Researchers push LED performance
Since the 1960s, the luminous efficacy of LED illumination has increased while the cost of LED illumination has decreased, both by about an order of magnitude per decade, according to plenary comments by George Craford, chief technical officer of Lumileds (San Jose, CA) during the 5th International Conference on Solid State Lighting at the Annual Meeting of the SPIE (July 31-Aug.
Since the 1960s, the luminous efficacy of LED illumination has increased while the cost of LED illumination has decreased, both by about an order of magnitude per decade, according to plenary comments by George Craford, chief technical officer of Lumileds (San Jose, CA) during the 5th International Conference on Solid State Lighting at the Annual Meeting of the SPIE (July 31-Aug. 4; San Diego, CA).
The LED flux coming out of a single package has grown by 35% a year for more than 30 years, he said. With outputs from new devices currently on the order of 100 lm, Craford noted that the growth rate soon will begin to level off as the devices approach the theoretical limits for total light extraction (see figure). But the light-extraction efficiencies and internal light-generation efficiencies also need to approach theoretical limits to reach projected targets for general illumination. At the same time, initial purchase prices will need to drop by almost two orders of magnitude to make a significant market impact, even though lifetime LED costs are already competitive. Whether green LEDs that match current levels of red and blue efficiency can be developed will ultimately be a major factor in determining whether RGB color combinations, as opposed to phosphor-stimulated white light from blue or UV sources, will become more-important sources of white-LED illumination, Craford said. The phosphor approach is simpler but loses some quantum efficiency in the conversion process. The RGB approach, while not sacrificing quantum efficiency, requires the additional complexity of feedback circuitry to maintain color mixture. Ideally, combining a phosphor approach with RGB can not only yield high color-rendering-index (CRI) values on the order of 96, but also provide tunable white light that could be adjusted during the day to match daylight conditions.
Areas of study that may yield significant efficiency and cost improvements include quantum-dot sources and phosphors, as well as photonic-crystal LEDs, to provide higher light outputs while focusing the light output entirely in a forward direction. The current U.S. performance goal is 150 lm/W by 2012.
With a primary focus on significant energy savings over the long term (lighting accounts for 21% of U.S. energy consumption) the Department of Energy is currently funding 42 solid-state lighting projects with a cumulative contract value of $63.6 million, according to James Brodrick, who spoke after Craford and described the solid-state-lighting R&D effort in the DOE Building Technologies Program. Of the 42 funded projects, 23 account for $29.8 million in total funding ($23.5 million from the DOE and $6.3 million from applicants) and concern LED technology. The remaining 29 projects account for $33.8 million in funding ($23.3 million from DOE and $10.5 million from applicants) and concern OLED research.
High-flux LEDs enable architectural displays such as this rainbow facade at the Kuo Hua Commercial Insurance Building (Taipei, Taiwan).
Major OLED R&D projects include a materials and devices designs project at Los Alamos National Laboratory (Los Alamos, NM) focused on key materials challenges for OLED use in general illumination. A polymer-OLED white-light development program at OSRAM Opto Semiconductors hopes to initially develop, fabricate, and fully characterize a 12 × 20-in. white-light OLED prototype based on multiple 2 × 3-in. white-light devices fabricated on glass substrates and to ultimately produce a color-balanced OLED white light source with luminous efficacy of 20 lm/W at 800 cd/m2and an operating half life of 3000 hours.
Also with DOE funding, the Pacific Northwest National Laboratory (Richland, WA) is exploring state-of-the-art phosphorescent organic light emitters to increase the power efficiency of blue OLEDs to ultimately integrate a UV or blue OLED with a thin-film phosphor and demonstrate 30-lm/W white light with a CRI in excess of 85 at 800 cd/m2. The University of California-Santa Barbara is working on surface-plasmon-enhanced phosphorescent white and colored OLEDs to demonstrate high efficiency at large injection-current densities, operational lifetimes in excess of 10,000 hours, true daylight color temperature, and luminous efficiency close to 50 lm/W for white OLEDs. General Electric (Fairfield, CT) is working with Dow Chemical (Midland, MI) to develop polymer OLEDs with CRI greater than 90, correlated color temperature between 3000 K and 6000 K, luminous efficacy in excess of 45 lm/W, and compatibility with high-speed processing.
Juni Kido described a three-year project at Yamagata University (Yamagata, Japan) funded by the Ministry of Economy, Trade and Industry (METI; Tokyo, Japan) for US$8 million to produce white OLEDs for lighting. As with other OLED development efforts, Kido’s team is looking beyond the theoretical 25% quantum efficiency (QE) available through singlet spin states from fluorescent materials, to approach the potential 100% QE by the combination of singlet states and triplet states in phosphorescent materials. They are also vertically stacking transparent OLED structures including multiple charge-generation layers to produce multiple photons for each incident electron. Current quantum efficiencies are on the order of 4%, with 16-lm/W efficacies at 100 cd/m2 for a 30 × 30-cm single-pixel source. Kido’s group is working with lighting companies in Japan to improve the efficiencies.
Hassaun A. Jones-Bey