Mar 16, 2017: A cost-effective method of increasing the efficiency of OLEDs through increased light extraction has been developed by the researchers from the Karlsruhe Institute of Technology and the InnovationLab in Heidelberg, in collaboration with Osram OLED GmbH.
An article titled “Large-Area Screen-Printed Internal Extraction Layers for Organic Light-Emitting Diodes”, stated that the researchers screen-printed a combination of high refractive index TiO2 nanoparticles embedded into a multicomponent polymer matrix onto the glass substrate before it receives a full OLED stack.
To get the full potential of white OLEDs as large-area light sources, guided optical modes were efficiently out-coupled, and internal extraction layers (IELs) were integrated into a scalable manufacturing process. The researchers 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.
By optimizing the initial mixture composition, the researchers achieved screen-printing-compatible rheological properties as well as tailored light scattering and transmission over the visible spectrum. The devices were finalized by depositing a high-efficiency white OLED stack atop the IEL.
The screen-printed composite lends to high-throughput roll-to-roll manufacturing processes, and can be easily scaled up. This can be applied to large OLED panels.
“The weak light extraction from OLEDs stems from the coupling of the generated photons with substrate, waveguide, and surface plasmon-polariton modes and subsequent parasitic absorption” the article said.
The composite layer operates as a volumetric light, and raises the luminous efficacy of white OLEDs by 56%.
The researchers designed a 5×5 array of 13.4×13.4mm2 white OLEDs on a 15×15cm2 substrate. While the devices had efficiencies of 35lm/W (68cd/A), the white OLED with a screen-printed scattering layer’s performance increased to 57lm/W (102cd/A) at a luminance of 2000cd/m2.