The concept of the Beer-Lambert law is fundamental across all forms of electromagnetic radiation and applies universally to materials that absorb light, whether they are gases, solids, liquids, molecules, atoms, or ions. It states that the proportion of light absorbed by a transparent medium is independent of the intensity of the incoming light, meaning that each equal thickness segment of a medium will absorb light at the same proportional rate along its optical path.
Recently, a team at Ghent University in Belgium has made a breakthrough in hybrid LED technology, leveraging quantum dot innovations. Their design features a blue LED light source paired with a hybrid fluorescent film. This film combines red cadmium selenide and cadmium sulfide (CdSe/CdS) quantum dots with green fluorescent material doped with europium (Eu). Given the quantum dot's excellent light conversion efficiency, tunable luminescence spectrum, and narrow spectral bandwidth, the researchers believe this hybrid model could offer both cost-effective and highly efficient Solutions.
In their research methodology, the scientists crafted fluorescent films using red CdSe/CdS quantum dots and green SrGa₂S₄:Eu²⺠(STG) materials, dissolving them in a methyl ethyl ketone solution containing a specific amount of toluene. They then applied this mixture onto a thin glass disc with an 18mm diameter via drop-casting. To assess the performance of these fluorescent films, various structures were designed:
1. A simple combined red-green fluorescent film structured as |RG|.
2. Two separate fluorescent films 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 films to improve index matching, structured as |R||G| or |G||R|.
Regarding their findings, the hybrid fluorescent film structure demonstrated impressive results. With a |RAGA| structure, the LED emitted white light at 7082K, with CIE coordinates of (0.299, 0.345) and an internal quantum efficiency (IQE) of 80%, where individual |R| had an IQE of 71% and |G| reached 93%.
Interestingly, the researchers noted differences in light intensity attenuation among the hybrid structures. While the STG material showed negligible attenuation, the quantum dots exhibited a consistent rise in luminous intensity throughout the decay process, thanks to the direct excitation from the blue light source and indirect excitation of the STG material.
Analyzing the separate fluorescent film configurations, the team observed a clear order effect. When quantum dots were placed closer to the light source (|RA||GA|), the STG material’s luminous intensity was significantly suppressed. Conversely, positioning the STG material nearer to the light source (|GA||RA|) suppressed the quantum dots' luminous intensity. This phenomenon aligns with the Beer-Lambert law, where the bottom light-converting material typically exhibits the highest luminous intensity upon blue light exposure.
Comparing air-filled versus ethylene glycol-filled discrete structures, the latter provided better index matching, enhancing the optical coupling rate of the STG material and converting more green light into red light. Additionally, under such high refractive index conditions, green light could enter the quantum dot material from multiple angles, increasing the average light path within the quantum dots.
In summary, the study concluded that for a fixed fluorescent film structure, the luminous efficiency of a hybrid LED largely depends on the intermediate material in the phosphor layer, such as the use of ethylene glycol. Key factors include the quantum dots' re-absorption of green light and the self-absorption inhibition of red light by fluorescent powder crystals. Of the three fluorescent film structures, the most economical and effective option is the separate structure with high refractive index matching.
This research opens new possibilities for improving LED efficiency while reducing costs, offering promising applications in lighting and display technologies.
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