dc.description.abstract | The aim of this work is to investigate the long-term durability of highly flexible thin film components for use in flexible optoelectronic devices. In the first phase, effects of cyclic deformation and annealing treatment on conductive durability of indium tin oxide (ITO) thin film deposited on polyethylene terephthalate (PET) substrate are investigated. In-situ electrical and mechanical tests of ITO/PET sheet under various combinations of cyclic and static loadings are conducted at room temperature. Experimental results show that the number of cycles to failure is significantly decreased with an increase in displacement amplitude, given a specific extent of electrical resistance change of ITO/PET sheet. A static holding period of 1000 s in various loading modes plays a role in influencing the failure of ITO/PET sheet. Cyclic bending combined with a static tensile holding generally generates more damage and a smaller number of cycles to failure than does that combined with a compressive holding, neutral holding, or no holding. Under a small fatigue loading, the conductive durability of ITO/PET sheet is increased with an increase in annealing temperature. However, there is little effect of annealing temperature on ITO/PET fatigue life under a larger displacement amplitude of fatigue loading. Using a surface-based cohesive modeling technique, a simplified three-dimensional (3D) finite element analysis (FEA) micromodel subjected to tensile and compressive loadings is numerically analyzed to clarify the failure mechanism of interfacial and buckling-like delamination which governs the change in electrical conductivity of ITO/PET sheet. Modeling results indicate that buckle height of the ITO/PET micromodel subjected to tensile loading is significantly greater than that of compressive loading, providing more evidence of the aforementioned effect of loading mode. In addition, buckle height of the ITO/PET micromodel subjected to tensile loading is significantly increased with an increase in ITO thickness and size of preexisting crack. Buckle height of the ITO/PET micromodel subjected to an angle of 0° between tensile loading and major axis of an elliptical crack is much smaller than that of other loading directions. Buckle height is also increased with an increase in aspect ratio of elliptical crack.
In the second phase, effects of pure cyclic and combined cyclic-static bending on the encapsulation properties of a barrier thin film are investigated. Water vapor transmission rate (WVTR) of the given barrier film is measured after variously cyclic bending conditions using a calcium (Ca) corrosion test technique. Experimental results show that microcracks in the barrier thin film are found after applying a certain number of bending cycles. They are responsible for an increase in WVTR. Given a bending radius, a greater number of bending cycles leads to a larger amount of damage, and consequently a greater extent of moisture ingress. A smaller bending radius produces a greater amount of damage than a larger one, in a short period of loading time (? 104 cycles). However, after 105 cycles of cyclic bending, the amount of damage reaches a saturated level regardless of bending radius, as all the WVTR values become comparable. A simplified 3D FEA model is established in microscale to analyze the moisture diffusion mechanism. Numerical results show that, with the presence of cracking and delamination, the WVTR value increases significantly. Good agreement between the simulation and experimental measurements on WVTR confirms that the failure mechanism involves cracking and delamination under cyclic bending. The 3D FEA modeling developed could offer a method to predict the change of WVTR in correlation with cracking and delamination in the barrier thin film. For a short holding period of static bending in the combined cyclic-static case, there is no obvious effect of static bending on barrier performance. Cyclic bending takes a main role in damaging the given barrier film. However, given a longer holding period of static bending, the contribution of static bending to deterioration in the encapsulation performance of barrier thin film is clear for a smaller bending radius. Moreover, a cyclic bending combined with static holding under a smaller bending radius generates a larger amount of damage than that of a larger bending radius. | en_US |