Encapsulating fabric-based OLEDs
Publishing their results in Advanced Electronic Materials under the title “Reliable Actual Fabric-Based Organic Light-Emitting Diodes: Toward a Wearable Display”, the researchers disclose a multi-step planarization and encapsulation method including both organic and inorganic multi-layered moisture barriers.
The researchers started with polyester-fibre-based woven fabric substrates, 100μm thick, which they planarized via a dual-step lamination process. To smooth out the rough fabric structure (the wefts and warps of the interwoven yarns), they used a 20 to 40μm thick PU film made of a low viscosity layer on the bottom side laying against the bare fabrics, and a higher viscosity film on the upper side to create a reliable flat surface onto which to deposit their OLED structure.
First, the researchers proceeded to lay a multilayer barrier film on top of the prepared fabric, consisting of aluminium oxide (Al2O3) deposited by atomic layer deposition (and poly(vinylalcohol) (PVA) used as further planarization layers, with respective thicknesses of 30 and 250 to 300nm. Inverted top-emitting OLEDs were then stacked by thermal evaporation. To further protect the OLED and prevent contamination during their fabrication, multilayer capping layers consisting of N,N′-Bis(naphtanlen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB)/zinc sulfide (ZnS)/NPB/ZnS were inserted in the stack by in-situ thermal evaporation before the deposition of the first Al2O3 layer of the top multilayer barrier film.
In this experiment, turn-on voltage was 4 to 4.25V for a luminance over 1cd m−2 and current efficiency was around 5cd A−1. At 8.5V, the luminance was around 1300cd m−2 out of a prototype with an active area of 9mm2. The double-sided encapsulated devices showed stable operation for 1000h when driven at a constant current density of 11.1 mA cm−2, they still operated after 3500h, with very few dark spots when kept in ambient air. Their lifetime tendencies were similar to those of the glass-based control devices, the researchers wrote in their paper.
While the bending length slightly increased from 15 to 21mm after coating with the PU films, they noted that OLEDs with the multilayer capping barely degraded the flex stiffness. But the multilayer barrier films (seven layers of Al2O3 and six layers of PVA) increased the bending length to 40mm. In the whole, the 122.2μm thick fabric-based OLED had exhibited a flex stiffness characteristic similar to that of 50μm thick PET films, something they ought to improve in order to make these fabric-based OLED a compelling display proposition.
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