In order to achieve luminous efficiency of more than 200 lm / W, white led is constantly improving. However, the features that white LEDs must have are not just luminous efficiencies. In the process of popularization of white LEDs, there are more and more factors that can open the difference, such as temperature stability and color rendering index.
The luminous efficiency is steadily increasing at an average annual rate of 15%, and will soon reach 200 lm/W... How long will the white LED luminous efficiency increase and to what extent?
In fact, the luminous efficiency at the R&D level is almost at its limit (Figure 1). It is considered that a white LED such as a blue LED and a yellow fluorescent material capable of obtaining the highest luminous efficiency is used in combination, and the limit is about 260 lm/W. At present, it has exceeded 200 lm/W at the research and development level, and there is little room for growth. “There is no doubt that the speed of efficiency through technological innovation will be slower and slower†(Osram Japan). Many people believe that before 2020, the luminous efficiency of white LEDs will reach its peak. However, a few years later, the luminous efficiency of white LED products is still an indispensable indicator of the strength of LED manufacturers. Because the luminous efficiency of existing products is nearly twice the difference compared with the limit.
Figure 1: The luminous efficiency limit of reaching the ultimate white LED is considered to be around 260 lm/W.
It should be noted that while the use of white LEDs continues to expand, the "light-emitting efficiency supremacy" situation is gradually disintegrating. The level of luminous efficiency reflects the level of energy use efficiency, and is also an important indicator for reducing the unit price per unit of brightness (yen/lm). However, in the case where white LEDs are widely used in various applications, it is impossible to judge whether the requirements of white LED users such as product manufacturers can be met by the luminous efficiency alone. In the future, this tendency will be further enhanced.
In this case, LED manufacturers will accelerate the development of other features of white LEDs while developing technologies that can achieve luminous efficiencies of 200 lm/W or more. Let's take a look at the latest developments in white LED development.
There is still room for improvement in internal quantum efficiency
Regarding the improvement of luminous efficiency, there are many LED manufacturers who have room for improvement in blue LED chips and fluorescent materials (Fig. 2(a)).
Figure 2: Focus on achieving luminous efficiency above 200lm/W
In the future, in order to increase the luminous efficiency to 200 lm/W or more, it is necessary to improve the internal quantum efficiency in the blue LED chip and the wavelength conversion efficiency of the fluorescent material. (Fig. (b) produced by the site based on Philips Lumens)
The process of converting the power input to the white LED into light and outputting it to the outside is as follows: 1 inputting power to the blue LED chip, emitting blue light; 2 converting the wavelength of part of the blue light into long-wavelength visible light by the fluorescent material; The wavelength-converted light is mixed and becomes white light output to the outside of the package. In order to improve luminous efficiency, it is necessary to reduce the energy loss in each link.
Each link requires the following technologies: 1 reducing the resistance loss, improving the internal quantum efficiency of photons generated by electrons in the luminescent layer, improving the light extraction efficiency of outputting photons out of the blue LED chip; 2 improving the wavelength conversion efficiency; 3 improving the out-of-package Output light extraction efficiency of white light, and so on. So far, various manufacturers have proposed various improvement measures and achieved remarkable results.
For example, the indicator of resistance loss - the forward voltage of the white LED is significantly lower than the original. The 1W product commonly used for lighting applications has a forward voltage of nearly 4V, but has now dropped to around 2.9 to 3V (Figure 3). From the energy of blue light (2.75 eV), 20 to 30% of the previous input power will be lost due to resistance, but now only about 5%. The light extraction efficiency has also been greatly improved, with the highest product being around 70% and the research and development level reaching 90%.
Figure 3: The forward voltage is basically at the lower limit
Nichia's white LED with an input power of 1W for lighting applications has significantly reduced its forward voltage in recent years. The new product in 2011 is 3V, which is quite close to the lower limit of the forward voltage (2.75V). The difference between the lower limit value and the forward voltage produces an energy loss that does not contribute to luminescence.
In order to achieve luminous efficiency of 200 lm/W or more, it is necessary to improve the internal quantum efficiency of 1 and the wavelength conversion efficiency of 2 on the basis of implementing all the above improvements. Because the increase in the two is relatively large. For example, internal quantum efficiency is currently at a level of 50 to 70%. Philips Lumileds Lighting announced that it will achieve a high output of 1000 lm and a high efficiency white LED for each LED chip, thereby increasing the internal quantum efficiency to over 80% (Fig. 2(b)).
There is room for improvement in crystal quality and luminescent layer structure
How will internal quantum efficiency and wavelength conversion efficiency improve? In terms of internal quantum efficiency, there is a need to improve the luminescent layer material - the crystal quality of the epitaxial crystallization of GaN-based semiconductors and the structure of the luminescent layer; in terms of wavelength conversion efficiency, it is necessary to improve the fluorescent material and develop new materials. These initiatives have been in progress since the launch of the white LED.
In addition to this, there is a method of changing the crystal surface of the GaN-based crystal from the current polar surface to the non-polar surface, thereby greatly improving the internal quantum efficiency. This method has been in the research and development stage, but has recently begun to move towards the practical stage. For example, South Korea's Seoul Semiconductor plans to begin sampling at the end of 2011, and Mitsubishi Chemical plans to be practical as early as 2012. Seoul Semiconductor said that by increasing internal quantum efficiency, it is expected to increase luminous efficiency by 40 to 70% compared to existing products.
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