Quantum dot hybrid LED

Glossary

What is Beer-Lambert's law?

Beer-Lambert's law is the basic law of light absorption and applies to all electromagnetic radiation and all light-absorbing substances, including gases, solids, liquids, molecules, atoms and ions.

The proportion of light absorbed by the transparent medium is independent of the intensity of the incident light; each equal weight of light is absorbed by every equal weight on the optical path.

Recently, a researcher at Ghent University in Belgium found a hybrid LED using quantum dot technology consisting of a blue LED light source and a non-contact hybrid fluorescent film material. The fluorescent film is mainly composed of red cadmium selenide and cadmium sulfide (CdSe / CdS) quantum dot material and europium (Eu) doped green fluorescent material. Due to the quantum dot structure has a good light conversion for light conversion, emission spectrum adjustable, narrow band characteristics. Therefore, the researchers believe that this hybrid structure has good performance in terms of cost and efficiency.

Research methods

The researchers used a red cadmium selenide and cadmium sulfide (CdSe / CdS) quantum dot material and a green SrGa2S4: Eu2 + (STG) material to make a fluorescent film, and dissolved these materials in methyl ethyl ketone (containing a certain amount of toluene ) Solution. At the same time, the mixed solution will be drop-cast on a thin glass plate having a diameter of 18 mm.

For the fluorescent film, the researchers designed a variety of different fluorescent film structure to evaluate the performance of LED.

1. Simple red-green mixed fluorescent film, structure such as | RG |;

2. Separate fluorescent film using two glass substrates (mid-filled with air). Structures such as | R || G | or | G || R |;

3. Using a structure similar to the second one, but filled with ethylene glycol in the middle, this structure can alleviate the problem of index matching, such as | R || G | or | G || R |.

research content

Mixed fluorescent film structure

As shown in FIG. 1a, the color temperature of the | RAGA | hybrid LED is 7082K and the coordinates in CIE (X, Y) are (0.299, 0.345) with an internal quantum efficiency (IQE) of 80 % (| R | alone 71%, | G | 93%).

At the same time, researchers found that the light intensity attenuation of this hybrid structure is different from that of a single material. According to Figure 1d, there is almost no change in the attenuation of the STG material, but the emission intensity of the quantum dots is guaranteed to increase during the entire attenuation. This comes mainly from the direct excitation of blue light sources and the indirect excitation of STG materials.

Fig.1 Luminescence spectra of hybrid white LEDs

 

a | RA || GA |

 

b | RA || GA | and | RA || GA | structures

 

Separated fluorescent film structure

By comparing Figure 1b1c, the researchers found that there was a strong sequence of relationships between these separate structures. When the quantum dot material is near the side of the light source (| RA || GA |), the luminescence intensity of the STG material is strongly suppressed. Similarly, if the STG material is on the side of the light source (| GA || RA |), the luminescence intensity of the quantum dot material is also suppressed. This phenomenon is mainly due to Beer-Lambert's law, the bottom of the light conversion materials in the blue light source will usually have the highest light-emitting intensity.

The ethylene glycol-filled | RA || GA | and | GA || RA | structures have a higher red light emission intensity and a lower green light intensity than the air-filled separated structure due to more A good index matching results in an increase in the optical coupling of the STG material, with more green light converted to red light. At the same time, with this high refractive index matching condition, the green light can enter the quantum dot material from any angle, thereby increasing the average path length of light in the quantum dot material.

in conclusion

The researchers found that for a fixed fluorescent film structure, the luminous efficiency of a hybrid LED depends on the intermediary material in the phosphor layer (as mentioned previously for ethylene glycol). Among them, the quantum dot material for green light reabsorption and fluorescent powder crystals for the red self-absorption inhibition is the main factor. Among the three kinds of fluorescent film structures, the most economical structure is a separate structure with a high refractive index matching.

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