Transparent high barrier coatings for electronic encapsulation

Kaabeche, Nessima (2017) Transparent high barrier coatings for electronic encapsulation. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Barrier coatings are a category of functional films designed to enhance enduse properties to the underlying substrate. When used for electronic applications (such as flexible displays, digital paper, lighting, OLEDs and solar cells), the barrier characteristics are meant to protect the device from environmental influence, especially the permeation of oxygen and water vapour that can degrade and corrode the active layers of the devices (causing mal-functioning). In this project, silicon oxide barrier layers were deposited onto a non-treated BO-PET via plasma enhanced chemical vapour deposition, using a pilot scale roll-to-roll coater. The aim was to optimise the deposited coatings by understanding the effect of the deposition parameters on the barrier properties (oxygen and water vapour barrier) and the surface properties (i.e. topography, chemistry, structure, thickness, mechanical properties) of the coatings. For encapsulation in electronic devices such as OLEDS and photovoltaics cells, the barrier coatings must remain transparent and flexible, which is one of the challenges of this project. This project has demonstrated that the moisture barrier performance of silicon oxide coated BO-PET is dependent on film structure (i.e. porosity), which are linked to the plasma conditions of the deposited film. Lower WVTR could be reached (10-2×g/m2/day) for film produced at high input power (1.6kW), low webspeed (<0.5m/min) and low ratio oxygen:monomer (between 2 and 3). In these conditions, the coating was rather stoichiometric and exhibited a relatively low carbon content (30% atomic) and similar contents in O and Si (about 30% each). The film type was found to have an influence on the final barrier level, as coatings on planarised and adhesion treated substrates showed better barrier performances than coatings on standard untreated PET. Despite all the development done, barrier levels did not match the requirements (10-5×g/m2day) for electronic encapsulation but showed some promising improvement. Thinner coatings were found to have better barrier against moisture permeation, although a threshold of 800nm was identified as critical thickness above which the WVTR dramatically increased. As far as changing the plasma composition via the addition of CO2 as a reactive gas, a slight decrease in WVTR (improvement in barrier) was observed at low CO2 flow rates (up to 200SCCM). WVTR doubled, however, when increasing the amount of CO2. This increase was associated with a decrease in hardness and increase of carbon content. Alternative power generation, using squared waves instead of sinusoidal, was not successful and deteriorated the barrier performances (higher WVTR). Finally, the route of topcoats to enhance the barrier by filling the pores showed promising results in the case of Al2O3 ALD topcoats but didn’t show significant improvement in the case of acrylate topcoats, partly due to a lack of adhesion of the acrylate on the surface of the SiOₓ coating.