Recently, scientists at Ghent University in Belgium have made a breakthrough in the field of LED technology by developing a hybrid LED that incorporates quantum dot technology. This innovative LED combines a blue LED light source with a unique hybrid fluorescent film. The fluorescent film primarily consists of red cadmium selenide and cadmium sulfide (CdSe/CdS) quantum dot materials, along with green fluorescent materials doped with europium (Eu). Given the exceptional light conversion efficiency, spectral tunability, and narrow spectral bandwidth of quantum dots, researchers believe this hybrid design holds significant promise in terms of cost-effectiveness and performance.
The research team employed red cadmium selenide and cadmium sulfide quantum dots alongside green SrGa₂S₄:Eu²⺠(STG) materials to fabricate their fluorescent films. These materials were dissolved in a methyl ethyl ketone solvent containing a specific amount of toluene. The resulting mixture was then applied to a small glass substrate with a diameter of 18mm using a drop-casting technique.
To assess the performance of the LEDs, the researchers experimented with several different fluorescent film configurations:
1. A simple mixed fluorescent film with a structure resembling |RG|;
2. Separate fluorescent films on two glass substrates, separated by air, structured as |R||G| or |G||R|;
3. Similar to the second configuration, but with ethylene glycol filling the gap between the substrates, which helps mitigate issues related to index matching, structured as |R||G| or |G||R|.
In terms of hybrid fluorescent film structure, Figure 1a illustrates that the |RAGA| hybrid structure produces a color temperature of 7082K with coordinates (0.299, 0.345) in the CIE (X,Y) system, boasting an internal quantum efficiency (IQE) of 80%. Individually, the |R| component achieved an IQE of 71%, while |G| reached 93%.
Additionally, the researchers observed distinct differences in light intensity attenuation across these hybrid structures. According to Figure 1d, the STG material showed minimal attenuation, whereas the quantum dots exhibited sustained increases in luminous intensity throughout the decay process. This phenomenon can be attributed to the direct excitation of the blue light source and the indirect excitation of the STG material.
When examining the separate fluorescent film structure, the research revealed a pronounced ordering effect. In the |RA||GA| configuration, the luminous intensity of the STG material was significantly suppressed when the quantum dot material was positioned closer to the light source. Conversely, placing the STG material nearer to the light source in the |GA||RA| configuration suppressed the quantum dot's luminous intensity. This behavior aligns with the Beer-Lambert Law, which predicts that the bottom light-converting material typically experiences the highest luminous intensity after exposure to a blue light source.
Comparing air-filled discrete structures to those filled with ethylene glycol, the latter demonstrated higher red light intensity and lower green light intensity. This improvement arises from better index matching, enhancing the optical coupling efficiency of the STG material and facilitating greater conversion of green light into red light. Furthermore, under such high refractive index matching conditions, green light can penetrate the quantum dot material from multiple angles, thereby increasing the average path length of light within the quantum dot material.
In conclusion, the study found that the luminous efficiency of a hybrid LED depends significantly on the intermediate material in the phosphor layer, such as the aforementioned ethylene glycol. Key factors influencing efficiency include the re-absorption of green light by quantum dots and the suppression of red light self-absorption by fluorescent powder crystals. Among the three fluorescent film structures tested, the most economical and efficient option was the separate structure with high refractive index matching.
Intern Editor: Yang Zhiwei
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