Sub-electrode grid boosts OLEDs' EQE by 50%

January 26, 2017 // By Julien Happich
Seeking a way to improve OLEDs' light extraction efficiency, researchers from the University of Michigan investigated the use of a reflective and scattering sub-electrode grid to reduce light losses through waveguide and surface plasmon polariton (SPP) modes.

In a paper published in the ACS Photonics journal under the title "Top-Emitting Organic Light-Emitting Devices Using a Reflective Subelectrode Grid", they detail a novel outcoupling scheme for top-emitting OLEDs, replacing both the anode and cathode with indium zinc oxide (IZO)/ molybdenum trioxide (MoO3) transparent contacts and placing a reflective and scattering corrugated metal-coated dielectric mirror beneath the electrically active organic region. They completed their design optimization with a low refractive index antireflection (AR) layer on top of the stack to reduce microcavity effects.

Schematic cut away view of top-emitting
organic light emitting devices with indium zinc oxide
(IZO)/MoO3 electrodes and a metal-coated
sub-electrode grid.

"The design spaces the active region away from the metal reflector to minimize coupling to SPP modes while scattering out the waveguided optical power without disturbing the planarity of the device itself" they write in the article.

"The silver reflector is a patterned grid of raised rectangles whose periodicity is on the order of several wavelengths to avoid angle- and wavelength-dependent effects. A dielectric spacer fills in the depressions and extends above the rectangular grid, providing a planar surface for the subsequent deposition of the electrodes and organic layers. The thick and thin spacer regions couple differently to the microcavity modes by locally creating both thick and thin cavity regions beneath the electrode" explains the paper.