Quantum Dots: a growing success story

Latest IDTechEx report analyses changing and expanding applications landscape

Quantum dots (QD) are a success story in displays but they are also finding applications in lighting, NIR/SWIR QD-Si hybrid photosensors, agriculture colour conversion films, and security tagging. IDTechEx Research has been analysing the market in a new detailed report 'Quantum Dot Materials and Technologies 2019-2029: Trends, Markets, Players'. It has outlined some of the findings in the following article.

Quantum dots in displays: roadmap

In displays, the main mode of integration is film type (enhancement film). The technology can be integrated into large displays (TVs) from 27-inches to 98-inches and prices range from $370 (55-inch) to $3k (82 to 90-inch). ​Some would argue that the net cost of adding QD enhancement film is now close to zero since QDs help to make brightness enhancement films redundant.

The way QDs are integrated into displays is now changing.

Some companies are seeking to develop QD films into QD-on-glass technology. Here, the QDs are coated directly on the light guide plate and, on one side, encapsulated in a thin-film. This will change the value chain at the expense of roll-to-roll (R2R) filmmakers. It will lead to thinner solutions since the additional substrate is eliminated and the QD layer itself can be made thinner.

The next evolution is for QDs in-pixel colour converters (often referred to as QD colour filters or QDCFs). The red and green QDs can be applied to sub-pixels by inkjet or via photolithographically-patterned QD photoresist. This will require high blue absorbance to ensure colour purity and thin layers. It will also require good dispersion at high loading levels into the resin.

The QDs must also survive the process. In the case of photopatterned resists, the QD must survive the soft bake, the etching, the hard bake and so on, with little or no quantum yield, FWHM (full width at half maximum), or emission wavelength change. In the case of inkjet, the QDs should be formulated into printable-inks, must print well within the black matrixes and must survive the curing (most likely thermal) process. According to IDTechEx, material suppliers, in close collaboration with QD suppliers, have developed good QD photoresists and inkjet inks. This is in an advanced stage.

The QDCF can be applied both to LCD and OLED. In LCDs, the need for an in-cell polariser is a major challenge, requiring technology development and new process adaption. For OLEDs, red and green QDs need to be inkjet-printed atop a continuously applied (un-patterned) blue OLED (now fluorescent, but this might change in time to TADF for efficiency gains if and when TADF or hyper TADF becomes commercial). The use of printing here may give a cost-effective process for achieving high-quality, large-sized QD-OLED hybrid displays that exceed the performance and cost of WOLEDs, which use standard colour filters. IDTechEx says that the use of printing will also represent critical strategic learning towards the ultimate goal of solution-processed emissive QLEDs.

QLED: ultimate display?

QLEDs are a major long-term topic of R&D. They can enable thin/flexible high-contrast, efficient, and wide colour gamut displays, but there are many challenges on the way. IDTechEx predicts that it will be some years before we see full-colour commercial products. A new QD toxicant-free chemistry is likely to be needed for efficient blue at the desired wavelength. Green and red InP QDs will likely require better shell coverage and graded alloying to eliminate internal CTE (coefficient of thermal expansion) mismatches. The right organic hole transport layers (HTL) with a sufficiently deep valence band will be needed to ensure a good charge balance. The electron transport layers (ETL), likely based on a printable metal oxide nanoparticle, will need to be optimised. And, of course, all production and scale-up processes will be engineered too.

Image Sensors

Lead sulphide QDs can be tuned across a wide range of wavelengths, enabling NIR (near infrared) or SWIR (short-wave infrared) sensing. Interestingly, they can be integrated with a silicon ROIC (read-out integrated circuit) to create a hybrid QD-Si NIR/SWIR image sensor. This could lead to high-res small-pixel silicon-based NIR/SWIR sensors, thus doing away with the need for the heterogeneous hybridisation of GaAs sensors with silicon ROIC.

The first generation of products is already on the market. A leading consumer electronics company was also active in this area. It had made significant acquisitions and was understood to have commissioned a UK QD supplier to scale up its production. This company has reportedly recently pulled the plug.

The promise of this technology remains strong but there are big challenges. Stability is a critical issue. Some QD- or device-level encapsulation will be required. The photostability is even more of an issue. Today, sensors can handle low light-level indoor conditions, but going outdoors in applications, such as cars, will require further developments and, potentially, breakthroughs.

Furthermore, product optimisation will be required. QD supply with high batch-to-batch consistency will be needed. The QD film will need to be cast and probably patterned. Curing is likely to be important to ensure good carrier transport but not the loss of yield due to too much compactness. This will likely require some in- or ex-situ ligand exchange. Nonetheless, this remains an exciting area with a promising roadmap for future development and improvement.

Lighting

Lighting is a high-volume and promising application. The focus has mainly been on red QDs because an efficient narrowband red at the right wavelength can boost the CRI without compromising efficiency.

The challenge has been to develop QDs with sufficient heat, humidity and light stability to survive conditions close to the LED. It will also be very helpful to render the QD as much of a drop-in solution as possible. For this to be possible, it needs to be mixed with other colour converters, e.g., phosphors, and to be processed using existing tools and procedures.

Early products are on the market. These are probably cadmium-based and are made stable using a silica shelling procedure. Some have also demonstrated sufficient stability with InP for low light level conditions and remote on-chip phosphors, but these are not yet commercially ready.

On-chip types can also be useful in displays. This is because they could replace the phosphors which are used in today’s LED-lit displays. The challenge here, too, is ensuring good humidity, heat and light stability. In displays, both green and red QDs might be required.

Other (phototherapy, solar, agriculture films, security tagging, etc.)

Many other applications are in the pipeline. QDs as colour conversion films in agriculture is an interesting one. The idea is that the film will modify the sun’s spectrum in a way that will boost growth and yield. Here, there is a debate as to whether narrow or broadband emitter will be best-suited, and the answer is likely to be plant-specific, with no universal one-size-fits-all solution. The first generation of products is close to being launched. These are likely to be based on broadband CIS QDs. Multiple field trials have been conducted to demonstrate the RoI and value proposition.

QD stability remains a challenge, especially as high sun exposure is expected and will probably require additional encapsulation layers. The competition here includes dye-based films and LEDs, e.g., magenta, lights. The former can be lower cost, but its spectrum is linked to chemistry and not particle size.

Another application is in security tagging. Companies are developing broadband graphene or carbon QDs that can be added, in very small traces, to liquids, including petroleum products, to act as liquid-level security taggants. Some are proposing the use of QDs in phototherapy. The idea is to use the QDs to modify the colour spectrum to meet a specific medical need. This technology could lead to comfortable, portable and wearable phototherapy solutions. There is still work underway on QD solar cells. This, however, remains a difficult value proposition in the medium term, unless there is a major breakthrough.

The QD market is changing. The use in displays is rapidly evolving. These transitions will enable many material innovation opportunities. There are many other non-display applications in the pipeline too. This will sustain a strong market for years to come concludes IDTechEx Research.