05-06-2013, 11:56 AM
Improvement of emission efficiency of high-power LED by controlling size of phosphor particles
Improvement of emission 1.pdf (Size: 461.2 KB / Downloads: 35)
Abstract
White light can be produced by a combination of red, green and blue emitting diode chips or by the combination of a single diode chip with phosphors. Presently, more single chip white light-emitting diodes (LEDs) than multi-chip one are used because of their low cost, easily controlled circuitry, ease of maintenance and favorable luminescence efficiency. Since phosphors must be used as light converting materials in a single diode chip to obtain the desired emission, this study considers the problems encountered in using phosphors in LEDs. The proper application of phosphors in the package of LED can improve its efficiency, color rendering and thermal stability of luminescence. For example, a uniform size distribution of phosphors with red, green and blue emission helps to improve luminescence efficiency by preventing cascade excitation; the change in color with temperature can be overcome by counter-balancing redshifting and blue-shifting phosphors; larger particles help to ensure the high efficiency of high-power LEDs, and costs can be reduced by using small particles size in low-power LED packaging because allows less phosphor to be used to obtain a particular efficiency.
INTRODUCTION
The operation of LEDs is based on the spontaneous emission of light in semiconductors that is caused by the radiate recombination of excess electrons and holes that are generated by injection of a current [1]. LEDs are not limited by the fundamental factors that restrict conventional incandescent lamps and compact fluorescent lamps [2]. Therefore, LED light sources have superior efficiency, lifetime and reliability, and a small volume. They are energy-saving and environmentally friendly. They generate less thermal radiation than incandescent lamps and compact fluorescent lamps; use no mercury, and are used in slim size of packages [3,4]. Scholars, engineers and manufacturers all over the world
have become involved in research into, and the development of LEDs [3,4]. LEDs have had a great impact on our everyday lives in, for example, voltage signal indicators, liquid crystal display (LCD) panel backlights for mobile phones and TV sets, automobile lights, traffic lights, street lighting, outdoor decoration, and other applications [1–4].
PHOSPHORS FOR HOME-LIGHTING AND LED DISPLAYS
The first commercially available white LED, which was produced by Nichia Corporation, was prepared by combining the blue-emitting InGaN diode chip with Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor, which yield yellow luminescence [10]. One of the most important merits of YAG-based white LEDs is their high efficiency, and most high-power LEDs are this type. However, the main drawbacks of these YAG-based WLEDs are poor color rendering and serious thermal quenching of luminescence. As an alternative, the combination of red, green, and blue phosphors with near-ultraviolet/ ultraviolet (nUV/UV) InGaN diode chips to produce white light is highly favored [3,4].
SIZE OF PHOSPHOR PARTICLES AND LUMINOUS EFFICIENCY
Fig. 3a presents the emission spectra of Y3Al5O12:Ce3+ (YAG) with four distributions of particle sizes [19]. The relative emission intensity from samples A, B, C and D increases with D50 (the media size of the particles in the sample) from 2.2, 4.1, 7.2 to 15.6 lm, and D that are mixed into silicone to achieve similar emission efficiency with a commercially available low power (<1 W) LED lamp versus D50 particle size, from which we can conclude that less amount of phosphors was used if the LEDs were packaged by using phosphors with smaller particles. Correspondingly, cost was saved.
The enhancement of luminescence efficiency and the reduction in cost achieved by improving the manufacturing process are extremely important for LED applications. Luminescence efficiency depends on the size of the phosphor particles. Usually, the luminous efficiency of larger particles is higher than that of small particles. Accordingly, larger particles of phosphor are suitable for packaging high-power LEDs. However, the cost of LEDs can be reduced by reducing the amount of phosphor used, without compromising the efficiency of the final LED package by using small particles in low-power LEDs.
CONCLUSIONS
The luminescence properties of white LEDs, including power
efficiency, luminescence intensity, thermal stability, and color rendering, can be proved by improving the design of the LED lamp, properly selecting basic components (such as, diode chip, thermal conductive glue, phosphors, heat sink substrate, transparent silicone, and others) and properly optimizing the technological process of LED package. However, with respect to the use of phosphor, luminescence efficiency can be maximized by controlling the size of the particles in the LED packaging, improving the color rendering by adding an appropriate red phosphor into a YAG-based LED (which is a blue LED chip that is combined with yellow phosphor) or by combining green and red phosphors with blue LED chips, and overcoming the change of color with temperature by counter-balancing the red-shift and blue-shift of emission of phosphors. The above results demonstrate that efficiency of LEDs similar to that of commercially available one can be achieved by using less amount of phosphor but with smaller phosphor particles. The uniform size distribution of phosphor particles with red, green and blue emission helps to avoid cascade excitation and thereby promotes luminescence efficiency.