(1) The working temperature of the LED screen on building exceeds the load-bearing temperature of the chip, which will cause the luminous efficiency of the LED screen on building to decrease rapidly, resulting in obvious light attenuation and damage;
(2) LED screens on building are mostly encapsulated with transparent epoxy resin. If the junction temperature exceeds the solid phase transition temperature (usually 125°C), the packaging material will transform into rubber and the thermal expansion coefficient will rise sharply, resulting in open circuit and failure of the LED screen on building.
The lifespan of the LED screen on building is shown by its light decay, that is, as time goes by, the brightness will decrease until it finally goes out. It is common to define the LED display luminous flux decay time of 30 as its service life.
Generally, the reasons for the light decay of LED screen on building are as follows:
(1) The defects in the LED screen on building chip material will rapidly multiply at higher temperatures, until they invade the light-emitting area, forming a large number of non-radiative composite centers, and seriously reducing the luminous efficiency of the LED screen on building. In addition, under high temperature conditions, micro-defects in the material and fast-expanding impurities from the interface and the plate will also be introduced into the light-emitting area, forming a large number of deep energy levels, which will also accelerate the light decay of the LED screen on building.
(2) The transparent epoxy resin will denature and turn yellow at high temperature, which will affect its light transmission performance. The higher the working temperature, the faster this process will proceed. This is another main reason for the light decay of the LED screen on building.
(3) The light decay of phosphors is also a major factor affecting the light decay of LED screen on building, because the attenuation of phosphors is very serious at high temperatures.
Therefore, high temperature is the main source of light decay of LED screen on building, which may shorten the service life of the LED screen on building.
The reasons why temperature affects the luminous efficiency of the LED screen on building include the following aspects:
(1) As the temperature increases, the concentration of electrons and holes will increase, the band gap will decrease, and the electron mobility will decrease.
(2) As the temperature rises, the probability of radiative recombination of electrons and holes in the potential well decreases, resulting in non-radiative recombination (heat generation), thereby reducing the internal quantum efficiency of the LED screen on building.
(3) The temperature rise causes the blue wave peak of the chip to shift to the long wave direction, which makes the emission wavelength of the chip and the excitation wavelength of the phosphor do not match, and also reduces the external light extraction efficiency of the white light LED screen on building.
(4) As the temperature rises, the quantum efficiency of the phosphor decreases, the light output decreases, and the external light extraction efficiency of the LED screen on building decreases.
(5) The performance of silica gel is greatly affected by ambient temperature. As the temperature increases, the thermal stress inside the silica gel increases, resulting in a decrease in the refractive index of the silica gel, which affects the light efficiency of the LED screen on building.