# Fabrication of Stretchable Transparent Electrode by Utilizing Self-Induced Vacuum Force
## 3 Results and Discussion
### Effect of Heating Methods on the Fabrication Process  
As previously mentioned, the evaporation of the solvent in the slurry should be completed before the viscosity of the elastomer reaches the critical value. Hence, it is preferable to heat the wet electrode prior to thermsetting the elastomer during the curing process. For this reason, the curing process was performed under various heating conditions: either in a convection oven or on a hot plate. In the former condition, the thermal energy was delivered by convection from hot air both to the elastomer and the silicon wafer. While, in the latter condition, the silicon substrate/wafer was heated first. Therefore, the evaporation of the solvent occurred earlier than the curing (see Figure 11). Hence, the penetration of the elastomer into the electrode was observed to accelerate when the silicon substrate was cured on the  
Figure 10: Electrical resistance of the electrode pattern depending upon the fraction of the solvent (**a**) Acetone and THF (**b**) Acetone (**c**) THF in the conductor slurry. hot plate. Owing to this, it was considered that using the hot-plate curing, electrode fabrication could be achieved, even with low-porous carbon or excessive solvent in the slurry. To demonstrate this hypothesis, the electrodes were fabricated using the slurry synthesized with Denka Black. In Section 3.2, it was confirmed that only the vacuum force is insufficient to enable the full penetration of elastomer into the electrode slurry because of the poor absorption/desorption characteristics of Denka Black (recall Figure 7). Consequently, an asymmetric electrode was obtained using the Denka-Black-based slurry. However, a well-formed electrode was obtained by curing on a hot plate (see Figure 12). In Figure 12b, the uniformly distributed particles in the electrode area and along the straight boundary between the electrode and elastomer are confirmed. In the case of curing on a hot plate, the good fabrication quality can be attributed to the fact that heat transfer began from the bottom of the silicon substrate on the hot plate, following this, the solvent evaporation, which occurs prior to the curing of elastomer, resulted in the full penetration of the elastomer. To demonstrate the effect of the heating method used, the fabrication process was also examined by changing the amount of the solvent in the electrode slurry. In this study, the stretchable electrodes were fabricated using slurries synthesized with 10, 13, and 23 mL acetone. First, the stretchable electrode was cured in a vacuum oven for convective heat transfer. As discussed in Section 3.3, a well-fabricated electrode was obtained using the slurry synthesized in 10- and 13-mL acetone due to its high volatility (see Figure 13a,b). However, the bubble defect was observed in the fabrication using 23 mL of acetone; note the bubbles in the elastomer shown in Figure 13c. Although, the volatility of acetone is superior, its excessive amount in the slurry hinders its complete evaporation (changing case A to case B in Figure 8). Therefore, the remaining acetone (i.e., the unevaporated acetone) in the electrode slurry was captured with the highly viscous elastomer during the curing process. Here, the electrode fabricated using the slurry synthesized with 23 mL of acetone was cured on a hot plate (100 \({}^{\circ}\)C). As Figure 14 depicts, the bubble defect that disappeared in the stretchable electrode, cured  
Figure 11: Thermal curing process using (**a**) convection heating from all directions and (**b**) conduction heating to the bottom. Figure 12: SEM images to show the characteristic of elastomer penetration into porous conductive materials with different thermal curing methods: curing (**a**) in convection oven and (**b**) on hot plate. The heating of the bottom of the silicon substrate results in the solvent evaporation preceding the elastomer curing, thereby leading to superior penetration of the elastomer.