Advances in white LEDs signal a switch to solid-state lighting
08/01/2006
Advances in thin-film stacks of inorganic materials, along with improvements in chip-level integration, deposition techniques, and optimized plastic packages, have enabled light-emitting diode (LED) makers to greatly increase luminous efficacy. As a result, leading suppliers of high-brightness white LEDs have announced new products that promise visual light utilization efficiency equal to or slightly better than that of mainstream fluorescent fixtures.
A half century ago, solid-state technology began as small-signal devices, aimed primarily at replacing bulky vacuum tubes in switching and amplification applications. Over the decades, transistors, ICs and other solid-state circuits have chipped away and eliminated vacuum tubes from most systems applications. Even cathode-ray tubes (CRTs) today are succumbing to flat-panel displays. Next on the solid-state hit list is none other than the grandfather of vacuum tubes-the incandescent light bulb (along with its fluorescent-tube lighting cousins).
Accelerated developments in compound semiconductor materials and manufacturing technologies have pushed LEDs to the verge of being viable replacements of fluorescent lamps and incandescent lights in indoor lighting applications. Many LED suppliers and technologists consider this to be the next “holy grail” for solid-state light engines. What’s more, the huge potential of solid-state lighting could not come at a better time for white LED manufacturers, which have prospered in recent years from the spread of backlighting applications in displays and cell-phone pads, but recently have seen those growth rates level off.
Throughout 2005 and into 2006, leading suppliers of high-brightness white LEDs have announced a stream of new products that promise visual light utilization efficiency equal to or slightly better than that of mainstream fluorescent fixtures now used in most offices. Advancements in thin-film stacks of inorganic materials-most of which are based on gallium-nitride (GaN) compounds-along with improvements in chip-level integration, deposition techniques, and optimized plastic packages have enabled LED makers to greatly increase luminous efficacy, which is measured in lumens per watt (lm/W).
By now, most leading LED companies have reached “wall-plug efficiency” levels of 60-80lm/W in new devices, while white LED pioneer Nichia Corp. in Japan recently disclosed the development of a highly luminous GaN-based device with 100lm/W efficiency. This level of efficiency is considered a critical milestone for high-brightness white LEDs because it is equal to the performance of standard fluorescent lights. (Power-hungry but cheaper incandescent light bulbs typically put out about 14-16lm/W.) Most important, the latest LED efficiency levels now indicate that solid-state lighting technology is running two to three years ahead of most industry-roadmap targets.
Projections made in 2005 by the US Department of Energy (DOE) showed LED laboratory devices reaching 150lm/W by 2012 and 200lm/W by 2020, but commercially available devices were expected to lag (Fig. 1). However, significant increases in R&D spending have enabled LED manufacturers to accelerate development and potentially make solid-state lighting a major contender in the $13 billion global lighting industry before the end of this decade. IC Insights Inc. is predicting that high-brightness LEDs, including those used in room lighting systems, will account for about 86% of the world’s $6.7 billion total revenues for light-emitting diode sales in 2010 (Fig. 2).
The solid-state lighting movement has also become a high priority for energy-conscious government agencies worldwide, which are increasing funding of LED development. One US energy study has concluded that solid-state lighting in the foreseeable future could save 3.5 quadrillion BTUs of electricity, which would be more than three times the annual energy consumed in the state of Oregon [1]. Additionally, global carbon emissions from future power plants could be reduced by 300 million tons/year, if LED-based lighting takes hold in the next decade, according to another US estimate [2].
White light
White light-emitting diodes are relatively new in the LED arena. It took the industry nearly three decades of R&D to reach a full rainbow of LED colors after the first 655nm wavelength red-light emitting devices were put into production in the late 1960s using a compound of gallium, arsenic, and phosphor (GaAsP). Other compounds and chemical structures have been applied to LEDs in recent decades to produce visible light with yellow, green, orange, and amber colors. The major leap in achieving full-color LED display capability occurred in the early 1990s, when blue-light devices were finally perfected, initially using silicon-carbide (SiC) and then GaN compound materials. From there, white LEDs literally came out of the blue technology during the late 1990s.
One basic approach to turn a blue LED structure into a white LED uses chemical-vapor deposition (CVD) to place special phosphors in an InGaN-based device. The blue light excites the yellow phosphor compound, causing the LED to glow white light. Another basic approach combines materials for red, green, and blue LEDs to achieve white light. Some manufacturers have added phosphor materials to UV-LEDs to generate white light at high levels of brightness and power, while others have tried to make more powerful white LEDs without using rare-earth phosphors.
Potentially disruptive technologies for high-output white LEDs continue to be reported by research labs worldwide. In October 2005, a graduate student at Vanderbilt University reported a technique using quantum dot crystals (measuring a few nanometers) on blue LEDs to produce “true” white light twice as bright as a 60W light bulb. Also in 2005, researchers at Cree Inc. in Durham, NC, used GaInN-on-SiC for a white-light emitter technology that demonstrated efficacy of 70lm/W at 350mA of current. According to the Lighting Research Group at Lawrence Berkeley National Laboratory (which collaborated with Cree), this level of performance had moved white LEDs “at least two years closer to achieving the ‘holy grail’ of 150 lm/W.” Elsewhere, Rohm Co. Ltd. in Japan is attempting to lower the price of white LEDs by using zinc-oxide (ZnO) instead of more expensive GaN compounds.
White LED suppliers are also aiming to improve luminous efficacy, boost output levels, and lower costs by using new types of GaN wafers with larger diameters of up to 100mm (4 in.) compared to standard 50mm (2-in.) substrates grown on sapphire, which can suffer lattice mismatches between material layers and result in defects. Continuous improvements in metal organic CVD (MOCVD) growth technology has opened the door for reduced defects in epitaxial films on larger GaN substrates. Work is also underway to tailor dry-etch techniques for GaN-based devices using reactive ion etching (RIE) and inductively coupled plasma.
While progress continues in high-output white LEDs, suppliers face a range of issues before solid-state lighting can become a serious threat to conventional illumination products. First, manufacturers must prove that high-brightness LEDs will deliver on their promise of long life, lower operating costs, and reduced power consumption when compared to cheaper light bulbs and fluorescent lamps. LEDs do not have filaments that burn out, but they do wear down and lose brightness over long periods of electroluminescence. Estimates show many white LEDs will begin to measurably fade after 35,000-50,000 hours of use (roughly 4-6 years of continuous 24/7 operation). However, since many high-output white LEDs have not been on the market that long, there is no established track record.
The industry also needs standards to measure the quality of solid-state white light. Sunlight has generally been used to benchmark “perfect” interior light, which enables humans to see “true colors” of objects. Sunlight has a color-rendering index (CRI) value of 100. Homes and offices typically use conventional lights with CRI ratings of 80-90. Currently, most LED-based fixtures have CRI ratings in the 70-80 range. Various combinations of materials, manufacturing techniques, and LED packages can vary the output of devices, which can cast a warm “manila” glow or “cool” white light.
The threat of patent fights between LED suppliers has concerned some light-fixture manufacturers. High purchase prices for LED-based lights are another stumbling block. Depending on the efficiency of LEDs and the number of individual devices needed in a fixture, solid-state lighting systems can cost 10-50× that of fluorescent ceiling lamps. Average selling prices of white LEDs have been declining 20% per year, and projections by the Optoelectronics Industry Development Association anticipate that “lifetime ownership costs” of LED lighting systems will be less than incandescent lights by 2007 and lower than fluorescent lamps by 2012.
Assuming these targets are reached, the DOE has predicted that significant displacement of traditional lighting will start by 2010. However, many LED suppliers are aiming to start the transition to solid-state lighting a year or two sooner.
References
1. “Energy Savings Potential of Solid-state Lighting in General Illumination Applications,” Nov. 2003, Building Technologies Program, the Office of Energy Efficiency and Renewable Energy, US Dept. of Energy, prepared by Navigant Consulting Inc.
2. Sandia National Laboratories, Next-Generation Lighting Initiative (NGLI), http://lighting.sandia.gov/XlightingoverviewFAQ.htm#much.
Rob Lineback, the author of the 2006 edition of the Optoelectronics, Sensors, and Discrete [OSD] Report, is a senior market research analyst at IC Insights Inc., 13901 N. 73rd St., Suite 205, Scottsdale, AZ 85260; ph 972/447-0812, e-mail [email protected].