LCDs require 2 polarisers. But OLED should not need one. However I have seen some sketches where a polarizer is present. Why?
Indeed it is related to reflections. Every pixel (actually color subpixel) of an Active Matrix OLED (AMOLED) is made of a TFT transistor, a cathode and an emissive layer. It is customary to include a mirror layer under the TFT to enhance emission on the output direction. This setup has a problem: light coming from outside can get into the device, go through all the layers and be reflected by the mirror. This reduces dramatically the contrast. Placing a CIRCULAR polarizer on top (actually, a linear polarizer plus a quarter wave plate), the ambient light becomes circularly polarized inside the device, e.g. right-handed. The mirror inverts the handiness i.e., from right-handed to left-handed upon reflection. Then reflected light is then cut off by the top circular polarizer.
Summarizing, using a circular polarizer in OLEDs you lose brightness but increase significantly contrast.
You are probably right but I do not understand. a polarizer cuts the brightness by 50%, so any gain in reducing reflections would be done at the expenses of reducing brightness. is it really worth it?
Oh, reflections can be quite disturbing. Thus I can understand attempts to reduce them.
Although maybe not the first means of choice, one might apply some polarizer on top. Another means would be to use a matte cover glass. It also depends on whether what you saw was some 'accademic approach' or some industrial series solution. Anyway, the use of a polarizer is not prohibited. And if you have 'more than enough' light emission, cutting 50 % off does not matter.
Indeed it is related to reflections. Every pixel (actually color subpixel) of an Active Matrix OLED (AMOLED) is made of a TFT transistor, a cathode and an emissive layer. It is customary to include a mirror layer under the TFT to enhance emission on the output direction. This setup has a problem: light coming from outside can get into the device, go through all the layers and be reflected by the mirror. This reduces dramatically the contrast. Placing a CIRCULAR polarizer on top (actually, a linear polarizer plus a quarter wave plate), the ambient light becomes circularly polarized inside the device, e.g. right-handed. The mirror inverts the handiness i.e., from right-handed to left-handed upon reflection. Then reflected light is then cut off by the top circular polarizer.
Summarizing, using a circular polarizer in OLEDs you lose brightness but increase significantly contrast.
You nailed it Jose'!!!!!!
Do you have a reference for it? I am teaching a Unit and would love to give a source to the students.
Well, Max, a good didactic source for your students is the YouTube video
https://www.youtube.com/watch?v=izBWE8g6nVI
Enjoy!
I don't know what do you mean 'at the bottom'. Behind the mirror, of course it does not work. In front of the mirror, well, I guess a non-conductive layer in the middle of the structure would affect the OLED functioning. But it's just a guess. As far as I know, all people place the circular polarizer on top of the structure.
Regarding resources, indeed, there are plenty of them. For example, advertisements of circular polarizers for OLEDs like
https://www.eenewseurope.com/news/oled-circular-polarizer-45um-thin-flexible
Or a new PLED material that emits circularly polarized light, avoiding losing half of light in the outer polarizer
https://www.oled-info.com/researchers-develop-pled-materials-circularly-polarized-luminescence
And some patents where the circular polarizer is integrated within the structure.
https://patents.google.com/patent/US20040189196A1/en
https://patents.google.com/patent/US20180047945A1/en
Anyhow, from a didactic point of view, I think the best contribution is the YouTube presentation I recommended in a previous answer
https://www.youtube.com/watch?v=izBWE8g6nVI
Regards
JMO
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