Screen printing is one of the most cost effective methods for producing Si solar cells. Recently, the metal mesh technique, which utilizes the screen printing technology, is gaining attention as an alternative technology that can replace the indium-tin-oxide (ITO)-based touch panel sensors. Because the fabrication of ITO grid lines in touch sensors needs vacuum and wet/dry etching processes, the non-vacuum metal mesh printing technique is advantageous from the viewpoint of low-cost.
Flexible displays are one of the most attractive trends in flat panel display technology. Because the ITO is brittle, it cannot be used in flexible substrates such as poly ethylene terephthalate(PET), polyimide, etc. On the other hand, since the polymer-based flexible substrates are easily deformed by heat, a very low temperature heat treatment process is required and the temperature is generally limited being lower than 150℃. However, in order to obtain a good electrical conductivity of Ag paste, a thermal treatment at least 250℃ is necessary because sintering between the Ag nano particles occurs at this temperature, and the temperature is unacceptable for the polymeric substrates.
In this study, a transient liquid phase sintering (TLPS) process in Ag paste
was examined by using a fusible metal alloy of Sn58Bi, which has the melting
temperature of 138℃. Two kinds of Ag nano powders in different sizes (71 and 308 nm) and the Sn58Bi alloy powder were used as starting materials. In terms
of paste manufacturing, different amounts of the Ag and Sn58Bi alloy powders were mixed, and a relevant amount of binders, solvent (n-Methyl Pyrrolidone) and dispersant (BYK190) were added. The mixtures were homogenized using a 3-roll mill. After screen printing the Ag pastes on polyimide (PI) substrates, the electrodes were heat treated in the range of 150, 180, 200℃ for 60 min in air. The microstructure of the samples was observed using a scanning electron microscope (6701F, JEOL), and the thickness of the electrodes was determined by the Daktak XT Stylus Profiler (Bruker) using a step method.
Comparing the electrical conductivity of the Ag pastes after a heat treatment at 150℃ with and without the Sn58Bi alloy powder, the alloy definitely played a major role in increased conductivity.
When the alloy powder melts during the heat treatment above its melting temperature around 150℃, the melt will permeate into the Ag particles by the capillary force and provides electrical conduction paths between Ag particles, which in turn improved the electrical conductivity of the Ag paste. However, without the alloy powders, improvement of the electrical conductivity was not observed.