Chemist Markus Suta explains in the video how LEDs and OLEDs work.© scinexx
Let there be light: LEDs have become an integral part of our everyday lives. These light-emitting diodes are based on chemical substances that emit light when exposed to electricity. In our video, chemist Markus Suta explains exactly how this works, how modern LEDs are constructed and what distinguishes OLEDs from LEDs.
Some molecules can absorb light and then re-emit it. This ability is called luminescence. The phosphors, the so-called luminophores, convert the energy supplied into light. The energy supply can occur through light or other radiation, chemical reactions, electricity or even pressure. In nature, such molecules often occur in the bodies of animals, fungi, bacteria or plants and emit a characteristic glow. This is then referred to as bioluminescence because the substances occur naturally.
However, chemists can also produce such substances synthetically and in any desired color by giving them the appropriate optical properties via their often crystalline structure. The wavelength and color of the light depends on the material used. Such luminescent materials can now be found in a variety of forms in our everyday lives, but especially in lighting devices such as LEDs. Electricity first stimulates a semiconductor material to release energy in the form of light. In modern LEDs, this then stimulates the chemical substances to glow.
Why do LEDs appear white?
“One of the most common applications of inorganic phosphors is certainly white light lighting indoors or in car headlights,” says chemist Markus Suta from Heinrich Heine University Düsseldorf, who researches such substances. These LEDs, for example, contain coarse-grain ceramic materials from the “rare earth” group, such as europium or cerium, which glow red or green when stimulated. Together with a semiconductor made of indium gallium nitride, which emits blue light and UV light when powered, the rare earth compounds form a mixture of colors that appears white to our eyes.
But why do the substances glow in different colors? “Both phosphors absorb UV light and convert it into visible light, which we see as luminescence,” explains Suta. The reason why one substance appears red and the other green is because they absorb UV light differently and produce more or less luminescence. More absorption means brighter colors – in this case red.
What is different about OLEDs than LEDs?
OLEDs work in a similar way, but as the name suggests, they use organic instead of inorganic phosphors. The chemical substances are organic in nature and their powder grains are significantly smaller than inorganic substances. This means they can be applied in thinner layers. “That’s why most OLED displays are relatively small – such as in smartphones or smartwatches – or can even be bent,” says Suta.
Despite their thin layer thickness, OLEDs shine in various iridescent colors because the organic phosphors can also be used individually. Because of their size, the inorganic phosphors are preferably mixed and used for white light. However, their advantage is that they are more stable than organic substances and still produce intense light even at high temperatures, as Suta explains.
September 27, 2024 – Claudia Krapp
