BRILLIANT Researchers from the University of Illinois, Northwestern University, the Institute of High Performance Computing in Singapore, and Tsinghua University in Beijing report having found a better way of producing inorganic LEDs (ILEDs). Image: © D. STEVENSON AND C. CONWAY, BECKMAN INSTITUTE, UNIVERSITY OF ILLINOIS
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Lights and video displays made with energy-efficient light-emitting diodes (LEDs) are already making strong inroads in consumer and industrial markets long dominated by fluorescent bulbs and liquid crystal displays (LCDs). Although the majority of these LEDs get their electroluminescence from layers of film made from carbon-containing organic compounds, such organic LEDs (OLEDs) may be superseded by LEDs made from inorganic compounds that shine brighter and last longer than OLEDs.
The life spans of inorganic LEDs (ILEDs) can be 100,000 hours or more, much longer than OLEDs. (Some television-makers promote sets with OLEDs lasting 30,000 hours or so—that's about 3.5 years of continuous run-time.) Now, an international team of researchers is reporting success developing a process for making ILEDs that could put them on equal footing with OLEDs, the latter of which currently can be made smaller, cheaper and in a larger number of configurations. (ILEDs have so far be used primarily in large outdoor digital displays that do not require high resolution.)
View a slide show of experimental ILED devices
The new approach to ILED-making offers a more effective means of fabricating and assembling smaller, thinner ILEDs, says John Rogers, a University of Illinois at Urbana–Champaign professor of materials science and engineering. Rogers and colleagues from Northwestern University, the Institute of High Performance Computing in Singapore, and Tsinghua University in Beijing describe their work—funded by Ford Motor Co., the National Science Foundation, and the U.S. Department of Energy—in the August 21 issue of the journal Science.
LEDs generally are made up of several layers of organic or inorganic compounds that emit light when an electric current flows through them. These layers include the emissive layer that gives off the light, a conductive layer and a substrate, along with anode and cathode terminals that provide the electric charge. ILEDs are used today to make large display screens but cannot easily be made small enough to be packed into arrays that work well for indoor lighting or for video screens (several different brands of televisions, computer monitors and smart phones already use OLEDs).
ILEDs are "ridiculously bright, with the strongest emitting the equivalent of about one tenth the brightness of the sun," says Ioannis Kymissis, an assistant professor of electrical engineering at Columbia University in New York City who is also studying ways to make ILEDs more practical, separate from Rogers's research.
Whereas OLEDs can be made atop relatively inexpensive materials such as plastic or glass, ILEDs must be created on top of more expensive crystal wafers (made from, for example, gallium arsenide or gallium nitride, both of which are semiconductors). For ILEDs, thin films are grown to generate crystals on the surfaces of these wafers, which are typically circular in shape, with thicknesses a little less than 0.5 millimeter and with diameters between five and 10 centimeters.