Quantum dots: when will color filter or on-chip QD displays arrive?
Sep 19, 2018
Quantum dots in displays are already a commercial success story. They will grow and witness major technology transitions. In this article, we describe the emerging dynamic between the different QD approaches for displays: edge optic, film-type, color filter type, and on-chip- type. We will examine how, and when, these technologies are likely to commercially rise and fall. We leave the analysis of emissive QD display to a follow-up article.
This article is drawn from the IDTechEx Research report, Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players. This report provides a detailed analysis of QD technology and markets. It provides a ten-year technology roadmap, showing how various QD implementation approaches will rise and fall. It includes ten-year market forecasts, segmented by QD technology and application, showing how the market will grow and how the technology mix will be significantly transformed with time. In terms of displays, it considers edge type, film type, color filter type, on-chip type and emissive types of displays. This report also provides a thorough technical analysis, outlining many technical (material and device level) and market challenges that must be overcome to commercialize various QD technologies. Finally, it provides detailed overviews of the key industry players.
Quantum dots in displays: the story so far
The edge optic and film type integration approaches, both using QDs essentially as remote phosphors or downconverters, came first, pushed by the pioneering QD companies. The edge optic is now however essentially obsolete despite its early adoption. This is because its main proponent lost an IP litigation and sold its patent portfolio at a bargain price. It was also because the edge optic type had several distinct technology disadvantages. In particular, it required the integration of a space-consuming QD filled tube on the display edge. This went against the overriding trend in the industry which is to narrow the bezel.
The film type integration (e.g., QDEF) is growing. More companies, including multiple in China, have adopted it. Here, a transition from Cd or Cd free/less is now in full swing and will soon be complete. Here also the push to drive down total system cost is making progress. As described before, this push entails improving QDs' air stability to relax barrier film performance requirements, boosting QD efficiency and brightness to minimize consumption per sqm, and scaling up and innovating in the synthesis process. The fall in the cost of implementation is reshaping the pricing strategy of QD displays.
For now, film type QDs reign supreme in the QD display market, but for how long will this continue? This is a pertinent question because alternative, and better, integration approaches are emerging to unseat it. Bringing these new approaches to the market represent an enormous material innovation opportunity and may open the market door to new players who were previously denied access because they are either later and/or lacked patents.
Quantum dot color: is it still far away?
Traditional color filters (CF) have two major drawbacks: (1) each color only transmits a third of the light and (2) the wide and overlapping spectrums narrow the color gamut. In contrast, QDs promise to overcome both shortcomings. Here, QD formulations replace CF resins, essentially acting as downconverters to create color via their ultra-narrowband (narrow FWHM) re-emission. As such, this technology promises high efficiency and extremely wide color gamut with wide viewing angles.
Achieving QDCF (Quantum dot color filter) however is not straight forward. First, the QDs must be deposited and patterned as color filters. This can be achieved by photopatterning or inkjet printing. In the former case, the QDs must be dispersed at high loading levels in the photoresist and then must survive the photopatterning process which involves a deposition (e.g., spin coating) step, an exposure step, several high temperature baking/curing steps, and a development and cleaning step.
Making this possible requires extensive material engineering (e.g., ligand exchange, minimising temperature-induced stress through softening the interface in the core-shell-ligand structure, additional protective coatings, etc). This is an area of continued progress although lab results are increasingly promising.
In the inkjet printing case, air-stable QDs must be designed and synthesised, and formulated into an inkjet printable ink with the right solvent, viscosity level, surface adhesion, shelf life without agglomeration, etc. Furthermore, the QDs must also survive the curing step (thermal more likely than UV curing). Here too there is continued progress both in research and semi-commercial capacity (see report for an outline of the latest development).
But even achieving all this is not the end of the long road to market. The QDs must then have high blue absorbance to achieve high color purity (no leakage) and must be designed, for example via control of shell size and/or composition, to minimize self-absorption. Furthermore, an in-cell polarizer will be required since QDs (unlike the much less mature quantum rods) de-polarize the light, whilst a backpass filter will also be need so as not to waste the inward (into the display itself) re-emitted light.
These challenges are however now being solved. Indeed, the IDTechEx quantum dot technology roadmap, disclosed in Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players, assesses that QDCF will be the first to market after the film type QDs (QDEF) in the display market.
A schematic depicting the future trends of quantum dots in the display industry. For details see the IDTechEx report Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players. Source: IDTechEx Research (www.IDTechEx.com/qd)
On-chip QDs: ideal end game?
Another approach is on-chip integration. Here, QDs directly replace phosphors in pc-LED (phosphor converted LEDs) supplying the backlight in displays. This is the ideal scenario because the emitted backlight itself will become narrow band, because the material consumption will be lowered, and because QDs become a drop-in replacement for existing phosphors, eliminating the need for additional films (e.g., QDEF). This approach might also enable three-color microLED displays as well as efficient high-CRI LED lighting applications.
Achieving on-chip type is also not easy. This is because the QDs must withstand high heat and light flux stress without thermal or light quenching and other degradations. This instability criteria is in addition to those that require high quantum yield (QY), narrow FWHM, Cd free composition, proper dispersibility in silicones or other LED package resins, and also strong blue absorbance (at the LED wavelength).
Many approaches are being followed to enable this with already very promising results for mild heat and light stress conditions. In one general strategy, material makers seek to eliminate abrupt interfaces in the core-shell-ligand system to minimise thermally induced strains (CTE mismatch). This itself requires innovation in material design and also material synthesis (e.g., graded composition). In another general strategy, a variety of ligand systems are being explored to find the best for improved thermal and humidity stability. And, of course, different material compositions including an additional inorganic protective coating as well as growth conditions are being explored too to eliminate sources of instability. See the report for the latest results.
In general, today the ability to withstand mild heat, humidity and lighting stress conditions are demonstrated. This potentially open the market for low-power LED lights and some (not all) microLED display applications. This technology however requires still further improvement to be commercially ready for a wider application set. Nonetheless, our ten-year technology roadmap and market forecast, disclosed in Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players, suggests on-chip QD LED will also enter the market in medium to long term.
As stated above, we focus here was only on PL type of QD implementation in display, leaving the discussion of emissive QD displays to the follow up article. It is hoped that this article has demonstrated that the QD display market will grow and its technology mix will transform. This will be driven largely by material innovation, helping overcome the technical challenges that today prevent the immediate commercial update of better QD integration methods such as color-filter and on-chip type.
For a more detailed analysis please refer to Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players. This report provides a detailed analysis of QD technology, covering all the latest progress and innovation in enabling various applications. It covers different material technologies including perovskite QDs. The report offers our technology roadmap as well as ten-year market forecasts segmented by technology and applications. This report also provides detailed and up-to-date overviews of players in the QD value chain.