Screen-printed composite boosts OLED light extraction

March 13, 2017 // By Julien Happich
In collaboration with Osram OLED GmbH, a team of researchers from the Karlsruhe Institute of Technology and the InnovationLab in Heidelberg have found a cost-effective way to boost the efficiency of OLEDs through increased light extraction.

Detailing their method in the ACS Photonics journal under the title "Large-Area Screen-Printed Internal Extraction Layers for Organic Light-Emitting Diodes", the researchers screen-printed a mix of high refractive index TiO2 nanoparticles embedded into a multicomponent polymer matrix onto the glass substrate before it receives a full OLED stack.  Much simpler than using complex multi-periodic gratings or randomly disordered structures, the screen-printed composite lends itself to high-throughput roll-to-roll manufacturing processes and could easily be up-scaled to be applied to large area OLED panels.

The white OLED without (top) and with (bottom)
the screen-printed light extraction layer.

"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" they note in their paper, and the additional layer efficiently out-couples the photons propagating in those internal modes. Only 10μm thick and with only 8.82 weight % of TiO2 nanoparticles (21nm in diameter), the composite layer operates as a volumetric light scattering medium, raising the luminous efficacy of white OLEDs by 56% with spectral and angular invariance.