Quantum spin statistics once capped how brightly an OLED could glow
Inside an OLED pixel, electrons and holes pair into excitons that release light. Spin rules produce three triplet pairs for every singlet, and triplets decay only slowly because the transition is forbidden, starving simple fluorescent designs of efficiency. Phosphorescent materials exploit spin-orbit effects to let both kinds shine, and much of the progress since followed.
The basic structure is a sandwich: organic films between an anode and a cathode on a substrate. Conjugated molecules spread their electrons in a way that lets them conduct, making them organic semiconductors. With voltage applied, electrons enter at the cathode and holes at the anode; they meet, bind and fall to a lower energy state, and the energy gap sets the colour. Indium tin oxide is the usual anode because it is transparent, while reactive metals such as barium or calcium serve as cathodes under a protective aluminium cap that also reflects light forward. Because each pixel makes its own light, no backlight is needed, the key edge over liquid-crystal screens.
The idea is older than most realise. Andre Bernanose's team in Nancy observed organic electroluminescence in the early 1950s, and Martin Pope's group at New York University made anthracene crystals glow under direct current in 1963 using 400 volts. In Britain, Roger Partridge at the National Physical Laboratory built the first polymer LED, but government secrecy and a deliberately vague 1974 patent title buried his credit.
The practical breakthrough came in 1987, when two Kodak chemists, Ching Wan Tang and Steven Van Slyke, split the organic film into separate hole and electron transport layers, lowering operating voltage and raising efficiency. In 1990 J. H. Burroughes at Cambridge reported an efficient green polymer device, opening the field of plastic electronics.
Commercial steps followed slowly. Pioneer put one in a car stereo in 1997, Kodak fitted the first full-colour panel to a camera in 2003, and Sony's XEL-1 became the first OLED television in 2007. On 5 December 2017 JOLED began the first commercial shipments of inkjet-printed panels, and phones and high-end sets adopted the technology widely in the late 2010s.
Source: OLED
