OLED outcoupling layer is wavelength and viewing-angle independent

July 18, 2018 //By Julien Happich
OLED outcoupling layer is wavelength and viewing-angle independent
Improving on previous microlens-based approaches they had developed, researchers from the University of Michigan are now doing away with microlens arrays and demonstrated better OLED light outcoupling with a simpler to implement dielectric diffuse reflector.

The paper "Efficient Outcoupling of Organic Light-Emitting Devices Using a Light-Scattering Dielectric Layer" published in ACS Photonics details how the integration of a high index of refraction waveguide layer together with a rough, dielectric diffuse reflector at the bottom of a transparent, top emitting structure eliminates plasmonic, waveguide, and substrate modes without introducing wavelength and viewing-angle dependence.

A PHOLED stacked on top of a diffuse reflector, scattering
the light back into the viewing direction (following a Lambertian
profile in red). Light in the emission cone is scattered into the
viewing direction (green cone).

In this study, the authors replaced the conventional bottom metal electrode in top-emitting OLEDs with a transparent layer of indium tin oxide (ITO) deposited on a 240μm thick polytetrafluoroethylene (PTFE, Teflon) diffuse reflector (itself backed by a 20μm thick Al mirror). A planarized high index of refraction (nP = 1.8) polymer slab waveguide covers the rough reflector so as to create a smooth surface on which to fabricate the OLED, while helping maximize the light coupled into the diffuser.

This new approach was demonstrated to increase the external quantum efficiency from 15 ± 2% to 37 ± 4% compared to a similar device with a metal mirror, a 250% improvement.

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