# Stability of Silver-Nanowire-Based Flexible Transparent Electrodes under Mechanical Stress
## 2 Materials and Methods  
To observe the effect of external compressive and tensile stresses on the properties of flexible transparent electrodes, an ITO electrode on a PET film was purchased and AgNW-based flexible transparent electrodes were prepared. For the AgNW-based transparent electrodes, a AgNW-exposed electrode, where the AgNPs were exposed to air, and a AgNW-embedded electrode, where the silver nanowires were impregnated onto a polymer surface, were fabricated. The ITO electrode on the PET film was purchased from Sigma-Aldrich and used as received. The thicknesses of the ITO and PET film were 130 nm and 127 um, respectively. The AgNW-exposed electrode was manufactured by spin-coating a 0.3 wt% AgNW solution in deionized water (C3 nano Korea, Yonggin, Republic of Korea) on a PET film and drying it on a hot plate. The diameter and length of the AgNWs were 25 nm and 25 um, respectively. Spin-coating was performed at 1000 rpm for 60 s, followed by a drying process at 100 \({}^{\circ}\)C for 60 s. The AgNW-embedded electrode was fabricated by coating an anti-adhesive layer on a glass substrate, coating a silver nanowire on the anti-adhesive layer, coating an ultraviolet (UV)-curable polymer on the silver nanowire, curing it under UV illumination, and then peeling it off from the anti-adhesive layer. Poly(methyl methacrylate) (PMMA, Kayaku, Westborough, MA, USA) was used as the anti-adhesive material, which was spin-coated at 3000 rpm for 60 s and dried on a hot plate at 180 \({}^{\circ}\)C for 30 s to form an anti-adhesive layer. The AgNW solution was also coated and dried under the same conditions to form a AgNW layer as mentioned above. NOA63 (Norland Products Inc., Jameburg, NJ, USA) and NOA71 (Norland Products Inc.) were used as polymers for embedding the AgNPs. The viscosities of NOA63 and NOA71 were 2000 cps and 200 cps at 25 \({}^{\circ}\)C, respectively. The moduli of elasticity were 240,000 psi for NOA63 and 55,000 psi for NOA71, respectively. The tensile strengths were 5000 psi for NOA63 and 1300 psi for NOA71. NOA63 and NOA71 were spin-coated at 500 rpm for 60 s, followed by curing under 365 nm UV illumination for 90 min. This resulted in a 70 um-thick NOA63 layer and a 25 um-thick NOA71 layer, respectively. NOA63 and NOA71 have different viscosities; thus, the thicknesses of NOA63 and NOA71 were different under the same spin-coating conditions. They were then peeled off from the anti-adhesive layer to complete the fabrication of the AgNW-embedded electrode [32]. The AgNW thickness for the conduction path was the same in both AgNW-embedded electrodes because the spin-coating conditions for AgNWs were the same.  
After preparing various flexible electrodes, their electrical, morphological, and optical properties were examined by applying compressive and tensile stresses. Assuming that the conductive material was located on the top surface, the electrodes were bent upward to induce compressive stress and downward to induce tensile stress. The bending radius was 4 mm under each stress condition. The sheet resistance was measured to examine the effect of external stress on the electrical characteristics. After five samples of each electrode type were tested, the average value and standard deviation were calculated excluding the maximum and minimum values. A four-point probe meter (EDTM, RC2175) was used to measure the sheet resistance, and the change in the resistance depending on the number of stress applications was recorded. Next, the morphological changes were examined to investigate the factors that result in changes in resistance. Morphological characteristics were analyzed via SEM (FE-SEM, JSM-6500F, JEOL, Peabody, MA, USA) and AFM (XE-100, Park Systems, Suwon, Republic of Korea), and crack generation and crack density were observed. In addition, the optical transmittance was analyzed using UV-Vis spectroscopy (UV-2600, Shimadzu, Kyoto, Japan) to investigate the effect of external stress on the optical characteristics. The wavelength range for the transmittance measurements was 300-800 nm.