Yeah, science: how OLEDs work

OLED. The technology reigns supreme in the high-end TV market. Smartphone manufacturers, too, are increasingly placing their trust in OLED. The technology is unique in that its LEDs are organic and much smaller than synthetic LEDs. As a result, they can produce an image while self-illuminating.

One moment... OLEDs produce an image and their own light?

Yep, that's exactly it, I'll explain it all in the article.

Electron migration in organic layers

An OLED pixel consists of six layers. The two outer layers are made of very thin glass, the substrate. These two layers enclose the "OLED sandwich" and protect it. Between the two layers of glass are two electrodes: the cathode and the anode. An electrical voltage is created between the two electrodes. This is where the inner layers are located: the emissive layer and the transport layer. These are the organic layers that emit the light.

Warning, we're now going to get down to the nitty-gritty: molecular physics and all that.

Step 1: applying voltage

To light up an OLED pixel, we apply voltage to it: an electric current flows from the cathode to the anode through the different layers - an electric circuit is created.

Step 2: electrons and holes

As soon as the current flows through the pixel, the cathode receives what are known as electrons, i.e. negatively charged elementary particles, from the energy source. The anode then loses electrons as they pass through the pixel. The missing pixels then leave "holes" in the transport layer.

In physics, a hole is not like a pothole or a hole in your trousers, it is an "unoccupied location". Without electrons, such holes would not exist. It's like air bubbles in water: the water represents the electrons and the air in the bubbles represents the positive charge. The bubbles themselves represent the holes, they wouldn't exist without water around them.

If all this is too abstract for you, imagine electrons and holes as electrically charged elements. Electrons have a negative charge, while holes have a positive charge. That's enough to understand the rest.

Step 3: the great migration

The positively charged holes move towards the transport layer, while the negatively charged electrons go into the emissive layer. The transport layer is therefore positively charged and the emissive layer negatively charged.

Step 4: when holes and electrons meet

Holes are much more mobile than electrons and move from the transport layer to the emissive layer where the electrons are. When a positively charged hole collides with a negatively charged electron, the two opposing voltages cancel each other out. The impact triggers a brief emission of energy in the form of a luminous particle, a photon is created.

Hundreds of impacts occur every second, so an OLED pixel emits constant light for as long as it remains powered. If the power is cut off, the pixel switches off. OLED screens have no backlighting and can therefore display true blacks - the term "True Black" has become popular.

OLEDs don't just turn on and off, of course. The brightness of each pixel can be set by adjusting the voltage. The higher the voltage applied to the pixel, the more holes and electrons will travel between the organic layers. As there are more electrons and holes, there are more impacts and therefore also more photons generated - or light particles - and the more light particles there are... the brighter the OLED pixel!

Summary of the four stages of the physical phenomenon:

1. The OLED pixel is switched on to an electrical voltage.
2. Electrons pass through the cathode and leave holes in the anode.
3. Holes meet electrons.
4. The impacts generate luminous particles that affect the brightness of the pixel.

What about colours?

In order for an OLED pixel to emit coloured light, a filter of the desired colour is placed before or after one of the substrates. So if the pixel needs to emit red light, a red filter is added to the OLED sandwich.

How can pixels change colour? An OLED pixel is actually made up of four sub-pixels: one red, one green, one blue and one white. Depending on the brightness of each of the sub-pixels, different colours are obtained. For example, if all four sub-pixels have the same brightness, the OLED pixel appears white.

This is how several million pixels lined up next to each other can display different colours. Each pixel emits its own light and can influence its brightness and colour independently of the other pixels. This ensemble creates an image.