Department of Mehcanical EngineeringSilver nanowires (AgNWs) electrodes satisfy the conditions that transmittance is higher than 80% at a wavelength of 550 nm and surface resistance is less than 100 ?? / ???, which are required criteria for the application of transparent electrodes. In addition to this, AgNWs have high flexibility and electrical conductivity that are suitable for flexible organic light-emitting diodes (OLEDs), and thus has been of a great interest as an alternative transparent electrode being able to replace indium tin oxide (ITO) of conventional displays.
So far, for the development of alternative transparent electrodes, only transparency and sheet resistance criteria have been considered. However, when we consider flexible display devices in the near future, the transparent electrodes should be robust to folding and stretching in which conventional ITOs have intrinsic limitations. Therefore, a mechanical investigation on the structural stability during folding process is required for the application of AgNWs electrodes to flexible display devices. In this thesis, we performed the stress analysis for OLEDs that contain AgNWs or ITO electrode thin layer as one of components of flexible OLEDs displays. We compared the stress distributions of OLEDs under bending test and investigated the effect of the volume fraction of AgNWs on the structural robustness of the structures.
The first reference model consisted of five layers including ITO with a thickness of 200 nm. Other four layers were aluminum (Al, 70nm), super yellow light-emitting polymer (PDY-132, 80nm), poly (3,4ethylenedioxythiophene) doped with poly (styrene sulfonate) (PEDOT:PSS, 40nm), and polyethylene terephthalate (PET, 0.1mm) layers. The second reference model also consisted of five layers, but now the ITO layer was replaced by AgNWs composite with PEOT:PSS with a thickness of 72 nm while the materials of the other four layers are the same. AgNWs of which length and diameter are 25+-5 um and 36 +-5 nm are employed in real production. Second reference model has a sheet resistance of 12.63 ?? / ??? and a transmittance of 93% at a wavelength of 550 nm.
The real size of the flexible OLED device is 15 mm x 15 mm in the lateral directions, but we carefully reduced it to 7 mm x 1 mm without any significant change in the stress analysis to lower computational cost. We modelled the AgNWs composite layer as a conventional fiber-reinforced composite in which the AgNW acts as a fiber and the conductive polymer PEDOT: PSS acts as a resin. Then we determine the longitudinal Young???s modulus (E_L), transverse Young???s modulus (E_T), in-plane shear modulus (G_LT), and major Poisson???s ratio through a proper homogenization. Although AgNWs are arbitrary distributed in the real AgNWs composite layers, we assumed that they are distributed in orthogonal patterns in our numerical models.
AgNWs manufacturing process uses spin-coating. Accurate volume fraction of AgNWs is unknown because the spin rate and time can be changed according to situation or purpose of experiment. However, I can assume that the volume fraction of AgNWs is 10% and the thickness of this layer is twice diameter of AgNW through the SEM images of AgNWs. The Young???s modulus and Poisson???s ratio of AgNW are 176 GPa, and 0.225. The Young???s modulus and Poisson???s ratio of PEDOT:PSS are 1.9 GPa and 0.34. For the homogenization for the AgNWs with 10% volume fraction, the values of E_L,E_T,G_LT and major Poisson???s ratio are 10.81 GPa, 10.87 GPa, 0.85 GPa, and 0.127. As the volume fraction increases from 6% to 65%, the longitudinal Young???s modulus also steeply increases from 7.19 GPa to 95.29 GPa.
From the numerical analysis of OLEDs containing ITO or AgNWs with 10% volume fraction, Von-mises distributions in the other four layers such as Al, PDY-132, PEDOT: PSS, and PET layers in the both models were almost the same. However, the maximum Von-mises in the ITO layer, 6.99 GPa was found to be approximately 6.32 GPa larger than the maximum Von-mises in the AgNWs composite layer, 0.67 GPa. In addition to stress distribution, the fracture toughness of ITO and silver are about 2.59 MPa m^(1/2) and 40 MPa m^(1/2) at room temperature. Fracture toughness is used as a failure criterion of material and represents the resistance of the material to brittle fracture. If ITO is used for flexible OLEDs, it will be easy to break even under small deformation. As a result, it was confirmed that the AgNWs electrode is superior to the ITO electrode in the aspect of structural and flexural robustness during bending.ope
