History[edit]
André Bernanose and co-workers at the
Nancy-Université in France made the first observations of
electroluminescence in organic materials in the early 1950s. They applied high alternating voltages in air to materials such as
acridine orange, either deposited on or dissolved in cellulose or cellophane thin films. The proposed mechanism was either direct excitation of the dye molecules or excitation of electrons.
[5][6][7][8]
In 1960
Martin Pope and some of his co-workers at
New York University developed
ohmic dark-injecting electrode contacts to organic crystals.
[9][10][11]They further described the necessary energetic requirements (
work functions) for hole and electron injecting electrode contacts. These contacts are the basis of charge injection in all modern OLED devices. Pope's group also first observed direct current (DC) electroluminescence under vacuum on a single pure crystal of
anthracene and on anthracene crystals doped with
tetracene in 1963
[12] using a small area silver electrode at 400
volts. The proposed mechanism was field-accelerated electron excitation of molecular fluorescence.
Pope's group reported in 1965
[13] that in the absence of an external electric field, the electroluminescence in anthracene crystals is caused by the recombination of a thermalized electron and hole, and that the conducting level of anthracene is higher in energy than the
exciton energy level. Also in 1965, W. Helfrich and W. G. Schneider of the
National Research Council in Canada produced double injection recombination electroluminescence for the first time in an anthracene single crystal using hole and electron injecting electrodes,
[14] the forerunner of modern double-injection devices. In the same year,
Dow Chemical researchers patented a method of preparing electroluminescent cells using high-voltage (500–1500 V) AC-driven (100–3000 Hz) electrically insulated one millimetre thin layers of a melted phosphor consisting of ground anthracene powder, tetracene, and
graphite powder.
[15] Their proposed mechanism involved electronic excitation at the contacts between the graphite particles and the anthracene molecules.
Roger Partridge made the first observation of electroluminescence from polymer films at the
National Physical Laboratory in the United Kingdom. The device consisted of a film of poly(
N-vinylcarbazole) up to 2.2 micrometers thick located between two charge injecting electrodes. The results of the project were patented in 1975
[16] and published in 1983.
[17][18][19][20]
The first practical OLEDs[edit]
Hong Kong-born American physical chemist
Ching W. Tang and his co-worker
Steven Van Slyke at
Eastman Kodak built the first practical OLED device in 1987.
[21] This was a revolution for the technology. This device 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.
Research into polymer electroluminescence culminated in 1990 with J. H. Burroughes
et al. at the
Cavendish Laboratory in Cambridge reporting a high efficiency green light-emitting polymer based device using 100 nm thick films of
poly(p-phenylene vinylene).
[22]
Universal Display Corporation holds the majority of patents concerning the commercialization of OLEDs.[
citation needed]
