Challenges In OLED Research and Development II: Light Extraction

Light extraction is the process of removing light from the device itself. So when the device emits a photon, it gets trapped in the glass, in the organic layers themselves in the metals. And the-- what our challenge is, is to get all of that light into the viewing angle. So that when somebody sees the OLED they're seeing the full efficiency of that device. Rather than having light lost in the various segments.

You think of this device as occupying a plane. And the photons that you want to see are the ones that are coming more or less perpendicular to the plane. But if the light is emitted parallel to the plane then it's trapped and you have to extract that somehow. Get it into the glass. Once it gets into the glass, then we can take it out with microlens arrays.

(The Office of Energy Efficiency & Renewable Energy/ LEDinside)

The DOE (Department of Energy) puts in a grid of varying index of refraction contrast and it's sitting below the anode. It's just in the substrate so it's not impacting the device structure. And so the wave guided light that's running parallel to the plane, scatters into the glass modes. If you don't do anything you get 20% outcoupling. With the micro-lens you get about 40% and with the sub-anode grid, you get another 15%. So you're about 55%.

And now you're left with the hardest. And that's where the DOE is today, at 45%. It has some schemes that it thinks it can actually get around an 80% external quantum efficiency. If the DOE does that then OLEDs would become a spectacularly efficient light source either for displays or lighting. To have 80% external efficiency is just really quite remarkable and these things work at very low voltage and so on. So this is a challenge worth approaching. 

 

(Source of knowledge: The Office of Energy Efficiency & Renewable Energy, U.S.)

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