Organic Light-Emitting Diode (OLED)

Let's compare a shadow puppet show with a dazzling firefly show. A traditional liquid crystal display (LCD) is a lot like a shadow puppet show. A big light that works like a fluorescent lamp (the backlight) stays on behind the screen, and liquid crystals in front act as shutters, blocking or passing the light to show color.

An OLED, on the other hand, uses self-emission: millions of tiny pixels each make their own light. On a smartphone screen, every pixel has its own organic layers that give off red, green, or blue light, and each one glows independently when it receives electricity. In large TVs, a layer that gives off white or blue light is sometimes combined with color filters or quantum dots to make the colors. There is no lampshade to block the light. You just switch on only the parts you need.

That means the thick light panel behind the screen can be dropped entirely. The display gets as thin as a sheet of paper, and that makes foldable phones and rollable TVs possible, ones you can roll up like a sticker or fold freely.

The difference between the two methods is even clearer when you show a dark night sky. On a screen with a light always on behind it, no matter how well the light is blocked, some leaks through the gaps. Black looks like a hazy dark gray. It's like pulling the blinds in a room with the lights on: the room never gets completely dark.

A self-lit screen is different. To show black, it switches that pixel's power completely off. No light comes out at all, so it can make a deep, natural, "true black."

Once black is perfect, the brightness gap between the lightest and darkest parts (the contrast ratio) approaches infinity. That's why, in a movie set in outer space or a dark cave scene, the outlines and colors of objects pop out with surprising sharpness and life.

In OLED, "organic" means a compound that contains carbon. Light comes from running electricity through a very thin film of organic molecules built on a carbon backbone. Unlike inorganic semiconductors such as silicon, a thin film like this holds up well when bent, which is a big advantage for flexible screens.

When the power is connected, electrons (−) enter from the cathode, and holes (+), the empty spots left where electrons have gone missing, enter from the anode. They meet in the middle of the organic layer and combine, leaving the material in a high-energy state. When that energy drops back down, the leftover energy is released as light.

But organic material has a weakness. It wears out easily when it touches oxygen or moisture. The blue-emitting material wears out fastest, and as pixels age at different rates, an afterimage can stay behind if the same picture is left on for a long time. This is called "burn-in." Scientists today keep researching ways to wrap the organic material tightly and make it last longer.