conducting film especially for touch panel application. In this work, we investigated such inter-related issues as UV imprinting process to make trench layer pattern, synthesis of core-shell type Ag and Cu@Ag composite nanoparticles and their paste formulation, filling of Ag and Cu@Ag mixture nanoparticle paste to the trench layer and touch panel fabrication processes.