Screen-printed composite boosts OLED 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 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.
The researchers designed a 5×5 array of 13.4×13.4mm2 white OLEDs on a 15×15cm2 substrate and characterized them against reference devices designed without an internal extraction layer (IEL).
While the reference devices without an internal extraction layer had efficiencies of 35lm/W (68cd/A), the white OLED with a screen-printed scattering layer saw their performance increased to 57lm/W (102cd/A) at a luminance of 2000cd/m2. This 56% relative enhancement was demonstrated for both cd/A and lm/W efficiencies, indicating an efficient waveguide and substrate mode out-coupling.
Without introducing any electrical defects, the scattering layer also led to a better angular stability of the emission spectra.
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