The welding of silver nanowire via combined flash light process for flexible transparent electrode

Abstract

Transparent conductive electrodes (TCEs) with growing the wearable electronics industry has been increased over the past decades to fabricate the touch screens, active-matrix liquid crystal displays (LCDs), organic light emitting diodes (OLED) and solar cell. To apply the various electronics with TCEs, various materials have been employed with indium tin oxide (ITO), graphene, carbon nanotubes (CNTs) and silver nanowire. Among these, ITO for material to fabricate TCEs was generally used. However, the material has several drawbacks because of its brittleness, high material cost and the scarcity of indium. Also, in growing the flexible electronics industry, the brittle characteristic of ITO was not appropriate for flexible transparent conductive electrodes (FTCEs). Meanwhile, graphene and CNTs have significantly attractive bending characteristic. However, these materials exhibit a higher sheet resistance (100 ~1000 Ω/sq) and lower transmittance (< 80%) when compared to ITO. Therefore, these materials for FTCEs was insufficient. To overcome these drawback (i.e. high cost, high sheet resistance, low transmittance and low flexibility), silver nanowires have recently been utilized to fabricate FTCEs owing to the low cost, high ductility, high transmittance and low sheet resistance of the structures. To indicate the characteristic of low sheet resistance and high transmittance in the silver nanowire films, the welding in the junction of silver nanowire required with various welding techniques such as thermal heating, laser, electrical processes. However, each of these processes has several problems. First, thermal heating is not suitable for FTCEs because the welding of silver nanowires requires high temperature (150–300 °C) and long processing time (30–60 min) However, polymer substrates such as polyethylene terephthalate (PET), poly(methyl methacrylate), and polycarbonate have low glass transition temperatures of 150 °C. Second, laser process has short welding times without damage to the polymer substrate. Meanwhile it requires sophisticated equipment and small welding areas. Third, the electrical process, which is operated using electromigration between silver nanowires under current received from an electrode, can function over large welding areas and requires only short welding times. However, it is not applicable to roll-to-roll mass production processes because the electrode for supplying current must be attached to the coated silver nanowire film during the process. To solve these limitations, Kim et al. developed a combined flash light welding process. Combined flash light welding equipment was composed with xenon, ultra violet (UV) and infrared (IR) lamp. Its process can weld the silver nanowire for large scale area during a few milli-seconds at room temperature under the ambient condition without damage of polymer substrate. However, an in-depth study on the welding mechanisms of the silver nanowire via combined flash light irradiation has not yet been conducted. To analyze plasmonic welding effect of silver nanowire for the wavelength range, COMSOL simulation using finite element method (FEM) was conducted under various wavelength from deep UV light range (180 nm) to NIR light range (2500 nm). Also, to investigate the welding mechanisms of silver nanowire in-depth, the changing of sheet resistance of silver nanowire films during milli-seconds was monitored using in-situ monitoring system composed with a Wheatstone bridge electrical circuit and a high-rate data acquisition system. Furthermore, to improve the plasmonic welding effect of silver nanowire films, flash light which was wavelength controlled using optical filters (i.e. high-pass, band-pass, low-pass filters) was irradiated. Thereby, the wavelength effect of the silver nanowire was investigated. In conclusion, the welding mechanism of silver nanowire via the combined flash light process was demonstrated. Also, the welded silver nanowire films has low sheet resistance, high transmittance and excellent bending characteristic. Moreover, the fabricated FTCEs using silver nanowire and combined flash light process were employed in the various applications (i.e. flexible heat film, solar cell and touch panel). Therefore, it is expected that the newly developed combined flash light welding technique of silver nanowire films would be widely contributed in the flexible electronics industry.