Quantum dots: the ultimate emissive display material?

By guest author Dr Khasha Ghaffarzadeh, Research Director, IDTechEx

Many consider quantum dots (QDs) as the ultimate emissive (electroluminescent) material, one day representing the future of display and one day evolving emissive displays beyond the level that organic LEDs offer today. This is because potentially QD emissive displays offer extremely wide color gamut through their direct narrow band emission, high efficiency, high contrast, solution processing, and thinness. The latter attribute also gives a degree of future proofing as display technology finally transitions towards flexible and foldable screens.

But what is the reality? What is the status of performance and technology readiness? What are the challenges to overcome? And, whether, and when, will it reach the market? In this article, we provide a high-level overview of our assessment.

Our report, Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players, provides a more detailed analysis. It outlines the past and the present of this technology’s performance in terms of efficiency and lifetime; it assesses its time-to-commercialization; it examines the key challenges that remain to be overcome; and it outlines the latest progress made so far worldwide. In addition, it provides our technology roadmap and ten-year market forecasts segmented by application and technology, including emissive QD displays.

The ultimate display

The promise of QD emissive displays is great: printed, thin, high contrast, flexible/foldable, efficiency and extremely wide colour gamut. This explains why many consider it the ultimate display material. The challenges standing in the way of realising these potentials are, however, also great.

The chart below outlines the current status of emissive QD displays. Evidently, there is a performance gap between OLEDs and QDs. The gap is wider when we consider the leading Cd-free QD materials (InP based QDs) which are mandated by legislation. This gap is even wider for other emerging material system including champion in-lab inorganic green perovskite QDs. Consequently, much further effort is required to match the efficiency of existing organic materials to at least reach the point of parity with the incumbent.

Bridging this gap will not be easy. First, organic materials themselves will improve, even if in small increments as progress appears to have approached a plateau, and thus represent a moving goal post. Second, performance is a function of many parameters on both material and device levels. The material parameters include composition, core-shell-ligand system design, and so on. The device level parameters include all the other materials in the stack, the device architecture and geometry, and processing conditions. The optimal conditions are yet to be determined. This is bad and good news: bad because we are still in the early years of the long road towards commercialization; and good because there is still a significant upside potential.

Evolution of EQE of R G B quantum dots with time. Here, this represents the best data for each colour regardless of composition (leading ones including Cd). Note the y-axis is logarithmic. Inset: Evolution of OLED (black circles), green and blue QDs of all composition (green and blue circles) and InP QDs (in X) on a linear axis.

For further details and analysis see Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players

Another major shortcoming today is lifetime. Emissive electrically-excited QDs suffer from short lifetimes. For Cd based reds under low brightness conditions (e.g., 100 Cd/sqm) the lifetime can be around 100k hours. For higher and more realistic brightness conditions (1000 Cd/sqm) the lifetime drops by orders of magnitude (e.g., 1-3k hours). For InP QDs the lifetime is much lower even for low brightness conditions, hovering around the several tens of hours mark. It is even lower for perovskite QDs including the more stable inorganic green PeQDs.

Achieving commercially acceptable lifetimes will not be easy. The exact degradation mechanisms are still being actively debated, but it is certain that extrinsic encapsulation alone will not be sufficient. The solution will thus have to be a function of material development and device engineering. Here, much scientific work remains to be done before work on commercialization can be started in earnest.

Once these performance gaps are narrowed then the industry will face the usual engineering challenges. Today most demonstrators are still monochrome. Therefore, a reliable deposition and patterning technology achieving high yield, high PPI and low cost three-color displays will be needed. Inkjet printing and photopatterning are both being considered. Inkjet printed and photopatterned QDs are being developed already for color-filter and on-chip type QDs. These processes, however, currently degrade performance even for photoluminescent applications. Alternative patterning methods such as dry transfer printing processes are also being explored, and interesting academic results have been demonstrated.

Once the basics of the patterning process have been demonstrated, as viable proof-of-concepts, in labs and on champion results, then work on prototyping and small-scale narrow-sized pilot production will begin. This benign step itself will also not be easy and adds some years to the commercialization timeline of emissive QDs. Only after all these steps are successfully achieved then commercial production will gradually commence, starting from small displays.

The best comes to those who wait?

As stated above, however, the potential of emissive QD is simply too great to give up in the face of remaining challenges. Furthermore, material companies are accustomed to such long development timelines that test one’s patience. In fact, those successful in the display industry remember well that both liquid crystal and OLEDs took decades to succeed, and along the way material developers often came under immense pressure to cut their losses and prioritize short term wins. Yet those who persevered reaped, and continue to reap, great benefits.

Our report, Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players, provides a detailed analysis of emissive QD display technologies. It outlines the past and the present of the its performance in terms of efficiency and lifetime; it assesses its time-to-commercialization and technology readiness; it examines the key challenges that must be overcome along the way towards commercialization; and it outlines the latest progress made worldwide. In addition, it provides our technology roadmap and ten-year market forecasts segmented by application and technology including emissive QD displays.