1、外文翻译 VIDEOCASSETTE Before the videocassette recorder there was the movie projector and screen. Perhaps you remember your fifth-grade teacher pulling down a screenor Dad hanging a sheet on the wall, ready to show visiting friends the enthralling account of your summer vacation at the shore. Just as t
2、he film got started, the projector bulb often blew out. Those days did have one advantage, though: the screen was light, paper-thin and could be rolled into a portable tube. Compare that with bulky television and computer screens, and the projector screen invokes more than just nostalgia. Could yest
3、erdays convenience be married to todays technology? The answer is yes, thanks to organic light-emitting materials that promise to make electronic viewing more convenient and ubiquitous. Used in displays, the organic materials are brighter, consume less energy and are easier to manufacture (thus pote
4、ntially cheaper) than current options based on liquid crystals. Because organic light-emitting diodes (OLEDs) emit light, they consume significantly less power, especially in small sizes, than common liquid-crystal displays (LCDs), which require backlighting. OLEDs also offer several exciting advant
5、ages over common LEDs: the materials do not need to be crystalline (that is, composed of a precisely repeating pattern of planes of atoms), so they are easier to make； they are applied in thin layers for a slimmer profile； and different materials (for different colors) can be patterned on a given su
6、bstrate to make high-resolution images. The substrates may be inexpensive glass or flexible plastic or even metal foil. In the coming years, large-screen televisions and computer monitors could roll up for storage. A soldier might unfurl a sheet of plastic showing a real-time situation map. Smaller
7、displays could be wrapped around a persons forearm or incorporated into clothing. Used in lighting fixtures, the panels could curl around an architectural column or lie almost wallpaperlike against a wall or ceiling. LEDs currently have longer lifetimes than organic emitters, and it will be tough to
8、 beat the widespread LED for use in indicator lamps. But OLEDs are already demonstrating their potential for displays. Their screens put out more than 100 candelas per square meter (about the luminance of a notebook screen) and last tens of thousands of hours (several years of regular use) before th
9、ey dim to half their original radiance. Close to 100 companies are developing applications for the technology, focusing on small, low-power displays see box on page 80. Initial products include a nonflexible 2.2-inch (diagonal) display for digital cameras and cellular phones made jointly by Kodak an
10、d Sanyo, introduced in 2002, and a 15-inch prototype computer monitor produced by the same collaborative venture. The global market for organic display devices was about $219 million in 2003 and is projected to jump to $3.1 billion by 2009, according to Kimberly Allen of iSuppli/Stanford Resources,
11、a market-research firm specializing in displays. 一、 What LED to OLED CRYSTALLINE semiconductorsthe forerunners of OLEDstrace their roots back to the development of the transistor in 1947, and visible-light LEDs were invented in 1962 by Nick Holonyak, Jr. They were first used commercially as tiny sou
12、rces of red light in calculators and watches and soon after also appeared as durable indicator lights of red, green or yellow. (When suitably constructed, LEDs form lasers, which have spawned the optical-fiber revolution, as well as optical data storage on compact discs and digital video discs.) Sin
13、ce the advent of the blue LED in the 1990s see “Blue Chip,” by Glenn Zorpette； Scientific American, August 2000, full-color, large-screen television displays made from hundreds of thousands of LED chips have appeared in spectacular fashion on skyscrapers and in arenas see “In Pursuit of the Ultimate
14、 Lamp,” by M. George Crawford, Nick Holonyak, Jr., and Frederick A. Kish, Jr.； Scientific American, February 2001. Yet the smaller sizes used in devices such as PDAs (personal digital assistants) and laptops are not as practical. LEDs and OLEDs are made from layers of semiconductorsmaterials whose e
15、lectrical performance is midway between an excellent conductor such as copper and an insulator such as rubber. Semiconducting materials, such as silicon, have a small energy gap between electrons that are bound and those that are free to move around and conduct electricity. Given sufficient energy i
16、n the form of an applied voltage, electrons can “jump” the gap and begin moving, constituting an electrical charge. A semiconductor can be made conductive by doping it； if the atoms added to a layer have a smaller number of electrons than the atoms they replace, electrons have effectively been remov
17、ed, leaving positively charged “holes” and making the material “p-type.” Alternatively, a layer that is doped so that it has an excess of negatively charged electrons becomes “n-type” see box on opposite page. When an electron is added to a p-type material, it may encounter a hole and drop into the
18、lower band, giving up an amount of energy (equal to the energy gap) as a photon of light. The wavelength depends on the energy gap of the emitting material. For the production of visible light, organic materials should have an energy gap between their lower and higher conduction bands in a relativel
19、y small range, about two to three electron volts. (One electron volt is defined as the kinetic energy gained by an electron when it is accelerated by a potential difference of one volt. A photon with one electron volt of energy corresponds to the infrared wavelength of 1,240 nanometers, and a photon
20、 of two electron volts has a wavelength half as much620 nanometersa reddish color.) 二、 A Surprising Glow ORGANIC semiconductors are formed as aggregates of molecules that are, in the technologies being pursued, amorphousa solid material, but one that is noncrystalline and without a definite order. T
21、here are two general types of organic light emitters, distinguished by “small” and “large” molecule sizes. The first practical p-n-type organic LED, based on small molecules, was invented in 1987 by Ching W. Tang and Steven A. Van Slyke of Eastman Kodak, after Tang noticed a surprising green glow co
22、ming from an organic solar cell he was working on. The duo recognized that by using two organic materials, one a good conductor of holes and the other a good conductor of electrons, they could ensure that photon emission would take place near the contact area, or junction, of the two materials, as i
23、n a crystalline LED. They also needed a material that held its electrons tightly, meaning that it would be easy to inject holes. For the light to escape, one of the contacts must be transparent, and the scientists benefited from the fortunate fact that the most widely used transparent conducting mat
24、erial, indium tin oxide, bound its electrons suitably for p-type contact material. The structure they came up with has not changed much over the years and is often called “Kodak-type,” because Kodak had the basic patent see box on opposite page. Beginning with a glass substrate, different materials
25、are deposited layer by layer. This process is accomplished by evaporating the constituent materials and letting them condense on the substrate. The total thickness of the organic layers is only 100 to 150 nanometers, much thinner than that of a conventional LED (which is at least microns in thickness) and less than 1 percent of the thickness of a human hair. Because the molecules of the materials used are relatively lightweighteven lighter
