## 3 Results and Discussion
### Thickness-Enhanced Inkjet Lines by Overprinting  
Increasing the thickness of inkjet-printed lines needs the overprinting of a number of ink layers on top of each other. In the overprinting process, temperature condition is a critical factor since it changes the solidification rate of ink droplets before their spreading by increasing a solvent evaporation rate. The proper control of temperature enables the buildup of ink layers without their flowing down. Therefore, this study explored the ways to stably produce thick inkjet-printed lines with narrow width. The experiment was performed on \(S\) 50 substrate heated to \(T=125\)\({}^{\circ}\)C and each ink layer was printed every 2 min on the substrate. First, overprinting by continuous printing procedure was attempted. It was sequentially performed by increasing \(N\) from 1 to 4 at 40% overlap ratio (Figure 12a). The morphology of the resulting silver lines was investigated by measuring the cross-sectional profiles after drying (Figure 12b) and after sintering (Figure 12c) respectively and comparing them each other. According to the measurement, there is almost no increase of the line width during the overprinting (\(N\geq 2\)). This is because the overprinted droplets were dried on the substrate heated to 125 \({}^{\circ}\)C before their spreading. If an ink flow occurred throughout the overprinting, the width of the inkjet-printed lines would increase with the number of printings (\(N\)). The spreading of the deposited droplets is also greatly influenced by the surface morphology of as-printed lines. The smooth and concave surface of as-printed lines helps stable landing and stacking of ink droplets. The combination of these effects enabled the successful printing of the stack-like geometry with \(\sim\)9 \(\upmu\)m thickness and \(\sim\)70 \(\upmu\)m width at \(N=4\). However, coffee ring effect was observed from all morphologies of the printed lines. Figure 12c shows the morphology change of the lines after sintering. The inkjet lines experienced a significant decrease of thickness after 1 h heating at 220 \({}^{\circ}\)C in furnace. It is due to the aggregation and densification of the Ag nanoparticles along with grain growth, and the decomposition of organic additives within the ink. The coffee ring effect became more dominant because of the central collapse which was resulted from the movement of the silver particles toward the contact line during the sintering. For further details, the change of thickness of the overprinted lines by each of the drying and the sintering, and the coffee ring effect were re-evaluated in terms of three parameters: (1) thickness increase ratio; (2) vertical shrinkage ratio and (3) valley-to-peak ratio. The results are given in the Figures 13 and 14. The first ratio of the overprinted lines increased linearly with \(N\) for both the two  
Figure 12: Microscopic images (**a**) and morphologies (**b**) after drying and (**c**) after sintering of inkjet-printed lines on \(S\) 50 substrate for different numbers of printings (\(N\)) at 120 \({}^{\circ}\)C substrate temperature when using continuous printing procedure. processes, due to the stacking behavior of the lines. The second ratio was constant regardless of the overprinting, since the densification of Ag nanoparticles in the ink occurred in all the stratified lines during the sintering process. Additionally, the surface non-uniformity of the inkjet-printed lines by coffee ring effect was also investigated by calculating the change of the third ratio with the overprinting (Figure 14). As \(N\) increased, the value of the valley-to-peak ratio increased too. It presents that the surface of the inkjet-printed lines could get uniform through the overprinting even at the high substrate temperature. The valley-to-peak ratio after the sintering was overall lower than after the drying, indicating that the coffee ring effect became more dominant due to higher shrinkage at the middle surface region.