How does this sandwich of layers make light?
To make an OLED light up, we simply attach a voltage (potential difference) across the anode and cathode.
As the electricity starts to flow, the cathode receives electrons from the power source and the anode loses them (or it "receives holes," if you prefer to look at it that way).
Now we have a situation where the added electrons are making the emissive layer negatively charged (similar to the n-type layer in a junction diode), while the conductive layer is becoming positively charged (similar to p-type material).
Positive holes are much more mobile than negative electrons so they jump across the boundary from the conductive layer to the emissive layer. When a hole (a lack of electron) meets an electron, the two things cancel out and release a brief burst of energy in the form of a particle of light—a photon, in other words. This process is called recombination, and because it's happening many times a second the OLED produces continuous light for as long as the current keeps flowing.
We can make an OLED produce colored light by adding a colored filter into our plastic sandwich just beneath the glass or plastic top or bottom layer. If we put thousands of red, green, and blue OLEDs next to one another and switch them on and off independently, they work like the pixels in a conventional LCD screen, so we can produce complex, hi-resolution colored pictures.This is the OLED display light.