3 mm mm33 mm FEATURE LIGHTING
Quantum dots of different sizes display different colours of emission
Snapshots of monochromatic devices Red
Snapshots of monochromatic devices Red
Green Green
3 mm 3 mm
Snapshots of the four monochromatic QD-LEDs g
quantum dots of different sizes, ranging from three to 30 nanometres, they were able to change colour and accurately mimic white light. In general, when generating white light, a
mixture of red, green and blue is used, but the Cambridge team found that by using a larger range of colours, white light could be more accurately represented. “We are expecting to make people more
comfortable in daylight,” Jung continues. “And we focused on the circadian rhythm of the human during daytime.” The study showed excellent CRIs for
daylight of up to 92 per cent, with a wide range of shades of white light and excellent flexibility in its output colour range, providing colour temperature variation from 1612K to 8902K, using only four primary quantum dots. “The main motivation that led us to
start this programme is that LED lighting is commonplace, but there is room for improvement in the spectrum of light that is available and the programmability of that light, it’s customisation,” echoes Professor Gehan Amaratunga, co-author and co-PI on the Cambridge study. “And that is what we foresaw as the next step, and it’s easier to achieve with QD because you can vary the colour they emit by size.” “In terms of colour variation and CRI, QD-LEDs have married the two,” Jung concludes. The team refer to this as a world-first
smart white lighting system. But many research questions remain. “We need to ensure we get the power
down,” Amaratunga adds. “LED lighting is considered very good because of its energy
16 Electro Optics November 2022
saving, but with multicoloured lighting the energy goes up. The main driver now is to reduce power consumption.” To do this, the team must address the injection of current into their device. QDs emit light through a process called electron-hole recombination. The QDs are sandwiched between several layers, and a pair of electrodes inject charge carriers: electrons and holes. These electrons and holes recombine in the QD layer giving out light. The team now are looking to make this process more efficient. “This will also help us with reliability,” Amartunga says. Amaratunga also stresses that one of
the challenges with QD-LED systems is the difference in devices with an academic shelf-life and commercially viable solutions. He reiterates that industry expertise is responsible for driving academic growth, particularly in this field of research, crediting his co-author on the study Professor Jong-Min Kim. “Our success in achieving what we
did are in no small part due to [Kim] and his team, who have experience from industrialisation from Samsung. We’ve done something new, but it is built on the back of industrial knowledge,” he says. “To have repeatability and confidence in what you achieve without the cross fertilisation of industrial knowledge and academic knowledge in the applied science area is not possible.”
Market leader Quantum Dots enable high-colour rendering and very energy-efficient LED components for the next generation of lighting applications, Ams Osram writes on its website.
The company, which is the only player in
the lighting industry to sell a commercial product featuring QD-LEDs, has a small number of relevant products, with their newest model being the Osconiq E 2835 CRI 90 QD, a hybrid-technology which uses an in-house developed QD-phosphor hybrid. This enables their CRI 90 product to deliver ‘outstanding efficacies, even at high colour rendering indexes.’ The creation of such a product is no
mean feat. The development of commercial QD-LEDs faces significant challenges, largely because of device stability considerations. “There are strong reasons why you would
want to use QD-LEDs in lighting just as in displays. But in display technology the temperatures are lower, and the light flux is lower because the QDs are removed from the LED chip itself,” says Peter Palomaki, a consultant specialising in the use of QDs in industry, for display and lighting applications. “In lighting, the consensus is that you have to be able to survive ‘on-chip’ conditions: the QD directly is on top of the LED and silicon, where phosphors usually reside on an LED.” QD materials easily degrade when in
J. Mater. Chem. C, 2022, 10, 10728–10741 J. Mater. Chem. C, 2022, 10, 10728–10741
such ‘on-chip’ conditions. On chip, the QDs are exposed to high temperatures (sometimes > 100°C); exposure to the dampness and humidity of the atmosphere; and requiring a high flux of light into the QD, both in terms of absorption and re- mission. “These three things can cause
degradation of the QD, often by way of its surface,” Palomaki continues. QDs are nanoparticles with many atoms at or
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J. Mater. Chem. C, 2022, 10, 10728–10741
J.J. Mater. Chem. C, 2022, 10, 10728–10741 Mater. Chem. C, 2022, 10, 10728–10741
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University of Cambridge
University of Cambridge
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