Forgive the pun, but the future looks bright for solid-state lighting. Light-emitting diodes (LEDs) lead the way, while organic LEDs (OLEDs) promise even greater lighting efficiency and more energy savings in the years ahead. In fact, we'll soon see more LEDs in indoor and outdoor lighting applications than traditional light bulbs, incandescents, fluorescents, and halogen lamps.
LEDs offer longer life and greater efficency than those other technologies, which is why they're seeing rapid adoption in buildings, parking lots, plants, streets, stadiums, airports, and other large facilities. The U.S. Department of Energy (DoE) is putting some muscle (read: funding) into this growth with its Solid State Lighting (SSL) plan, which spans the years 2000 to 2020. The program consists of core technology research, product development, and commercialization support activities.
Increases in solid-state lighting luminous efficiencies are quite dramatic when compared to other conventional light sources, which have reached roughly even plateaus with no significant additional growth expected (Fig. 1). Huge electrical energy savings are expected from solid-state lighting, too. According to the DoE, 22% of the electricity produced in the U.S. goes to power light sources.
Progress on the white-light LED front is moving faster than originally anticipated. According to the DoE, researchers have already improved the efficacy of white LEDs available on the market to about 50 to 60 lm/W. That's almost four times more efficient than incandescent sources, furthering energy savings nationwide.
Based on the U.S. government's Energy Star ratings, the use of efficient solid-state lighting all over the U.S. (where it has been shown to be profitable) would cut the nation's demand for electricity by more than 10%. This translates into a $17 billion savings in energy costs and pollution reduction of 202 million metric tons of carbon dioxide—the equivalent of taking 15 million cars off the road.
Though light source costs are still high, we're seeing significant drops from approximately $250/klm in 2004 to around $50/klm in 2006 (based on manufacturer estimates for volume purchases). By comparison, conventional light sources (incandescent, fluorescent, etc.) cost around $1/klm.
Developmental versions of white LEDs from Japan's Nichia have produced 150 lm/W of output light. Now, commercial versions are ready for market introduction. Market data by Lumileds Lighting (a joint venture by Philips Lighting and Agilent Technologies) supports the favorable efficacy rating of white-light LEDs compared to other conventional light sources (Fig. 2).
According to DoE projections, the market penetration and energy savings potential of solid-state lighting will be driven primarily by economics that incorporate the initial light source price, operating cost, maintenance, and lifetime (see "Major Challenges Lie Ahead For LED Acceptance").
The DoE expects more of the 348 terawatt-hours (TWh) of energy saved by 2027 to be derived from LED lamps and fixtures. They will displace incandescent lamps with very high color-rendering indexes (CRIs), particularly in commercial and residential sectors (Fig. 3). For outdoor and industrial areas, savings in energy will be seen from LEDs with low and medium CRIs, respectively, for the same year.Civic Service
Municipal governments have taken notice of the LED's potential for huge energy savings in general illumination. For example, Raleigh, N.C., will become the first "LED City" in the U.S., using LEDs supplied by Cree Inc. Raleigh will deploy LED lighting through its living laboratory initiative to serve a number of applications, including garage and parking lot sites, street lights, architectural and accent lighting, portable lighting, and pedestrian and walkway lighting over the next 14 months (Fig. 4).
"LED technology provides a clear benefit to a municipal infrastructure, as well as the citizens it serves," says Charles Meeker, the mayor of Raleigh. "The economic benefits for municipalities to invest in LEDs are clear. They save energy, reduce environmental impact, and improve the quality of light."
Cree's latest LED lamps, the warm-white XLamps, can produce 124 lm of output at a correlated color temperature of 300K when driven at 700 mA.
Raleigh isn't the only city embracing solid-state lighting. Two years ago, LED lights from Ledtronics began illuminating San Pedro's mile-long Vincent Thomas Bridge over Los Angeles Harbor, the third-longest suspension span bridge in California (Fig. 5). The bridge is lit by 160 lamps, with 360 LEDs per lamp. Taking green lighting even further, they're powered by a 4.5-kW solar panel system.
Beyond the streets and parking lots lie lucrative automotive applications. Solid-state exterior lighting for automotive applications offers significant reductions in energy requirements for cars and trucks, as well as a significant environmental impact from fuel savings.
The DoE estimates that the potential savings in passenger vehicles alone would account for 1.4 billion gallons of fuel per year if every vehicle in the U.S. used all-LED lights. The savings potential for trucks is more than double that amount. Add to that the reliability and uptime improvements, and the value proposition of LED lighting becomes even greater.
Market research firm Yole Développement predicts that exterior high-brightness LEDs for automotive applications will constitute 60% of a vehicle's total lighting by 2009, with the other 40% consisting largely of solid-state light sources as well. The report sees Cree, Nichia, Osram, Philips Lumileds, and Toyoda Gosei as major competitors in this arena. Lumileds' Luxeon LEDs will find homes in high- and low-beam headlamps on the Audi R8 sports car, which also will use Osram's TopLED high-brightness LEDs for its daytime running lamps.
R&D efforts continue to home in on longer-lasting, more efficient white-light LEDs. But that's not the only area of research in the LED realm. Engineers at Rensselaer Polytechnic Institute's Lighting Research Center have shown that the present method of placing the phosphorous material within an LED's package is much too close to the die, leading to light backscattering and lots of light loss. They're proposing a scattered photon extraction technique that situates the phosphor away from the die, resulting in less heat loss and greater white-light output and efficacy (Fig. 6).
Innovations in LED design, such as chip shaping, better lens materials, wafer bonding, surface roughening, and flip-chip mounting have contributed substantially to LED brightness and efficacy levels, as well as lower prices. Silicones have emerged as a material of choice for high-brightness LED packaging, delivering better electrical, mechanical, and thermal performance than the epoxy and cyclo-olefin copolymers generally used to seal lower-brightness LED packages.
OLEDs: The Dark Horse
OLED lighting, an integral part of the DoE's SSL plan, is already much more efficient than incandescent lighting and should surpass the efficiencies of fluorescent light sources within a year or two. Still, major roadblocks must be overcome, like inadequate lifetimes for high-brightness versions and high capital costs. Most lighting experts don't see any significant market penetration for high-brightness OLEDs much before 10 years from now. Nevertheless, R&D efforts continue in OLEDs.
Last year, Royal Philips Electronics and Novaled achieved a breakthrough in high-brightness OLED efficiency levels and lifetimes. They collaborated to produce a device with a power efficiency of 32 lm/W with color coordinates of 0.47/0.45 and a CRI of 88 at a brightness level of 1000 cd/m2.
The device showed a lifetime of 20,000 hours.
"This result combines for the first time ultrahigh power efficiency and high operational stability and can pave the way to a bright future for OLED lighting," says Jan Blochwitz-Nimoth, chief technology officer of Novaled.
Other impressive OLED achievements include Universal Display's white-light OLED. Scientists recently demonstrated the OLED with an external quantum efficiency of 30% operating at 850 nits. At OSRAM Opto Semiconductors, initial R&D efforts produced white-light OLEDs with luminous efficiencies of 25 ml/W. Even at their typical 18-lm/W levels, these devices exceed the 12-lm/W levels of conventional light sources and are on par with the 20- to 26-lm/W levels of halogen lamps.
Researchers at Princeton University and the University of Southern California have devised a means of producing high-brightness white-light OLEDs. Their hope is that these OLEDs will lead to significantly higher lifetimes, in addition to raising the efficiency bar higher than ever before.
The research is part of a 14-year DoE-funded effort that also involves funding from Universal Display. The researchers found that they could substitute a fluorescent dye for the OLED's blue light source with blue, green, and red dyes to generate light without significantly sacrificing the OLED's superior properties.