One of the cutting-edge flatscreen display technologies that has generated considerable interest is OLED, or organic light emitting diode technology. This technology has been used for some time for small displays such as cell phones and automobile instrument panels. OLED sets, priced at around $2,000, were sold by Sony in Japan for about the past two years, but recently the company announced that owing to lack of demand, sales of the sets would be discontinued in that country (about 2,000 OLED TV sets were sold last year, according to the New York Times). However, Sony's Sony XEL-1 OLED TV continues to be sold in the United States and LG Electronics recently announced that its 15-inch EL9500 OLED TV set, now only available in Korea, will be sold in this country by this summer.
OLED technology has some significant advantages that make it attractive as a television display, but there are some significant disadvantages as well.
THE ORIGIN OF OLED
Organic light emitting diodes are LEDs made from organic substances. As we remember from our chemistry classes, organic chemicals are those found in entities that are or have previously been alive, which are also known as organisms. When we consider such organisms as fireflies, or any number of ocean-dwelling organisms large and small that can self-fluoresce, it is not difficult to make the leap to electroluminescent organic molecules that can be fabricated into a display screen. In an OLED display, a film of electroluminescent organic compounds is sandwiched between two electrodes, one of which is transparent.
Sony's 11-inch XEL-1 OLED model HDTV As it says in a book that has been with us for many centuries, there is nothing new under the sun. Electroluminescence in organic molecules was produced in the early 1950s by André Bernanose and his associates at Nancy-Université in France. They deposited acradine orange, a fluorescent organic dye, and quinacridone, an organic molecule used as a pigment in paints, on thin films of cellulose or cellophane, and applied high-voltage AC fields to these deposits, causing fluorescence. In 1960, dark injecting holes and electron injecting electrode contacts to organic crystals were discovered; these form the scientific basis for all OLED devices. Based on this work and work that followed through the 1970s, the first organic diode device was developed at Eastman Kodak in the 1980s, which gave rise to the term "OLED."
Subsequently, this work was applied to polymers. A polymer, like many familiar organic molecules, is a long chain of repeating structural units typically connected by covalent bonds, which are chemical bonds in which two atoms "share" one or more electrons, as opposed to ionic bonds, in which one or more electrons are transferred from one atom to another. Ionic bonds, such as those found in sodium chloride (table salt) are common in inorganic compounds, while covalent bonds are common in organic molecules.
A typical OLED contains an emissive layer, a conductive layer, a substrate on which the layers are deposited, and positive and negative terminals. The layers are made up of organic molecules with conductivities ranging from conducting to insulating, and they may thus be thought of as organic semiconductors.
We can see that if we fabricate a display using OLEDs that fluoresce with the appropriate red, green and blue colors, we can, at least theoretically, make a television display from OLEDs. Some of the advantages for such an application include the fact that OLED displays typically emit bright, well-saturated colors, and the fact that they emit their own light obviates the need for a backlight, which in turn makes it easier to achieve deeper blacks than can be reached with a backlit display such as an LCD. In backlit displays, the backlight is always on, so the subpixel devices must become as opaque as possible when they block light. With a self-luminescent device such as an OLED, providing there is no after-glow, when a subpixel is off, it emits no light, so theoretically at least, the OLED should be able to provide a blacker black than an LCD. There are, as mentioned, a number of small-display devices like cellphone screens that use OLEDs, and those displays can look very good.
What are the disadvantages of OLEDs? One of the major technical problems has been the lifespan of the organic materials that are used, particularly those that generate blue light. Uneven degeneration of the colors in OLEDs causes shifting color balance in the display. This somewhat rapid and uneven deterioration also generates the potential for image burn-in on the screen, said to be more severe than burn-in on CRT screens. Also, OLED displays, being emissive, are less suitable in high ambient light conditions, such as outdoors, than are such transmissive and partially reflective displays as LCDs. We are reminded that another emissive display, plasma, also does not typically perform as well in high ambient light situations as does an LCD display.
Why was there so little demand for OLED TVs in Japan? First, it is still difficult and expensive to manufacture large-screen OLED displays. The only OLED TV actually sold in Japan is Sony's 11-inch screen, seemingly somewhat anachronistic in this big-screen HDTV age. Despite its small size, this set sells for about the equivalent of $2,000. With screens getting both larger and cheaper, it appears that big-screen OLED displays will be moved to the back burner for the present.
Randy Hoffner, a veteran of the big three TV networks, is a senior consulting engineer with AZCAR. He can be reached throughTV Technology.
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