Light Tape UK's History of Electroluminescence

Light Tape UK's "History of Electroluminescence"


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Electroluminescence (EL) or the generation of light by the electrical excitation of light emitting phosphors has been around for many years. Electroluminescent was first observed in silicon carbide (SiC) by Captain Henry Joseph Round in 1907. Round reported that a yellow light was produced when a current was passed through a silicon carbide detector. An employee of the Marconi Company and a personal assistant to Guglielmo Marconi, Round was an inventor in his own right with 117 patents to his name by the end of his life.

The next recorded observation of Electroluminescence of any great significance came at the time of the Second World War, though there had been various reports of work done in this area during the 1920’s and 1030’s. In1936 George Destriau, once again noted that Electroluminescence could be produced from, this time, zinc sulfide (ZnS) powder after applying an electrical current to it so producing light. It was said that it was Destriau, who first coined the word "electroluminescence" to refer to the phenomenon he observed. Destriau, who worked in the laboratories of Madame Marie Curie in Paris (the Curies being early pioneers in the field of luminescence because of their research on radium), published a report of his findings.

During World War II, a considerable amount of research was done on phosphors in connection with work on radar displays (which was later to benefit the television industry in the form of better cathode ray tubes). Wartime research also included work on the deposition of transparent conductive films for de-icing the windshields of airplanes. That work was later to make possible a whole generation of new electronic devices.

In the 1950s, GTE Sylvania fired various coatings, including EL phosphors onto heavy steel plates to create ceramic EL lamps. During this period, most research focused on powder EL phosphors to get bright lamps requiring minimal power and with a potentially long lifetime. Research funding was cut back when it was determined that product lifetimes were too short (approximately 500 hours).

The first thin-film EL structures were fabricated in the late 1950s by Vlasenko and Popkov. These two scientists observed that luminance increased markedly in EL devices when they used a thin film of Zinc Sulfide doped with Manganese (ZnS:Mn). Luminance was much higher in thin film EL (TFEL) devices than in those using powdered substances. Such devices however were still too unreliable for commercial use.

Several large U.S. companies were also conducting research on ELDs in the 1970s, including: IBM, GTE, Westinghouse, Aerojet General, and Rockwell. All of these companies realised that ELDs had potential advantages over existing LCD technology in the following areas: Contrast, Multiplexing, and Viewing angle.

The most important problem that had to be solved before mass production of ELDs could begin was increasing the reliability of the EL thin film stack. Since the devices operated at very high field levels -- about 1.5 MV/cm -- there was a high probability that they would break down, especially if there was insufficient uniformity in the stack. Sharp, Tektronix, and Lohja Corporation in Finland were able to solve this problem between 1976 and 1983 using slightly different approaches.

The introduction to the market in 1985 of Grid and Data General laptops with EL displays from Sharp and Planar respectively helped to build the foundations for the nascent laptop computer industry at a time when LCDs did not have sufficient brightness or contrast to be used in commercial products. Both Planar and Sharp monochrome ELDs used a phosphor layer made from zinc sulfide doped with manganese (ZnS:Mn). These displays gave off an amber (orange-yellow) colour that was bright but also pleasing to the eye.

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