# Stability of Silver-Nanowire-Based Flexible Transparent Electrodes under Mechanical Stress
## 3 Results and Discussion  
To investigate the effect of compressive stress on the electrical properties of various types of AgNW-based electrodes, sheet resistances were measured depending on the number of compressive stresses applied to the electrodes. Figure 1 shows the sheet resistance depending on the number of compressive stresses applied to an ITO electrode on a PET film, a AgNW-exposed electrode, and a AgNW-embedded electrode. As shown in Figure 1a, the sheet resistance of the ITO electrode on the PET film increased linearly with the application of compressive stress. While the initial sheet resistance was 93 \(\Omega\)/sq, the sheet resistance increased to 2214 \(\Omega\)/sq after compressive stress was applied 500 times and to 3173 \(\Omega\)/sq after compressive stress was applied 1000 times. This indicates that the ITO electrode on the PET film is no longer suitable for electronic device applications. In contrast, the AgNW-exposed electrode exhibited relatively stable electrical characteristics under compressive stress. Although the sheet resistance increased according to the number of compressive stresses, it was only approximately 572 \(\Omega\)/sq when compressive stress was applied 500 times and 1178 \(\Omega\)/sq when compressive stress was applied 1000 times (Figure 1b). The sheet resistance was approximately 1/3 of that of the ITO electrode on the PET film at an identical number of compressive stresses. The AgNW-embedded electrode exhibited better stability under compressive stress than the AgNW-exposed electrode. First, when NOA63 was used as a polymer for embedding AgNWs, the sheet resistance was approximately 325 \(\Omega\)/sq when compressive stress was applied 500 times and approximately 441 \(\Omega\)/sq when it was applied 1000 times (Figure 1c). These were approximately 1/10 of the sheet resistance of the ITO electrode on the PET film and 1/2 of that of the AgNW-exposed electrode, indicating the superiority of the AgNW-embedded electrode. Even when the polymer for the embedding process was changed to NOA71, there was no significant change in sheet resistance characteristics upon application of compressive stress. As shown in Figure 1d, the sheet resistance was approximately 334 \(\Omega\)/sq when stress was applied 500 times, and approximately 343 \(\Omega\)/sq when stress was applied 1000 times. Therefore, as long as the polymer used for the embedding process is sufficiently flexible, sufficient stability under compressive stress can be ensured for a AgNW-embedded electrode. To determine why the sheet resistance of the ITO electrode on the PET film increased rapidly and why the AgNW-based electrodes showed good electrical stability with the application of compressive stress, the surface of each electrode was observed using SEM and AFM after applying compressive stress 1000 times. Figure 2 shows SEM and AFM images before and after applying compressive stress to the ITO electrode. The SEM image shows the smooth surface of the ITO electrode before applying compressive stress (Figure 2a). In contrast, a large crack is observed in the SEM image of the ITO electrode after the application of compressive stress (Figure 2b). The SEM results indicate that the compressive stress generated cracks on the rigid ITO surface. The AFM image also shows the changes on the surface upon applying compressive stress. As shown in Figure 2c, the ITO electrode on the PET film exhibits a flat surface with an RMS roughness of 1.9 nm. However, after applying the compressive stress 1000 times, the ITO electrode appeared to float on the PET film and exhibited an RMS roughness of 170.0 nm. This indicates that cracks occurred on the ITO surface due to compressive stress, and the sheet resistance continued to increase with crack growth. Figure 3 shows SEM and AFM images of the AgNW-exposed electrode. First, large cracks, which were observed in the ITO electrode, were not found on the AgNW-exposed electrode; however, local bending or breaking of the AgNW s was observed in the SEM image.