Solid-state light sources based on laser diode are becoming great alternative for LEDs. Improvement of the thermal characteristics of InGaN LD is very important for realizing reliable devices. In this investigation the influence of the temperature of diode on light parameters was studied. White light was obtained by coupling blue light of diode with yellow phosphors: YAG:Ce 3+ and GYAG:Ce 3+ with nitride. For three values of the temperature of LD’s stem, regulated by Peltier module, CCT, CRI and chromaticity coordinates were measured by spectroradiometer. The importance of emission characteristics of materials was shown. Subsequently, the influence of temperature on laser diode intensity was investigated for 120 hours. This experiment was repeated for different levels of current and temperature. Finally, the steady state of thermal finite element analysis was performed to reveal the distribution of the temperature. The analysis showed the importance of heat sink and also temperature control.
In past few years InGaN-based semiconductors have attracted much attention for application in solid-state light sources. Recently, their usage is constantly increasing in worldwide market. High-brightness white LEDs have been used due to their size, long life and energy saving. Also, white light can be generated from LEDs in three major ways: mixing blue, red and green diode [
Unfortunately, LEDs used in these devices suffer a loss in external quantum efficiency as operating current increases, known as a droop. This may lead to shift in a peak emission wavelength and broadening of the emission spectrum. Variations of spectrum of LED and phosphor change the ratio of the light emitted by each component, resulting in shift in the color coordinates of the emitted light and decrease in overall device efficiency [
An alternative to light-emitting diodes for obtaining white light can be blue laser diode (LD). In contrast to LEDs, they do not exhibit efficiency loss. The output power and external quantum efficiency of laser diode increase linearly with current and maintains color stability of laser emission. Additionally, laser-based devices can be operated in reflection mode, allowing for the phosphor to be placed on a reflection substrate that may also act as a heat sink to effectively dissipate heat away from the phosphor.
Common problem of SSL based on semiconductors is their heating [
The white light was generated by coupling blue laser diode and two different types of materials. For the production of the materials a two-part high optical transparency silicon was used as a matrix. Silicon was filled in with different powders to obtain two types of material: YAG:Ce3+ (NYAG4354 Intematix) powder and mixture of green GYAG:Ce3+ (GYAG3856-01-13 Intematix) and nitride in ratio 4.5:1. Phosphors were incorporated into silicon plate of diameter 10 mm and thickness of 1mm. The materials were prepared in ICMCB laboratory in Bordeaux, France. As an excitation source the InGaN blue laser diode OSRAM TB450B 450 nm was used. Laser was thermally coupled with a Peltier module, which was connected to PID temperature controller TEC 2000 from Thor Labs. A sensor AD590 was placed under a stem of the laser to measure the temperature. The measurements were performed in an optical sphere (
and chromaticity coordinates. The temperature of LD was fixed at three different levels: first at 25˚C, then at 45˚C and finally 55˚C. For each, we measured firstly the spectrum of the LD itself, and then the spectrum of laser’s light converted by the phosphor and its parameters.
Below, on
This variation is connected to the characteristics of light emission by materials. The optimal emission for YAG can be found at level of 442 nm (
If the emission of converted light drops, at the output of our light source we find more blue light. It is caused by the drop of the efficiency of the phosphor emission. In emission characteristics we are not anymore at the maximum, which means that more blue light passes through plate. Rise of blue color can cause perturbation of human circadian rhythm [
The color rendering index (CRI) of light source defines the ability of the source to render the true colors of an object [
Temperature | YAG:Ce3+ | GYAG:Ce3+ with nitride |
---|---|---|
25˚C | 65 | 85 |
40˚C | 65 | 84 |
55˚C | 65 | 84 |
Since the efficiency of converted material changed, CCT and chromaticity coordinates should be affected. In
Emission by YAG:Ce3+ does not change radically. The small shift into green can be noticed. In contrary, the shift into blue is clearly seen on the figure above for GYAG:Ce3+, which is caused by drop of the emission efficiency.
Variation of the diode temperature not only has an influence on light parameters. Operating on high value of temperature can lead to reduction of life duration of semiconductor. To obtain a long lifetime of a laser, appropriate cooling is required. The important factor to prolong diode operation is junction temperature not exceeding 100˚C. In our study, we investigated the temperature distribution of the operating diode by performing Finite Element Method Analysis.
The Laplace heat equation was numerically solved for this thermal analysis [
Q c = I V (2)
I = W L J (3)
V = h l J σ (4)
where Qc is the heat generation, V the voltage drop, J the density of current, W the stripe width, L the cavity length, h the high of the layer, σ the electrical conductivity.
Below, the model designed in FEM software is presented (
Based on thermal conductivity of each material [
The maximum temperature reached on the chip and substrate was 190.15˚C, which exceed the recommended operating junction temperature almost twice.
Temperature | YAG:Ce3+ | GYAG:Ce3+ with nitride |
---|---|---|
25˚C | 6653 | 3927 |
40˚C | 6354 | 4974 |
55˚C | 6254 | 4974 |
The heat dissipation for a packaged semiconductor is mainly by conduction. From the heat source, through the substrate, finally to heat sink. Heat can be also removed by convection via a gas, as in surrounding air. Although effective heat transfers from source to ambient is an aim, it is more usual to define heat dissipation by considering inverse of the heat flow―thermal resistance between these two points. It is calculated by equation:
R t h = Δ T Q t o t a l (5)
where ΔT is a difference of the temperature between junction and ambient, Qtotal is a total heat generation. Calculated Rth was 46 K/W. However, the thermal resistance value calculated by this equation is a lumped value and will differ from depending on the special distribution of the heat source. Subsequently, we simulated the conditions, where we fixed our temperature of stem on 25˚C, and then 55˚C respectively, which represents thermal coupling with Peltier module.
The thermal resistance, while connected thermally to the copper plate, results in the lower value of 15.5 K/W (Equation (5)).Studies show [
The dependency of the temperature on the emission of a laser, which results in variation of light parameters, when used as excitation source in down converted phosphor method was investigated.
To study the influence of this changing on white light parameters, the phosphors absorption and emission characteristics were performed. Investigation shows that shift of wavelength, which is a result of temperature rise has an influence on light parameter. In case of our blue laser diode coupled with YAG:Ce3+, it came out that more optimal results, closer to “daylight” parameters, are achieved when the shift to higher wavelengths appeared. On the other hand―when the laser was coupled with GYAG:Ce3+, we observed deterioration of the results. This part of studies showed the importance of keeping the temperature of diode on a constant level and finding an optimum of light conversion by phosphor.
From thermal analysis in FEM software we found out, that operating without cooling system leads to exceeding the junction temperature almost two times. Also, the studies show, that the material of stem in used laser diode could be optimized. We calculated the value of thermal resistance between heat generator and air for conditions with and without cooling module. Investigation proves, that without appropriate cooling module, the value of thermal resistance can increase even three times, which leads to fast diode degradation.
Czesnakowska, A., Ledru, G., Glorieux, B. and Zissis, G. (2018) Thermal Analysis of Blue Laser Diode for Solid State Lighting Application. Optics and Photonics Journal, 8, 40-49. https://doi.org/10.4236/opj.2018.83005