Effects of Flexural Deformation on the Functional Properties of Conductive and Encapsulation Thin Films in Flexible Electronics(2/3)

Flexible electronic devices have great potential for widely novel applications as they can conform to a desired shape, or flex during its use. As organic semiconducting materials have the ability to flex such that they are the key materials in flexible electronic devices such as organic light-emitting diode (OLED), organic photovoltaic (OPV), and organic thin-film transistor (OTFT). In a typical flexible electronic assembly, transparent conductive thin-film electrode and encapsulation thin-film barrier are two of the major components, e.g. in OLED and OPV. A highly transparent, conductive electrode is needed for light transmitting and an effective encapsulation barrier is needed to protect the device from exposure to the environment. In practical applications of such flexible electronic devices, they might be subjected to long-term static and/or cyclic flexural deformation which may cause damages in their components and degrade their performance. In particular, flexural-deformation induced damages (microcracking and/or delamination) would reduce the electronic conductance of transparent conductive thin-film electrode and increase exposure to water vapor and oxygen leading to adverse oxidation of functional layers. In this regard, structural reliability is one of the greatest challenges which must be addressed prior to wide spread commercial application of flexible organic devices. For this reason, it is necessary to investigate the mechanical behavior of transparent conductive thin film and encapsulation thin film for use in flexible electronic devices and how their functional properties are affected, when subjected to long-term static and/or cyclic flexural deformation.The aim of this three-year study is thus to systematically characterize the effects of long-term static and/or cyclic flexural deformation on the electric conductance of transparent conductive thin films as well as on the water vapor transportation resistance of encapsulation thin films used in flexible electronic devices. Flexural fatigue, creep and fatigue-creep combined tests are to be conducted on indium-tin-oxide (ITO) thin film on two types of polymeric substrate, namely polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). The change of electrical resistance will be monitored simultaneously during mechanical testing so as to investigate the effects of flexural deformation on electrically conductive properties of the ITO film under applied loadings. A finite element method (FEM) modeling is to be developed to simulate the mechanical behavior and offer more understanding of the failure mechanism that governs the change in electrical conductivity of ITO thin film. Similarly, static, cyclic, and cyclic-static combined bending tests will be conducted on two encapsulation thin films of different barrier structures to investigate the effects of long-term flexural deformation on their water vapor transportation rate (WVTR). Calcium corrosion test is to be employed to measure the WVTR for the given encapsulation thin films after each given flexural loading. Moreover, an FEM model is to be established to simulate the moisture permeation mechanism in a damaged encapsulation thin film so as to predict the WVTR. It is hoped that results of this study could provide useful information for flexible electronics developers and users to prevent failure of ITO thin film on polymeric substrate and of encapsulation thin film, to find methodologies for improving their performance and mechanical integrity, and to reach competitive cost and engineering requirements.