The purpose of The Light for the 21st Century Project (the Akari Project) for solid-state lighting is to contribute to the reduction of greenhouse gases by reducing the amount of energy spent on artificial illumination. The high efficiency of white-light LEDs means that the active potential exists for enormous energy savings.
The Akari Project has focused research on the development of high-brightness blue and UV devices based on III-nitrides for the purpose of white-light LED sources. This solid-state approach is connected with the development of new phosphor-converted white LEDs and could be improved by scientists and engineers involved in the compound semiconductor, phosphor, and lighting communities.1generating white light
There are currently two methods commonly used for LED-based white light generation: individual red-green-blue (RGB) LED combinations that mix to generate white light, and InxGa1-xN-based blue and near-UV (NUV; 380 to 410 nm) LED systems incorporating fluorescent phosphors that down-convert some of the emission to generate a mix of light. The RGB approach requires at least three LEDs, and each device must be adjusted by individual supply circuits to balance the emission intensity of each color for proper white light generation.
Several problems currently exist with white-light devices composed of blue LEDs and Ce3+-doped yttrium aluminum garnet (Ce:YAG) yellow phosphors that mix blue and yellow light to produce what appears to be white light. These include the halo effect of blue/yellow color separation, strong temperature and current dependence of chromaticity, and poor color rendering caused by the lack of green and red components.
A lighting source requires high-quality light radiation because when we look at objects, we see the reflected light. The spectrum of the illumination source affects the appearance of objects in a phenomenon we call color rendering. If the illumination source does not include a spectrum close to that of incandescent bulbs or the sun, then the color of objects will be different than what we are accustomed to. If we can match the appropriate multicolor phosphor and encapsulation material to the NUV region, then we can obtain white LEDs with both high color rendering and high luminous efficacy. avenues to efficiency
To develop efficient high-brightness white LED light sources, project research has focused on fundamental studies of emission mechanisms in ZnS- and GaN-based wide-bandgap compound semiconductors; improvement of epitaxial growth methods of multiple quantum wells (MQWs) and of external quantum efficiency of NUV LEDs; production of large substrates for homoepitaxial growth; development of multicolor, UV-excited phosphors that generate white light; and realization of illumination sources and fixtures using white LEDs.
Japan's national program on white lighting research and development set the goal of reaching an external quantum efficiency of 40% and a luminous efficacy of 60 to 80 lm/W by 2003, rising to 120 lm/W by 2010. In a roadmap of the national project for developing the high-efficiency NUV LED, there are several basic research issues concerning the reduction of defect densities and the identification of recombination centers for improvement of the external quantum efficiency and luminous efficacy.
In general, the wall-plug efficiency (ηwp) of a p-n heterojunction LED under a forward-bias condition is expressed by voltage efficiency ηv, internal quantum efficiency ηint, and extraction efficiency ηext:
ηwp = ηv ηint ηext 
where ηv is controlled by the resistance and voltage barrier, ηint is controlled by non-radiative recombination processes, and ηext is controlled by a loss by internal absorption.
Recently, we fabricated InGaN-based NUV LEDs on patterned sapphire substrates using lateral epitaxy. The structure consists of the four periodic MQWs of InGaN/GaN, and the barrier layers are n- and p-type Al0.2Ga0.8N. Cathodoluminescence measurement revealed that the dislocation densities involved in the active region of the devices are much reduced compared to those fabricated on unpatterned substrates. We can obtain high external quantum efficiency ηe by flip-chip bonding the device onto a silicon submount.
Figure 1. Room temperature performance plots for 43% efficient NUV LED show gradual decrease in external quantum efficiency at drive currents above 20 mA. The device structure (inset) consists of an MQW LED structure on a lateral epitaxy on a patterned substrate (LEPS) and a flip-chip-type LED mounted on a silicon substrate.
In order to improve the extraction efficiency of the NUV LEDs, we optimized the patterned structural configuration by means of photoluminescence measurements. When we operated the optimized LED at 20 mA, the device produced an estimated 26.1 mW of output power Po at 405 nm, for a 43% efficiency (see figure 1). With increasing current, Po increases linearly to an estimated 150 mW at 100 mA. phosphor challenges
The NUV white LED approach is analogous to three-color fluorescent lamp technology, which is based on the conversion of NUV radiation to visible light via the photoluminescence process in phosphor materials. In the blue/YAG process, a sharp blue light from the blue LED source is an essential component of white light, and is strongly affected by temperature and drive current. UV light, on the other hand, is not included in the white light generation from NUV-based devices. This technology can thus provide a higher quality of white light than the blue and YAG method.2
Figure 2. Typical luminescence spectra of day-white (red) and high-Ra LEDs (blue) show broad emission across the visible spectral region.
A typical photoluminescence spectrum of RGB phosphor materials excited at 382 nm contains at least three main peaks located at 447, 528, and 626 nm, respectively (see figure 2). The 447-nm blue emission, 528-nm green emission, and 626-nm red emission bands originate from the fluorescent emission of (Sr, Ca, Ba, Mg)10(PO4)6CL:Eu2+, ZnS:Cu,Al, and L2O2S:Eu3+ phosphors, respectively. The blue phosphor indicates two absorption peaks at about 330 and 380 nm and which occur at the 4f 7 → 4f d5d1 optical transition in Eu2+ ions. The green phosphor indicates an absorption peak at about 400 nm and occurs at a donor-acceptor pair transition. The red phosphor indicates an absorption peak at 350 nm and occurs at both the f-f transition in Eu3+ ions and a charge transfer process.
The two spectra in figure 2 are produced by day-white LEDs and high-color-rendering-index (Ra) LEDs having different components of red, green, and blue phosphor materials. The value of Ra >= 90 is close to that of a three-band emission fluorescent lamp. We estimated the luminous efficacy of radiation as about 30 lm/W for the present high-Ra white LED. Both components of green and red emission are very important to improve the color temperature and the color-rendering index. The illuminance distribution from the high-Ra-type white LED indicates the full radiation as described by cos θ. We estimate the lifetime as more than 6000 hrs at a forward bias of 20 mA.
The realization of high-performance white-light LEDs requires new phosphor materials. We have developed an orange, yellow, green, and blue (OYGB) white LED consisting of OYGB phosphor materials and an NUV LED. The device generates three peaks located at 450, 520, and 580 nm, which are produced by fluorescent emissions of strontium- and ZnS-based long-wavelength phosphor materials, respectively. In testing, the devices demonstrated white luminescence with Tc = 3700K, Ra >= 93, K = 40 lm/W, and chromaticity (x, y) = (0.39, 0.39). The OYGB device generates a luminescence spectrum broader than that of an RGB white LED, and a better color-rendering index. all about efficiency
Figure 3. If the Akari Project is to keep to its roadmap (purple), it will need to continue rapid improvement in external quantum efficiency ηe (red) and luminous efficacy K (blue).
Following the technological roadmap, we achieved a peak external quantum efficiency of over 40% at the end of 2002, and the Akari National Project finished the first phase of its research and development programs (see figure 3). The new targets for the external quantum efficiency are 60% by 2006 for phase two of the program and 80% by 2010 for the third and final phase.
The program has achieved a significant increase in luminous efficacy, for example demonstrating a record 30 lm/W in a practical RGB white LED having high color-rendering index, no halo effect, and a 6000-hr lifetime. It seems that this type of device can replace the conventional blue/yellow design soon.
In March 2003, we obtained 60 lm/W performance for greenish-white phosphors. In order to achieve the luminous efficacies in excess of 80 lm/W, we will need new phosphor materials and novel structures that are suitable for the excitation wavelength at 380 to 410 nm; for example, a novel type of white LED light source developed in the project can produce high radiation flux using high-current operation and a high-reflection mirror. Our target in phase two of the Akari Project will be 80 lm/W by 2006 and more than 120 lm/W by 2010. These goals are driven by practical requirements of general lighting applications. Our project teams have developed white LEDs with a range of color temperatures.
The Akari Project has been jointly carried out by the New Energy and Industrial Technology Development Organization (NEDO) and the Japan Research and Development Center for Metals through a subsidy provided to NEDO by the Ministry of Economy, Trade, and Industry. We have carried out the research and development with the cooperation of seven universities, 13 enterprises, and the Japan Electric Lamp Manufacture Association.
Project researchers have demonstrated an NUV LED with an external quantum efficiency in excess of 43% for emission centered at 400 nm. In conjunction with phosphor blends, such devices can offer superior color uniformity, high color-rendering index, and excellent light quality for many illumination applications. oe
This work was supported by the Japan national project, The Light of the 21st Century, from METI/NEDO/JRCM and the White LEDs for Medical Applications cluster project from MEXT.
Shedding light on LEDS
Tsunemasa Taguchi received his PhD in electrical engineering from Osaka University in Japan in 1974. After becoming a senior lecturer at his alma mater, Taguchi went to Britain, where he spent 1981 and 1982 as a visiting research scientist at the physics division of Sussex University (Brighton, England). He became a full professor of the department of electrical and electronic engineering at Yamaguchi University in Japan in 1994.
Taguchi has concentrated on research into wide-gap semiconductor materials such as ZnS and GaN, seeking new applications in the form of short-wavelength lightemitting devices and white LED lighting systems. "About 30 years ago, I was studying ultraviolet LEDs using compound semiconductors based on ZnS. I came up with the idea of using near-UV LEDs and multicolor phosphor, which is similar to fluorescent lamps, to make an efficient white LED lighting system without either Hg or deep-UV radiation. My idea resulted in the first white LED in the world, even preceding Nichia," Taguchi says.
Taguchi led a national project called The Light of the 21st Century, which he says will enter its second phase in 2010, and currently leads the Japanese National Project for applying white LEDs to medicine. His LED patents have led to links with several companies in Ube, near the university. "I am engaged in LED businesses and am especially interested in venture businesses having to do with medical applications," Taguchi says.
Taguchi is a member of Applied Physics, the Institute of Electrical Engineers of Japan, and the Illuminating Engineering Institute of Japan. -Charles Whipple
1. T. Taguchi, Proc. of Int. Conf. in EL 2002, Ghent, 245-250 (2002).
2. T. Taguchi, Proc. SPIE 4445, 5-12 (2001).
Tsunemasa Taguchi is a full professor in the department of electrical and electronic engineering at Yamaguchi University, Yamaguchi, Japan, and project leader of the Japanese National Project application of White LEDs for Medical Applications from the MEXT CLUSTER Program.