The History of OLED
A. Bernanose and co-workers at the Nancy-Université, first produced electroluminescence in organic materials in the early 1950s by applying high-voltage alternating current (AC) fields in air to acridine orange and quinacridine either deposited on or dissolved in cellulose or cellophane thin films. They proposed a mechanism of either direct excitation of the dye molecules or excitation of electrons.
In 1960, Martin Pope and his group made the seminal discovery of ohmic, dark injecting electrode contacts to organic crystals,[9] and described the necessary energetic requirements (work functions) for hole and electron injecting electrode contacts. Dark injecting hole and electron injecting electrode contacts are the basis of all current OLED devices, molecular and polymeric, as will be pointed out in the description of the requirements for the construction of successful OLEDs.
In 1963, Martin Pope and his group made the first observation of direct current (DC) electroluminescence, under vacuum, on a pure, single crystal of anthracene, and also on anthracene crystal doped with tetracene. The injecting electrode was a small area silver electrode, at 400 V DC, and the proposed mechanism was field accelerated electron excitation of molecular fluorescence.
In 1965, Martin Pope and his group refined their experiment and showed that in the absence of an external electric field, the electroluminescence in anthracene single crystal was caused by the recombination of a thermalized electron and hole. This paper proved conclusively that the conducting level of anthracene is higher in energy than the exciton energy level.
Also in 1965, W. Helfrich and W.G. Schneider produced double injection recombination electroluminescence for the first time, in an anthracene single crystal using hole and electron injecting electrodes whose work functions satisfied the requirements specified by Pope’s group. Electroluminescent materials can be insulators or doped insulators. The Helfrich and Schneider paper is the forerunner of all double injection induced OLED devices.
In 1965, researchers at Dow Chemical developed high voltage (500-1500 V) AC-driven (100-3000 Hz), electrically insulated thin (1 mil) layers of a melted phosphor consisting of ground anthracene powder, tetracene, and graphite powder. Their proposed mechanism was electronic excitation at the contacts between the graphite particles and the anthracene molecules.
Conductivity of such materials limited light output until more conductive organic materials became available, especially the polyacetylene, polypyrrole, and polyaniline “Blacks”. In a 1963 series of papers, Weiss et al. first reported high conductivity in iodine-doped oxidized polypyrrole. They achieved a conductivity of 1 S/cm. Unfortunately, this discovery was “lost”[clarification needed], as was a 1974 report[18] of a melanin-based bistable switch with a high conductivity “ON” state. This material emitted a flash of light when it switched.
In a subsequent 1977 paper, Hideki Shirakawa et al. reported high conductivity in similarly oxidized and iodine-doped polyacetylene. Alan J. Heeger, Alan MacDiarmid & Hideki Shirakawa received the 2000 Nobel Prize in Chemistry for “The discovery and development of conductive organic polymers”. The Nobel citation made no reference to the earlier discoveries.
The first attempt to create a polymer LED was by Roger Partridge at the UK’s National Physical Laboratory. The project succeeded, being patented in 1975 though publication was delayed until 1983.
The first diode device was invented at Eastman Kodak by Dr. Ching W Tang and Steven Van Slyke in the 1980s. This diode, giving rise to the term “OLED” used a novel two-layer structure with separate hole transporting and electron transporting layers such that recombination and light emission occurred in the middle of the organic layer. This resulted in a reduction in operating voltage and improvements in efficiency, and started the current era of OLED research and device production.
Later, this concept was adapted for use with polymers culminated in the Burroughes et al. 1990 paper in the journal Nature reporting a very-high-efficiency green-light-emitting polymer.