The reason for the light decay of white LEDs: the decline of phosphor performance

The reason for the light decay of white LEDs: the decline of phosphor performance

So far, the rapid decline of the luminous performance of white light LEDs, especially low-power white light LEDs, has become increasingly recognized by people. In fact, blindly exaggerating the publicity can only lead the LED industry astray, and not facing up to the problems of white light LEDs can only delay the development of white light LED applications. Only by facing up to the problems, studying the problems, and solving the problems as soon as possible can white light LEDs develop healthily and rapidly.

One of the main problems facing white LEDs is the lifespan. Since the price of white LEDs is still very high, if they want to gain a foothold in the lighting market and allow users to save both electricity and money, they must rely on the electricity savings from long lifespan to make up for the high price of lamps. The reality is that the lifespan of white LEDs makes it difficult to do this. Therefore, we must find the factors that affect the lifespan of white LEDs and then find solutions. I am not very knowledgeable and can only put forward some superficial views, test data and analysis for reference by those with lofty ideals.

From the normal use of LED, the most important factor affecting the life of LED is heat. The source of heat includes Joule heat generated by the normal resistance of the material when it is powered on, heat generated by the PN junction, Joule heat generated by the parasitic resistance brought by the process, and heat generated after light is absorbed. The accumulation of heat causes the temperature to rise, and the temperature rise causes the performance of the chip to decline, the material to age, and the property to change. Now I will analyze the decline of the performance of the phosphor, which is the cause of the light decay of white LED.

1. Causes of white light LED light decay: Decline of phosphor performance

There seems to be no relevant data on the relationship between the light excitation efficiency of LED phosphors and temperature. However, there are sufficient facts to prove that the increase in temperature does affect the performance and life of phosphors.

A phosphor manufacturer has done a test and found that at a temperature of 80 degrees, the excitation efficiency of the phosphor decreased by 2%. It recovered after cooling. This was just a short-term test. It shows that the performance of the phosphor decreases with increasing temperature. As for the irreversible performance degradation, it is a cumulative process that takes a certain amount of time.

We often encounter this situation: after using or aging a white light LED for a period of time, we find that the LED becomes brighter, as shown in Figure 1. At present, this situation usually occurs within 1000 hours for low-power LEDs [Note 1] (here, the luminous flux after 1000 hours may still be

Greater than the initial value, products after mid-2008 and SMD products can approach or reach 1000 hours). For high-power packaged LEDs, this condition may last up to 2000 hours. This condition may be caused by the following situations:

Figure 1 Low-power white light LED lifespan diagram

a. The performance of the phosphor is reduced by the action of the phosphor and the mixed glue, and the performance of the phosphor is restored under the initial action of temperature;

b. The phosphor and the mixed glue work together to improve the performance of the phosphor;

c. The performance of the blue light chip is enhanced for an initial period of time.

In the test, it was found that in the initial period of time, the luminous flux of the white light LED either increased or decreased. This situation also occurred when the same red light chip was packaged by different manufacturers. Therefore, it is difficult to determine whether it is a problem with the phosphor or a problem with the packaging material and packaging process based on a short-term test.

However, in the life test of low-power blue LEDs, it was found that the luminous flux generally increased during the initial period, as shown in Figure 2. The luminous flux increase period is generally around 200 hours. The luminous flux increase period of plug-in white light is generally around 100 hours [Note 1].

It can also be inferred from this that the performance of the phosphor in white light LEDs should be the first to decline.

After the short period of luminous flux increase, the situation of low-power white light LEDs is not optimistic. They start to be like a plane without wings, and the result is self-evident. High-power white light LEDs generally increase their luminous flux after about 100 hours, and then they are in a not very stable state from 6000 hours. Over time, the luminous flux of some products has a large fluctuation of rise and fall. After 6000 hours, they basically start to decline unswervingly. At present, general high-power white light products reach the end of their life in 15,000 to 20,000 hours (light decay reaches 50%).

Since the bracket material of the plug-in LED is iron, the thermal conductivity cannot meet the heat dissipation requirements of the blue light chip, so the temperature generated by heat accumulation is not suitable for the survival of the blue light chip and phosphor.

There are several phenomena that also prove that the life of white light LEDs is mainly shortened due to the performance degradation of phosphors.

First, after the life test, we can see that the LED is no longer white light, but a seriously bluish light. This means that the phosphor has lost its function. (Note that it is the end of the life test, not the aging test. Please refer to Note 2.)

Secondly, someone did an experiment where they took out a white LED chip that had decayed by more than 30% and found that the chip had only decayed by a few percent, which also shows that the phosphor had lost its function.

Third, for the same small chip, the lifespan of a high-power packaged white light is much better than that of a low-power packaged white light. (This will be discussed in detail later.)

Generally speaking, phosphors will decay under the influence of temperature. Moreover, it is expected that the performance decay will be drastic above 80-100 degrees. The higher the temperature, the more serious the phosphor performance decay. Therefore, the main reason for the light decay of white light LEDs is the first and rapid decay of phosphor performance under the action of heat. Whether the phosphor itself decays at the normal working temperature of the LED or the decay is caused by the influence of the mixed glue, more sufficient experimental verification is needed. The verification method is envisioned as follows:

The same phosphor is divided into several portions, one of which is not treated in any way for comparison, while the other portions are heated at different temperatures and for different times.

In short, whether it is the phosphor itself or the combination of phosphor and colloid, the performance of the phosphor will decay rapidly at a certain temperature. Therefore, this is the first reason for the light decay of white light LEDs.