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.
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.
The researchers then implemented the SEMLA with a number of phosphorescent OLEDs, attaching an external microlens array to the SEMLA substrates to further enhance outcoupling. They report an external quantum efficiency of up to 70 ± 4% (for green phosphorescent OLEDs) and up to 50 ± 3% for white PHOLEDs, enhancing light output by a factor of 2.8 and 3.1 compared to similar devices built on conventional glass substrates. This wavelength-independent enhancement can be used with regular glass substrates in place of sapphire substrates, to benefit from both higher efficiency and lower manufacturing costs, conclude the authors, seeing their development suitable for both display and lighting applications.
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