Large-Area Screen-Printed Internal Extraction Layers for Organic Light-Emitting Diodes

Abstract

To unleash the full potential of white organic light-emitting diodes (OLEDs) as large-area light sources, guided optical modes have to be efficiently outcoupled, which calls for internal extraction layers (IELs) that can be easily integrated into a scalable manufacturing process. To realize such IELs, we developed a high refractive index scattering polymer:TiO2-nanoparticle mixture that can be deposited onto a large area by using the cost-effective screen-printing method. We exploited this approach to produce a 10 μm thick IEL covering the exact area of active pixels distributed over a 15 × 15 cm2 glass substrate. By optimizing the initial mixture composition, we achieved screen-printing-compatible rheological properties as well as tailored light scattering and transmission over the visible spectrum. The spatial homogeneity of those optical properties was obtained by additional substrate treatments to improve the wetting behavior and to allow reflow after printing. The devices were finalized by depositing a high-efficiency white OLED stack atop the IEL. We demonstrated a luminous efficacy increase up to 56% due to the scattering layer. The IEL also ensured a Lambertian emission profile without any angular color shift.