Recently, scientists at Ghent University in Belgium have uncovered a novel hybrid LED utilizing quantum dot technology. This LED combines a blue LED light source with a non-contact hybrid fluorescent film material. The fluorescent film primarily features red selenized cadmium and cadmium sulfide (CdSe/CdS) quantum dot materials alongside green fluorescent substances doped with europium (Eu). Given the quantum dot structure’s excellent light conversion rates, luminescence spectrum adjustability, and narrow spectral bands, researchers believe this hybrid design offers significant advantages in terms of both cost and efficiency.
The research team employed red cadmium selenide and cadmium sulfide (CdSe/CdS) quantum dot materials along with green SrGa2S4:Eu2+ (STG) materials to create fluorescent films. These materials were dissolved in a methyl ethyl ketone solution (with a specific quantity of toluene) and then applied onto a thin glass disc measuring 18mm in diameter through a drop-casting method.
To evaluate the performance of these LEDs, researchers experimented with various fluorescent film configurations. One configuration was a simple red-green mixed fluorescent film structured as |RG|. Another involved separate fluorescent films placed on two glass substrates with air filling the gap, structured as |R||G| or |G||R|. A third configuration mirrored the second but used ethylene glycol instead of air to fill the gap, improving index matching, structured similarly as |R||G| or |G||R|.
In their study, the researchers examined hybrid fluorescent film structures. As illustrated in Figure 1a, the color temperature of the |RAGA| hybrid structure LED was 7082K, with coordinates in CIE(X,Y) at (0.299, 0.345), and an internal quantum efficiency (IQE) of 80%. Individual components showed efficiencies of 71% for |R| and 93% for |G|.
Additionally, they observed distinct differences in light intensity attenuation across these structures. Figure 1d shows that while the STG material experienced minimal changes in light intensity, the quantum dots exhibited a consistent rise in luminous intensity throughout the decay process. This was attributed to the direct excitation of the blue light source and the indirect excitation of the STG material.
Further analysis of the separate fluorescent film structures revealed a clear 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 reduced. Conversely, placing the STG material near the light source (|GA||RA|) suppressed the luminous intensity of the quantum dot material. This phenomenon aligns with the Beer-Lambert Law, where the bottom light-converting material typically exhibits the highest luminous intensity after exposure to a blue light source.
When comparing air-filled to 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. This also led to more green light being converted into red light. Under such high refractive index matching conditions, green light could enter the quantum dot material from any angle, 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 previously mentioned ethylene glycol. Key factors include the quantum dot material's re-absorption of green light and the inhibition of red light self-absorption by fluorescent powder crystals. Among the three fluorescent film structures tested, the most cost-effective structure was the separate structure with high refractive index matching.
Intern Editor: Yang Zhiwei
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