Screens that glow

Many phone screens are millions of tiny lights, each built from layers of carbon-based molecules. They are organic light-emitting diodes, or OLEDs.

In an OLED the energy to lift an electron comes from an electric current. The molecules sit in thin layers between two electrodes, and all the layers together are less than a thousandth of a millimetre thick. One electrode is clear, or thin enough, to let the light out.

The negative electrode pushes electrons into the layer next to it. The positive electrode pulls electrons out of the layer on its side, leaving gaps called holes. Electrons and holes hop from molecule to molecule toward each other. Where an electron and a hole meet on one molecule in the glowing layer, that molecule is left excited, and as its electron drops it gives out light. The colour depends on which molecule sits in the glowing layer.

Pixels and black

Each pixel of an OLED screen is a red, a green and a blue OLED side by side, and the screen sets how brightly each one glows. A liquid crystal screen shines a lamp behind every pixel all the time and blocks the light where the picture is dark. An OLED pixel makes its own light, so a black pixel is switched off. The layers are thin enough to lay on plastic, so OLED screens can bend, as in phones that fold.

Many OLED screens light their red and green pixels with molecules built round an iridium atom. The metal atom lets nearly every electron that arrives make light. Blue OLEDs wear out fastest, and chemists are designing tougher blue molecules.