Japan sets white LED targets as technology improves

The program has around 25 active projects and received some 100 proposals for work in 2003 and a similar number in 2004. Coinciding with the event, the DoE announced awards totaling around $20 million for 11 new projects that will focus on core technology areas such as advanced phosphors, novel device concepts, and manufacturing technology for OLEDs (see Compound Semiconductor September p11). The DoE is also close to reaching an agreement with the Next-Generation Lighting Industry Alliance to act as its industrial partner and to assist with planning and agenda setting.

The purpose of the DoE program is to achieve energy savings that potentially could be made by replacing current lighting sources, such as incandescent and fluorescent lamps, with LED-based lighting. The table "Room for improvement" compares the cost and efficacy of current lighting technologies. One recent study suggests that solid-state lighting could reduce US energy consumption for lighting by around 30% by 2025, saving more than $125 billon in consumer electric bills. These figures represent the predicted level of progress with an investment of $100 million per year, which the DoE hopes to secure from Congress. At present around $6-7 million is directed annually to the Solid-State Lighting Program.

Brodrick outlined some of the successes of the US program, including a project involving Lumileds and Sandia that has achieved quantum efficiencies of 76% using quantum dots for down-conversion in white LEDs. The DoE has also funded work at Cree Lighting to improve LED packaging efficiency and brightness through the development of new structures and materials, resulting in a white LED demonstrator with an efficacy of 74 lm/W.

The Japanese government is just as keen on LED lighting as its US counterpart. Its initial five-year project, Light for the 21st Century, ended in March 2003, with partners demonstrating white LEDs with an efficacy of 60 lm/W, as well as practical RGB white LEDs with a high color-rendering index (CRI), an efficacy of 30 lm/W and a lifetime of 6000 hours. Like the US program, the Japanese project sought to benefit from the energy savings and the reduction of greenhouse gas emissions resulting from the development of high-performance white LEDs.

With funding from Japan s Ministry of Economy, Trade and Industry, phase two of the program is under way, according to Tsunemasa Taguchi of Yamaguchi University, Japan. Targets include white LEDs with efficacies of 80 and 120 lm/W by 2006 and 2010, respectively, and LED chips with external quantum efficiencies of 60 and 80% in the same time frames.

Taguchi also leads a national project in Japan entitled White LEDs for Medical Applications, which aims to develop products such as endoscopes for internal use, as well as surgical lighting and sterilization equipment. Taguchi says that among the requirements for medical applications are: avoiding strong blue emissions at around 450 nm, which it is suggested, could cause retinal detachment; avoiding equipment heating to above 40 °C for internal use; and making a product easy to irrigate to avoid causing infection.

For white LED lamps that use phosphor down-conversion of blue or UV light, the development of new phosphors is a key requirement to enable improvements in color temperature and color rendering, and to open up new markets for LEDs. Several speakers from GE Global Research and GELcore described efforts to develop improved phosphors - notably GELcore s phosphor technology manager Emil Radkov, who unveiled a range of new phosphor blends that he described as having "unprecedented" performance. Radkov reported a specific white LED with an output of 25 lm at 23 lm/W, which had a CRI of 97 and color temperature of 3000 K. Figure 1 shows the emission spectrum of this LED. The color co-ordinates were x = 0.441 and y = 0.410, and the direct contribution of LED light to the down-converted spectrum was less than 2%.

Unlike deluxe linear fluorescent lamps, which have a broad spectrum, the blue LED/YAG phosphor combination has distinct deficiencies in the violet and blue-green regions. Both types of lamp are also deficient in the red region, which makes them poor at rendering deep-red colors. Compared to a traditional blue LED with a YAG phosphor, the combination of blue, cyan and orange phosphors pumped with a near-UV chip represents a major improvement in lumen output at color temperatures of below 4000 K, while further CRI enhancement is achieved by adding a fourth phosphor emitting in the deep-red region.

The highest CRI value for commercial white LEDs is about 90, which falls just short of the value of 100 defined for incandescent lamps. Radkov described a range of white LED blends with four-color phosphors, which had CRI values exceeding 95 over the entire range of color temperatures from 2500 to 8000 K. In some cases the CRI values (particularly the R9 component, which refers to the rendering of deep-red colors) can approach the theoretical maximum value of 100. Although there is a trade-off between luminous efficacy and color rendering, improvements continue to be made to both parameters through optimization of phosphor blends.

A number of novel and diverse applications were discussed during the meeting. LED manufacturers were encouraged to start producing LEDs for plant cultivation by a research team from Vilnius University, Lithuania. Gintautas Tamulaitis from the team described how replacing conventional light sources with LEDs enhances photosynthetic productivity and leads to better plant morphology. Key wavelengths include 640 and 660 nm - where light is absorbed by chlorophyll - as well as 455 and 735 nm.

Jack Curran of US-based industrial safety specialist Wheelock Inc described the potential replacement of xenon gas flashtubes in visual fire-alarm signals, a market that has grown exponentially in the US due to the 1990 Americans with Disabilities Act. LEDs offer a comparable output for a smaller footprint, although at a higher cost. In an office environment, Curran found that the response times of test subjects were similar for both types of fire signal, although obstacles remain for the introduction of a new technology such as LEDs into such a safety-driven application.

Andrew Siegel of MIT Lincoln Labs, near Boston, conducted a practical demonstration of covert communication using ultraviolet LEDs, transmitting data to a non-line-of-sight (NLOS) detector with a sharp cut-off solar-blind filter. LEDs with wavelengths of around 275 nm, developed as part of DARPA s SUVOS program (see Compound Semiconductor May p20), have been used to demonstrate NLOS links over distances of 2.1 km.

LEDs are also starting to penetrate the aviation lighting business, according to Jeff Singer of Honeywell, whose company s Astreon lights have been designed as replacements for position lights on business jets manufactured by Cessna, Gulfstream and Learjet.

Red and green position lights are used on the left and right wingtips, respectively, with white lights used at the rear of the aircraft. The color, light distribution and intensity levels of such lights are closely regulated through Federal Aviation Regulation documents. LEDs offer several advantages - most notably in that they are robust, do not require a colored lens and do not produce much heat. Problems include dealing with the different drive currents of red and green LEDs, the variations of properties within the same LED bins, and lifetime issues as the output of the LED degrades with time.

An emerging application for high-brightness LEDs is in projection displays. Won Yong Lee of Samsung Electronics in Korea - a firm that recently released a range of LCD monitors featuring LED backlights - described efforts to utilize RGB LEDs in a 40 inch projection TV. Four LEDs of each color were used to illuminate separate LCD panels, the output from which was projected onto the viewing screen. Crucial to Samsung s design was the use of parabolic reflectors, polarization converters and integration rods to collect and collimate the light for each LCD screen.

RGB LEDs offer a number of attractive features, notably a large color gamut (130% of the color space defined by the US National Television System Committee), a dynamic white point and a large dimming range. RGB LEDs also eliminate the need for color wheels, which are rotating filters used in color sequencing, and this makes the optical system much smaller and simpler. Also they can be turned on and off instantly, and have a relatively long lifetime.

Among the drawbacks of using LEDs are the need for very accurate thermal management and color compensation, and the problems caused by a relatively low light-output density produced by large high-power chips.

Despite a total LED flux of 745 lm, the system itself had a flux of only around 25 lm onto the screen; with an efficiency of 3.3%, this was deemed insufficient for the application. "For these systems, we need a large flux from a small chip with high light-extraction efficiency," said Lee.

In high-power LEDs, a key requirement is to increase the drive current without compromising reliability. Stanton Weaver of GELcore described various ways to manage thermal issues, which include ensuring efficient current spreading at the chip level, and designing the chip and its package as an integral process. Weaver also used thermal modeling and high-resolution IR imaging to demonstrate that the placement and area of bump bonds has a strong influence on thermal gradients. Also, there is much less bump-to-bump variation using SiC compared with sapphire, due to the higher thermal conductivity of the former substrate material.

Yoshi Ohno of the National Institute of Standards and Technology in the US spoke about the issues involved with using CRI, the only internationally recognized metric for the way in which light sources bring out the appearance of colored objects. Unfortunately, there are many issues with using CRI - for example, the CRI value of a white LED source does not indicate how far the color co-ordinates are located from the blackbody locus.

Ohno also pointed out that there is a trade-off between color rendering and luminous efficacy, which are arguably the two most important parameters for light sources. Incandescent sources have (by definition) a CRI of 100, but the maximum luminous efficacy of radiation is 240 lm/W. For a green LED, efficacy can reach 640 lm/W, since the eye is most sensitive at 550 nm, but with a green source all objects appear green, and the CRI value can be as low as -28.

Ohno has developed a simulation program that allows metrics such as CRI to be calculated for different LED types - for example by altering the wavelengths of the individual LEDs in an RGB combination. He demonstrated that a well designed RGB LED can give good color rendering, while a 4-chip device has an excellent CRI value. However, the deficiencies inherent in the CRI mean that a new metric needs to be devised for the LED industry.

Kevin Dowling of lighting-system designer Color Kinetics discussed how color is perceived by the human eye, and the issues that are peculiar to using LEDs. The human eye has a huge dynamic range but also has a nonlinear response to luminance. Small changes are very noticeable at low brightness levels, but much larger changes can be tolerated at high brightness. One issue for LED lifetime, which is defined by lumen depreciation, is whether or not the change in lumen output can actually be observed by the user. Other light sources also depreciate, but for these there tend to be different ways of defining lifetime.

Another issue is color consistency between LEDs: "Customers can perceive differences and they care," said Dowling, whose company manufactures LED-based lighting systems. "It is sometimes possible to perceive differences between LEDs in the same bin, and color mixing can be dramatically affected by improper binning." Noting that the different LED manufacturers have different ways of binning their products, Dowling raised the question of the need for standardization in this area.

Another DoE-funded project, this time to develop white OLEDs for lighting applications, was described in a talk by Anil Duggal of GE Global Research. Until now, the main focus of OLED development has been for the display market, but OLEDs can also be used as a diffuse light source suitable for large-area applications. The key advantages of OLEDs are high efficiency, the ability to construct the devices on thin and flexible substrates, and low potential cost - GE Global Research is involved in another project to develop a roll-to-roll production machine for OLED manufacturing.

Duggal explained that the company has already demonstrated an OLED panel measuring 2 x 2 ft, which produces a total of 1200 lm with an efficacy of 15 lm/W. This is equivalent to an 80 W incandescent bulb. A color temperature of 4000 K and a CRI of 88 - which exceeds the performance of most fluorescent tubes - were also reported. However, together with cost, the big drawback for OLEDs is lifetime: the 1200 lm panel had a lifetime of about 400 hours, which is significantly shorter than equivalent white LEDs experience.