QD films in displays: major technology and market trends

This article will focus on some of the key frontiers of short-, medium- and long-term change in quantum dot (QD) films. In particular, we will argue that the transition away from Cd-based QDs will soon be complete; we will show how QD material improvements are translating into total cost reductions and how that is reshaping the market’s pricing strategies; and finally we will consider whether film type QDs can continue their supreme reign, or whether relentless innovation soon renders them obsolete.

The story so far

The primary market of quantum dots (QDs) today, beyond research uses, is in displays. Here, the driver is achieving wide colour gamut displays thanks to the ultranarrowband emission of QDs. This attribute is helping position LCDs as a viable competitor against large-sized, premium-priced, high colour quality OLED displays.

Up to now, the QDs’ material characteristics have defined their use case. The QDs have been essentially only integrated into displays as ‘remote’ phosphors. In one implementation, the QDs are loaded into a resin which is then coated onto a film that is then sandwiched by two barrier (encapsulation) layers. This arrangement is called film-type QD or QD enhancement film (QDEF). In the other implementation, the QDs are put into tubes which are placed at the edge of displays. This is generally called edge optic QDs.

In both cases, QDs re-emit the light from the phosphor-converted backlight LEDs and in doing so narrow the emission spectrum, thus achieving wide colour gamut and thus pushing the display industry further towards the Rec2020 standard. In both cases, remote phosphor integration was adopted because QDs could not tolerate harsher heat or light flux stress conditions which a closer proximity to the light source (LED) would have imposed on them.

The edge optic QD implementation is now essentially obsolete. Its main proponent (QD Vision) lost an IP battle against the market leader (Nanosys), and consequently had to sell its patent portfolio at a bargain price to a leading QD user (Samsung). The film-type QD implementation, however, is continuing its growth.

Transition away from Cd-based QDs will soon be complete

Several key trends today characterise the market and technology dynamics of film-type QDs (or QDEFs). Below we will discuss two key trends: transitioning away from Cd-based and reducing implementation cost.

The ban on cadmium (Cd) in Europe has finally been announced for October 2019. Even prior to this announcement, Cd QDs had appeared only as a transitory solution, a technology on borrowed time.

To respond, the industry has been developing InP-based QDs as an alternative to Cd-based ones. In the early days, the performance gap between Cd and InP QD technologies was vast. Now, however, the quantum yield (QY) difference has all but disappeared, while the FWHM gap has also been notably narrowed (note: QY affects efficiency and FWHM affects colour gamut).

The FWHM gap, however, stubbornly persists, with InP QDs today achieving around 40 nm commercially. Pushing it down towards 38 nm commercially (or even 35 nm in the lab) remains a challenge. This is because the emission bandwidth is not just a function of the control of the monodispersity of the QDs during the synthesis, it is also a function of the shape and crystalline structure of the particles.

Despite this continued performance discrepancy, the market transition appears largely complete, with the market share of Cd-based QDs having dropped substantially over time and hovering at around 20% today.

Approaches for reducing implementation costs

As with all display components, the pressure to achieve more for less is relentless and perennial. QDs are no exception. In the case of QD films, it means lowering the cost of the entire solution, including QD materials, substrate, coating, barrier, etc.

In fact, the barrier layers are a significant cost contributor. Their cost, however, can be lowered if barrier performance requirements are relaxed, leading to simpler barrier film structures. This, in turn, can only be achieved if QD materials become more air stable.

Progress continues to be made here, with the barrier requirements appearing to have been relaxed from 1E-4 g/day/m² to 1E-3 g/day/m² and 1E-2 g/day/m², and now even the target of 1E-1 g/day/m² seems feasible in the near future.

The QD material cost should also be reduced. One way is to increase the efficiency and the brightness of the QDs, leading to a lower consumption/m² for a given output. Another method would be to lower the cost of QD production/kg. This, of course, will stem from greater scale and also from innovation in the synthesis procedure.

As to the latter, companies have explored a low-temperature molecular seeding process, a continuous (vs batch) process in heated microcapillary reactors, a one-pot synthesis process for core-shell (or graded core-shell) structures, and so on. In all cases, however, it remains challenging to balance reaction time and temperature against the need to maintain high size and shape monodispersity, to achieve excellent shelling process, to minimise built-in stresses and CTE mismatches, and to obtain full ligand surface coverage (more so when ligands are exchanged away from as-grown ones to promote compatibility with a resin, a photoresist or an ink, since this is often a time-consuming process). There is continued progress all around, but often of an incremental nature.

Pricing strategy on the cusp of major change

With cost reduction comes renewed questions about the pricing strategy of QD-enabled display makers. Originally, display makers positioned QD TVs as ultrapremium. This reflected the need to counter the value proposition of the (then) emerging white OLED TVs and also the actual cost of implementation.

Now that the implementation cost has fallen, and is expected to fall further, new pricing options have become viable. Some makers are seizing this opportunity to create a product range covering a spectrum of prices, whereas others are still struggling to maintain the ultrapremium aura with the help of price signals. This latter position, however, increasingly appears untenable in the long term. This long-term pricing trend will also present a challenge to large-sized WOLED unless that too can reduce its production costs.

Overall, we find that film-type implementation will continue its volume (m² of displays sold) growth in the short and medium terms. But a major question is whether this trend will last, and if so for how long? This is because alternative implementation approaches are being developed.

The pending decline of film type QDs

We have developed a 10-year technology roadmap and market forecasts (segmented by QD integration type), assessing how the QD technology will transition from edge optic (now almost obsolete) and film type towards colour filter type, on-chip type and finally emissive type. These emerging approaches have vastly different technology readiness levels as well as requirements and material- and device-level challenges.

These alternative approaches are not limited (vs film type) by value proposition, but by technical challenges. Therefore, as these challenges are overcome, film type QDs will first begin to lose their exclusive control of the QD display market, and will then enter into a decline phase. Indeed, we find that film type will have to share the market first with colour filter type, then with on-chip type (eg, microLED displays), and finally with emissive type (long-term development but ultimate prize).

Innovate or die

These transitions may open the market to newcomers who either came too late or were barred by IP issues. In any case, they condemn the suppliers and users to continue to invest in R&D so as not to be left behind as these transitions occur. This is exciting because many material improvements are needed: reduce self-absorption, increase air/heat/light stability, improve dispersion and processability in photoresists or inkjets without too much degradation, boost blue absorbance, find optimal device stack and so on.

The IDTechEx report ‘Quantum Dot Materials and Technologies 2018–2028: Trends, Markets, Players’ provides a detailed assessment of all these directions of material improvements and outlines the latest progress. For more, see www.IDTechEx.com/qd.

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