LED luminous efficiency development trend

LED luminous efficiency development trend
Another factor that LED backlight modules can rapidly expand their application range is that LED backlight modules can achieve high brightness per unit power consumption, that is, the number of luminous lumens per unit watt lm/W. As shown in Figure 2, the light source of LED backlight modules can be divided into the following four types:

‧Pseudo-white LED

Basically, it is composed of blue light LED and yellow phosphor, and uses the complementary principle to produce white light when in operation. This type of LED structure is very simple and has high luminous efficiency. Therefore, it is used as a backlight source for small LCDs and is widely used in mobile phones. The disadvantage is that the intensity of the red component is relatively weak.

‧Near-ultraviolet white light LED

It is a combination of an LED that can produce near-ultraviolet light and a phosphor that can produce three colors of RGB. Since it uses the three colors of RG to mix into white light, the color reproducibility is very high. However, this white light LED is based on the consideration that ultraviolet light will deteriorate the packaging resin and phosphor, so it is necessary to develop UV-resistant resin and phosphor separately.

‧Single RGB white LED

Since the heat dissipation structure of the single RGB white light LED can be designed for each single LED, it is easier to achieve a high output effect. However, the chips of the RGB single LED are physically separated from each other, so a dedicated light guide must be designed so that the light of the RGB single LED can be evenly mixed into white light, so as to avoid the backlight module from becoming thicker.

‧Integrated RGB white LED

Integrated RGB can directly mix colors into white light, so there are no problems such as dedicated light guides and backlight module thickness restrictions. However, the amount of current applied is limited, so it is not easy to obtain high output effects.

Figure 2 Common LED types for LED backlight modules

Regarding how to improve the luminous lumens per unit watt (lm/W) of LED backlight modules, in addition to improving the luminous efficiency of the LED itself, how to extract light more effectively is also one of the key points. According to past experience, LEDs with simple structures are more likely to improve the luminous lumens per unit watt. For example, Japan's RIGHTS company uses pseudo-white light LEDs and special light guides to obtain the same luminous lumens per unit watt as traditional cold cathode tube backlight modules; Nichia Chemical uses integrated RGB white light LEDs to test the backlight module of liquid crystal displays. Its luminous lumens per unit watt is about 80% of that of traditional cold cathode tube backlight modules. If a single RGB white light LED is used instead, the luminous lumens per watt will be relatively low because white light must be generated through a color mixing light guide. In general, based on the technical level in 2003, the luminous lumens per unit watt of the LED backlight module of Japan's RIGHTS company was about 1/3 to 1/2 of that of the traditional cold cathode tube backlight module. In other words, in addition to improving the luminous efficiency of the LED itself, it is also necessary to find ways to improve the luminous lumens per unit watt of the LED backlight module (Figure 3).

Figure 3 Lumens per Watt of LED backlight module

Figure 4 shows the evolution of LED luminous efficiency. It can be seen from the figure that the luminous efficiency of LED has increased by about 2 times every year in the past 2 to 3 years. Although it is questionable whether it can continue to maintain such a high growth rate in the future, most Japanese LED manufacturers believe that LEDs are expected to maintain the above level in the next 2 to 3 years, and the luminous lumens per watt in 2005 should be able to achieve a level of 50lm/W, which is almost the same as that of cold cathode lamps. In fact, Nichia Chemical, which leads in pseudo-white light LEDs, has achieved laboratory results of 60lm/W as early as 2002. In contrast, Toyota Gosei was confused about the choice between pseudo-white light LEDs and near-ultraviolet white light LEDs due to the luminous efficiency problem of 50lm/W. Germany's OSRAM Opto Semiconductors officially announced that it would join the ranks of LCD backlight development and set the luminous efficiency at 50lm/W. Regarding the future development trend of RGB white light LED's luminous efficiency, Nichia Chemical basically estimates that the luminous efficiency of RGB white light LED is about 1.2 times that of quasi-white light LED. Although the prediction after 2005 involves many factors, Nichia Chemical believes that it should be able to reach the level of 70lm/W, but it is unlikely to exceed 70lm/W.

Figure 4 The development of LED luminous efficiency

LED manufacturers have been keen on improving the structure of LEDs for a long time in order to improve the luminous efficiency of LEDs. As for how to improve the luminous efficiency of LEDs, basically all manufacturers have almost the same thinking mode. In other words, improving the luminous efficiency of LEDs basically depends on how to get the light generated by the light-emitting layer to the outside of the LED, because the light generated by the light layer will be repeatedly reflected inside the component, and even absorbed by the component and converted into heat energy. In order to suppress the multiple reflections of light, all manufacturers have started to improve the shape of the substrate and electrodes. For example, Germany's OSRAM tilts the end face of the SiC substrate and then makes concave-convex electrodes to change the incident angle of light and achieve the effect of suppressing light reflection. The company plans to flip the components upside down for flip chip connection in the future, and set up a mirror on the light-emitting layer so that the light can be collected forward (Figure 5); Nichia Chemical forms concave-convex shapes on the surface of the sapphire substrate to scatter the light, and uses mesh electrodes to increase the light collection efficiency of the electrode part (Figure 6).

Figure 5 OSRAM's GaAlP LED improves component structure and increases light extraction efficiency

Figure 6 Nichia Chemical's method for improving component structure and light extraction efficiency

As mentioned above, the driving force for the advancement of LED backlight modules is not only the significant improvement in the luminous efficiency of the LED light source itself, but also the optimal design of the light guide path. The light guide path of the LED backlight module is easier to design than the traditional backlight module, because the structure of the light-emitting component can be changed to make the light easily gather in the same direction, so the light uniformization components such as the reflector sheet can be omitted. According to experimental results, the luminous efficiency of the pseudo-white light LED is only 1/2 of that of the cold cathode lamp. If it is matched with the optimized light guide path, the unit watt luminous lumen (lm/W) can be raised to the same level as the cold cathode lamp. It is worth mentioning that in the case of single RGB white light LED, in order to make the light generated by each single LED evenly mixed and turned into white light, a specially designed light guide path similar to that shown in Figure 7 must be used.

Figure 7 Backlight module using RGB colors

If the LED backlight module can overcome the problem of luminous lumens per unit watt (lm/W), the LCD panel can fully utilize the characteristics of the LED backlight module such as "high color reproducibility" and "high-speed response".

※ In order to thoroughly pursue the high color reproducibility in item (1), directly using the RGB chip (chip) to mix the emission colors to produce white light is more advantageous than using a pseudo-white light LED that uses a phosphor to produce white light. Therefore, Mitsubishi of Japan uses RGB three-color LEDs to produce a liquid crystal display with an NTSC ratio of up to 104.4%, and its color reproducibility even exceeds that of CRT displays of the same level. Although the red light component of pseudo-white light LEDs using phosphors is weak, near-ultraviolet white light LEDs that use RGB three-color phosphors to emit light can obtain light with a fairly balanced RGB component, thus making up for the above-mentioned disadvantage of weak red light components. In fact, Toyoda Gosei has already started a small-scale trial production of near-ultraviolet white light LEDs with the same brightness as pseudo-white light LEDs, and officially entered the mass production stage in September 2003 (Figure 8).

Figure 8 White light LED spectrum using phosphor

※ Regarding the high-speed response of item (2), the Field Sequence LCD panel of Samsung SDI in South Korea introduced in the previous section has proven that the use of three types of RGB LEDs can replace the high-priced color filters. In addition, the luminous characteristics of LEDs are stable, and the response speed of turning on and off is calculated in nanoseconds. Its response speed of turning on and off is three digits less than the 10ms of traditional cold cathode lamps. If it is used with high-speed LCDs, a full-color display effect with no switching time difference can be obtained. In addition, the on-off characteristics of LEDs are very beneficial to the in-pulse driving method, because the in-pulse driving method can improve the image blur phenomenon that is common in the boundary area of ​​dynamic images. However, if the response speed of the light source is too slow, the above effect cannot be fully exerted. The high-speed lighting characteristics of LEDs just meet such needs.

Although LED backlight modules have many advantages, they have not been popularized. The main reason is that the cost of LED backlight modules remains high. For example, if a single RGB white light LED is used for a 23-inch LCD TV, the production cost in 2003 was 2.5 times that of a traditional cold cathode backlight module (Figure 9), and the cost of the LED light source was 10 times that of a cold cathode light source, making the only advantage of the LED backlight module that it does not require a high-voltage inverter. In order to achieve the goal of making the cost of LED backlight modules the same as that of traditional cold cathode backlight modules in 2006, the cost of LED light sources must be reduced to less than 1/6, and at the same time, the luminous efficiency of LEDs must be greatly improved and the number of LEDs used must be reduced, so that traditional cold cathode backlight modules can be fully replaced.

Figure 9 Cost analysis of LED backlight modules

Figure 10 shows the low-cost Xe planar lamp structure developed by OSRAM of Germany for 30-inch LCD TVs. Since it is a planar light source, it does not require a light guide plate and a diffuser sheet. Xe planar lamps use pulse voltage to excite Xe (Xenon) gas to convert ultraviolet light into visible light. Although its luminous efficiency is only 1/2 of that of traditional cold cathode lamps, since Xe planar lamps do not have light loss due to light guide plates, their luminous lumens per unit watt (lm/W) characteristics are the same as those of cold cathode lamps. The biggest feature of Xe planar lamps is that their lifespan is 2 to 3 times that of cold cathode lamps, and they will not deteriorate due to high temperatures, or cause aging of plastic components around the backlight module due to ultraviolet rays. According to OSRAM, the brightness half-life of Xe planar lamps is 100,000 hours, and the color change is very small. The brightness remains almost unchanged at -300C~+850C, and the stand-up time when lit is less than 5ns. The price of early Xe planar lamps was 2 to 3 times higher than that of cold cathode lamps. Therefore, OSRAM adopted four countermeasures to try to reduce the production cost of Xe planar lamps. The four countermeasures are as follows:

(i) Make a conical groove on the front glass to replace the spacer.
(ii) Reduce the thickness of the glass substrate.
(iii) Use spray coating to coat the phosphor.
(iv) Eliminate the dielectric structure.
The company has already started small-scale production in 2003 and plans to start mass production in 2005. The price of 30-inch grade is 100-120 USD, which is generally considered to be quite competitive.