OLED

Organic Light-Emitting Diodes (OLEDs) are made from organic (carbon-based) compounds with properties similar to inorganic semiconductors (silicon and germanium). The first OLED was made of poly(p-phenylene vinylene). The OLED material is layered between electrodes, one of which is (usually the anode) transparent. As current flows through the material, holes flow in from the anode, and electrons flow in from the cathode. As the electrons and holes combine, the electrons lose energy, which is emitted as visible light.

As in inorganic semiconductors, there is a range of energy levels in OLED material in which electrons cannot exist; electrons are either captured in the crystal structure (lower energy level) or free to move (higher energy level). These energy levels are called the valence band and the conduction band, respectively, and the range between them is called the bandgap. The width of the bandgap determines the color of light emitted by the material. Tris(8-hydroxyquinolinato)aluminum is commonly used as a green emitter, and fluorescent dyes are often used to modify the color output. Some OLED displays produce white light and use colored filters to produce red, green, and blue.

OLED anodes are commonly made of indium tin oxide, which is transparent to visible light and efficiently injects holes into the OLED material. Barium and calcium are commonly used in the cathodes because they efficiently inject electrons. The cathodes are often coated with aluminum to protect the reactive metals from degradation and to reflect light emitted in that direction. A thin-film transistor backplane, similar to the control plane of LCD panels, controls the OLED cells.

Depending on the material, OLED layers may be vacuum-deposited or spin-coated onto a substrate. Both methods may be used on a single panel as the different methods suit different materials. The substrates are often flexible, making curved and foldable screens possible.

Curved-screen OLED television[1]

Phones with foldable OLED displays

OLED displays require no backlight as the OLEDs emit their own light. This allows an entirely black screen in dark environments (unlike LCD screens that allow a significant amount of light through when in the black state). OLEDs can also produce a power-saving dim image when appropriate, activating only the necessary cells. LCD panels with LED backlights can reduce the backlight brightness--some can even light only necessary parts of the display. However, OLEDs can control the power of individual cells and do not have the complexity, rigidity, bulk, and weight of an LCD panel. OLEDs also respond much faster than LCDs.

Unfortunately, OLEDs have a limited lifespan. Early OLED displays degraded by 7 to 12 percent after 1,000 hours of use, with blue cells degrading the most. Exposure to moisture and oxygen accelerates degradation. However, newer OLED displays have significantly improved lifespans. OLED displays are often manufactured with blue cells that are optimized to use less current, somewhat equalizing the lifespans of the colors.