One might then be tempted to assume that QDs are now a stagnant technology with slow and unchanging commercial prospects. This assumption would however be very wrong.
Largely inspired from the IDTechEx Research report “Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players”, this article sets out to make this point, demonstrating that QDs have now entered a time of growth, and crucially, rapid technological change.
Quantum dots: transitions so far from the past to present
QDs’ first success beyond research uses came in the display industry. Here, first high-performance Cd-based QDs were adopted in LCDs either in edge-optic or film-type implementations. The industry however has already evolved beyond that status: the edge optic has largely become obsolete since its main proponent sold its patent portfolio after IP litigations, whilst the industry has already transitioned away from Cd based towards Cd-free/less QDs with the latter expected to reach 80% market share in 2018. Note that this transition in material composition was driven largely by legislatures who finally announced a ban (effective Oct 2019) on toxic cadmium.
The transition however still comes at a performance penalty: the alternative InP QDs still suffers from a wider emission band (FWHM) whilst having largely bridged the quantum yield (QY) discrepancy. Today, CdSe already achieves 35nm and <20nm in commercial and laboratory settings, respectively; whereas InP QDs are at c. 40nm commercially but struggle to go to 35nm even in labs.
This gap is hard to rapidly fill because controlling the shape and size monodispersity of InP particles is challenging. QDs have also become more stable. This has already relaxed barrier requirements in film type implementation and is anticipated to continue to do so. This trend therefore leads to simpler and lower cost barrier films. Efforts to improve production either via larger scales or via innovative processes such as low-temperature molecular seedings, continuous reactions in microcapillaries or one-pot synthesis process all seek to lower per Kg production costs.
Increased QD brightness will also result in lower per sqm consumption. All these factors are driving down total QD implementation costs. In turn, this is opening new pricing strategies to display makers. This has led to an interesting dynamic in which some, including many in China, are expanding product range to cover a spectrum of prices, whereas others are struggling to maintain the high-priced ultra-premium aura of QD displays in defiance of new cost realities.
Quantum dots: Advanced material innovators’ dream
InP chemistry is not the only alternative to Cd QDs. There is also the perovskite QDs (PeQDs). These QDs offers emission wavelength (color) control via size and composition. For example, varying the X from iodide to bromide and chloride in organic MPbX or inorganic CsPbX shifts the emission from red to green and blue. PeQDs also exhibit a higher tolerance of disorder, making it easier to achieve narrowband emission. They are however still a young technology with significant drawbacks. The red PeQDs are chemically highly stable, restricting the choice mainly to inorganic green today (CsPbBr3) which gives <20nm FWHM.
As such, their future, at least in the medium term, will likely be hybrid with QDs used in conjunction with red phosphor or non-perovskite QDs. The work on materials does not end with this transition away from Cd QDs. In fact, the opposite: it is just entering an exciting time with many directions of development. Indeed, further material developments will dictate the fate of various QD implementation methods in displays.
Today, film type QDs in displays reign supreme because they keep QDs sufficiently away from sources of heat and light stress. They are however likely only a transiently solution because, as shown in our technology roadmap, improved materials will in time enable color filter QDs, on-chip QDs and finally emissive QD displays. Below, we briefly mention what is needed for each implementation.
We provide details in the report.
Color filter: QDs must be better dispersed in resins or inks and must survive the baking/curing process without too much degradation. The QDs must limit their self-absorption and boost their blue absorbance. The surrounding system must also change around them, adopting in-cell polarizers and potentially a backpass filter. Would it be worth the effort? The prize is improved efficiency, widened color gamut, and expanded viewing angle.
On-chip: QDs must be much more tolerant of light and heat stress. This is critical and the industry is exploring many strategies such as softening the interface in the core-shell-ligand system, new inorganic protective coatings, novel ligands, and so on. Today, QDs tolerate mild conditions found in low power LEDs and some (not all) microLED applications, but going beyond that requires extensive sustained development.
Emissive: QDs are in many ways the ultimate optical material. They give wide color gamut, high efficiency, high contrast, solution processing, and thinness (thus flexibility). They are however at an early stage of development with much scientific work still to be done. The efficiency gap with OLED, particularly for Cd free QDs, must be bridged; the lifetime must be drastically prolonged; the optimal device stack composition and geometry must be established; methods to manufacture high PPI three-color displays must be developed and scaled; and so on.
The potential is therefore great but so are the challenges. As typical, the best will go to those who wait in the display material supply business.
The IDTechEx Research report Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players details the merits of different QD integration approaches, the challenges facing each, and the latest progress. The report also provides our technology roadmap as well as ten-year market forecasts segmented by technology, showing whether and when each technology will rise and fall.
The overall market will grow in the coming decades although costs reductions can cause a plateauing or a decline near the end of our forecast period. More importantly, the technology mix will be significantly transformed over the years as film-type will no longer reign supreme, giving its market share to color filter QDs, on-chip QDs, emissive QDs, and so on.
Finally, the industry will not be limited to displays, potentially moving beyond towards lighting, sensors and photovoltaics. There are many other applications such as lighting, sensors, photovoltaics and so on as shown in our report Quantum Dots 2018-2028: Technologies, Markets, Players.
QDs in lighting
Lighting is an attractive application not least because lighting is the largest application for LEDs. Here, the driver in the general lighting sector is to increase CRI of LED lights without sacrificing efficiency. This can be made possible with the narrow FWHM of QDs. This industry will make further progress as cost fall and, more crucially, as QDs become more stable enabling integration into more types of LEDs.
Prior to that however, companies have proposed film-type QDs to optimize the emission light. However, market response has thus far been lukewarm. In parallel, some seek to deploy QD lights in specialize applications as horticulture in which QDs are used to fine-tune the emission spectra.
QDs in sensors
Sensors are also a promising proposition. Here, the focus is on QDs’ broad absorption characteristics. We can divide the work into two categories: visible and IR/NIR image sensors. In the former, QDs can be cast onto silicon read-out circuits to enable high resolution (small pixel) and highly sensitive images sensors with a global shutter and a large pixel capacitor.
This hybrid QD-Si sensor is made possible because of the high sensitivity of the QD layer (if properly fused) and its ability to separate the photosensitive and processing circuits. In the latter approach (IR/NIR sensor), the right QD chemistry (e.g., PbS) can tune the absorption spectra to be sensitive to NI/IR. The QDs can also be added directly on the Si read-out circuit. As such, there will not need to un-monolithic integration of different semiconductor systems with silicon, limiting resolution.
QDs in photovoltaics
Photovoltaics are another interesting application. Here, the QDs can be complementary, helping extend the absorption range. Furthermore, perovskite QDs may hold potential but they are very immature and suffers from various instability issues (Note perovskite thin film PVs are the fastest improving PV technology ever and have now breached the 27% efficiency mark for champion cells). Novel PV technologies, such as perovskites, also enter a fiercely competitive landscape dominated by China and wafer-based Si PV technology.
In parallel, others are exploring the use of QDs (e.g., CnInSe) to enable luminescent solar concentrator in which the QDs absorb and reemit the light over transparent window surfaces. Other: QDs are of course already used in research particularly for imaging. Many other applications such as security applications are also being proposed.
We expect that as the technology matures new applications will inevitably be established.
About the author:
Dr Khasha Ghaffarzadeh is Research Director at IDTechEx – www.IDTechEx.com