FEATURE LIGHTING
‘By changing the particle size, the colour or peak wavelength can be changed easily with quantum dot technology’
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near the surface, and photooxidation can easily cause QDs to go dark. Combined with a high manufacturing cost and the fact that their component LEDs can last between 10 and 25 years, degradation and long-term operational stability present significant challenges for those trying to commercialise QD-LEDs. So far, Ams Osram is the only company to have circumvented stability considerations to produce a commercially viable device. To do this, the company acquired Pacific Light Technology (PLT) in 2018. PLT’s main focus was developing red quantum dots in on chip conditions for LED lighting applications. This began with developing a new fabrication process for QDs, producing them as powders that could easily be integrated into standard manufacturing processes. It was in collaboration with PLT that the first QD-LED for lighting applications, the Luxeon 3535L HE Plus, was released by Lumileds in 2018, demonstrating that red quantum dots can be integrated into on- chip conditions. With PLT’s technology, the quantum
dots are encapsulated to protect them from moisture and humidity, so they can withstand the harsh conditions of ‘on chip’ operation inside the LED component, Ams Osram says on its website. While the commercialisation of the device is an achievement, Ams Osram themselves have pointed to some limitations through conferences and other discussions of their QD-LED products. When considering the output spectrum for their broadband QD- LEDs, you can easily note that the peak produced by the quantum dots is masked in a broader band phosphor peak. The quantum dot is only used for red, while other phosphors must be relied on to produce other colours. In terms of competition, this is still a
win for Ams Osram, even compared to using standard phosphor-based LEDs for broadband white light generation. This is partly because they get higher quality red light, than other competing applications, with a high R9 value. The R9 value describes the quality of a saturated red colour, descriptive of how realistic reds
18 Electro Optics November 2022
Typical output spectrum from the Ams Osram Osconiq E 2835 CRI 90 QD showing peaks in the red at around 630nm, alongside peaks around 400-450nm
look under this lighting. The efficiencies they obtain are also high, with the company referring to their QD-LED system as “[closing] the efficiency gap between CRI 80 and CRI 90”, lighting products with typically high colour rating index. “It’s one of those colours where it’s difficult to make it look good,” continues Palomaki “But with those LEDs they do look better. And they’ve shown that they do that while maintaining a high efficacy. Usually there is a trade-off, but QDs have eliminated this.”
Tough competition With the attractive possibilities that QD-LEDs have to offer the lighting community, it perhaps appears odd that there hasn’t been a surge in related research for improving and developing QD-LED lighting devices. Part of this is down to stiff competition from industry alternatives. For all their disadvantages, some phosphors perform extremely well for lighting applications. In particular, the red phosphor KSF, manufactured by General Electric, naturally provides an efficient, narrowband conversion from blue LEDs into the red. It emits a spectrum containing five narrow peaks centred around 630 nm, and it already performs well in an on-chip configuration. Since its advent, over 40 billion KSF containing LEDs have been sold worldwide, primarily to the display industry. Because of its ease of integration onto LED chips, it is also a major player in lighting. Research groups such as the white light
team at Cambridge are also not the first movers in smart white lighting. Philips, who has divested much of its product
offering to focus solely on lighting, offers a smart lighting product. Philips’ smart bulbs, known as Philips Hue bulbs, work using LEDs that can be controlled from a smartphone. The technology boasts 16 million colours of light and colour temperatures between 2,000K and 6,500K. Philips technology is also efficient as compared with standard LED systems. There is, however, space for QD-LEDs to edge them out in terms of efficiency, in systems like Cambridge’s. This is because, for phosphor-based LEDs, in converting from blue light into red or green light, you have to throw away energy. By directly controlling colour through QD size, it may be possible to improve overall device performance. But limitations on electro- optical conversion efficiency remain a stopping block to this benefit. “QDs are an alternative approach,”
Amaratunga reassures us, when asked about Philips Hue. “It’s an additive technology and gives the industry another avenue to pursue in having this type of customised lighting.” “Any technology QD or otherwise that helps improve the efficiency, consumer trust (for example in terms of reliability), and the uptake of energy efficient technology is good for everybody,” concludes Palomaki. So, when it comes to the future of QD-LEDs, it appears the technology is at the precipice of a revolution. As for when this revolution will happen, watch this space. EO
References 1. Samarakoon, C., Choi, H.W., Lee, S. et al. Optoelectronic system and device integration for quantum-dot light-emitting diode white lighting with computational design framework. Nat Commun 13, 4189 (2022). https://doi. org/10.1038/s41467-022-31853-9
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