LEDs shine in solid-state lighting

Posted : 17 Mar 2008

Related Content • LED driver with inductive boost ready for sampling (2008-03-17) • Keyboard backlight requires less power, fewer LEDs (2008-03-12) • Four-channel white LED drivers peg 98% efficiency (2008-03-10) • Illuminated pushbutton switches offer more cap styles (2008-03-07) • Leynard puts up giant LED panels at Olympics site (2008-03-07) More Related Content The use of LEDs for solid-state lighting, rather than as indicators, is poised to make inroads in applications in the automotive, architectural and general illumination markets, according to market analysis firm Yole Developpement. But for the growth to pan out, particularly in the general illumination market, there is still technical work to be accomplished. Although LEDs are prized for their energy efficiency, a great deal of color performance and design and cost optimization remains to be done, and semiconductor companies need to keep improving manufacturing processes. Billion-dollar potential Yole projects a market size for all types of LEDs of $10.3 billion by 2012. High- and ultrahigh-brightness LEDs, combined, will be responsible for about $4.45 billion of that total—almost 5.5 times the $783 million market size, based on packaged LEDs, estimated for 2007. With such a large part of the growth driven by bright-LED varieties, the two key criteria for the new market segments are luminous efficacy in lumens per watt and cost efficiency in dollars per lumen. Manufacturing advances Until now, LED manufacturers have focused on light efficiency and light output, said a spokesman for Philips Lumileds. "They are critical, but only two parts of the system," he said. Upcoming issues to address include thermal management, drive electronics and consistency, and range of color temperature. Resolving them will require manufacturing advances in optics, packaging, testing and binning. Positioning light LED-based lighting systems are often seen as an alternative to incandescent, halogen and fluorescent lights. The technology's lack of mercury and low power consumption are obvious pluses. However, LEDs are still an emerging technology in this segment. "It is a misconception that LEDs will take the place of the bulb for general lighting," said Tom Pearsall, general secretary of the European Photonics Industry Consortium. "The risk of promoting LEDs before they have reached a level of efficacy at least as good as the compact fluorescent bulb is that early adopters will be disappointed," said Rainer Beccard, director of marketing at Aixtron AG, a German supplier of manufacturing equipment for the compound semiconductors used to make LEDs. Nonetheless, "LEDs have a niche market opportunity—literally and figuratively," said Pearsall. "They can be used for lighting up shelf space or counter nooks, or the insides of drawers in a kitchen, for example." Zumtobel's Tempura LED spotlight is a 442 unit that emits 1,000 lumens of projected light, equivalent to a 100W halogen's output. Click to view full image) Emerging niches for solid-state lighting technology include the illumination of architectural elements—a staircase, for instance—as well as less glamorous but no less technology-appropriate applications such as refrigerated display units. New concepts needed Pearsall went on to add an important condition to growth in general illumination: "Until I see manufacturers coming out with a new concept for lighting and not producing LED lamps that are shaped for and fit into fixtures suitable for incandescent bulbs, then even that niche market will remain unexploited." For Pearsall, moving away from the glass envelope of the incandescent bulb should be a liberating experience. In other words, it is not a matter of designing products that enable swapping out an old incandescent bulb for an LED-based one. Such thinking leads to strange-looking creations, such as bulb-shaped LED lights encrusted with bulky heat sinks. Even in the higher end of the lighting market, industrial designers still have to go to some lengths to draw heat away from drivers and semiconductor components so that longevity and peak performance are maintained. Needed, said Pearsall, is a "revolution in lighting design" that takes advantage of the unique properties of LEDs, such as their ability to support digital color control, light shaping, and rapid and frequent on/off switching, as well as their excellence as a point-source light. - Valerie Thompson EE Times Link

Thermoelectric Breakthrough in Silicon Nanowires

Contact: Lynn Yarris (510) 386-5375, lcyarris@lbl.gov

Jan. 9, 2008

BERKELEY, CA — Energy now lost as heat during the production of electricity could be harnessed through the use of silicon nanowires synthesized via a technique developed by researchers with the U.S. Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley. The far-ranging potential applications of this technology include DOE’s hydrogen fuel cell-powered “Freedom CAR,” and personal power-jackets that could use heat from the human body to recharge cell-phones and other electronic devices. Rough silicon nanowires synthesized by Berkeley Lab researchers demonstrated high performance thermoelectric properties even at room temperature when connected between two suspended heating pads. In this illustration, one pad serves as the heat source (pink), the other as the sensor. “This is the first demonstration of high performance thermoelectric capability in silicon, an abundant semiconductor for which there already exists a multibillion dollar infrastructure for low-cost and high-yield processing and packaging,” said Arun Majumdar, a mechanical engineer and materials scientist with joint appointments at Berkeley Lab and UC Berkeley, who was one of the principal investigators behind this research. “We’ve shown that it’s possible to achieve a large enhancement of thermoelectric energy efficiency at room temperature in rough silicon nanowires that have been processed by wafer-scale electrochemical synthesis,” said chemist Peidong Yang, the other principal investigator behind this research, who also holds a joint Berkeley Lab and UC Berkeley appointment. Majumdar, who was recently appointed director of Berkeley Lab's Environmental Energy Technologies Division (EETD) and is a member of the Materials Sciences Division, is an expert on energy conversion and nanoscale science and engineering. Yang is a leading nanoscience authority with Berkeley Lab's Materials Sciences Division and with the UC Berkeley Chemistry Department. Majumdar and Yang are the co-authors of a paper appearing in the January 10, 2008 edition of the journal Nature, entitled “Enhanced Thermoelectric Performance of Rough Silicon Nanowires.” Also co-authoring this paper were Allon Hochbaum, Renkun Chen, Raul Diaz Delgado, Wenjie Liang, Erik Garnett and Mark Najarian. The Nature paper describes a unique “electroless etching” method by which arrays of silicon nanowires are synthesized in an aqueous solution on the surfaces of wafers that can measure dozens of square inches in area. The technique involves the galvanic displacement of silicon through the reduction of silver ions on a wafer’s surface. Unlike other synthesis techniques, which yield smooth-surfaced nanowires, this electroless etching method produces arrays of vertically aligned silicon nanowires that feature exceptionally rough surfaces. The roughness is believed to be critical to the surprisingly high thermoelectric efficiency of the silicon nanowires. “The rough surfaces are definitely playing a role in reducing the thermal conductivity of the silicon nanowires by a hundredfold, but at this time we don’t fully understand the physics,” said Majumdar. “While we cannot say exactly why it works, we can say that the technique does work.” Nearly all of the world’s electrical power, approximately 10 trillion Watts, is generated by heat engines, giant gas or steam-powered turbines that convert heat to mechanical energy, which is then converted to electricity. Much of this heat, however, is not converted but is instead released into the environment, approximately 15 trillion Watts. If even a small fraction of this lost heat could be converted to electricity, its impact on the energy situation would be enormous. From left, Renkun Chen, Arun Majumdar, Peidong Yang and Allon Hochbaum were co-authors of a Nature paper that described a wafer-scale electrochemical synthesis technique for producing rough silicon nanowires that can convert heat into electricity with surprisingly high efficiency. “Thermoelectric materials, which have the ability to convert heat into electricity, potentially could be used to capture much of the low-grade waste heat now being lost and convert it into electricity,” said Majumdar. “This would result in massive savings on fuel and carbon dioxide emissions. The same devices can also be used as refrigerators and air conditioners, and because these devices can be miniaturized, it could make heating and cooling much more localized and efficient.” However the on-going challenge for scientists and engineers has been to make thermoelectric materials that are efficient enough to be practical. The goal is a value of 1.0 or more for a performance measurement called the “thermoelectric figure of merit” or ZT, which combines the electric and thermal conductivities of a material with its capacity to generate electricity from heat. Because these parameters are generally interdependent, attaining this goal has proven extremely difficult. In recent years, ZT values of one or more have been achieved in thin films and nanostructures made from the semiconductor bismuth telluride and its alloys, but such materials are expensive, difficult to work with, and do not lend themselves to large-scale energy conversions. “Bulk silicon is a poor thermoelectric material at room temperature, but by substantially reducing the thermal conductivity of our silicon nanowires without significantly reducing electrical conductivity, we have obtained ZT values of 0.60 at room temperatures in wires that were approximately 50 nanometers in diameter,” said Yang. “By reducing the diameter of the wires in combination with optimized doping and roughness control, we should be able to obtain ZT values of 1.0 or higher at room temperature.” The ability to dip a wafer into solution and grow on its surface a forest of vertically aligned nanowires that are consistent in size opens the door to the creation of thermoelectric modules which could be used in a wide variety of situations. For example, such modules could convert the heat from automotive exhaust into supplemental power for a Freedom CAR-type vehicle, or provide the electricity a conventional vehicle needs to run its radio, air conditioner, power windows, etc. When scaled up, thermoelectric modules could eventually be used in co-generating power with gas or steam turbines. Figure (a) is a cross-sectional scanning electron microscope image of an array of rough silicon nanowires with an inset showing a typical wafer chip of these wires. Figure (b) is a transmission electron microscope image of a segment of one of these wires in which the surface roughness can be clearly seen. The inset shows that the wire is single crystalline all along its length. “You can siphon electrical power from just about any situation in which heat is being given off, heat that is currently being wasted,” said Majumdar. “For example, if it is cold outside and you are wearing a jacket made of material embedded with thermoelectric modules, you could recharge mobile electronic devices off the heat of your body. In fact, thermoelectric generators have already been used to convert body heat to power wrist watches.” The Berkeley Lab researchers will be studying the physics behind this phenomenon to better understand and possibly manipulate it for even further improvements. They will also concentrate on the design and fabrication of thermoelectric modules based on silicon nanowire arrays. Berkeley Lab’s Technology Transfer Department is now seeking industrial partners to further develop and commercialize this technology. This research was funded by the U.S. Department of Energy's Office of Basic Energy Science, through the Division of Materials Sciences and Engineering. Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. Visit our Website at www.lbl.gov. http://www.lbl.gov/Science-Articles/Archive/MSD-silicon-nanowires.html Additional Information Arun Majumdar can be reached via e-mail at majumdar@me.berkeley.edu Peidong Yang can be reached via e-mail at p_yang@uclink.berkeley.edu For more information on the research of Arun Majumdar, visit his Website at http://www.me.berkeley.edu/faculty/majumdar/ For more information on the research of Peidong Yang, visit his Website at http://www.cchem.berkeley.edu/pdygrp/main.html If interested in licensing the silicon nanowire technology for commercial development, please contact Berkeley Lab’s Technology Transfer Department at TTD@lbl.gov