Hybrid Materials and Structure with Anti-Moisture and Stress-Release for Flexible Thin Film Encapsulation

Abstract

The rapid development of flexible electronics is leading to a paradigm shift from rigid-form technology to the trans-form technology. Among of them, OLEDs has moved towards commercialization with large area curved display applications in 2017. Next generation display based on OLEDs has a great attention for future display such as foldable, rollable and stretchable display. One of main challenges for next generation display is high efficient thin film encapsulation under mechanical stress. In order to accomplish these requirement, several previous studies for flexible moisture barrier using various techniques (ALD, CVD, Sol-Gel, and PVD) and structure since Vitex inorganic/organic structure was announced in 2003. Even though inorganic/organic structure is effective for improving gas barrier properties and flexibility, it has never achieved for ultimate requirements. The main purpose of this thesis is that understanding the moisture permeation mechanism in single or multi-stacks layers, and development of moisture barrier materials and structure with low moisture permeability under mechanical stress. Firstly, in order to understand multi-structure effect for low moisture permeability, the inorganic/inorganic multi-stacks films using Al2O3/ZnO layers grown by ALD at extremely low growth temperature (60oC) with various structures are investigated. The multi-laminated structure with a thinner ZnO and Al2O3 had better barrier property than that of single ZnO and Al2O3 layers, showing that the water vapor transmission ratio of multi-laminated ZnO/Al2O3 layer was 10 times lower than that of the single layer Secondly, a novel moisture barrier films which consists of inorganic Al2O3 layer and GO nano-sheets were investigated. While selective ALD growth was observed on CVD-grown graphene along defective sites, smooth and continuous films were grown on GO without selective growth. Linear growth of Al2O3 on GO was observed without a nucleation region or growth selectivity. Highly aligned, multiple-stacked, three-dimensional Al2O3/GO structures were fabricated using ALD and solution process, which showed much better moisture barrier characteristics than single Al2O3 thin films of the same thickness. Thirdly, in-situ inorganic/organic laminated thin films using ALD and MLD process were examined as moisture barriers. The sensitivity of individual organic layer deposited by MLD process with respect to ambient exposure was found to be related to moisture permeation and hydration reactions, of which the mechanism is studied by density functional theory calculations. The inorganic/organic hybrid moisture barrier showed better moisture barrier property with mechanically stability than single Al2O3 layer when chemically stable structure was formed. Finally, in order to suggest optimized moisture barrier structure by stress engineering, I investigated moisture barrier properties with bending stress as a function of substrate, thin film and additional layer thickness for optimized flexible moisture barrier structure. Also, ex-situ inorganic/organic hybrid structure was adopted because the compliant organic layer between stiff inorganic layers absorbs the stressed induced in the inorganic layer due to its elastic moduli different by order of magnitude. As a result, optimized moisture barrier structure using inorganic/organic layer is not only to improve moisture barrier properties, but also exhibited mechanically robust behavior after bending stress.