Flash white light sintering of copper nanoparticle inks for highly conductive and flexible electrode patterns

Abstract

Printed electronic techniques (e.g. functional inks formulation, printing, and sintering) are a low-cost solution to the production of electronic devices such as flexible displays, radio frequency identification (RFID) tags, and wearable electronics, since these techniques can replace the expensive and time-consuming photolithography technique. Conventionally, novel nanotechnology-based conductive inks made with gold or silver nanoparticles have been widely employed because of their excellent conductivity, stability and low melting point. However, these noble metals are too expensive for widespread commercialization. Recently, for this reason, copper nanoparticle inks have received the increased attention as a low-cost alternative to gold and silver nanoparticle inks for printed electronics. However, most copper nanoparticles can be easily oxidized in the air, so that they cannot be sintered via conventional thermal sintering method under ambient conditions. For these reasons, several approaches, such as the plasma and laser processes, have been developed to sinter copper nanoparticles without the oxidation. However, these approaches have constraints in production on a large scale, because of their low throughput, high complexity, and considerable environmental obstacles (e.g., vacuum conditions and chamber). Therefore, a low temperature and rapid large area sintering technique is needed to realize mass production and flexible electronic devices on polymer or paper substrates. To overcome these limitations, Kim et al. developed a flash white light sintering method combined with PVP functionalized copper nanoparticle inks (Cu NP-inks). The flash white light sintering method can instantly reduce the copper oxide shell and sinter copper nanoparticles at room temperature while under ambient conditions in just a few milliseconds without damaging the substrate. Moreover, a large area of Cu NP-ink film can be sintered by flash white light from a xenon lamp. However, an in-depth study on the reducing and sintering mechanisms of the oxidized Cu nanoparticles combined with PVP functionalization and flash white light irradiation has not yet been conducted. To investigate the mechanisms in-depth, the millisecond flash white light sintering process of Cu NP-inks was monitored using a Wheatstone bridge electrical circuit and a high-rate data acquisition system. Using the devised in-situ monitoring system for flash light sintering, the effects of the amount of poly (N-vinylpyrrolidone) (PVP) in Cu NP-ink and the flash white light irradiation conditions (irradiation energy density, pulse number, pulse duration, and pulse gap) on the sintering were investigated. To enhance the photonic sintering of Cu NP-ink films, an ultra-high speed photonic sintering method involving flash white light (FWL) combined with near infrared (NIR) and deep ultraviolet (UV) light irradiations was also developed in this study. Flash white light irradiation energy and the power of NIR/deep UV were optimized to obtain high electrical conductivity of Cu NP-ink films. Furthermore, multi-walled carbon nanotubes (MWNTs) were employed to improve the conductivity and fatigue resistance of FWL sintered Cu NP-ink films for flexible electrode. The effects of the weight fraction and the length of CNTs on the FWL sintering of Cu NP/CNT composite films were investigated. In conclusion, FWL-induced photonic reduction and sintering method of the oxidized Cu NP-inks was demonstrated, and highly electrical conductive Cu films were obtained by employing Cu NP/CNT composite inks. Therefore, it is expected that the newly developed photonic sintering technique of Cu NP films would be a strong alternative to realize in situ sintering in electronics printed at room temperature.