The luminous efficiency of white LEDs is constantly increasing, and products exceeding 100 lm/W (comparable to high-efficiency fluorescent lamps) have also appeared. When a white LED is used on a lighting fixture, the loss of the power supply circuit, the reflector, the lens, and the like of the lighting fixture generally causes the appliance efficiency (efficiency after loss of the power supply circuit or the like) to be lower than that of the simple light source, but a device has recently appeared LED lighting fixture with an efficiency of 100 lm/W. What kind of brightness will white LEDs achieve in the future, and what characteristics will they have? Recently, the reporter interviewed Ruediger Mueller, President and CEO of OSRAM Opto Semiconductors GmbH, a high-power, high-efficiency white LED business. He asked him about the technical outlook for white LEDs and the expectations of the company's OLED lighting.
Reporter: What is the current luminous efficiency of white LEDs?
Mueller: The product with color temperature up to 5000~9000K has exceeded 100lm/W, and the bulb color product that is popular for lighting purposes is around 80~85lm/W. In addition, the high color rendering product with an average color rendering index (Ra) of 90 is around 70 lm/W. Although the luminous efficiency of white LEDs has reached more than 100 lm/W, many factors can not reach 100 lm/W due to the conditions of color temperature, color rendering and input current.
From the research and development level, the energy conversion efficiency (the ratio of input power to output light energy) of the blue LED chip based on white LED reaches 50%. When the chip is used, it is expected to realize a white LED that greatly exceeds 100 lm/W. However, the energy conversion efficiency cannot exceed 100%. Because there are physical limits. Just as semiconductor memory has limits on the improvement of integration, the energy conversion efficiency and the luminous efficiency of white LEDs are not likely to increase in the future. A more reasonable view is that the luminous efficiency of the product will stay between 150 and 200 lm/W.
Reporter: For LED manufacturers, what characteristics of white LEDs need to be improved in the future?
Mueller: What's important in the future is how to maintain a high level of luminous efficiency at high current densities. The LED has a Droop phenomenon in which the current density input to the chip is higher and the luminous efficiency is lower. Although the recession cannot be eliminated, it can be alleviated. If the recession is alleviated, the brightness will be higher when the current density is increased. That is to say, this reduces the number of LED chips that can be obtained when the same brightness (light flux) can be obtained with the same input current. If a large number of LED chips were used, the same amount of LED chips can be used to obtain the same luminous flux in the future.
The higher the brightness per unit area of ​​the LED chip (the luminous flux), the lower the unit price of the brightness, and the stronger the cost competitiveness of the white LED. The key to improving this cost competitiveness depends on the technical competitiveness of LED manufacturers. The lighter the recession, the higher the cost competitiveness. The reason is that reducing the area of ​​the LED chip can reduce the cost of the chip. And it is good for customers. By increasing the current input to each white LED, the brightness of each LED can be increased, so that the number of white LEDs used by the lighting fixture and the LED backlight source can be reduced.
Reporter: If the cost is reduced, it is indeed a good news for LED backlights and LCD TVs.
Mueller: Over time, LEDs used in LCD TV LED backlights and lighting fixtures require lower cost products. LED backlights were originally equipped with RGB three LEDs, and the high image quality achieved with excellent color expression is a selling point. But nowadays, it can be said that LEDs are used to sell ultra-thin rather than image quality. To reduce costs, you need to use white LEDs instead of RGB three LEDs. Moreover, in order to further reduce costs, the demand for reducing the number of white LEDs is becoming stronger and stronger. It can be said that cost is placed in a more important position than quality. A reduced number of white LEDs is best suited to meet these requirements.
Although it is a personal point of view, I think that in the next two to three years, LED lighting will also have the same trend. When it comes to LED lighting, it is currently popular for high color rendering products with Ra reaching 80 to 90. Although high-quality white LEDs are subject to more consultations, it is estimated that there will be a large number of LED lightings that are cost-priority rather than quality-first. Ra has 70, and the demand for white LEDs will increase as long as they are cheap.
Reporter: OSRAM Opto Semiconductors is also involved in lighting OLEDs and has begun small-scale sales. Please talk about the progress.
Mueller: At present, the company is working to improve luminous efficiency and extend component life. It is expected to achieve a luminous efficiency of 50 lm/W and a lifetime of 10,000 hours in the near future.
Compared with the characteristics of OLEDs, manufacturing costs are not so much more important than big ones. Due to the simple structure of the components, OLEDs achieve low prices after the manufacturing technology is established. However, at this stage, LEDs can be combined with light guide plates to realize a low-cost surface light source that OLED is good at. The LED+ light guide plate has made technical progress as a liquid crystal television LED backlight, and the mass production is also constantly improving. If it is just a simple surface illumination, it may not be able to compete with the LED+ light guide.
In the future, in order to promote OLED in the lighting market, in addition to the use of competition with LED+ light guide plates, it is necessary to develop applications that LED+ light guide plates cannot achieve. For example, it can be embedded in the window glass to use. At this time, since it is transparent when the lamp is turned off, it can be used as a normal door and window glass, and can be lit at night to make the door and window a lighting fixture. Since a flexible backplane can be used, it is also conceivable to introduce OLED lighting that can be used as a curtain. Without the use of these organic lighting, the OLED lighting market will not expand. These new uses may not appear on the market until five years later.
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