LIGHTING FEATURE
you have to be able to survive ‘on chip’ conditions’
Sung-min Jung, an academic at Cambridge University working on the development of QD-LEDs for lighting. “This is a major property of quantum dots.” Varying the quantum dot size is simple
in terms of their fabrication and requires little more than extra growth time for the QD. The composition of the QDs in terms of the semiconductor material from which they are fabricated is also identical. This means for materials like cadmium selenide (CdSe), a common material from which QDs are made, a 2 nm QD will emit in the red, but after a slightly longer fabrication time, a five nanometre CdSe QD will emit in the green. This is a big advantage over, say, phosphors, whose emission and stability are specific to material type, meaning that spectral flexibility is limited. As a result, QDs also provide a specific
advantage in the so-called ‘green gap’, an area of the spectrum well known to optical physicists, where there is a lack of high quality, high efficiency LED technology in the green region of the spectrum. In addition to tunability, QDs also offer
advantages in terms of efficiency, especially towards the red end of the spectrum. Typically, when using phosphor-based LEDs for red light, an inability to control the bandwidth of the output emission spectrum of the phosphor means that some infrared light is emitted. For visible LEDs, this light is effectively wasted. For QD-LEDs, a narrow emission spectrum means that a sharp cut-off can be obtained in the red, providing higher efficiencies in lighting- based applications. QD-LEDs also provide a route to LEDs
visible light upon excitation from deep blue or ultraviolet radiation. The phosphor coating on a blue LED enables energy down-conversion, taking energetic blue photons and instead converting them to less energetic red or green ones. By tuning the properties of a given phosphor, the output light can also be carefully tuned. But the process is inefficient: in converting from blue to red, for example, phosphors waste energy. QD-LEDs are not all that different. The QDs are also situated on top of an LED chip, in a slurry with a silicone
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Quantum dot lighting could transform commercial lighting, such as in offices
encapsulating material. The LED drives the emission process in the QD. There are, however, significant advantages to using quantum-dots over conventional phosphor- based LEDs. Perhaps most importantly, QDs offer the option of wide spectral tunability. “By changing the particle size, the colour or peak wavelength can be changed easily with the quantum dot technology,” says Dr
with high colour rendering index (CRI) values. This means that a measure of the colours illuminated by the QD-LED lighting performs well in comparison to daylight (CRI = 100). This is what Jung and his colleagues at
the University of Cambridge demonstrated in a recent paper1
which developed a new
device using QD-LEDs for generating white light. The team looked at smart lighting, which can be controlled by the user and allows lighting to modulate based on mood and circumstance. They created a device that combined QD-LEDs with system-level colour-control and a colour optimisation algorithm. By using multiple layers of
‘In lighting, the consensus is that
g November 2022 Electro Optics 15
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