The luminous efficiency and service life of white light LED

In order to obtain sufficient white light LED beams, large-size LED chips were developed in an attempt to achieve the desired goal. In fact, when the power applied to the white light LED exceeds 1W, the beam will decrease and the luminous efficiency will be relatively reduced by 20%~30%. The problems that must be overcome to improve the input power and luminous efficiency of white light LEDs are: suppressing temperature rise; ensuring service life; improving luminous efficiency; and equalizing luminous characteristics.

Increasing the power will reduce the thermal impedance of the white light LED package to below 10K/W, so foreign countries have developed high-temperature resistant white light LEDs in an attempt to improve the temperature rise problem. Because the heat generated by high-power white light LEDs is more than dozens of times higher than that of low-power white light LEDs, even if the white light LED package allows high heat, the allowable temperature of the white light LED chip is certain. The specific method to suppress temperature rise is to reduce the thermal impedance of the package.

The specific method to increase the service life of white light LEDs is to improve the chip shape and use small chips. Because the emission spectrum of white light LEDs contains short-wavelength light with a wavelength below 450nm, traditional epoxy resin sealing materials are easily damaged by short-wavelength light, and the large amount of light from high-power white light LEDs accelerates the degradation of sealing materials. Using silicon sealing materials and ceramic packaging materials instead can increase the service life of white light LEDs by a single digit.

The specific method to improve the luminous efficiency of white light LEDs is to improve the chip structure and packaging structure to achieve the same level as low-power white light LEDs. The main reason is that when the current density is increased by more than 2 times, it is not only difficult to extract light from the large chip, but the luminous efficiency will be lower than that of low-power white light LEDs. If the electrode structure of the chip is improved, the above-mentioned light extraction problem can be solved in theory.

The specific method to achieve uniform luminous characteristics is to improve the packaging method of white light LEDs. It is generally believed that the above problems can be overcome by improving the uniformity of the concentration of the phosphor material of the white light LED and the manufacturing technology of the phosphor.

The specific contents of reducing thermal impedance and improving heat dissipation are:

① Reduce the thermal impedance from chip to package.

② Suppress the thermal impedance from the package to the printed circuit board.

③ Improve the heat dissipation of the chip.

In order to reduce thermal impedance, many foreign LED manufacturers place LED chips on the surface of heat sink fins made of copper and ceramic materials, as shown in Figure 1. The heat dissipation wires on the printed circuit board are connected to the heat sink fins that are forced to be air-cooled by cooling fans by welding. The experimental results of OSRAM Opto Semiconductors Gmb in Germany confirmed that the thermal impedance from the LED chip to the welding point of the above structure can be reduced by 9K/W, which is about 1/6 of that of traditional LEDs. When 2W of power is applied to the packaged LED, the temperature of the LED chip is 18℃ higher than the welding point. Even if the temperature of the printed circuit board rises to 500℃, the temperature of the LED chip is only about 700℃. Once the thermal impedance is reduced, the temperature of the LED chip will be affected by the temperature of the printed circuit board. For this reason, the thermal impedance from the LED chip to the welding point must be reduced. On the other hand, even if the white light LED has a structure that suppresses thermal impedance, if the heat cannot be conducted from the LED package to the printed circuit board, the increase in LED temperature will reduce its luminous efficiency. Therefore, Panasonic has developed a technology that integrates printed circuit boards and packages. The company encapsulates square blue light LEDs with a side length of 1mm on a ceramic substrate in a chip-on-chip manner, and then sticks the ceramic substrate on the surface of a copper printed circuit board. The overall thermal impedance of the module, including the printed circuit board, is approximately 15K/W.

(a) OSRAM LED packaging

(b) CITIZEN LED packaging method

Figure 1 LED heat dissipation structure

To address the issue of extending the life of white light LEDs, the current countermeasures taken by LED manufacturers are to change the sealing material and disperse the fluorescent material within the sealing material, which can more effectively suppress the rate of material degradation and the reduction in light transmittance.

Since epoxy resin absorbs 45% of light with a wavelength of 400-450nm, while silicon sealing material absorbs less than 1%, the time for epoxy resin brightness to halve is less than 10,000 hours, while silicon sealing material can be extended to about 40,000 hours (as shown in Figure 2), which is almost the same as the design life of the lighting equipment, which means that the white light LED does not need to be replaced during the use of the lighting equipment. However, silicon sealing material is a highly elastic and soft material, and the processing must use a manufacturing technology that will not scratch the surface of the silicon sealing material. In addition, silicon sealing materials are very easy to adhere to dust during the process, so it is necessary to develop technologies that can improve surface properties in the future.

Figure 2 Effects of silicon sealing material and epoxy resin on LED optical properties

Although silicon sealing materials can ensure that white light LEDs have a service life of 40,000 hours, the lighting equipment industry has different views. The main debate is that the service life of traditional incandescent lamps and fluorescent lamps is defined as "the brightness drops below 30%." The time it takes for white light LEDs to halve their brightness is 40,000 hours. If the brightness drops below 30%, it will only have about 20,000 hours left. There are currently two countermeasures to extend the service life of components, namely:

① Suppress the temperature rise of the entire white light LED.

② Stop using resin packaging.

The above two measures can achieve the requirement of 40,000 hours of service life when the brightness drops to 30%. The temperature rise of white light LED can be suppressed by cooling the printed circuit board of white light LED package. The main reason is that the packaging resin will deteriorate rapidly under high temperature and strong light exposure. According to Arrhenius's law, the service life will be extended by 2 times when the temperature drops by 100℃.

Stopping the use of resin packaging can completely eliminate the deterioration factor, because the light generated by the white light LED is reflected in the packaging resin. If a resin reflector is used to change the direction of the light on the side of the chip, the reflector will absorb the light, so the amount of light taken out will drop sharply. This is also the main reason for using ceramic and metal packaging materials. The structure of the LED packaging substrate without resin is shown in Figure 3.

Figure 3 LED package substrate without resin structure

There are two ways to improve the luminous efficiency of white light LED chips: one is to use a large LED chip with an area 10 times larger than that of a small chip (about 1mm2); the other is to use multiple small high-luminous efficiency LED chips to combine into a single module. Although large LED chips can obtain large beams, increasing the chip area will have negative effects, such as uneven light-emitting layers within the chip, limited light-emitting parts, and severe attenuation of light generated inside the chip when radiated to the outside. In response to the above problems, by improving the electrode structure of white light LEDs, using chip-on-chip packaging, and integrating chip surface plus technology, a luminous efficiency of 50lm/W has been achieved. The packaging method of large white light LEDs is shown in Figure 4. Regarding the uniformity of the light-emitting layer of the entire chip, since the emergence of comb-shaped and grid-shaped P-type electrodes, the electrodes have also been developed in the direction of optimization.

Figure 4: Packaging of large LEDs

Regarding the chip-on-chip packaging method, since the light-emitting layer is close to the package end and is very easy to discharge heat, and there is no electrode shielding when the light from the light-emitting layer is emitted to the outside, the American Lumileds Company and Toyota of Japan have officially adopted the chip-on-chip packaging method. The chip surface processing can prevent the light from being reflected at the interface when it is emitted from the inside of the chip to the outside of the chip. If a concave-convex structure is set on the sapphire substrate where the light is taken out, the light extraction rate outside the chip can be increased by about 30%. The improved large LED chip packaging entity can make the light emitted from the side of the chip move toward the reflector above the package. The package size for efficiently taking out the light inside the chip is about 7mm×7mm. The final packaging method of the large LED is shown in Figure 5.

Figure 5 Final packaging method of large LED

The improvement of the luminous efficiency of small LED chips seems to be more effective than that of large LED chip modules. For example, CITIZEN of Japan combines 8 small LED chips to achieve a high luminous efficiency of 60lm/W. If a 0.3mm×0.3mm small LED chip made by Nichia is used, a package module can use up to 12 such chips. Each LED chip adopts the traditional gold wire bonding package method, and the applied power is about 2W.

For the uneven brightness and color temperature of white light LEDs, it is necessary to select white light LEDs with similar optical properties. In fact, it is very important to reduce the unevenness of white light LED luminous characteristics, make the luminous characteristics of LED chips consistent, and implement uniform management of phosphor material concentration distribution.

Regarding the luminous characteristics of LED chips, various manufacturers are actively conducting chip screening and equalization of luminous characteristics to reduce the problem of uneven LED luminous characteristics. For example, Panasonic has achieved the goal of uniform characteristics through chip screening. The company uses chip-overlapping to package 64 LED chips on a substrate, and then covers them with phosphors. During processing, the LED chips are first packaged on the secondary substrate to test the luminous characteristics, and then the chips with consistent luminous characteristics are transplanted and packaged on the main substrate. 8 LED chips are packaged on a substrate. Even if the luminous characteristics of the LED chips are uneven, the unevenness of the luminous characteristics of the 8 LED chips combined between the packages will become very small. The effect of using a combination of multiple small LED chips to improve the uniformity of the luminous wavelength is shown in Figure 6.

Figure 6: Using a combination of multiple small LED chips to improve the uniformity of the emission wavelength

White light LEDs are usually directly coated with sealing resin containing fluorescent materials. At this time, the concentration of fluorescent materials in the sealing resin may deviate, which will eventually cause uneven color temperature distribution of white light LEDs. Therefore, resin sheets containing fluorescent materials can be combined with LED chips. Since the thickness of the sheets and the concentration of fluorescent materials are strictly managed, the uneven color temperature distribution of white light LEDs is reduced by 4/5 compared with traditional methods. The industry believes that by using fluorescent sheets, matching the light-emitting characteristics of LED chips, changing the concentration of fluorescent materials and the thickness of the sheets, the color temperature changes of white light LEDs can be controlled within the expected range.

Although the possibility of applying white light LED in the field of lighting is increasing with the gradual improvement of the luminous efficiency of white light LED, it is obvious that the luminous flux of a single white light LED is low, so it is unlikely to achieve the required number of lumens for lighting with a single white light LED in the current packaging form. In response to this problem, the main solutions can be roughly divided into two categories: one is to use multiple LEDs to form a light source module in a more traditional way, and the driving power required for each white light LED is the same as that used in general (20~30mA); the other method is to use a larger chip, in which the traditional 0.3mm2 chip is no longer used, but a 0.6~1mm2 chip is used, and a high driving current is used to drive such a light-emitting component (generally 150~350mA, currently the highest reaches 500mA or more). However, no matter which method is used, it will have to deal with extremely high heat in an extremely small LED package. If the component cannot dissipate this heat, in addition to the various packaging materials having product reliability issues due to the different expansion coefficients between each other, the chip's luminous efficiency will also drop significantly as the temperature rises, causing a significant shortening of the service life. Therefore, how to dissipate the heat in the component has become an important issue in the current white light LED packaging technology.

For white light LEDs, the most important thing is the output luminous flux and light color, so one end of the white light LED must not be shielded from light, but must be covered with a highly transparent epoxy resin material. However, current epoxy resins are almost all non-thermal conductive materials, so for the current white light LED packaging technology, the main method is to use the metal foot under the white light LED chip to dissipate the heat generated by the component. According to the current trend, the metal foot material is mainly composed of materials with high thermal conductivity, such as aluminum, copper and even ceramic materials, but the thermal expansion coefficients of these materials and the chip are very different. If they are directly in contact, it is likely to cause reliability problems due to stress between the materials when the temperature rises. Therefore, an intermediate material with appropriate conductivity and expansion coefficient is generally added between the materials as a spacer. Panasonic Electric has made many of its white light LEDs on a multi-layer substrate module made of metal materials and metal composite materials to form a light source module. The high thermal conductivity of the light source substrate is used to ensure that the output of the light source can remain stable during long-term use. The material used in the white light LED substrate produced by Lumileds is copper with a high conductivity coefficient. By connecting it to a special metal circuit board, it can take into account both circuit conduction and increased heat conduction effects.

The chip manufacturing technology and packaging technology of high-power white light LED products seem to have become the mainstream technology of high-brightness white light LEDs. However, the manufacturing technology and packaging technology related to large chips are not just about increasing the chip area. If we hope to apply white light LEDs to the field of high-brightness lighting, the relevant technologies still need further research.

There are still many problems to be solved in the application of white light LEDs in the field of general lighting. The first is to improve the efficiency of white light LEDs. For example, the efficiency of GaInN-based green light, blue light, and near-ultraviolet light LEDs still has a lot of room for development. In addition, the improvement of the internal quantum efficiency of the overall energy efficiency is the most important project. The internal quantum efficiency is determined by the non-luminescent recombination percentage and the luminescent recombination percentage of the active layer. Therefore, the focus can be locked on the non-luminescent recombination part and try to reduce the crystal defects. Reducing the translocation density of ultraviolet LEDs can indeed significantly improve the internal quantum efficiency. In the future, it is necessary to further reduce the translocation density of ultraviolet LEDs. However, this countermeasure has no obvious effect on green and blue LEDs.

Green and blue LEDs have the highest quantum efficiency at low current density (about 1A/cm2), but the quantum efficiency decreases at high current density, as shown in Figure 7. From a cost perspective, it is hoped that LEDs can be driven at high current density while increasing the output power of the components as much as possible. Therefore, it is necessary to unravel the mechanism and cause of the decrease in quantum efficiency of green and blue LEDs at high current density as soon as possible. This research is not only necessary for exploring the physical properties of materials, but also plays a key role in future applications. Current research shows that the quantum efficiency of purple LEDs (wavelength 382nm) will not decrease even if a high current density (50A/cm2) is applied.

Figure 7 Relationship between quantum efficiency and current density of GaInN LED

Traditional white LEDs are square chips with a side length of 200~350μm packaged into a round head column shape, and then in order to obtain the light beam required for lighting, the packaged multiple white LED components are arranged in a matrix. White LEDs with an area 6~10 times larger than previous chips and an external dimension of up to 500μm~1mm, which are specially developed for the purpose of high output power, can obtain an output power of hundreds of milliwatts (tens of lumens) after packaging, but increasing the chip's external dimensions will increase the internal light absorption ratio of the white LED and reduce the external light extraction rate. Take AlGaInP LED as an example. After the chip's external dimensions are increased from 0.22mm×0.22mm to 0.50mm×0.50mm, the external light extraction rate is reduced by about 20%. If the TIP structure is used instead, the result of multiple internal reflections will reduce the internal light absorption rate and significantly increase the external light extraction rate. GaInN LEDs have the same effect. How to improve the external light extraction rate of LED chips is the key to the application of LEDs in the field of general lighting. In addition, high thermal impedance (150~200K/W) is quite unfavorable for high brightness output. The internal quantum efficiency of LED is highly dependent on the temperature of the active layer, so in addition to low thermal impedance packaging technology, using heat sinks to remove heat flow from the active layer will become a hot spot for future research and development.