Thin Film Encapsulation
With the rise of wearable devices, flexible active matrix OLEDs have been seen as the next-generation display technology. To ensure protection of flexible devices, conventional encapsulation methods are not suitable due to their inherent rigidity, and Thin Film Encapsulation (TFE) has been considered as the most promising technology.
Thin Film Encapsulation is based on a multi-layer film, made of alternating organic and inorganic layers. The inorganic layers are typically made of metal oxides and act as barriers for moisture. These layers are virtually extremely good barriers, but they are mechanically rigid and brittle. Moreover, these layers alone would naturally present pinhole defects, that in the long run would let water and oxygen in. In standard TFE structures, organic planarization interlayers are employed, in order to improve the mechanical properties of the multilayer (flexibility without delamination) and to limit, to some extent the water permeation through the pinholes.
The organic layers are usually deposited by ink-jet printing, while CVD (Chemical Vapor Deposition) or ALD (Atomic Layer Deposition) can be used for the deposition of the inorganic layers, which are usually made of nitrides or oxides (e.g. SiNx, SiOx, Al2O3). These layers must be deposited at low temperature, without damaging the OLED stack beneath. If perfectly integrated, TFE could enable truly flexible, lightweight devices, entirely based on plastics.
Thin Film Encapsulation is thus a very complex design, where each component has to be chemically, physically, mechanically and optically optimized, in order to assure several years of lifetime to the OLED device, without damages. From the materials design point of view, TFE is extremely challenging and continuous research is on-going, to optimize both the organic and inorganic layers materials and the corresponding deposition processes.
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