The Beer-Lambert law is a fundamental principle governing the absorption of electromagnetic radiation by various materials, including gases, solids, liquids, molecules, atoms, and ions. According to this law, the proportion of light absorbed by a transparent medium remains constant regardless of the intensity of the incoming light. Each equal thickness of the medium absorbs the same fraction of light along its optical path.
Recently, researchers at Ghent University in Belgium have developed a hybrid LED using quantum dot technology. This LED combines a blue LED light source with a non-contact hybrid fluorescent film. The fluorescent film contains red cadmium selenide and cadmium sulfide (CdSe/CdS) quantum dots, along with green fluorescent material doped with europium (Eu). Quantum dots offer excellent light conversion rates, luminescence spectrum adjustability, and narrow spectral bands. Consequently, the researchers believe this hybrid structure offers significant advantages in terms of both cost and efficiency.
The research involved creating fluorescent films using red CdSe/CdS quantum dots and green SrGa₂S₄:Eu²⺠(STG) materials, which were dissolved in a solution of methyl ethyl ketone containing a specific amount of toluene. These Solutions were then applied to thin glass pieces with a diameter of 18 mm using a drop-casting method.
To evaluate the performance of these LEDs, the researchers designed several different fluorescent film structures:
1. A simple red-green mixed fluorescent film with a structure like |RG|.
2. Two separate fluorescent films separated by air, structured as |R||G| or |G||R|.
3. Similar to the second design, but with ethylene glycol filling the gap between the films to improve index matching, structured as |R||G| or |G||R|.
One of the key findings was related to the hybrid fluorescent film structure. As illustrated in Figure 1a, the color temperature of the |RA||GA| hybrid structure LED was 7082K, with coordinates in CIE(X,Y) of (0.299, 0.345), and an internal quantum efficiency (IQE) of 80% (individual |R| was 71% and |G| was 93%).
The researchers also noted differences in light intensity attenuation between this hybrid structure and the individual materials. Figure 1d shows that the attenuation of the STG material remained stable, while the luminous intensity of the quantum dots increased throughout the decay process. This is primarily due to the direct excitation of the blue light source and the indirect excitation of the STG material.
Further analysis of the split fluorescent film structure revealed a strong ordering effect. When the quantum dot material was positioned closer to the light source (|RA||GA|), the luminous intensity of the STG material was significantly suppressed. Conversely, when the STG material was closer to the light source (|GA||RA|), the luminous intensity of the quantum dot material was also suppressed. This phenomenon aligns with Beer-Lambert's law, where the bottom light-converting material typically exhibits the highest luminous intensity after being irradiated by a blue light source.
Comparing air-filled and ethylene glycol-filled discrete structures, the latter showed higher red light intensity and lower green light intensity due to better index matching, which increased the optical coupling rate of the STG material. Under these conditions, green light could enter the quantum dot material from multiple angles, thus increasing the average path length of light within the quantum dot material.
In conclusion, the researchers determined that for a fixed fluorescent film structure, the luminous efficiency of a hybrid LED depends on the intermediate material in the phosphor layer, such as the aforementioned ethylene glycol. The re-absorption of green light by the quantum dot material and the self-absorption of red light by fluorescent powder crystals are major factors influencing efficiency. Among the three fluorescent film structures, the most cost-effective option is the separate structure with high refractive index matching.
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