How To Play a Greater Role Of Led Light Source In The Field Of Lighting
Writer：Jane Time：2021-07-30 Browse：204 Semiconductor lighting source (mainly refers to the LED light source here) has been batch into the lighting field, but there are still many problems, mainly energy efficiency, reliability, light quality and cost issues. There are a lot of things related to energy efficiency and color quality, such as visual comfort and intelligent dimming control, which we will not describe here. This paper will discuss the main technical problems that need to be solved urgently, which can be summed up as "three high and one low", namely, high light efficiency, high color rendering, high reliability and low cost. Realizing low cost is essentially a technical problem. To solve these four technical problems, we need to take a series of measures in each link of the semiconductor lighting industry chain, such as the adoption of new technology, new structure, new process, new materials, etc. Here we only mention the technical route and direction that should be taken, hoping to be helpful to THE product innovation of LED enterprises.
I. How to achieve high light efficiency
The light efficiency of semiconductor lighting, or energy efficiency, is an important indicator of energy saving effect. At present, the industrialization level of light efficiency of LED devices can reach 120 ~ 140LM /W, and the total energy efficiency of lighting can be greater than 100LM /W. This is still not high, the energy saving effect is not obvious, and there is still a great distance from the theoretical value of 250LM /W of semiconductor devices. To really achieve high light efficiency, it is necessary to solve the related technical problems from each link of the industrial chain, mainly to improve the internal quantum efficiency, external quantum efficiency, packaging light efficiency and lamp efficiency. This paper will discuss the technical problems to be solved in several links, such as epitepitzation, chip, packaging and lamp.
1. Improve internal quantum efficiency and external quantum efficiency
The following measures should be taken to improve the internal quantum efficiency and external quantum efficiency.
(1) substrate surface coarsening and non-polar substrate
GaN is grown on nanoscale plot substrate, "oriented" plot substrate or non-polar or semi-polar substrate to reduce dislocation and defect density and the effect of polarity field, and improve the internal quantum efficiency .
(2) generalized homogeneous substrate
GaN can be grown on Al2O3 sapphire substrate by HVPE (hydride liquid phase epitaxial) as a mixed homogeneous substrate GaN/Al2O3, and the dislocation density can be greatly reduced to 106 ~ 107cm-2, and the internal quantum efficiency can be greatly improved. Japan, Cree and China's Peking University are all under research and development .
(3) Improve the structure of quantum well
The internal quantum efficiency can be improved by controlling the change mode and amount of In component, optimizing the quantum well structure to improve the probability of overlapping between electron and hole, increasing the probability of radiation recombination, and adjusting the transport of non-equilibrium carriers.
(4) Chip with new structure
In order to improve the external quantum efficiency of the chip, the new structure requires the chip to emit light from six sides, and the new technology is adopted to carry out various surface coarsing methods on the interface of the chip to reduce the reflection probability of the photon on the interface of the chip and increase the surface transmittance.
2. Improve the encapsulation light efficiency and reduce junction temperature
(1) phosphor efficiency and coating process
The light excitation efficiency of phosphor is not high at present, the yellow powder can reach about 70%, the efficiency of red powder and green powder is low, which needs to be further improved. In addition, the phosphor coating process is very important, it has been reported that the chip surface coated with 60 micron thickness of phosphor, high excitation efficiency.
(2) COB encapsulation
At present, the light source of semiconductor lighting adopts various forms of COB packaging, and it is urgent to improve the light efficiency of COB packaging. It is reported that the second generation (some say the third generation) COB matrix structure packaging, its light efficiency can reach more than 120LM /W. If the flip-chip and hexahedral light emitting body are used for total reflection structure, the light efficiency can reach more than 160lm/W.
(3) Reduce junction temperature
When the junction temperature is 25℃, the luminescence amount is set to 100%, when the junction temperature rises to 60℃, the luminescence amount is only 90%, when the rise to 140℃, only 70%, so in the packaging to increase heat dissipation measures, maintain a low junction temperature, maintain a high luminescence efficiency.
3. Improve the light taking efficiency of lamps
The efficiency of different LED lamps differs greatly. Generally, the efficiency of LED lamps is greater than 80%, and some of them can be greater than 90%. According to the characteristics of LED light source and different application occasions, fine secondary optical design of lamps and lanterns, but also to consider the heat dissipation and glare of lamps and lanterns, improve the light efficiency of LED lamps.
Two, to achieve high color rendering
The light color quality of white LED has many contents, including color temperature, color rendering, light color fidelity, light color naturalness, tone recognition, visual comfort, etc. . The US SSL plan proposes that the spectral distribution of LED lighting products should be similar to that of sunlight. To meet these requirements is very difficult and requires a lot of basic research work, which will certainly be achieved in the future. Here we only discuss the problems of color temperature and color rendering which need to be solved urgently. Energy Star standards require a CRI≥80 for indoor lighting, but some high-end applications require a CRI≥90. High color rendering LED light sources will lose more light efficiency, so take these two factors into consideration when designing.
Here, it is necessary to explain the evaluation of CRI . CIE (TC1-62) Technical report 177 concluded that "CIE CRI is not suitable to represent the color rendering range of white LED light sources". At present, there are many proposed revisions to CRI calibration, such as CQS color quality index, GAI pangamut index, RF Flutley index, CPI color preference index, CDI color resolution index, etc. At present, it has not been decided which revision to use in CIE. NIST (National Institute of Standards) proposed the use of CQS to evaluate the quality of light source color, the test sample expanded to 15, including part of the sample with high color saturation, so much better, many people agree. To improve color rendering, in principle to consider RGB three primary color combination to achieve, currently there are three ways.
(1) multi-color phosphor
LED light source uses LED blue light chip plus aluminate yellow powder and nitrous oxide red powder, green powder combination LED white, its CRI can reach 80 ~ 90. It has been reported that the CRI of RGBY can reach 98.
(2) RGB multi-chip combination
Using RGB multi-chip effective combination of LED white, CRI can also reach 80 ~ 90, may be due to driving mode and cost factors, less application at present.
(3) phosphor plus chip
LED light source uses blue light chip plus aluminate yellow powder plus red chip, effective combination of LED white, its color rendering index can reach more than 80, light efficiency is high, the cost is ok, is currently widely used combination.
Third, improve reliability
The reliability, failure rate, life and other indicators of LED devices and light sources (lamps) have different understandings and descriptions in practical applications, so it is necessary to make some explanations.
1. The reliability
Reliability refers to the ability of a product to complete specified functions under specified conditions and within specified time . LED failure categories mainly include severe failure (key parameters change to leds are not bright) and parameter failure (photoelectric parameters change from the initial value to a certain limit). The failure curve includes early failure (high failure rate in the early days of service and then decreases rapidly), occasional failure (low but stable failure rate) and wear failure (wear failure increases with the longer use).
Life of 2.
Life is a characteristic value of product reliability. Due to the different meanings of the life of the product, it is easy to cause confusion, so it is described in detail here. There are many kinds of life descriptions. Here are 10 different meanings of life: life, average life, median life, characteristic life, expected life, reliable life, working life, optical maintenance life, mean fail-free working time and mean pre-failure working time. The common expression of LED life is:
Life: Generally refers to the statistical average, for a large number of components, LED device life is the meaning of this description.
Working life: refers to the effective working time of LED products, which is different from ordinary life.
Median life: For 50% lamps (light sources), the time when the light flux drops to a certain value (such as 70% of the initial value) is defined as the median life L70/B50. The standard expression of some lighting lamps is median life.
Light pass maintenance life: refers to the time when the light flux of light-emitting devices (LED) or lamps (light source) drops to a certain value, which is called the light pass maintenance life (at this time, the change of color parameters is not considered).
IEC and Energy Star statements:
The IEC organization proposes the use of maintenance rates to describe failure rates, life expectancy, etc.
EPA (The United States National Environmental Protection Agency) announced: Energy Star luminaires V1.0 technical specification "technology neutrality", stipulates that the life of the matching luminaires is 10,000 hours, that is, the rated light maintenance rate life, integrated LED luminaires require 15,000 to 25,000 hours.
Energy Star standard proposed LED light source (lamps) in the rated current of 6000 hours, the measurement of the product's light maintenance rate, and calculate the working life of the product (refers to the effective working time) at the same time in the life of the color space uniformity in CIE1976u 'V' figure within 0.006. This approach is widely accepted in the industry, but it is difficult to implement in an enterprise because it requires more than 250 days of testing.
3. Improve reliability
Improving LED reliability is an industry-wide goal. Various factors affecting the reliability of LED products, such as chip manufacturing, packaging, thermal resistance, heat dissipation, etc., have been described in detail before . In this paper, we hope that enterprises should make the following two requirements on the basis of comprehensive quality control for LED products:
(1) Reduce failure rate
At present, in practical application, product failure often occurs, some even very serious. It is hoped that the relevant units can find out the reasons of failure through various tests, and take effective process screening methods to eliminate the early failure and accidental failure of bad products, in use as much as possible to reduce the failure rate.
(2) Prolong the loss time
It is hoped that related enterprises can carry out a long time aging test (or accelerated aging test) on typical LED products, find out the cause of product loss and failure through analysis, and improve the process, material selection and other aspects to extend the loss time, which is also an effective way to improve the life of LED.
At present, all kinds of LED products have not been mass-produced and promoted. The main problem is that the price is too high. LED products cannot be developed by long-term subsidies, and continuous efforts should be made to reduce the cost. Related departments and units all over the world are paying attention to reducing the cost of LED products, for example, THE United States SSL plan to 2015 LED light source (lamps) cost up to $2 / KLM. China also proposed that the cost of LED light source (lamp) should be within 15 yuan/KLM in 2015. Taiwan has also proposed a target of $2 per KLM by 2014. To reduce the cost, in addition to mass production, mainly from the technical measures to reduce the cost of the method, the way.
To reduce the cost, we should start from each link of the LED industry chain and make joint efforts. First, the cost structure of LED products is about 4:4: 2, that is, the light source accounts for 40%, drive and heat dissipation accounts for 40%, other accounts for 20%, the main link is the epitaxial chip, packaging, drive, heat dissipation and other aspects of the cost has a large reduction space, to take effective measures from the technical, fundamentally solve the cost of LED products.
1. Discussion on the method of reducing the cost of epitaxial chip link
Currently, chips account for about 40% of the cost of LED products, and reducing the cost of chips is the most important task.
Veeco, the world's leading epitaxial manufacturer, proposed to improve the equipment, process and architecture of the existing MOCVD, and improve the volume production efficiency, to reach the epitaxial wafer price of $0.2 /cm2 by 2014. ($1 /cm2 in 2009)
Using 4 "and 6" MOCVD equipment and supporting chip manufacturing equipment, the efficiency of epitaxial and chip production capacity is greatly improved, so as to reduce the cost of epitaxial chip.
At present, many powerful companies in the world are developing GaN directly on 8 "silicon wafer, and have achieved gratiing results. Recently, Osram released a GaN chip growing on a 6 "silicon wafer with an area of 1mm2. The light efficiency can reach 127LM /W at 350mA and a color temperature of 4500K. It will be put on the market in 2 years. It is estimated that this method can reduce the cost of the chip by 50%.
GaN substrate was grown by HVPE liquid phase epitaxy. Growing GaNLED products here can not only improve THE PERFORMANCE of LED, but also greatly reduce the cost of epitaxial chip. This method can greatly shorten the epitaxial growth time and save a lot of MO sources. It is estimated that the cost of chip can be reduced by 50%.
LED devices using large current density work, such as the area of 1mm2 chip, the general working current is 350mA, several foreign large companies have developed can provide the working current of 1 ~ 2A power LED devices, such a chip can obtain light flux up to 400 ~ 500LM, of course, will lose part of the light efficiency, They have solved the Droop phenomenon well, and can greatly reduce the cost of the chip with the same light flux.
2. Discussion on the method of reducing the cost of packaging
The light source used in semiconductor lighting will adopt COB and module packaging form, which will greatly reduce the cost of LED light source packaging.
LED light source adopts COB packaging form, currently mainly adopts medium power multi-chip, matrix integrated packaging, known as the third generation of COB packaging form, and there are many different structure types, such as MCOB, COFB, MCOMB, etc., and can reduce the packaging cost by 30%. At present, a bulb lamp has been developed in China. 9 medium power chips integrated with COB package, small volume, no heat sink, power of 7W, light flux up to 500 ~ 700LM, its cost is only 16 yuan.
Modular standard packaging LED products, which will be the direction of semiconductor lighting source development. It is the chip, drive, control part, heat dissipation, parts of the package together to form a module, and standardized production, can be used for different lighting products, can greatly reduce the packaging cost, according to the estimate can reduce the packaging cost 50%, the main manufacturers are actively invested in research and development. The Zhaga alliance has started to develop related standards, mainly optical engine interface standards, which will cover the physical size as well as optical, electrical and thermal characteristics of the optical engine, and eventually achieve compatibility and interchangeability between different manufacturers in the alliance.
3. Methods to reduce the cost of lamps and lanterns
On the premise of ensuring the light efficiency and heat dissipation performance of the secondary optical design, new materials and new lighting methods are adopted to design and produce light, beautiful and inexpensive new lamps, so as to reduce the cost of lamps. In addition, the performance and reliability of domestic power supply have been greatly improved. On this basis, effective technical measures can be taken to greatly reduce the cost of power supply, from the current 2.5 ~ 3 yuan /W to 1.5 ~ 2 yuan /W.
4. Reduction of other supporting costs
If MOCVD epitaxial furnace and chip manufacturing equipment can be manufactured locally, the chip manufacturing cost will be greatly reduced. LED product inspection