Hyun-Jun Hwang

In?situ monitoring of flash-light sintering of copper nanoparticle ink for printed electronics

In this work, a flash-light sintering process for Cu nanoinks was studied. In order to precisely monitor the milliseconds flash-light sintering process, a real-time Wheatstone bridge electrical circuit and a high-rate data acquisition system were used. The effects of several flash-light irradiation conditions (irradiation energy, pulse number, on-time, and off-time) and the effects of the amount of poly(N-vinylpyrrolidone) in the Cu nanoink on the flash-light sintering process were investigated. The microstructures of the sintered Cu films were analyzed by scanning electron microscopy. To investigate the oxidation or reduction of the oxide-covered copper nanoparticles, a crystal phase analysis using x-ray diffraction was performed. In addition, the sheet resistance of Cu film was measured using a four-point probe method. From this study, it was found that the flash-light sintered Cu nanoink films have a conductivity of 72 Ωm/sq without any damage to the polyimide substrate. Similar nanoinks are expected to be widely used in printed and flexible electronics products in the near future.

In situ monitoring of a flash light sintering process using silver nano-ink for producing flexible electronics

In this work, a flash light sintering process using silver nano-inks is investigated. A silver nano-ink pattern was printed on a flexible PET (polyethylene terephthalate) substrate using a gravure-offset printing system. The printed silver nano-ink was sintered at room temperature and under ambient conditions using a flash of light from a xenon lamp using an in-house flash light sintering system. In order to monitor the light sintering process, a Wheatstone bridge electrical circuit was devised and changes in the voltage difference of the silver nano-ink were recorded during the sintering process using an oscilloscope. The sheet resistance changes during the sintering process were monitored using the in situ monitoring system devised, under various light conditions (e.g. light energy, on-time and off-time duration, and pulse numbers). The microstructure of the sintered silver film and the interface between the silver film and the PET substrate were observed using a scanning electron microscope, a focused ion beam and an optical microscope. The electrical sheet resistances of the sintered silver films were measured using a four-point probe method. Using the in situ monitoring system devised, the flash light sintering mechanism was studied for each type of light pulse (e.g. evaporation of organic binder followed by the forming of a neck-like junction and its growth, etc).The optimal flash light sintering condition is suggested on the basis of the in situ monitoring results. The optimized flash light sintering process produces a silver film with a lower sheet resistance (0.95 Ω/sq) compared with that of the thermally sintered silver film (2.03 Ω/sq) without damaging the PET substrate or allowing interfacial delamination between the silver film and the PET substrate.

Highly conductive copper nano/microparticles ink via flash light sintering for printed electronics

In this study, the size effect of copper particles on the flash light sintering of copper (Cu) ink was investigated using Cu nanoparticles (20-50 nm diameter) and microparticles (2 μm diameter). Also, the mixed Cu nano-/micro-inks were fabricated, and the synergetic effects between the Cu nano-ink and micro-ink on flash light sintering were assessed. The ratio of nanoparticles to microparticles in Cu ink and the several flash light irradiation conditions (irradiation energy density, pulse number, on-time, and off-time) were optimized to obtain high conductivity of Cu films. In order to precisely monitor the milliseconds-long flash light sintering process, in situ monitoring of electrical resistance and temperature changes of Cu films was conducted during the flash light irradiation using a real-time Wheatstone bridge electrical circuit, thermocouple-based circuit, and a high-rate data acquisition system. Also, several microscopic and spectroscopic characterization techniques such as scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy were used to characterize the flash light sintered Cu nano-/micro-films. In addition, the sheet resistance of Cu film was measured using a four-point probe method. This work revealed that the optimal ratio of nanoparticles to microparticles is 50:50 wt%, and the optimally fabricated and flash light sintered Cu nano-/micro-ink films have the lowest resistivity (80 μΩ cm) among nano-ink, micro-ink, or nano-micro mixed films.

All-photonic drying and sintering process via flash white light combined with deep-UV and near-infrared irradiation for highly conductive copper nano-ink.

We developed an ultra-high speed photonic sintering method involving flash white light (FWL) combined with near infrared (NIR) and deep UV light irradiation to produce highly conductive copper nano-ink film. Flash white light irradiation energy and the power of NIR/deep UV were optimized to obtain high conductivity Cu films. Several microscopic and spectroscopic characterization techniques such as scanning electron microscopy (SEM), a x-ray diffraction (XRD), and Fourier-transform infrared (FT-IR) spectroscopy were employed to characterize the Cu nano-films. Optimally sintered Cu nano-ink films produced using a deep UV-assisted flash white light sintering technique had the lowest resistivity (7.62 μΩ·cm), which was only 4.5-fold higher than that of bulk Cu film (1.68 μΩ•cm).

Copper Nanoparticle/Multiwalled Carbon Nanotube Composite Films with High Electrical Conductivity and Fatigue Resistance Fabricated via Flash Light Sintering

In this work, multiwalled carbon nanotubes (MWNTs) were employed to improve the conductivity and fatigue resistance of flash light sintered copper nanoparticle (NP) ink films. The effect of CNT weight fraction on the flash light sintering and the fatigue characteristics of Cu NP/CNT composite films were investigated. The effect of carbon nanotube length was also studied with regard to enhancing the conductivity and fatigue resistance of flash light sintered Cu NP/CNT composite films. The flash light irradiation energy was optimized to obtain high conductivity Cu NP/CNT composite films. Cu NP/CNT composite films fabricated via optimized flash light irradiation had the lowest resistivity (7.86 μΩ·cm), which was only 4.6 times higher than that of bulk Cu films (1.68 μΩ·cm). It was also demonstrated that Cu NP/CNT composite films had better durability and environmental stability than those of Cu NPs only.

Photonic welding of ultra-long copper nanowire network for flexible transparent electrodes using white flash light sintering

Copper nanowire (Cu NW)-based flexible transparent electronics represent an enormous breakthrough for the development of efficient, scalable and facile processing techniques. From the standpoint of commercialization, a cost-effective and eco-friendly procedure for welding nanowires is imperative to fabricate Cu NW network-based transparent electrodes. In this study, a photonic welding method using a white flash light (WFL) technique was developed in-house for welding Cu NWs in order to produce highly conductive and transparent electrodes under ambient conditions. Flexible Cu NW films with sheet resistances of 128 Ω □−1 and transmittances of >95% at 560 nm on PET substrates were obtained by using light intensity of 1.0 J cm−2 with single pulse and an irradiation time of 5 ms. The WFL technique facilitates localized surface heating and subsequent welding at the junction of the Cu NWs with excellent stability, low sheet resistance and no damage to the polymer substrates. It is expected that the WFL technique will be widely applied to flexible printed and optoelectronic devices such as touch panel displays and solar cells.

Photonic sintering of a ZnO nanosheet photoanode using flash white light combined with deep UV irradiation for dye-sensitized solar cells

The present report details research work on the photonic sintering of ZnO nanosheets (ZnO NSs), which were synthesized via a solid-state synthesis method. The sintering was performed using flash white light (FWL) combined with deep UV irradiation (photonic sintering) under ambient conditions at ultra-high speed, which is a superior process over the conventional thermal-sintering process. Furthermore, the application of this method was demonstrated in dye-sensitized-solar cells (DSSCs), where a power conversion efficiency (PCE) of 2.9% was achieved when the photoanode was annealed using photonic sintering with a FWL of 20 J cm−2 combined with deep UV irradiation of 30 mW cm−2 irradiation power. This PCE is higher than that of the pristine ZnO NSs (PCE = 1.5%) and of the thermally sintered ZnO NS photoanode (PCE = 2.0%). The superior performance of the dye cell with the photonic-sintered ZnO NSs is attributed to the better interconnection, higher effective electron diffusion coefficient (Dn), higher electron diffusion length (Ln) and a higher amount of dye loading than that of the pristine ZnO NS photoanode. The improved PCE suggests that the photonic-sintering method, as well as being extremely simple, is highly effective and enables a fast annealing for photoanodes in DSSCs and could be particularly beneficial for low-temperature-based solar cells.

TiO2/silver/carbon nanotube nanocomposite working electrodes for high-performance dye-sensitized solar cells

In this study, we developed a new way to increase the efficiency of dye-sensitized solar cells by using TiO2/silver/carbon nanotube composites as the working electrode. Silver nanoparticles and multi-walled carbon nanotubes were mixed with TiO2 nanoparticles and used as working electrodes in a dye-sensitized solar cell. The effect of the silver nanoparticles and multi-walled carbon nanotubes on the efficiency of the dye-sensitized solar cell was studied as function of their volume fractions using several microscopic and spectroscopic characterization techniques such as scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultra-violet-vis and electrochemical impedance spectroscopy. It was found that the silver nanoparticles could induce surface plasmonic phenomena, where the light absorption was enhanced in the ultra-violet wavelength range. Additionally, the carbon nanotubes could increase the electron mobility in the working electrode due to their high surface-to-volume ratio and superior electrical conductivity. The efficiency of the silver/carbon nanotube/TiO2 nanocomposite working electrode was compared with that of a conventional TiO2 working electrode under one-sun illumination (100 mWcm–2, AM 1.5 G). The TiO2/Ag/carbon nanotube nanocomposite working electrode had a two-fold higher efficiency (3.76%) than the conventional pure TiO2 working electrode (1.88%).

Effect of copper oxide shell thickness on flash light sintering of copper nanoparticle ink

In this study, the effect of the thickness of a copper oxide-shell on flash light sintering of Cu nanoparticles (NPs) was investigated. The electrical properties of Cu nano-ink films with various oxide-shell thicknesses were examined by measuring the sheet resistance. Furthermore, the amount of PVP in the Cu NP-ink was varied to reduce the copper oxide-shell efficiently and enhance the flash light sintering of the Cu NPs. Also, to investigate the reduction and sintering phenomena of Cu NPs with respect to the copper oxide shell thickness, the sheet resistances of the Cu films were measured in real-time using an in situ resistance measuring system during the flash light sintering process. The results of this study established the maximum allowable thickness of the copper oxide shell that allows flash light sintering and also provided the optimal amount of PVP in Cu nano-ink for a particular copper oxide shell thickness.

Selective wavelength plasmonic flash light welding of silver nanowires for transparent electrodes with high conductivity

In this work, silver nanowires (AgNWs) printed on a polyethylene terephthalate substrate using a bar coater were welded via selective wavelength plasmonic flash light irradiation. To achieve high electrical conductivity and transparent characteristics, the wavelength of the flash white light was selectively chosen and irradiated by using high-pass, low-pass, and band-pass filters. The flash white light irradiation conditions such as on-time, off-time, and number of pulses were also optimized. The wavelength range (400-500 nm) corresponding to the plasmonic wavelength of the AgNW could efficiently weld the AgNW films and enhance its conductivity. To carry out in-depth study of the welding phenomena with respect to wavelength, a multiphysics COMSOL simulation was conducted. The welded AgNW films under selective plasmonic flash light welding conditions showed the lowest sheet resistance (51.275 Ω/sq) and noteworthy transmittance (95.3%). Finally, the AgNW film, which was welded by selective wavelength plasmonic flash light with optical filters, was successfully used to make a large area transparent heat film and dye-sensitized solar cells showing superior performances.