Cheap and wavelength-independent OLED light extraction

April 25, 2018 // By Julien Happich
Looking at optimizing the light extraction of OLEDs and their overall external quantum efficiency, researchers from the University of Michigan have demonstrated that a microlens array embedded between the OLED's bottom transparent ITO electrode and the glass substrate was cheap to implement, yet very effective for extracting trapped light in the active region of the OLED.

Fully transparent and with no apparent impact on the image sharpness when implemented in an OLED display, the sub-electrode microlens array (SEMLA) was fabricated using conventional photolithography prior to the OLED array deposition. It consists of a flat spacer layer on top of a hexagonal closed-packed array of 10μm diameter hemispherical lenses.

Described in the paper "Efficient, Nonintrusive Outcoupling in Organic Light Emitting Devices Using Embedded Microlens Arrays" published in ACS Photonics, the SEMLA's refractive index (nSEMLA) was chosen to be 1.8, close-enough to that of the organic layer and the ITO electrode.

An OLED stack built on top of a sub-electrode microlens
array (SEMLA) substrate.

Using ray tracing calculations for a device stacking a 40nm ITO anode, a 40nm hole transport layer, a 20nm emission layer, an electron transport layer and then an aluminium cathode, the authors reported that for such a high refractive index, the waveguide modes were reduced to almost zero for an electron transport layer less than 70nm thick, with the SEMLA structure extracting all radiated optical power except for the surface plasmon modes (along the metal−organic interface).

What calculations reveal is that the light extraction from the SEMLA into glass (nglass = 1.45) is more efficient than from an external MLA (nMLA = 1.4−1.5) into air (nair = 1) due to reduced reflection at the lens/glass interface with its larger critical angle. What's more, the SEMLA surface is smooth, eliminating optical scattering from the structure. And because it is still external to the device active region, it does not affect the actual design of the OLED itself.

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