Water vapor permeation in amorphous/polycrystalline inorganic layers on polyethylene terephthalate substrates

Abstract

The barrier performance of amorphous and/or polycrystalline inorganic oxide thin films deposited on flexible polyethylene terephthalate (PET) substrates is evaluated by a developed model that accounts for the diffusion of water vapor through the pinholes and grain boundaries, respectively, in amorphous and polycrystalline sublayers. Three-dimensional diffusion simulations are performed for modeling the influences of layer thickness, defect spacing and grain size on the barrier properties of single (multi) layer films. The water vapor transmission rate (WVTR) is calculated for single/multi-inorganic layers with different structures (amorphous and polycrystalline), numbers of inorganic layers and thicknesses. The simulation results show that the WVTR decreases exponentially with the increase of the inorganic layer thickness. According to the fitted results, the diffusion coefficient of water vapor in the defects (grain boundaries or pinholes) of inorganic materials is much smaller than that in PET, and the diffusion coefficient of water vapor in the grain boundaries of the polycrystalline inorganic layer (PIL) is close to that in the defects/pinholes of the amorphous inorganic layer (AIL). In comparison to PET/AIL, PET/PIL and PET/PIL/AIL films, the barrier performance of PET/AIL/PIL film is superior. As examples, PET/Al2O3/ZnO and PET/ZnO/Al2O3 films were prepared for verification of the developed model. The simulated WVTR values agree well with the experimental ones.