Engineer Sharing: Analysis of Phosphor-free Dual-wavelength White Light LED

Introduction

White LEDs can be roughly divided into three types according to the light-emitting method. The most common method is to use blue LED with a wavelength between 410 and 460 nm + yellow phosphor to produce white light. It is also the most efficient and mass-producible method. However, various LED manufacturers hold various patents, and the production management of the phosphor itself has not yet been established, so it is not easy to mass-produce at a low price.

In addition, the white light method of ultraviolet light with a wavelength between 365 and 460 nm + RGB phosphor, and the method of combining the three primary colors of RGB can also obtain white light. However, the packaging materials and peripheral components of the ultraviolet method are at risk of degradation due to long-term exposure to ultraviolet light, and the RGB method has problems such as wavelength drift and complex control. Therefore, foreign companies have developed dual-wavelength white LEDs.

Development History

Figure 1 compares the packaging structure of traditional blue LED + phosphor and dual-wavelength white LED. It can be seen from the figure that dual-wavelength white LED has no need for phosphor ratio conversion and content management, and can completely solve the RGB three-primary color LED driving circuit The brightness, wavelength, and color rendering obtained can also be freely adjusted within a certain range.

FIG2 is a comparison of the light-emitting principles of a traditional white light LED and a dual-wavelength white light LED. It can be seen from the figure that a traditional white light LED obtains white light by coating a GaN-based blue chip with a phosphor, while a dual-wavelength white light LED obtains white light by using a single chip.

The basic structure of dual-wavelength white light LED is to generate blue light and yellow-green light simultaneously on the chip, and then mix the blue light and yellow-green light at a very short distance to obtain pseudo-white light. This pseudo-white light has a high degree of transparency and purity that traditional white light LEDs cannot achieve.

Figure 3 is a comparison of the structure of traditional white light LED and dual-wavelength white light LED chips. Since LED manufacturers hold various patents, only a general description can be given here. As shown in the figure, the buffer Multi Dot Active of the dual-wavelength white light LED chip is mainly composed of:
˙Multi Dot Active layer˙Active layer˙Air
Ocean
layer

This part plays a very important role when blue light and yellow-green light of two different wavelengths emit at the same time. Therefore, researchers have established crystal growth technology for each mode layer, so that dual-wavelength white light LEDs can smoothly enter the practical stage.

Characteristics of dual-wavelength white LEDs

Figure 4 shows the comparison of the characteristics of the components of the current mainstream blue LED + yellow phosphor and dual-wavelength LEDs. The main characteristics of dual-wavelength LEDs are:

˙The process can adjust the uneven distribution of color tones
˙Can emit light other than white
˙Long life

The biggest problem with mainstream blue LED + yellow phosphor combination white LED is that its yellow phosphor life is shorter than that of blue LED chip. The service life of LED chip is about 80,000 to 100,000 hours depending on the use environment, while the service life of phosphor is only 10,000 to 40,000 hours.

In contrast, dual-wavelength white LED does not use phosphor at all, and the service life of the component is consistent with that of LED chip, which can be used for a long time. This means that dual-wavelength white LED will have a far-reaching impact on future lighting applications.

In addition, dual-wavelength white LED does not use phosphor at all, and it can also adjust the color distribution through the chip manufacturing process. Especially when traditional white LED packaging, the wavelength and main power of blue LED must be limited to a narrow range, and then the content of phosphor is adjusted to perform cumbersome and complex process control. In comparison, the new dual-wavelength white LED has an absolute advantage in the yield rate of the process.

Although the absolute production quantity has not yet reached the quantitative stage, the dual-wavelength white light LED is designed with a larger color range. If the production efficiency issues and component supply that will be affected by future popularization are taken into consideration, the cost of the new dual-wavelength white light LED has an absolute competitive advantage, especially when facing micro-packages and multiple component packages, it can easily meet the strict substrate light source optical characteristics required by customers.

The new dual-wavelength white light LED is composed of blue light and green light, so it can emit flexible colors other than white. The specific method is to control the wavelengths of the water color and blue-green color series. The color tone obtained in this way is purer than that of LEDs using phosphors, which is very suitable for LCD backlight modules that have very strict color requirements . The

above-mentioned luminous brightness has reached the practical stage. If compared with the high-brightness white light LED of blue light LED + yellow phosphor, the brightness of the new dual-wavelength white light LED is not inferior.

In addition, through the improvement of the component surface processing method, the brightness of the new dual-wavelength LED is better than that of traditional white light LEDs. Regarding the color rendering of the new dual-wavelength white light LED, since it is composed of blue light + yellow-green light, there is almost no problem of red light mixing.

Wavelength distribution characteristics

As mentioned above, the new dual-wavelength white light LED uses two different wavelengths of blue light and yellow-green light to emit white light simultaneously. Figure 5 shows the wavelength characteristics of the new dual-wavelength white light. It can be seen from the figure that the peak wavelength of blue light is about 405nm, and the peak wavelength of yellow-green light is about 570nm, so there is almost no red light component.

The new dual-wavelength white LED has to overcome the problem of expanding the wavelength range of yellow-green light in the future. In comparison, the current mainstream blue LED + yellow phosphor white LED combination contains red light components in the yellow phosphor components, so its color rendering is not as good as dual-wavelength white LED.

Figure 6 shows the wavelength characteristics of LEDs close to yellow-green light tones. It can be seen from the figure that the peak wavelength of blue light of the new dual-wavelength white LED is almost fixed, and the peak wavelength of yellow-green light is different from that of the mainstream blue LED + yellow phosphor white LED, which is very close to 530nm. This is also the main reason why the combination of yellow-green light and blue light can achieve soft color adjustment.

Another feature of the new dual-wavelength white LED is the wavelength distribution characteristics of the mixed yellow-green light color system. As shown in Figure 7, since the blue light becomes a strong hue, the peak wavelength of the blue light end is almost fixed, and the peak wavelength of the yellow-green light moves toward the short wavelength end . In contrast, the output of the blue end and the green end of other hues is almost constant.

换言之新型双波长白光LED，主要降低绿色端的输出，达成软调色彩的目的，此处要强调的是新型双波长白光LED，可以依照实际需要在一定范围内，自由调整黄绿光与蓝光的发光波长，获得前所未有的LED发光色。

以上介绍新型双波长白光LED的优点，在此同时研究任人员充分利用传统封装技术，试图改善新型双波长白光LED的缺点，藉此建立高辉度化、低成本的技术。

In addition to choosing a high-efficiency packaging method, epitaxial packaging can also effectively improve brightness and increase the added value of the new dual-wavelength white LED. Dual-wavelength white LEDs, like blue LEDs, are InGaN-based semiconductor

components that are very vulnerable to static electricity . Therefore, when using a multi-chip packaging method that can improve brightness, the static protection component must be packaged together (Figure 8).

Generally, the VF value of AlGaInP series LED is about 2V at 20mA, while that of InGaN series LED is as high as 3.3V. In other words, when InGaN series dual-wavelength white light LED is used in portable electronic devices, a dedicated driver IC must be used .

Dual-wavelength white light LEDs using a new packaging method do not adjust the current value of the LED, so researchers are developing packaging technology that uses constant voltage to drive light-emitting components and can improve the color rendering of LEDs.

Dual-wavelength LED chips are packaged together with red light LED chips and become white light after color mixing (Figure 9). However, in this way, the VF values ​​of each chip are completely different. As the brightness and wavelength distribution of each LED chip are different, complex current value limit adjustments must be made.

In addition, heat dissipation and silicone after packaging are also issues to be overcome, so researchers are developing new countermeasures.

Currently, ψ3 and ψ5 shell-shaped dual-wavelength white light LEDs have begun mass production, and 3mm and 5mm square SMD dual-wavelength white light LEDs will be launched in the future.

Figure 10 is an ultra-thin LCD backlight module made by suppressing color distribution and component height. As shown in the figure, the white light LED is directly fixed on the surface of the thin substrate, and then matched with a special shading cover and an ultra-thin light guide plate for side lighting. The total thickness of the module is only 0.25mm, which is very suitable for use in portable electronic devices such as mobile phones.

Conclusion

The above introduces the new dual-wavelength white light LED. Traditional UV light + RGB phosphors and RGB multi-chip white light LEDs all have problems such as easy degradation of peripheral components, wavelength drift, and complex control.

In addition to completely solving the above problems, the new dual-wavelength white light LED has an ultra-short color mixing distance and pure color tone, providing another choice for LED downstream application manufacturers. In the future, if the packaging technology and the heat dissipation problem after packaging are successfully improved, dual-wavelength white light LEDs are expected to play a very important role in the thinning of electronic machine systems.