# Stability of Silver-Nanowire-Based Flexible Transparent Electrodes under Mechanical Stress
## 3 Results and Discussion
Upon application of tensile stress 1000 times, the resistance of the AgNW-embedded electrode increased by approximately 1/500 over that of the ITO electrode and 1/10 over that of the AgNW-exposed electrode. This indicates that the AgNW-embedded electrodes exhibited the highest stability under tensile and compressive stresses. To investigate why the ITO electrode on the PET film showed a more significant increase in sheet resistance than the AgNW-based electrodes when applying tensile stress, and why the sheet resistance significantly increased under the application of tensile stress compared to compressive stress, the morphology of the ITO electrodes was examined via SEM and AFM before and after applying tensile stress 1000 times (Figure 6). Although the ITO surface did not have any cracks or defects before the application of tensile stress, a large crack was observed on it after the application of tensile stress. While the cracks were not wide and the ITO was slightly connected across the cracks under the application of compressive stress, the cracks were very wide and the ITO was completely separated  
Figure 5: Sheet resistance of (**a**) ITO electrode on a PET film, (**b**) AgNW-exposed electrode on a PET film, (**c**) AgNW-embedded electrode inside NOA63 and (**d**) AgNW-embedded electrode inside NOA71 depending on the number of tensile stresses. The bending radius was 4 mm. because of the cracks under the application of tensile stress. The AFM images indicated that tensile stress increased the RMS roughness of the ITO electrode. The RMS roughness of the ITO electrode was 1.9 nm prior to applying the tensile stress and 13.2 nm after applying the tensile stress 1000 times. When comparing the application of compressive stress, the RMS roughness increased less with the application of tensile stress. When a compressive stress was applied, the two areas divided by the crack pushed each other upward, resulting in a rough surface. In the case of tensile stress application, the two areas do not have the force to push each other upward. There is only force to enable the two areas to move away from each other, resulting in a relatively small increase in the RMS roughness. In addition, both the SEM and AFM images indicate that tensile stress generated large cracks on the ITO surface and significantly increased the sheet resistance compared to that observed when compressive stress was applied, and the ITO was completely separated into two regions by the cracks. Figure 7 shows the SEM and AFM images of the AgNW-exposed electrode before and after applying tensile stress. The SEM image of the AgNW-exposed electrode shows no noticeable cracks, unlike the ITO electrode on the PET film. However, similar to the application of compressive stress, the AgNPs were broken locally. The AFM images show a different trend from that observed when compressive stress is applied. The RMS roughness remained similar, regardless of the application of tensile stress. The RMS roughnesses were 9.8 nm and 9.4 nm before and after applying the tensile stress, respectively. This is because, as in the ITO electrode, the tensile stress did not generate the force required to push the nanowires or the two different regions upward. Consequently, the AgNW-exposed electrode exhibited a smaller sheet resistance increase than that observed in the ITO electrode. In addition, although the application of tensile stress showed little change in the RMS roughness of the AgNW-exposed electrode, the sheet resistance increase was relatively large compared to that observed when compressive stress was applied. The reason is that the driving force to divide the conductive region into two was greater than the compressive stress. The AgNW-embedded electrode exhibited a stable surface morphology, regardless of the application of tensile stress, similar to the application of compressive stress. As shown in Figure 8, regardless of the type of polymer used for the embedding process, there were few cracks or regions where the nanowires were broken.