Quantum-dot LEDs: the daylight of the future

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Anita Chandran on the commercial potential of quantum lighting

 

 

 

 

 

 

 

 

Many of us are familiar with the light-emitting diode (LED), a technology that now provides the most efficient lighting option to date. LEDs in a diverse range of colours are widely used for lighting across application sectors. But over the course of the last 10 years, one prospective technology has threatened to rock the boat of LED lighting: the quantum-dot LED (QD-LED). 

QD-LEDs are based on quantum-dot technology, comprised of quantum dots (QDs), semiconductor particles just a few nanometres in size. Because of their small domain, their behaviour is governed by quantum mechanics. By ‘exciting’ quantum dots using electrical currents or driving radiation, they can be induced to emit light, the colour of which is based on the size of the QD. 

The QD-LED is already a leading player in the field of display technology, with companies such as Samsung carrying commercial television ranges based on QD-LEDs. Competition is rife between microLEDs, organic LEDs (OLEDs) and QD-LEDs when it comes to displays. The QD-LED has yet to conquer the lighting sector, and at present only one company offers a commercial QD-LED for lighting-based applications: Ams Osram. But recent developments in the technology have suggested the QD-LED may yet make big strides in the lighting industry.

Quantum advantage 

Most modern LEDs work using a blue LED backlight and a phosphor coating; phosphors are solid materials that emit 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 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 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. 

Image credit: University of Cambridge

QD-LEDs also provide a route to LEDs 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 paper 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 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 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 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 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 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 

Sponsored: Lighting metrology – reviewing the tools of the trade

 

Konica Minolta Illuminance Photometer

Pro-Lite serves customers working in lighting and illumination with a complete solution to measuring the brightness, colour, colour rendering and spatial distribution of lighting products in the architectural, automotive, avionics and many other sectors. 

Light is all around us (as The Troggs nearly sang), but to verify that the light level and colour are up to scratch, we rely upon specialist photometric and colorimetric test equipment. In this case study, we’ll review the tools of the trade used in light metrology, in both laboratory and field environments.

Let’s start with the simple lux meter, more correctly known as an illuminance photometer. This is used to measure the amount of light that’s illuminating a surface and is about as simple as light metrology gets. You can purchase a “lux meter” app for your smartphone for under £1, but to be brutally honest, you’d be better off investing your money in a packet of crisps. The lux metre app is simply fancy software that exploits the camera sensor in your phone. Unfortunately, your camera sensor neither matches the spectral sensitivity of the human eye (the “photopic response”) nor is it equipped with the cosine diffuser necessary to correctly scale off-axis illumination. So while a modern smartphone is many things, a lux meter it is not. 

Good quality illuminance meters tend to cost in the many hundreds of pounds and are characterised by a photopically filtered sensor fitted with a good quality cosine diffuser. An example of the state-of-the-art is the Konica Minolta T-10A, stocked by Pro-Lite. Just remember that illuminance varies with the distance from the light source (this is called the “inverse squared” relationship) and you should be careful to stand behind the lux meter, especially when wearing light coloured clothing that can reflect stray light into the photometer. How strange it is that we photometrists like to ply our trade in the dark!

A variation on the theme is the illuminance colorimeter, but whereas the simple lux meter just records illuminance in lux, the colorimeter deploys three sensors that are spectrally matched to the tristimulus response of the human eye in order to measure chromaticity and correlated colour temperature. This is important if your “human centric” lighting needs to be tuned to cool white to keep you awake, or warm white to help send you to sleep. Step forward the CL-200A colorimeter, also from Konica Minolta.

From hand-held light meters to what is most definitely not even remotely portable – the goniophotometer. When planning a lighting installation in a shop or office, designers will exploit so-called “photometric files”. These are machine readable datafiles that contain the candela luminous intensity values over all angles from a luminaire, and are usually assigned either the .ies or .ldt file extensions. These invaluable pieces of information are created using a piece of laboratory equipment called a goniophotometer. The light source under test is mounted on a two-axis motorised motion platform and our old friend the lux meter records the illuminance over a range of angles. The photometric file so produced can be opened in lighting design software and the performance of the luminaire modelled in different sized rooms. Interestingly, the software will even take into account the reflectance of the floor, walls and ceiling. Hint – for the best performance, avoid black carpet and paint! Pro-Lite supplies the professional-grade goniophotometers made by SSL Resource in Finland.

We cannot neglect to mention the good old integrating sphere. First invented in 1894 by German engineer Richard Ulbricht, but based on a theory published in 1892 by English scientist W E Sumpner, the integrating sphere couples an illuminance photometer to a hollow sphere painted matte white. Together, these allow you to integrate and measure the total luminous flux (lumens) of a lamp placed within the sphere. Pro-Lite supplies the integrating sphere photometers made by Labsphere.

Returning to a more recent technology, the humble filter photometer or tristimulus colorimeter can be replaced by a spectroradiometer, and this adds the ability to measure the colour rendering of a light source and also to measure the radiometric performance of UV and horticultural lighting products. Although more expensive, a spectroradiometer can be relied upon for more accurate readings and some models – notably the Specbos from German firm JETI – can be used to measure both the illuminance from lighting but also the luminance (brightness) from a display.

Our tour of light measurement equipment ends with what for many is the future of photometry – the imaging photometer or colorimeter. This is a sensitive CCD or CMOS based camera adapted to the photopic or colorimetric spectral response of the human eye and which records 2D luminance, illuminance or colorimetric images. As such, it is the direct machine analogue of the human vision system and is proving popular in all areas of light metrology, from evaluating the beam pattern of automobile headlamps to checking that your new smartphone displays the right white point and doesn’t suffer from any dead pixels or other blemishes. Pro-Lite supplies the imaging photometers made by Canadian firm Westboro Photonics. 

Further information

www.pro-lite.co.uk/File/light_measurement_overview.php

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